Pressure effects on crystal and electronic structures of materials

In this dissertation, I present studies on electronic and structural properties under high pressures of three materials, which belong to different classes of solids at ambient conditions: Methyl Ammonium Lead Bromide (MAPbBr3), which is an Organic-Inorganic hybrid Perovskite (OIP) and a semiconductor, Lithium, which is a simple metal, and Benz[a]anthracene (BaA), which is a hydrocarbon and a wide bandgap insulator. It is organized as follows: Techniques and a variety of experimental methods used for the studies are described in the experimental techniques chapter and investigations of each material are described in three separate chapters. For MAPbBr3, experiments were performed using synchrotron X-ray diffraction, photoluminescence spectroscopy and photoconductivity under high pressure at room temperature under different pressure transmitting media (PTM). It was demonstrated that the phase transformations of this material under pressure is highly dependent on the PTM and uniaxial stress. We also find that the trend and values of different opto-electronic properties of MAPbBr3 strongly depend on the crystal structure. For Lithium, synchrotron powder X-ray diffraction measurements were performed under cryogenic conditions to explore the isotope effect in structural phase transitions. The phase diagram of two stable isotopes of lithium, 6Li and 7Li, were compared for pressures below 10 GPa. It was clarified that only the bcc structure undergoes a martensitic transition to 9R and demonstrated that fcc phase is the actual iv ground state. Most significantly, the studies showed that the two isotopes of lithium have differences in their structural boundaries, which is an indication of large quantum effects present in lithium lattice. Some preliminary transport measurements on potassium were presented with a search for possible superconductivity under pressure. The studies on BaA include synchrotron X-ray diffraction measurements, fluorescence spectroscopy, absorption, and photoconductivity measurements. Results show that BaA exhibits remarkable piezochromism without any structural phase transition prior to polymerization. Above ~15 GPa, an irreversible chemical reaction takes place and polymeric products of BaA become amorphous. Studies on BaA provide new insights into herringbone-type PAHs and their tunable electronic properties under pressure. High pressure studies presented in this dissertation have opened new and important effects on the structure of materials studied.

PRESSURE EFFECTS ON CRYSTAL AND ELECTRONIC STRUCTURES OF MATERIALS by Rong Zhang A dissertation submitted to the faculty of The University of Utah in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Physics Department of Physics and Astronomy The University of Utah August 2018 Copyright © Rong Zhang 2018 All Rights Reserved The University of Utah Graduate School STATEMENT OF DISSERTATION APPROVAL Rong Zhang The dissertation of has been approved by the following supervisory committee members: Shanti Deemyad , Chair 05/09/2018 Sarah Li , Member 05/09/2018 Orest Symko , Member 05/09/2018 Michael Vershinin , Member 05/09/2018 Luisa Whittaker-Brooks , Member 05/09/2018 and by the Department/College/School of Peter Trapa Date Approved Date Approved Date Approved Date Approved Date Approved , Chair/Dean of Physics and Astronomy and by David B. Kieda, Dean of The Graduate School. ABSTRACT In this dissertation, I present studies on electronic and structural properties under high pressures of three materials, which belong to different classes of solids at ambient conditions: Methyl Ammonium Lead Bromide (MAPbBr3), which is an OrganicInorganic hybrid Perovskite (OIP) and a semiconductor, Lithium, which is a simple metal, and Benz[a]anthracene (BaA), which is a hydrocarbon and a wide bandgap insulator. It is organized as follows: Techniques and a variety of experimental methods used for the studies are described in the experimental techniques chapter and investigations of each material are described in three separate chapters. For MAPbBr3, experiments were performed using synchrotron X-ray diffraction, photoluminescence spectroscopy and photoconductivity under high pressure at room temperature under different pressure transmitting media (PTM). It was demonstrated that the phase transformations of this material under pressure is highly dependent on the PTM and uniaxial stress. We also find that the trend and values of different opto-electronic properties of MAPbBr3 strongly depend on the crystal structure. For Lithium, synchrotron powder X-ray diffraction measurements were performed under cryogenic conditions to explore the isotope effect in structural phase transitions. The phase diagram of two stable isotopes of lithium, 6Li and 7Li, were compared for pressures below 10 GPa. It was clarified that only the bcc structure undergoes a martensitic transition to 9R and demonstrated that fcc phase is the actual ground state. Most significantly, the studies showed that the two isotopes of lithium have differences in their structural boundaries, which is an indication of large quantum effects present in lithium lattice. Some preliminary transport measurements on potassium were presented with a search for possible superconductivity under pressure. The studies on BaA include synchrotron X-ray diffraction measurements, fluorescence spectroscopy, absorption, and photoconductivity measurements. Results show that BaA exhibits remarkable piezochromism without any structural phase transition prior to polymerization. Above ~15 GPa, an irreversible chemical reaction takes place and polymeric products of BaA become amorphous. Studies on BaA provide new insights into herringbone-type PAHs and their tunable electronic properties under pressure. High pressure studies presented in this dissertation have opened new and important effects on the structure of materials studied. iv To my family and friends TABLE OF CONTENTS ABSTRACT ....................................................................................................................... iii LIST OF TABLES ........................................................................................................... viii LIST OF FIGURES ........................................................................................................... ix ACKNOWLEDGMENTS ............................................................................................... xvi Chapters 1. INTRODUCTION .......................................................................................................... 1 1.1 Electronic and Crystal Structures of Matter as a Function of Density ................ 2 1.2 Tuning Density Using High Pressure Techniques ............................................... 7 1.3 Reference ........................................................................................................... 10 2. EXPERIMENTAL TECHNIQUES .............................................................................. 14 2.1 Diamond Anvil Cell ........................................................................................... 14 2.1.1 Diamonds ............................................................................................... 15 2.1.2 Mounting and Alignment of the Diamonds ........................................... 17 2.1.3 Gasket .................................................................................................... 19 2.1.4 Pressure Transmitting Medium .............................................................. 22 2.1.5 Pressure Measurements .......................................................................... 24 2.2 Synchrotron X-ray Diffraction........................................................................... 27 2.2.1 Scattering Principles .............................................................................. 28 2.2.2 X-ray Diffraction Measurement ............................................................. 29 2.2.2.1 Single Crystal X-ray Diffraction................................................ 31 2.2.2.2 Powder X-ray Diffraction .......................................................... 32 2.2.3 XRD Measurements in DACs ................................................................ 33 2.2.3.1 X-ray Axis Access to the Sample in a DAC .............................. 33 2.2.3.2 Low Temperature Studies in a DAC in Beamlines.................... 35 2.3 Absorption, Photoluminescence and Photoconductivity Measurements in a DAC ......................................................................................................................... 37 2.3.1 Transport Measurements in a DAC ....................................................... 38 2.3.2 New Design DAC for Electrical Measurements .................................... 42 2.4 Reference ........................................................................................................... 66 3. STUDIES OF MAPbBr3 UNDER PRESSURE ........................................................... 73 3.1 XRD and Photoluminescence Measurements .................................................... 77 3.2 Photoconductivity Measurements ...................................................................... 84 3.3 Discussion .......................................................................................................... 87 3.4 Reference ......................................................................................................... 112 4. BENZ[A]ANTHRACENE: PRESSURE RESPONSE............................................... 116 4.1 Piezochromism of Benz[a]anthracene ............................................................. 118 4.2 Fluorescence and XRD Measurements ............................................................ 120 4.3 Absorption and Photoconductivity Measurement............................................ 124 4.4 Discussion ........................................................................................................ 125 4.5 Reference ......................................................................................................... 139 5. LITHIUM: PRESSURE AND TEMPERATURE RESPONSE ................................. 143 5.1 6Li and 7Li Under Pressure at Low Temperature ............................................. 147 5.2 Conclusion ....................................................................................................... 152 5.3 Potassium Under Pressure at Low Temperature .............................................. 153 5.4 Reference ......................................................................................................... 167 6. DISCUSSION ............................................................................................................. 171 6.1 Phase Transitions Under Pressure ................................................................... 172 vii LIST OF TABLES Tables 2.1. Relation between the adjacent planes distance with the Miller indices and lattice parameters for different crystal systems. .......................................................................... 65 3.1. Lattice parameters of MAPbBr3 for all measured pressure points compressed in helium. ............................................................................................................................ 107 3.2. Selected crystallographic and experimental parameters of MAPbBr3 compressed in helium. ............................................................................................................................ 108 3.3. Lattice parameters of MAPbBr3 for selected measured pressure points compressed in argon. .............................................................................................................................. 109 3.4. Selected crystallographic and experimental parameters of MAPbBr3 compressed in argon. .............................................................................................................................. 110 LIST OF FIGURES Figures 1.1. Range of pressures in universe. ................................................................................... 9 2.1. A chamber for pressurization of the sample formed by a diamond anvil and a gasket (right). Ruby serves as a pressure gauge. Left image is a Merrill–Bassett DAC from https://journals.iucr.org/j/issues/2008/02/00/aj5098/aj5098fig2.html. ............................. 44 2.2. Image of Boehler-Almax conical support seat/anvil and conventional support seat/anvil from: https://www.almax-easylab.com/TypeIIacBoehlerAlmaxdesign.aspx. .. 45 2.3. Image of aligned diamonds and a single beveled anvil. ............................................ 46 2.4. Oxford Plasmalab 80 as a PECVD tool and Cambridge Fiji F200 from https://coral.nanofab.utah.edu/lab/equipment. .................................................................. 47 2.5. Images of a symmetric DAC and a plate DAC. ......................................................... 48 2.6. Image of the EDM setup in Prof. Shanti Deemyad’s Lab. ........................................ 49 2.7. Electron energy levels of ruby (left) and basic setup for ruby pressure measurement in a DAC (right). DAC image from http://iopscience.iop.org/article/10.1088/13616633/80/1/016101/pdf. ...................................................................................................... 50 2.8. Image plates of 6Li together with pressure monometers at different pressure and temperatures. The diffraction lines of NaCl (bcc) (and solid helium at low temperature (fcc)) together with ruby fluorescence used to determine the pressure in the vicinity of the sample. Unlabeled patterns are cryostat and DAC background that has no pressure dependence. Diamond reflections are masked with yellow dots. ..................................... 51 2.9. Schematic image of electrons scattering by two lattice points and X-ray crystallogaphy in a DAC. .................................................................................................. 52 2.10. Schematic drawing of synchrotron facility. A is the electron gun. B is the electron accelerator. C is particle accumulator ring. D is the booster synchrotron. E is the storage ring. E1 is the insertion device, and E2 is the bending magnet. ....................................... 53 2.11. A schematic image of high pressure synchrotron for Angle-dispersive X-ray diffraction. The beam stop is used to avoid damge of the CCD dector by X-ray beams. 54 2.12. Single crystal scheme and Ewals construction. ....................................................... 55 2.13. Integrated diffraction image plate of single crystalline sample Mercury(II) bromide at 2.5 GPa at room temperature using helium as pressure medium (𝜆=0.434 Å, 2𝜃 is from 2 to 22 degree). Few large spots at larger angles indicate the diamond reflections. XRD image from Dr. Weizhao Cai’s sample. ............................................................................ 56 2.14. Cryostat system with low temperature ruby system in Prof. Deemyad’s lab. ......... 57 2.15. Schematic presentation of gas membrane pressurization method. (a) Cap-can assembly for symmetric DAC and membrane. (b) Dual double-membrane setup for compression-decompression experiments at room temperature. A–DAC decompression attachment, B–DAC, and C–double-diaphragm cap-can. ................................................. 58 2.16. The fundamental absorption processes (an direct and indirect gap) in semiconductors. ................................................................................................................ 59 2.17. Photoluminescence of different thicknesses of molybdenum diselenide. ............... 60 2.18. Kelvin probe or 4-probe arrangement for resistivity meaurements. Schematics dragram (left) and picture of quasi-probe arrangment (right) built on a gasket with 4 platinum probes (5 micron thick). The probes will be pressed using the opposite diamond to sit on the gasket hole on the background and touch the sample. 1, 2, 3, and 4 electrodes are connected to current or voltage source with copper wires. ......................................... 61 2.19. The schematic layout of the optical system for laser micro-machining system for diamond anvil cell. ............................................................................................................ 62 2.20. Patterned dimoand by Almax easylab...................................................................... 63 2.21. DAC assemly with metallic boron doped dimaond electrode for electrical transport measurement. .................................................................................................................... 64 3.1. Ideal perovskite structure ABX3 at ambient condition. ............................................. 89 3.2. Structures of various MAPbBr3 phases observed under high pressure. (a) Phase I, cubic phase having space group Pm3̅m viewed approximately along the [001] direction. (b) Phase II, cubic structure having space group Im3̅ viewed approximately along the [001] direction. (c) Phase III, which is observed here when He was used as the PTM and is isostructural to phase II, viewed approximately along the [001] direction. (d) Projection of phase IV orthorhombic structure with space group Pnma along the [010] direction. The Glazer symbols are added for the corresponding phases. Color code: green Pb atoms, orange Br atoms. For clarity, the MA molecules are not shown. ..................................... 90 3.3. Synthesis process of MAPbBr3 sample...................................................................... 91 x 3.4. Lattice parameters of MAPbBr3 compressed in He obtained from single crystal diffraction data. (a) Formula-unit volume (V/Z) as a function of pressure is shown in the bottom panel. The lines through the data points are second-order Birch–Murnaghan equation-of-states fit to the volume (V/Z) data in phases I-III. The unit-cell dimensions of phases I, II, and IV obtained from Ar, and phase I at ambient pressure from the literature are added for comparison. The insets show a prominent piezochromism behavior of a single crystal in a DAC chamber at 0.6 and 3.0 GPa compressed in He. (b) Evolution of Pb–Br coordination bond length and Pb–Br–Pb angle in MAPbBr3 as a function of pressure. The insets show the dihedral angle of Pb–Br–Pb relative to the ab plane and a distortion of the PbBr6 octhedra under pressure during the phase transition. Vertical dashed lines in both (a) and (b) indicate two phase transitions at ~0.85 and ~2.7 GPa of MAPbBr3 pressurized in He.............................................................................................. 92 3.5. Integrated XRD patterns of MAPbBr3 compressed in helium (He) up to 4.8 GPa. Phase I ((Pm3̅m), phase II, and phase III (Im3̅). .............................................................. 93 3.6. Le Bail fit of the X-ray data of MAPbBr3 compressed in (a) Ar at 1.6 GPa, (b) He at 1.6 GPa, and (c) without PTM at room temperature. The black circles are the measured scattering intensity, and the red solid line represents the fit to the data. The vertical bars indicate Bragg reflection positions of phase II and IV together with difference profiles (blue lines) shown at the bottom. ...................................................................................... 94 3.7. Integrated XRD patterns of MAPbBr3 compressed in argon (Ar) up to 11.9 GPa and released to 3.6 GPa measured at room temperature (λ = 0.4066 Å). (a) Le Bail fit of Xray data at 1.6 and 2.8 GPa. The black circles are the measured scattering intensity, and the red solid line represents the fit to the data. The vertical bars indicate Bragg reflection positions of the phases Im3̅ and Pnma together with difference profiles (blue lines) shown at the bottom. The apparent asymmetry in the peaks in 7-12 degrees at 0.8 GPa resulted from presence of large diffusive scattering from liquid pressure medium in this region. (b) Wide angle integrated XRD pattern of MAPbBr3 compressed in argon at room temperature (λ = 0.434 Å). ................................................................................................ 95 3.8. X-ray powder diffraction data withou PTM. (a) Diffraction images and (b) integrated XRD patterns of MAPbBr3 without PTM up to 4.0 GPa. The asterisks indicate the reflections from the gasket. (c) Data refinements of MAPbBr3 sample at 0, 1.1 and 1.6 GPa for phase I, II, and IV, respectively. The diamond reflections are close to the edge of diffraction images in (a). ................................................................................................... 96 3.9. Formula-unit volume (V/Z) of phase I (P𝑚3̅𝑚), II (𝐼𝑚3̅), III (𝐼𝑚3̅), and IV (Pnma) of MAPbBr3 as a function of pressure measured by single crystal X-ray diffraction (SXRD) and powder X-ray diffraction (PXRD) using He, Ar, and without PTM. The transition pressures are marked with dashed lines. ........................................................... 97 3.10. Selected XRD patterns of MAPbBr3 compressed in helium (He) at RT (λ = 0.4066 Å). The insets show the color changes of the crystal sample under compression. ........... 98 xi 3.11. Proposed phase boundaries of MAPbBr3 compressed in various PTM using λ= 488 nm excitation laser: (a) in argon (Ar), (b) in He, and (c) without a PTM at room temperature. The black squares indicate the excitonic transition energy obtained from the PL band (right panels). The insets in the left panels of (a) and (b) show the optical macrographs of a MAPbBr3 single crystal compressed in Ar and helium, together with the polycrystalline sample without PTM in (c)................................................................. 99 3.12. Integrated XRD patterns of MAPbBr3 compressed in nitrogen (N2) up to 6 GPa and released to 3.8 GPa measured at room temperature (λ = 0.4066 Å), and Le Bail fit of Xray data at 1.2 and 2.7 GPa. The black circles are the measured scattering intensity, and the red solid line represents the fit to the data. The vertical bars indicate Bragg reflection positions of the phases Im3̅.and Pnma together with difference profiles (blue lines) shown at the bottom. ....................................................................................................... 100 3.13. Proposed phase boundaries of MAPbBr3 compressed in nitrogen PTM using λ= 488 nm excitation laser at room temperature. The black squares indicate the excitonic transition energy obtained from the PL band (right panels). The insets in the left panels show the optical macrographs of a MAPbBr3 crystal compressed in nitrogen............... 101 3.14. Photoconductivity measurements. (a) Photocurrent of MAPbBr3 single crystal in mineral oil as a function of voltage under pressure. (b) Evolution of PL energy as a function pressure (excitation wavelength: 532 nm). ....................................................... 102 3.15. The band gap (eV) vs. pressure (GPa) for the perfect symmetry Pnma structure at 0 K...................................................................................................................................... 103 3.16. The minimum and average Pb-Br distance (Å) vs. pressure (GPa) for the perfect symmetry Pnma structure at 0 K. ................................................................................... 104 3.17. The band gap (eV) vs. pressure (GPa) for the perfect symmetry Pnma structure at 300 K............................................................................................................................... 105 3.18. Phase transformations in different pressure transmitting media. Phase I (P𝑚3̅𝑚), phase II ((𝐼𝑚3̅), phase III ((𝐼𝑚3̅), and phase IV (Pnma). .............................................. 106 4.1. Structure of BaA. (a) Molecular structure of BaA. (b) Herringbone-type crystal structure of BaA viewed along the [001] direction. ........................................................ 127 4.2. Optical macrographs of a BaA single crystal compressed in silicone oil to 14.2 GPa and released to 0.2 GPa. Another single crystal was compressed up to 40.5 GPa and decompressed to 0.1 GPa. Few ruby chips for pressure calibration lie on the top side of the chamber for (a) and (b). ............................................................................................ 128 4.3. Fluorescence measurements. (a) Microphotographs of a BaA polycrystalline sample upon compression to 14.3 GPa at room temperature. (b) Fluorescence spectra of polycrystalline BaA measured at different pressures. The 0-0 band was partially cut off xii by the Notch filter below 0.5 GPa. The excitation laser wavelength is 488 nm. (c) Deconvolution of the fluorescence spectra at 0.5 and 8.4 GPa. (d) Vibronic bands shift as a function of pressure. Decompression data at 0.2 GPa are shown by open symbols. ... 129 4.4. Raman spectrum of recovered BaA sample from 39.5 GPa at room temperature. .. 130 4.5. XRD patterns of polycrystalline BaA with liquid nitrogen as PTM up to 14.4 GPa measured at room temperature (λ = 0.4066 Å). (a) Asterisks and squares indicate the reflections from solid nitrogen (hexagonal phase) and gasket. (b) Le Bail fit of X-ray data at 1.4 GPa. The black circles are the measured scattering intensity, and the red solid line represents the fit to the data. The vertical bars indicate Bragg reflection positions of the phase P21 together with difference profiles (blue lines) shown at the bottom. The fitted R values are Rp = 0.26% and Rwp = 0.59%. ........................................................................ 131 4.6. Integrated XRD patterns of BaA upon compression up to 38.8 GPa (black) and released to 6.7 GPa (orange) measured at room temperature (λ = 0.4966 Å). The diffraction peaks marked with triangles and circles indicate peaks from NaCl and Re gasket, respectively. ........................................................................................................ 132 4.7. Changes of 2θ angles of (001) peak as a function of pressure in BaA. The inset indicates the d spacing of (001) peak as a function of pressure. The cycles indicate reflections from newly formed polymers. The decompression data are illustrated as unfilled symbols. ............................................................................................................. 133 4.8. Changes of herringbone angle φ as a function of pressure. The inset shows the definition of angle φ. ....................................................................................................... 134 4.9. Lattice parameters of BaA. (a) Evolution of lattice parameters of BaA as a function of pressure. (b) The molecular volume (V/Z) as a function of pressure. The solid line is the third-order Birch–Murnaghan equation of state fitted to the volume data. Experimental and calculated data are indicated as circles and triangles, respectively. .. 135 4.10. Calculated pressure-dependent enthalpies of BaA for molecular phase P21 and polymer I from ambient pressure to 300 GPa. The polymer I becomes more thermodynamically stable than the molecular phase at 30 GPa while the kinetic barrier for this transition vanishes at 117 GPa. .......................................................................... 136 4.11. Absorption measurements. (a) Pressure-dependent absorption spectra measured from BaA polycrystalline sample. (b) Band gap energy changes as a function of pressure. Three different experimental runs and DFT data are shown by solid and open symbols, respectively. The inset enhances the calculated band gap around the polymerization pressure. .......................................................................................................................... 137 4.12. Photocurrent as a function of voltage under high pressure in BaA at room temperature. The inset shows the photocurrent change as a function of pressure at 100V................................................................................................................................ 138 xiii 5.1. Argon glovebox system in Prof. Shanti Deemyad's lab. Microscope is used for viewing the sample while loading................................................................................... 156 5.2. Observed stable and metastable crystal structures of 6Li and 7Li measured along the identified P-T paths. (A) Isobaric cooling paths are connected by gray lines as guides to eye. Data points we collected during isothermal compression or isobaric warming are labeled in numerical order. We used mineral oil (crossed symbols) or He (dotted and solid symbols) as pressure transmitting media. Blue dotted lines show the onset of the bcc to close-packed transitions on cooling. (B) Isobaric results for 7Li. Open symbols are data from previous studies measured either using mineral oil or no pressure medium during isothermal compression and isobaric cooling. Points 3 and 7 are very close in P and T but were approached via different thermal paths — the resulting structures are 9R + bcc vs. fcc + bcc, respectively. (C) Experimental paths for 6Li in P-T space to examine the possibility of a reverse transformation from fcc→9R during decompression. Dotted lines are the transition lines from (A) and (B). During decompression, we observed the pure fcc structure deep in what was previously identified as the 9R stability region. (D) Experimental paths for 7Li in P-T space with the same observation of the fcc structure in the 9R stability region. Points 12–14 show the martensitic transition of 7Li during isothermal compression, followed by a transition to fcc. ............................................... 157 5.3. Synchrotron X-ray diffraction patterns of 6Li at variable pressures and temperatures. The angle-dispersive diffraction measurements were performed using a wavelength of 0.4066 Å. (A) Selected diffraction patterns of 6Li from three different cooling paths. (Points 1→4, 8→10 and 11→13 in Figure 5.3). The reflections from bcc (red) and martensite (blue) phases are labelled by their hkl indices, using the 9R structure for the martensite. Not all 9R peaks are visible because the sample recrystallized to a highly textured quasi-single-crystal. (B) Diffraction patterns of 6Li during cooling to the base temperature 17 K and isothermal decompression to 0.5 GPa (points 18→22 in Figure 5.3). Only pure the fcc phase (green) was observed (Fig. 5.2). For clarity, the Compton scattering of the diamonds and reflections from the cryostat window have been removed in both panels A and B. ................................................................................................... 158 5.4. Various thermal paths and the structures of the 6Li sample using helium as pressure transmitting medium. Numbers and arrows are guides to eye for following the thermal history of the sample. Open symbols are used for data acquired during warming and decompression................................................................................................................. 159 5.5. Image plates of 6Li together with pressure monometers at different pressure and temperatures. The diffraction lines of NaCl (bcc) (and solid helium at low temperature (fcc)) together with ruby fluorescence used to determine the pressure. Panel F shows lithium in mixed 9R+bcc phase where diffraction lines of lithium are clearly smeared and split showing highly texture features. Unlabeled patterns are cryostat and DAC background that has no pressure dependence. Diamond reflections are masked. .......... 160 5.6. Synchrotron X-ray diffraction patterns of 7Li during cooling under nearly isobaric conditions. ....................................................................................................................... 161 xiv 5.7. Various thermal paths and the structures of the 6Li sample using mineral oil as the pressure transmitting medium. Numbers and arrows are guides to eye for following the thermal history of the sample.......................................................................................... 162 5.8. Left pictutre shows a micrograph of the helium loaded gasket with piece of lithium and ruby surrounded by helium. On the right panel showing 2-D X-ray image of the lithium isotope samples loaded inside the twin chamber design gasket (red spots are samples). ......................................................................................................................... 163 5.9. Phase transitions in lithium as function of pressure and temperature. Experimental observations of bcc, fcc, and martensitic (9R and disordered) polytypes of 7Li on cooling and warming at zero pressure. ........................................................................................ 164 5.10. Resistance of potassium by varying temperature under different pressure. .......... 165 5.11. Image of potassium sample under 18.4 GPa at room temperature. ....................... 166 xv ACKNOWLEDGMENTS I would first like to thank my advisor, Prof. Shanti Deemyad, of the Department of Physics and Astronomy at the University of Utah for her valuable suggestions during all my experiments, constant and helpful guidance, and support throughout all the years of my PhD career. I am grateful for the opportunity of working in her group and learning so many techniques. Those years are invaluable treasures for me; and they are wonderful memories in my life. I would also like to thank our collaborators, Prof. Eva Zurek’ group from State University of New York at Buffalo, Prof. Graeme J. Ackland’s group from University of Edinburgh, and also Prof. Yansun Yao’s group from University of Saskatchewan for their support of our experiments by their calculations and valuable discussions. Special thanks to HPCAT (Sector 16) and GSECARS (Sector 13), Advanced Photon Source (APS), Argonne National Laboratory for providing synchrotron facilities and technical help during all the measurements. Research at the University of Utah was supported by NSF Division of Materials Research award 1351986 and grant 1121252. I would like to thank our group members Dr. Weizhao Cai, Dr. Ella Olejnik, and Dr. Mihindra Dunuwille for their friendly discussions and suggestions. I also want to acknowledge Prof. Z. Valy Vardeny’s group for providing MAPbBr3 materials for measurements. I would like to thank Dr. Eran Sterer, who worked in our lab during his sabbatical. He demonstrated to me how to work like a scientist; he kindly helped me in building a new laser system in our lab. I would like to thank all my committee members: Prof. Orest Symko, Prof. Sarah Li, Prof. Michael Vershinin, and Prof. Luisa Whittaker-Brooks who gave me a lot of advising and guide for finishing my PhD program. Last but not least, I want to thank my family and friends in China and in USA for their endless support and love; they gave me the courage to accomplish my degree. xvii CHAPTER 1 INTRODUCTION Pressure is a fundamental thermodynamic intensive variable. It is defined as ∂F P = − (∂V) 𝑇 (1.1) where F is the Helmholtz free energy; V is the volume; and T is the temperature. Pressure describes a force distribution normal to a surface; in other words, pressure is the average of the normal stresses for a given volume. The hydrostatic pressure is one third of the trace of the stress tensor. The stress tensor includes both normal and shear terms. Therefore, pressure is an example of normal stress. Figure 1.1 shows the range of well-known pressure variations in the universe. Sea level atmospheric pressure is about 1 atm. Pressure increases with depth toward the center. Center of Earth is about 360 GPa (3.6 million atm). Center of Jupiter is about 45 million atm. Center of the Sun is about 200 billion atm. Pressure can change the interatomic distance in a sample, tune the density of materials, cause structural and electronic phase transitions, and affect other property changes that follow from that. Pressure is a cleaner parameter compared to other variables, such as temperature or chemical changes, since it only alters the density of the materials. Therefore, pressure is a fundamental variable in experimental physics and it can provide information about materials and help us understand materials. In the 2 laboratory there are two approaches to generate high pressure: dynamic pressure generation, which is based on shock wave compression generated by gas guns, pulsed laser and explosives, and static pressure generation methods, which use a variety of pressure vessels and presses. The highest static pressures are generated in a device known as the diamond anvil cell (DAC), which is the tool used for the experiments described in this dissertation. With the technical development of new designs of diamond anvil cell and new generation of synchrotron radiation, more and more high pressure studies have been performed in the last decade. One of the most striking effects of pressure is the changes of the electronic structure or identity of materials. These changes are often accompanied by structural phase transitions. In this dissertation, I will describe experimental studies on three materials that at ambient conditions belong to three different electronic categories, namely semiconductors (CH3NH3PbBr3), insulators (Benz[a]anthracene), and metallic groups (Li and K). They show that density changes allow us to change the fundamental interactions in the materials as reflected in bulk property measurements. Using several complementary probes, the study of the structural, optical, and electronic properties of these materials as function of density will be presented. 1.1 Electronic and Crystal Structures of Matter as a Function of Density Pressure is one of the thermodynamic parameters that control the equilibrium and rate of transformation of matter. Under compression, the volume of most materials decrease; however, several inorganic materials show some unusual characteristics: 3 negative linear or area compressibility (the material is compressed in one direction while expanded in another direction at a constant temperature) such as LaNbO4 1, and also in some metal-organic materials such as silver(I) 2-methylimidazolate [Ag(mim)].2 The severe compression and increasing overlap of the electron clouds will raise the kinetic energy of the electrons and bring the system into a higher free energy. In order for the system to recover to a free energy minimum state, it will undergo a number of structural and electronic changes including structural phase transformation,3 polymerization,4-6 and amorphization.7-8 Therefore, pressure can tune and alter chemical, structural, and electronic properties such as the nature of chemical bonding and lead to electron delocalization phenomena. High pressure studies can contribute to understanding the phase diagram of materials at microscopic level.9 Mechanisms of structural phase transitions are usually described as reconstructive (diffusion) or displacive,10 and, from thermodynamic perspective, structural phase transitions can be categorized as first-order or second-order phase transitions. A reconstructive phase transition is a homogenous transformation where atom diffusion is the main mechanism that leads to breaking and reforming of bonds to cause the phase transition. On the other hand, displacive transitions only involve a shift of atom positions. Martensitic phase transition involves a tilt or distortion of structure, and it is a diffusionless transformation. These transitions usually happen in metals and alloys.11 Pressure can help us design new chemistry and new compounds.12-13 Upon compression, atoms are brought closer to each other and the interatomic repulsion 4 increases, therefore the electron distribution must change to adapt to this compression causing changes in electronic structure.14-16 As a result, new crystal structures can form and materials may exhibit new electronic properties. For example, pressure can lead to metallization of insulators and semiconductors during compression with or without or with structural changes.3, 17-19 The insulator/semiconductor-metal transition is mainly due to the increased interaction among filled and unfilled orbitals with increasing pressure. However, there is no universal picture for effects of pressure on materials, and the opposite effects (metal-insulator transitions) are common under high pressure. For simple metals such as Na, it is expected that they become better electrical conductors under compression. This assumption relies on expectation for increase in the width of valence and conduction bands under compression, which would allow electrons to adapt freer electron behavior. However, in the case of sodium just the opposite happens and it transforms into an insulating material at 200 GPa.20 Another well-known example is Li, which undergoes metal to semiconductor transition at 80 GPa, and semiconductor to metal at 120 GPa.21-22 High pressure also can cause electron delocalization. The hybridization happens when tightly bound orbitals are mixed with orbitals with neighboring lattice under compression.23-24 Materials undergo macroscopic deformations under compression. The deformations are called elastic deformation if they are reversible when the load is removed. Otherwise, permanent irreversible deformation will occur, which is called a plastic deformation. One important parameter in quantifying the response of materials to 5 high pressure is bulk modulus which is defined as 𝜕𝑉 (1.2) 𝐵 = −𝑉(𝜕𝑃) 𝑇 Bulk modulus is used to describe the stiffness of the materials and is usually determined by fitting an ideal equation of state (EoS) to pressure-volume data. Equation of State of the material is known by the following: Diffraction measurements at high pressures can provide the correlation between unit cell parameters with pressure, and therefore the volume of sample with pressure, and sometimes temperatures (P-V or P-VT). There are many forms of EoS, and the validity of each form is judged by whether the derived EoS can reproduce experimental data or not. The principle of deriving them is based on the equation 1.2. EoS is semiempirical since some parameters are determined from experiments. One example of EoS that is often used to describe many classes of solids is the Vinet equation, which is given by25 𝑃= where X=(V/V0)1/3 and 𝐵0′ = 3(1−𝑋)𝐵0 𝜕𝐵0 𝜕𝑃 𝑋2 3(𝐵0′ −1)(1−𝑋) exp[ 2 (1.3) ] . Zero in the subscript means this parameter is at ambient pressure. In addition, the Birch-Murnaghan equation of state is also widely used in high pressure studies with its third-order form shown as26 𝑃= 3𝐵0 2 (𝑋 −7 − 𝑋 −5 )[1 − 3(𝑋 −2 −1)(4−𝐵0′ ) 4 ] (1.4) Large values of bulk modulus indicate that the materials are hardly compressible. For example, diamond that is the hardest material has B ~400 GPa. For comparisons the bulk modulus of ice is 2.2 GPa. By applying pressure, some materials (silicates, oxides, and organic materials) 6 can undergo a transition to amorphous state. Amorphization is a state in which a material loses its long-range order. This state can be determined through many experimental measurements including XRD and Raman spectrum. Pressure induced amorphization has been reported in many materials.27-29 One phenomenon that high pressure studies have extensively used to investigate is superconductivity. Superconductivity was discovered by Kamerlingh Onnes in 1911; when mercury was cooled below 4.15 K, its resistivity vanished abruptly and was essentially zero.30 This temperature where the transition happens is called the critical temperature (Tc). At sufficiently high pressure, almost every ambient pressure structure becomes unstable and will transform into higher density and higher symmetry, even undergoing phase transformations by pressure. Therefore, new superconducting materials can be observed under high pressure. The pressure dependence of critical temperature can be derived for microscopic theories of superconductivity such as the strong coupling superconductivity (Eliashberg) theory31 and weak coupling BCS theory.32 Pressure can tune the transition temperature Tc and the superconductivity properties. However, the pressure dependence of Tc is very difficult to predict, and it depends many parameters in the system; based on Eliashberg and BCS theories, it depends mainly on two parameters: the electronic density of states and phonon energy. Most of the superconducting metallic elements show a decrease of transition temperature with pressure.33 This is attributed to the changing of electronic density of states at the Fermi energy and electron-phonon coupling,33 thereby suppressing the 7 superconductivity. However, some elements show a complex pressure dependence of transition temperature that is mainly due to the changes of the Fermi surface topology.3436 For decades, scientists have paid much attention toward discovering high temperature superconductors, equivocally since the discovery of superconductivity in LaBa-Cu-O compound system with the critical temperature of about 35 K;37 this temperature has increased above 40 K with pressure.38 1.2 Tuning Density Using High Pressure Techniques High pressure can be generated in many ways in laboratory. Apparatus that generate high pressures are diamond anvil cells (DAC), large volume presses, and shock wave devices. DAC’s routinely used to reach pressures of 100-300 GPa, and pressures as high as 700 GPa are recently reported to be achieved in a DAC.39-40 Large volume presses usually generate lower pressure, reaching about 100 GPa.41 Shock wave devices can reach extremely high pressures (few hundred megabars) in a short time scale, ranging from nanoseconds to femtoseconds.42 In this dissertation, diamond anvil cell is the technique we used to perform our experiments to generate high pressures. Many different probes including X-ray, Raman, Photoluminescence, Photoconductivity and resistivity measurements were performed on tiny samples inside a diamond anvil cell under extreme pressures to understand the correlation between the structural and electronic properties of materials and their dependence on density. The experiments explained within this dissertation will study in 8 detail the properties of materials that under ambient conditions belong to three distinctly different electronic categories: semiconductor Organic-Inorganic Perovskite system (MAPbBr3), metallic systems Li & K, and wide-band gap insulating hydrocarbon Benz[a]anthracene under high pressure. This dissertation is organized as follows: experimental techniques part (Chapter 2), and the principle and components of diamond anvil cell will be introduced in detail. In order to study the structure of different materials, X-ray diffraction measurements were performed; therefore, principles of X-ray diffraction are also described in Chapter 2 including the incorporation of diamond anvil cell in cryogenic systems. At the end of Chapter 2, absorption, photoluminescence and photoconductivity measurements are also introduced including some new designs of diamond anvil cell. In next three chapters, response of those three material systems to high pressure are described and presented in detail. 9 Figure 1.1. Range of pressures in universe. 10 1.3 Reference 1. Mariathasan, J.; Finger, L.; Hazen, R. High-Pressure Behavior of LaNbO4. Acta Crystallogr., Sect. B: Struct. Sci. 1985, 41 (3), 179-184. 2. Ogborn, J. M.; Collings, I. E.; Moggach, S. A.; Thompson, A. L.; Goodwin, A. L. Supramolecular Mechanics in a Metal–Organic Framework. Chem. Sci. 2012, 3 (10), 3011-3017. 3. Nayak, A. P.; Bhattacharyya, S.; Zhu, J.; Liu, J.; Wu, X.; Pandey, T.; Jin, C.; Singh, A. K.; Akinwande, D.; Lin, J.-F. Pressure-induced Semiconducting to Metallic Transition in Multilayered Molybdenum Disulphide. Nat. Commun. 2014, 5, 3731. 4. Murli, C.; Song, Y. Pressure-Induced Polymerization of Acrylic Acid: a Raman Spectroscopic Study. J. Phys. Chem. B 2010, 114 (30), 9744-9750. 5. Goncharov, A. F.; Manaa, M. R.; Zaug, J. M.; Gee, R. H.; Fried, L. E.; Montgomery, W. B. Polymerization of Formic Acid under High Pressure. Phys. Rev. Lett. 2005, 94 (6), 065505. 6. Mailhiot, C.; Yang, L. H.; McMahan, A. K. Polymeric Nitrogen. Phys. Rev. B 1992, 46 (22), 14419-14435. 7. Swainson, I.; Tucker, M.; Wilson, D.; Winkler, B.; Milman, V. Pressure Response of an Organic−Inorganic Perovskite: Methylammonium Lead Bromide. Chem. Mater. 2007, 19 (10), 2401-2405. 8. Cusick, A. B.; Lang, M.; Zhang, F.; Sun, K.; Li, W.; Kluth, P.; Trautmann, C.; Ewing, R. C. Amorphization of Ta2O5 under Swift Heavy Ion Irradiation. Nucl. Instrum. a Methods Phys. Res. B 2017, 407, 25-33. 9. Tonkov, E. Y. Phase Transformations of Elements under High Pressure. CRC Press: Boca Raton, Fla., 2005. 10. Buerger, M. Polymorphism and Phase Transformations. Fortschr. Mineral. 1961, 39 (9). 11. Barrett, C. S. A Low Temperature Transformation in Lithium. Phys. Rev. 1947, 72 (3), 245-245. 12. 857. McMillan, P. F. Chemistry at High Pressure. Chem. Soc. Rev. 2006, 35 (10), 855- 13. Meng, Y.; Von Dreele, R. B.; Toby, B. H.; Chow, P.; Hu, M. Y.; Shen, G.; Mao, H.-k. Hard X-ray Radiation Induced Dissociation of N2 and O2 Molecules and the 11 Formation of Ionic Nitrogen Oxide Phases under Pressure. Phys. Rev. B 2006, 74 (21), 214107. 14. Li, R.; Liu, J.; Bai, L.; Tse, J. S.; Shen, G. Pressure-induced Changes in the Electron Density Distribution in α-Ge Near the α-β Transition. Appl. Phys. Lett. 2015, 107 (7), 072109. 15. Donev, A.; Stillinger, F. H.; Chaikin, P. M.; Torquato, S. Unusually Dense Crystal Packings of Ellipsoids. Phys. Rev. Lett. 2004, 92 (25), 255506. 16. Goncharov, A. F.; Struzhkin, V. V.; Somayazulu, M. S.; Hemley, R. J.; Mao, H. K. Compression of Ice to 210 Gigapascals: Infrared Evidence for a Symmetric HydrogenBonded Phase. Science 1996, 273 (5272), 218-220. 17. Loa, I.; Adler, P.; Grzechnik, A.; Syassen, K.; Schwarz, U.; Hanfland, M.; Rozenberg, G. K.; Gorodetsky, P.; Pasternak, M. Pressure-Induced Quenching of the Jahn-Teller Distortion and Insulator-to-Metal Transition in LaMnO3. Phys. Rev. Lett. 2001, 87 (12), 125501. 18. Zhang, J.; Barker, A. L.; Mandler, D.; Unwin, P. R. Effect of Surface Pressure on the Insulator to Metal Transition of a Langmuir Polyaniline Monolayer. J. Am. Chem. Soc. 2003, 125 (31), 9312-9313. 19. Shimomura, O.; Minomura, S.; Sakai, N.; Asaumi, K.; Tamura, K.; Fukushima, J.; Endo, H. Pressure-Induced Semiconductor-Metal Transitions in Amorphous Si and Ge. Philosophical Magazine 1974, 29 (3), 547-558. 20. Ma, Y.; Eremets, M.; Oganov, A. R.; Xie, Y.; Trojan, I.; Medvedev, S.; Lyakhov, A. O.; Valle, M.; Prakapenka, V. Transparent Dense Sodium. Nature 2009, 458 (7235), 182-185. 21. Matsuoka, T.; Sakata, M.; Nakamoto, Y.; Takahama, K.; Ichimaru, K.; Mukai, K.; Ohta, K.; Hirao, N.; Ohishi, Y.; Shimizu, K. Pressure-Induced Reentrant Metallic Phase in Lithium. Phys. Rev. B 2014, 89 (14), 144103. 22. Naumov, I. I.; Hemley, R. J. Origin of Transitions between Metallic and Insulating States in Simple Metals. Phys. Rev. Lett. 2015, 114 (15), 156403. 23. Bradley, J. A.; Moore, K. T.; Lipp, M. J.; Mattern, B. A.; Pacold, J. I.; Seidler, G. T.; Chow, P.; Rod, E.; Xiao, Y.; Evans, W. J. 4f Electron Delocalization and Volume Collapse in Praseodymium Metal. Phys. Rev. B 2012, 85 (10), 100102. 24. Lipp, M. J.; Sorini, A. P.; Bradley, J.; Maddox, B.; Moore, K. T.; Cynn, H.; Devereaux, T. P.; Xiao, Y.; Chow, P.; Evans, W. J. X-ray Emission Spectroscopy of Cerium Across the γ-α Volume Collapse Transition. Phys. Rev. Lett. 2012, 109 (19), 195705. 12 25. Vinet, P.; Rose, J. H.; Ferrante, J.; Smith, J. R. Universal Features of the Equation of State of Solids. J. Phys. Condens. Matter 1989, 1 (11), 1941. 26. Murnaghan, F. D. The Compressibility of Media under Extreme Pressures. Proc. Natl. Acad. Sci. 1944, 30 (9), 244-247. 27. Goncharov, A. F.; Gregoryanz, E.; Mao, H.-k.; Liu, Z.; Hemley, R. J. Optical Evidence for a Nonmolecular Phase of Nitrogen above 150 GPa. Phys. Rev. Lett. 2000, 85 (6), 1262-1265. 28. Santoro, M.; Gorelli, F. A.; Bini, R.; Ruocco, G.; Scandolo, S.; Crichton, W. A. Amorphous Silica-Like Carbon Dioxide. Nature 2006, 441 (7095), 857. 29. Swamy, V.; Kuznetsov, A.; Dubrovinsky, L. S.; McMillan, P. F.; Prakapenka, V. B.; Shen, G.; Muddle, B. C. Size-Dependent Pressure-Induced Amorphization in Nanoscale TiO2. Phys. Rev. Lett. 2006, 96 (13), 135702. 30. Onnes, H. K. On the Sudden Rate at Which the Resistance of Mercury Disappears. Akad. Van Wetenschappen (Amsterdam) 14: 113, 818 (1911). 31. Eliashberg, G. Interactions between Electrons and Lattice Vibrations in a Superconductor. Sov. Phys. JETP 1960, 11 (3), 696-702. 32. Bardeen, J.; Cooper, L. N.; Schrieffer, J. R. Theory of Superconductivity. Phys. Rev. 1957, 108 (5), 1175. 33. Jennings, L. D.; Swenson, C. A. Effects of Pressure on the Superconducting Transition Temperatures of Sn, In, Ta, Tl, and Hg. Phys. Rev. 1958, 112 (1), 31-43. 34. Hatton, J. Effect of Pressure on Superconducting Transitions and on Electrical Resistance at Low Temperatures. Phys. Rev. 1956, 103 (5), 1167-1172. 35. Chu, C. W.; Smith, T. F.; Gardner, W. E. Superconductivity of Rhenium and Some Rhenium-Osmium Alloys at High Pressure. Phys. Rev. Lett. 1968, 20 (5), 198-201. 36. Torikachvili, M. S.; Bud’ko, S. L.; Ni, N.; Canfield, P. C. Pressure Induced Superconductivity in CaFe2As2. Phys. Rev. Lett. 2008, 101 (5), 057006. 37. Bednorz, J. G.; Müller, K. A. Possible High Tc Superconductivity in the La-BaCu-O System. Z. Phys. B Condens. Matter 1986, 64 (2), 189-193. 38. Chu, C.; Hor, P.; Meng, R.; Gao, L.; Huang, Z.; Wang; YQ. Evidence for Superconductivity above 40 K in the La-Ba-Cu-O Compound System. Phys. Rev. Lett. 1987, 58 (4), 405. 13 39. Akahama, Y.; Kawamura, H. Diamond Anvil Raman Gauge in Multimegabar Pressure Range. High Press. Res. 2007, 27 (4), 473-482. 40. Dubrovinsky, L.; Dubrovinskaia, N.; Prakapenka, V. B.; Abakumov, A. M. Implementation of Micro-Ball Nanodiamond Anvils for High-Pressure Studies above 6 Mbar. Nat. Commun. 2012, 3, 1163. 41. Yamazaki, D.; Ito, E.; Yoshino, T.; Tsujino, N.; Yoneda, A.; Guo, X.; Xu, F.; Higo, Y.; Funakoshi, K. Over 1 Mbar Generation in the Kawai-Type Multianvil Apparatus and its Application to Compression of (Mg0.92Fe0.08) SiO3 Perovskite and Stishovite. Phys. Earth Planet. Inter. 2014, 228, 262-267. 42. Smith, R.; Eggert, J.; Jeanloz, R.; Duffy, T.; Braun, D.; Patterson, J.; Rudd, R.; Biener, J.; Lazicki, A.; Hamza, A. Ramp Compression of Diamond to Five Terapascals. Nature 2014, 511 (7509), 330. CHAPTER 2 EXPERIMENTAL TECHNIQUES In this chapter, I will discuss the details of the experimental techniques that we used during the measurements described within this dissertation. 2.1 Diamond Anvil Cell One of the main technical tools used in static high pressure research is Diamond Anvil Cell (DAC). DAC is a Bridgman type cell 1 in which diamonds are used as anvils. In a DAC, large number of techniques can be applied under extreme conditions. Bridgman cells are composed of two main parts: anvils and gasket. Various anvil types are used in Bridgman cell. For generation of high pressure, anvils are made of hard materials such as diamond, sapphire, and tungsten carbide. In a DAC, diamond anvils are supported by strong materials such as beryllium and tungsten-carbide supports (backing seats). The principle of generation of pressure with DAC is straightforward: Two opposite diamonds squeeze a sample, which is contained in ductile and hard gasket material (typically made of metals). The gasket that is inserted between those two diamonds serves as a sample chamber holding the sample from extruding during compression. Pressure generated by 15 pressing two diamonds creates an indentation in the gasket. In the center of this indentation area, a hole (sometimes double holes may be drilled based on the purpose of experiments) is drilled and filled with pressure transmitting medium where sample and pressure calibrants such as ruby chips are placed (see Figure 2.1). In the following parts I will explain in detail the various components of a DAC. 2.1.1 Diamonds Diamonds are the main components for generation of pressure in a DAC.2-3 Diamonds can withstand both high pressure and extreme temperatures,4 and diamond anvils have been used in large number of high pressure experiments for over 40 years.5 Diamonds are extremely hard materials (bulk modulus is about 443 GPa at 4 K6) with exceptional properties such as very high thermal conductivity (900-41000 W·m−1K · −1 at 104 K to room temperature7) and large energy bandgap (5.45 eV at room temperature), which makes them excellent electrical insulators and transparent in large domains of electromagnetic spectra. For applications in a DAC, different designs of diamonds are used. These include traditional brilliant cut designs and more specialized designs such as conical diamond cut, which are optimized for applications in high pressure as shown in Figure 2.2.8 In a DAC, typically diamonds are 1/6- 1/3 carat and have a table diameter which is ~3mm.9 Typical diamond anvils have small culets on their tip and a larger bases (table faces) on the opposite end (see Figure 2.2). The pressure limitation depends on several factors. The maximum pressure has inverse relation to the size of culet.9 In practice usually, culet size 16 that is larger than 600 µm cannot withstand pressures above 50 GPa. Beveled anvils, as shown in Figure 2.3, are usually used when higher pressure is in demand. The presence of bevel relieves the stress on the sharp edges of the diamonds. This would lower the deformation of the diamonds at the edges where the damage typically happened when cupped diamonds cut through the gasket or experimental fail due to large stress. Therefore, use of beveled diamonds allow diamonds to reach higher pressure before they get damaged. Pure diamonds are transparent to X-rays. Therefore, XRD measurements can be performed in a DAC. However, in the presence of defects such as nitrogen, which is the main impurity of natural diamonds, most properties are going to be affected. IR absorption spectra of the diamond can reveal those impurities. Although diamonds are generally considered to be chemically inert and stable at high pressure,10 some samples are reactive with carbon even in form of diamond.11 For example, lithium is known to chemically react with diamonds, especially under pressure.12-14 Therefore, for some of the X-ray diffraction measurements in this dissertation, where high pressure studies of lithium are performed and to avoid the chemical reaction between the sample and the diamonds in several runs, we deposited a thin aluminum oxide layer on the diamond anvils to prevent direct contact between samples and diamonds. The alumina film coating was carried out in the Utah Nanofab. Prior to any process in the Nanofab, all the surfaces of diamonds were careful cleaned with acetone and transported to the Nanofab. We first etched the diamonds in argon plasma in Oxford Plasmalab 80 PECVD (plasma-enhanced chemical vapor deposition). Etching is the process of material being removed from its surface and make diamonds 17 cleaned thoroughly. Following the etching process, a thin layer of 𝐴𝑙2 𝑂3 was deposited on the surface of diamonds using Cambridge Fiji F200 atomic layer deposition instrument dry etching machine see Figure 2.4. To minimize the diffraction of X-Ray by aluminum oxide, the thickness of this layer was minimized to the level that still can provide good coverage of the diamonds. Since the largest lattice constant (hexagonal crystal structure) of aluminum oxide is about 13 Å at ambient condition, we coated a 15 nm thickness layer. This would count for 10 atomic layers, which would only minimally affect the X-rays. 2.1.2 Mounting and Alignment of the Diamonds Majority of the measurements that were done in our lab used a new design of anvils that was introduced in 2004, known as Boehler-Almax diamond anvils.8 In this design, the diamond is supported on the backing seat on its crown. The crown of the diamond is completely rounded and is perfectly matched with the backing seats, as shown in Figure 2.2. The advantage of this design is that it provides a much larger axial aperture (up to 90°) than the conventional diamonds supported in their flat seat (<40°).15 Some of the measurements described here were done in diamonds with conventional designs in which the diamond is supported on its table directly mounted on top of the backing seat (Figure 2.2). In this case, properly securing the diamonds on the backing seats is extremely critical as diamonds may come off the seat due to the forces created during alignment operations. Before mounting the diamonds, all the surfaces of diamonds and seats are cleaned 18 using acetone or methanol. Sonic bath cleaning method is mainly used in cleaning diamonds. Several precautions must be made. These include placing only one diamond in the container during ultrasonic cleansing. If more than one diamond is placed together during sonication, the diamonds can touch and break each other. Moreover, generally one has to always be careful to avoid diamonds coming into direct contact with each other. Diamonds are centered on the backing seats, so the tip of their culet is centered with respect to the optical access of the back seats. We then apply moderate force to make sure the diamonds are perfectly in contact with the backings seat and apply a ring of strong epoxy such as Stycast 2850 FT/KT, which is thermally conductive (about 1 W/ (m. k)) and it can also provide electrical insulation as well as excellent properties at low temperature, around the diamonds. After it is cured, it has good strength to hold the diamonds in place. Some cleaning solvents such as acetone can weaken the epoxy; and therefore once the epoxy is used, one should no longer use acetone for cleaning the diamonds. After mounting two diamonds on two backing seats (and installing the backing seat if it was removed during the process), the diamonds need to be aligned with respect to each other. When perfectly aligned, the pressure is evenly distributed between the diamonds and they can sustain the loading to generate high pressure. Because of the fragileness and brittleness of the diamonds, most of the failures are due to the inaccuracy and bad alignment instead of high pressure. Alignment procedure includes two diamonds tilting alignment and side to side alignment. For aligning symmetric “Mao type” DAC as shown in Figure 2.5, first we clean the culet of the two diamonds’ surfaces using cotton 19 buds. We then assemble the cell and carefully bring the two diamonds close enough to see both of their culets at the same time viewing them through the microscope. Then we laterally align the diamonds by translating bottom diamond and adjusting its side screws until the culets of the diamonds are concentric. Next, very carefully, we bring the diamonds closer until we can see some fringes through the microscope. Those fringes are due to the light passing through diamonds and reflecting off the surfaces of both diamonds. The gap between two diamonds with different path lengths will have constructive and destructive interference. Therefore, a series of bright and dark bands are seen. By adjusting the tilting screws, two diamonds become more and more parallel, and interference fringes are reduced until there is no tilt between two diamonds. Since diamonds are fragile and expensive, to avoid breakage during the alignment operations, transparent paraffin films can be inserted between the two diamonds while adjusting diamonds’ positions. 2.1.3 Gasket The purposes of a gasket are to provide a cavity to hold samples and to provide support to the anvils. The materials of gaskets are usually metals that are both hard and ductile. Metal gaskets were first used in DACs by Van Valkenburg at 1965.16 The pressure that gaskets can support depends on the diameter of the pressure chamber (hole in the gasket), the thickness of the gasket, and also the material of the gasket. When the gasket is squeezed between the two diamonds, it deforms plastically and extrudes outwards. During this process, the central part of the gasket clamped 20 between the diamonds is strengthened under deformation. The pressure is due to the frictional force between the metal and the anvils. This force is limited by the shear strength of the metal.16 From the gasket edge to the center, the pressure increases exponentially and this gradient is proportional to the shear strength and inversely to the thickness of gaskets.17-18 The relation can be descried as 𝜕𝑃 𝜕𝑟 𝜎 ~𝑙 (2.1) where P is pressure along the gasket radius r, σ is the shear strength, 𝑙 is the thickness of the gasket. According to the equation 2.1, the material of gaskets must be strong enough to provide a thick final layer under the highest pressure. Stainless steel and rhenium or tungsten are among the commonly used gasket materials. Composite gaskets can also be constructed. In a composite gasket, in order to optimize for different measurements, some parts of gaskets can be made from nonmetallic materials, e.g., insulating inserts (Al2O3, MgO, cubic BN19) can be added to metallic gaskets for making electrical measurements at high pressure. As equation 2.1 shows, we can see that by using composite gaskets (such as packed diamond powder inserts20), we can increase the shear strength and consequently increase the gasket thickness, which in turn will effectively increase the sample volume. When using DACs with X-rays, the gasket materials are chosen not only based on their strength, but also their low scattering background. For example, amorphous boron epoxy/metal composite gaskets21 and high strength beryllium gaskets22 21 have been used in practice. Typically, the original thickness of gaskets is about 250-300 µm. During preindentation, the central part of the gasket is compressed below 80 µm depending on the maximum desired pressure. Maximum pressure can be achieved using the smallest culet size and thinnest gaskets. The thickness of the gasket decreases under pressurization, so usually sample height should be about 2/3 of the initial thickness of the gasket after preindentation to avoid bridging of the sample during the pressurization. Before drilling holes in the gaskets, the gaskets need to be preindented. Preindentation leads to compression of gasket material and formation of a thick mound of metal outside of the preindented area that prevents further thinning of the gasket under pressure. This also gives diamonds the massive support17 and increases the maximum pressure that the DAC can reach for a given gasket thickness.23 After preindentation, a hole will be drilled in the center of the prenidented region. For drilling holes in metal gaskets in our lab, we use micro EDM (electric discharge machining), as shown in Figure 2.6, which is designed mainly to drill the metal gaskets for DAC. Electric discharges are initiated across a narrow gap between the EDM tool and the gasket that is immersed in a dielectric fluid (light mineral spirits). The materials on the gasket are removed due to melting. When the voltage between the EDM tool and gasket increases, the strength of the electric field can break down the materials of gasket. To achieve smooth surfaces, lower discharge energy should be used. Sometimes, multiple holes24-25 in gaskets are drilled for simultaneous studies on multiple samples or to avoid chemical reactions between sample and pressure monometer. For larger diameter holes, higher peak power is 22 required. For achieving higher pressure in a DAC, the ratio of hole diameter to the culet diameter should be small.9 The rule of thumb for the ratio of the hole diameter to the culet size typically is 1 3 1 to 2 of culet diameter, i.e., 150-200 µm for a 500 µm culet face 2.1.4 Pressure Transmitting Medium Pressure transmitting medium (PTM) is a material that fills sample chambers and can transform axial force in all directions to the sample. The pressure transmitting medium can be solid or liquid. In the case of solid soft media (NaCl, KBr),26 we can place the medium into the hole and prepress it. Next, we remove some of the solid medium to form a cavity to place the sample and ruby chips. Use of solid pressure medium provides the easiest means to perform measurements in which the samples are needed to be contacted directly with anvil cells. However, using solid PTM always creates nonhydrostatic pressure distribution in the sample chamber due to the nonzero shear strengths. When samples are surrounded by fluid media during compression, samples are submitted to hydrostatic pressure conditions, which means the stress is uniform in all directions. The most common use is methanol-ethanol (4:1). Methanol-ethanol medium can provide hydrostatic conditions up to 10 GPa.27 The cell needs to be closed and pressure chamber needs to be sealed quickly before the mixture is evaporated. The best media are inert gases such as Helium, Argon, Neon, which are loaded into sample chambers in their liquid phase at low temperatures (cryogenic loading) or in high pressure dense form (pressure bomb). For the cryogenic loading, the procedures of 23 loading liquid nitrogen in the DAC are described first. For this type of loading, it is necessary to cool the whole DAC below the boiling temperature of the liquid after sample and ruby chips are loaded in the pressure chamber. At the starting point, the gasket is securely placed on top of one of diamonds and the other anvil is separated from the gasket with a small gap. We then immerse the DAC into a foam box, which has enough liquid nitrogen (boiling point 77 K) inside of it to cool the entire body of the DAC and cover the whole body of the DAC, making sure liquid is flowing into the pressure chamber and is filling the pressure cavity. Then we tighten the DAC till both diamonds come into contact with the gasket and seal it so liquid nitrogen will be trapped in the pressure cavity. We then take the DAC out of the liquid nitrogen and make sure there are no bubbles trapped inside the sample chamber and warm it to room temperature, and measure the pressure in the DAC to make sure nitrogen is actually sealed in the cell. Similar procedures can be used for variety of PTM, which are in gas phase at ambient conditions. However, since liquid argon is typically sold only as special order, for loading argon we use a modified procedure. Here I will simply describe how we load argon in the DAC. Here we use a sealed metal container, which is connected to an argon gas cylinder and can hold argon gas (boiling point 87 K). We then immerse the container in a liquid nitrogen bath and allow argon to cryogenically condensed inside the container. Once sufficient liquid is formed, we open the container that is otherwise immersed in a bath of liquid nitrogen and submerge the DAC in the liquid argon. The rest of the process is similar to loading liquid nitrogen into the DAC as described above. Different pressure media have different hydrostatic limits.28 When the sample is immersed in fluid PTM, the 24 pressure is fully hydrostatic since fluids do not support shear. However, every PTM will solidify at sufficiently high pressures, even at room temperature, and eventually uniaxial stress and shear stress would appear. Pressure is then transmitted through the solid medium, which leads to quasi or nonhydrostatic conditions with pressure anisotropy. Helium, nitrogen, argon, and mineral oils are used in the experiments that I am going to describe in this dissertation. Argon solidification pressure is about 1.4 GPa at 300 K. It can stay in quasi-hydrostatic condition up ~9 GPa. Helium freezes at 12.1 GPa at 300 K and provides quasihydrostatic condition above 100 GPa.29-30 Nitrogen freezes at 2.4 GPa at 300 K and provides quasi-hydrostatic condition up to 13 GPa.31 An alternative way to load DAC with a gas PTM is to use gas at high densities similar to liquid produced in large volume gas pressure vessel.32 The gas is pumped into the vessel to a certain pressure range (0.1 GPa to 0.2 GPa). Under such pressure range, the gases have densities close to those of liquids. There are two ways to achieve primary pressures. One way is to use a gas compressor that allows immediate compression to reach this desired pressure.33 Another way is to achieve compression in a piston-cylindertype high pressure vessel.34 We do not possess a pressure bomb system in our lab, and therefore we used mail in service facilities of Argonne National Lab where this method was required. 2.1.5 Pressure Measurements Two main techniques for pressure calibration in high static pressure science using DACs are based on pressure-shifts of ruby fluorescence peaks and pressure-volume 25 Equation of State (EOS) of calibrant materials. A wide range of materials can be used as pressure calibrant materials including nickel dimethylglyoxime35 based on the shifts of an optical absorption band, and BeAl2O436 due to it fluorescence properties. The reason that ruby can be widely chosen as a pressure sensor material is because ruby has a high luminescence intensity, sharp peaks with small halfwidths, which causes less uncertainties, and a strong pressure dependence of the wavelength shifts. It is first proposed by R. A. Forman37 that Ruby is 𝐴𝑙2 𝑂3 with a small amount of Chromium (Cr) doping, where the Cr atom substitutes for the Al atoms. Tiny ruby chips (few microns) are added into sample chamber. The process of pressure determination requires two series of measurements: one fluorescence spectrum measured from the ruby at ambient pressure, and one is ruby fluorescence spectrum measured with the sample under certain pressure. The shift of R1 line gives us the pressure in the sample. The schematic diagram of the ruby system and the mechanism of it are shown in Figure 2.7. The electronic energy levels of ruby are described by ligand-field theory.38 Excitation of the R-line fluorescence is an electronic excitation of U or Y bands.39 U and Y absorption bands shift at higher pressure, and the energy of U band becomes higher than green laser at pressure above 50 GPa. Therefore, in order to pump the excitations, higher energy laser is needed.40 The emission of R lines of ruby happens from the metastable 2E state to the ground state under blue-green excitation. The ruby R lines show red shift with applying pressure.37, 41 Temperature also influences the shift of R lines by ~0.0062 nm/K, and it is important to correct this effect when the temperature is above 100 K since 6 K temperature change has the same effect as 0.1 GPa pressure 26 change.42 Ruby Calibration can be described by equation43 up to 150 GPa. 𝐴 𝜆 𝑃 = (𝐵) [(𝜆 ) − 1] 0 (2.2) where the unit of pressure P is GPa, 𝜆 is the ruby R1 line wavelength in nm, A is equal to 1876±6.7, and B is equal to 10.71±0.14. 𝜆0 =694.24 nm is the zero-pressure value at 298 K. Ruby has strong fluorescent emission doublet of peaks: R1 is around 694.2 nm, and R2 is around 692.8 nm under ambient condition. Both peaks will shift with increasing pressure. No hydrostatic stress in the pressure transmitting medium will broaden ruby fluorescence lines.27 The R1 line shifts remarkably in the nonhydrostatic conditions, whereas the R2 line is independent of nonhydrostatic stress.44 Although the behavior of ruby spectra under pressure is thoroughly studied, there are still some problems left. One of them is that the intensity of its emission reduces under high pressure. When the pressure is above ~100 GPa, the intensity decreases drastically. Moreover, the diamonds also have strong fluorescence at megabar pressure range, which makes it harder to detect the ruby peaks. Positions of ruby lines also shift with temperature. Large chips of ruby can have preinduced strain. Therefore, ruby powder should be annealed to eliminate those stresses for precise measurements. It is also important to measure and compare ruby chips at different locations in the sample chamber in order to determine the pressure gradient. The second method used for pressure calibration technique is EOS calibrant. Suppose that the equation of state V=V (P, T) of some materials (e.g. MgO, NaCl, Pt) is obtained, then pressure can be obtained with the help of neutron or X-ray techniques to 27 get the volume. In order for a material to be used as pressure calibrant, it must meet some basic requirements. These include high symmetry, which gives less interference with the sample diffraction pattern, significant volume variation with pressure, no chemical reaction with PTM, and structural stability in interested pressure range. The most widely used calibration material in high pressure is gold. Gold has superior properties that makes it a suitable calibrant. It is soft, and it is a high symmetry (cubic) material; and it has high Z, which means the density of X-ray diffraction is strong. Another most popular calibrant is sodium chloride, and it also can be used as pressure transmitting medium due to its elastic properties. Moreover, NaCl undergoes B1 to B2 phase transition at about 30 GPa.44-45 The disadvantage of using EOS to calibrate pressure is a possible overlap or too close of diffraction peaks of the pressure calibrant and sample diffraction patterns, such as NaCl and sample Li, as shown in Figure 2.8. The other disadvantage is that EOS method is only suitable when you are performing diffraction measurements, and in most experiments is not a useful method. 2.2 Synchrotron X-ray Diffraction Pressure can change interatomic distance and lead to various structural and electronic changes. There are several methods of X-ray diffraction that can be used for high pressure studies. X-ray diffraction can be used to study polycrystalline and single crystalline samples or amorphous samples in different beamline setups,46-49 in addition radial XRD50-51 X-ray spectroscopy,52-56 and X-ray imaging57-58 can be used under 28 pressure. Techniques that we used in our studies are high pressure powder and single crystalline XRD. 2.2.1 Scattering Principles In X-ray scattering by electrons, we only consider the coherent scattering, which means the scattered X-rays have the same frequency as the incident beam. Incoherent scattering (Compton scattering) is not taken into account, because there is no interference effect. When the incident X-ray beam is scattered by one electron, according to the Thomson equation,59 the intensity of the scattered X-rays is 𝑟 2 1+(cos 2𝜃)2 𝐼 = 𝐼0 ( 𝑟𝑒) [ 2 ] (2.3) 𝐼0 is the intensity of the incident beam, 𝑟𝑒 is the electron radius in classical electromagnetic theory, which is about 10-15 m, 𝑟 is the radial distance from the electron to the point where the field is evaluated, and 2𝜃 is the scattering angle. The last term is also called polarization factor. When the X-ray beam is scattered by a single atom consisting of numbers of electrons, each electron produces scattering intensity given by equation 2.3. Phase difference should be considered when calculating the total intensity of this single atom, as show in Figure 2.9. Atomic scattering factor 𝑓𝑒 is defined as ⃗⃗ 𝑟 𝑓𝑒 = ∫ 𝑒 𝑖𝑠. 𝜌 𝑑𝑉 ⃗ -𝑘 ⃗ 0 , and 𝜌 is the density of the electron cloud. where 𝑆=𝑘 (2.4) 29 Therefore, the total intensity of scattering beam is 𝑟 2 1+(cos 2𝜃)2 𝐼~𝑓𝑒2 ( 𝑟𝑒) [ 2 ] (2.5) The primary aim in this section is to investigate the scattering from a crystal, which is usually the case for actual experiments when the scattered X-rays and the incident beams are completely in phase to produce a detectable diffraction beam. We first sum over all the electrons in an atom based on equation 2.5, and then sum over all the atoms in the lattice. Therefore, there will be another term for all the atoms in the crystal that should be added. X-ray diffraction by crystals is the case when scattered beams and the incident X-rays are completely in phase and reinforce to produce the diffraction beam. The most familiar description of this effect is known as Bragg law which is 2𝑑 sin 𝜃 = 𝑛𝜆 (2.6) where d is the interplanar spacing in the crystal lattice. The adjacent planes distance is related to the Miller indices and lattice parameters for different crystal systems (Table 2.1).60 2.2.2 X-ray Diffraction Measurement There are three methods used for collecting X-ray diffraction data: rotation-crystal method, the Laue method, and the powder method. The rotation-crystal method is used for analyzing the structure of single crystals. The source is a monochromatic beam. Using this method, the shape and size of the unit cell as well as the arrangement of atom inside the cell can be determined. The Laue method is used to determine the symmetry and orientation of single crystals. The 30 difference from the previous method is that the Laue source is a white beam. Therefore, it is not possible to obtain the actual size of the unit cells. Powder diffraction is used to determine the crystal structure if the sample is not a single crystal. The sample usually consists a large number of small crystallites, which are randomly oriented, therefore, there is enough of them to have proper orientation to satisfy Bragg’s Law. Powder diffraction has some advantages over single crystal diffraction. It can used for any samples since some materials are hard to grow as single crystal samples. All of these methods can also be divided into two main categories: one is known as Angle Dispersive X-ray diffraction, which source is monochromatic, i.e., the incident wavelength is constant. Another method known as energy dispersive X-ray diffraction uses polychromatic beams and a point detector at a fixed angle. The energy dispersive technique provides fast data acquisition, but it has relatively low resolution and generally can provide adequate resolution for materials with high symmetry. This method is usually used for studying liquid and amorphous materials.61 The angle dispersive X-ray diffraction provides reliable intensity information and high resolution, which is widely used in high pressure research recently.62-64 X-ray sources used for XRD in high pressure research need to have high brilliance and typically can be only achieved in synchrotron facilities where x-ray beam can be focused on small samples. The Advanced Photon Source (APS) is a third generation synchrotron facility that we have made all the structural measurements that I describe in this dissertation. Synchrotron radiation is intense and highly collimated, and it has horizontally polarized in the plane of the electron orbits and circularly polarized 31 above and below the orbits, whereas the electrons in laboratory sources are polarized in all directions perpendicular to its propagation. The size of the beam at its focus is around 3 × 5 𝜇𝑚, which is a critical property for high pressure studies since too large size of a beam can illuminate materials from DAC, and we will get too much unrelated signals from background. The anatomy of the synchrotron is shown in Figure 2.10. Electrons from electron gun (part A) are accelerated and confined in a storage ring (E) using bending magnets (E2). Insertion devices (E2) (undulators or wigglers) are used to produce X-ray emission. Insert devices are periodic magnetic materials that force the electrons to oscillate and deflect while passing through them, therefore radiation can be emitted by dipole magnets. Undulators have less periods than wigglers and produce a brighter light source.65 The generated light gets focused and goes through the collimator. The collimated beam is directed toward the sample. Then, diffraction beams are hitting on scintillator to convert the X-ray energy to electrical charges, and finally reach the CCD detector as can be seen in Figure 2.11. The basic information that can be extracted from XRD pattern of a sample are unit cell parameters, atomic positions, thermal parameters, preferred orientations, and lattice strain.66 2.2.2.1 Single Crystal X-ray Diffraction Crystals are periodic arrangements of atoms. Figure 2.12 schematically shows single crystal diffractometer. This method has been rapidly developed in recent years in high pressure research.67-71 As discussed before, the materials that support diamonds must be strong to withstand the high stress. In addition, for the purpose of high pressure 32 crystallography, the support material needs to allow the X-ray beam to reach the sample. Beryllium (Be) is used as support material and also gasket material in many high pressure crystallography experiments. This is because Be has low density (1.85 g/cm3) and Z (Z=4), so it is transparent to X-rays with low absorption. Two modes of diffraction geometry are available for high pressure single crystal XRD research. One is the transmission mode, which is used in most situations. Here the incident X-rays pass through one diamond, single crystal, and then the opposing diamond. In the other geometry, which is referred to as transverse mode, both incident and scattered X-rays pass through the same diamond. Crystal selection is the first step of single crystal XRD measurement. The size of crystal should be sufficiently large to allow diffraction from a large number of planes, but the sample cannot be bridged between two diamonds at highest desired pressure. Nice single crystals are free of stress and defects and have higher quality diffraction pattern, as shown in Figure 2.13. After the XRD, data is collected and the data needs to be analyzed and refined, and the unit cell and space group of the sample can be determined. In our analysis of the single crystal XRD, the diffractogram is analyzed with refinement program SHELXL. 2.2.2.2 Powder X-ray Diffraction Under high pressure typically single crystals get destroyed due to the applied forces, and maintaining the single crystallinity becomes exceedingly hard. Therefore, despite the superiority of single crystal XRD for most reliable structural studies, in 33 practice powder X-ray diffraction becomes a primary technique for determination of crystal structure and unit cell parameters as a function of pressure or even temperature. Moreover, naturally samples are rarely formed as single crystals. Powder diffraction obeys the same laws of physics as single crystal diffraction. Location of diffraction peaks is given by Bragg’s law equation 2.6. A perfect powder sample consists of an infinite number of randomly oriented crystallites. So, we will get many overlapping reciprocal lattice, resulting in a sphere of reciprocal lattice points. All these points fulfill the Bragg’s law. As a result, we will observe powder rings in diffraction patterns. The diffractogram is analyzed and compared with a known crystal structure using Rietveld analysis.72 Rietveld analysis usually includes only the unit cell parameters and provides no information about the atomic positions. Le-Bail analysis allows further analysis about the atomic positions. Powder XRD has many limitations. Samples may exhibit preferred crystalline orientation. In severe cases, only some diffraction peaks are observed, and others are absent. Some approaches can be taken to address this problem. For example, the samples can be ground into fine powder or one can mix the sample with some inert, soft materials to prevent the sample from recrystallizing. 2.2.3 XRD Measurements in DACs 2.2.3.1 X-Ray Axis Access to the Sample in a DAC As mentioned before, diamonds are transparent to a broad range of electromagnetic radiations.73 Therefore, a DAC can be used for variety of experimental 34 studies such as Raman and IR spectroscopy variety of X-ray techniques and Mossbauer spectroscopy where optical/electromagnetic access to the sample is required. In addition, in a DAC many other methods can be used to study material properties under pressure. These include but are not limited to Neutron diffraction, NMR, Magnetic susceptibility, and transport measurements. Most of the measurements described within this manuscript involve XRD in DACs. The thickness of diamonds that we used in DAC measurements are typically 1.5 to 2 mm, and culets diameter are about 250 to 600 µm. Low absorption of the X-rays through the DAC is important for collecting sufficient signal from the sample for XRD measurements. X-rays must reach the sample and the detector. In many designs, Be support seats are used, which allow transmission of the X-rays both through the diamonds and the support seat. In addition, in some experiments, special 90° geometry is used where the incident X-rays reach the sample through the Be gasket and collected through the diamonds. In our experiments, we were using tungsten carbide support seats and stainless steel or rhenium gaskets, which would block the X-rays, and therefore the only possible way for X-rays to access the sample was through the opening angle of the diamonds. The variability of the diffraction angle in the geometry used here is dependent on the DAC’s opening angle and the DAC’s position related to incident and diffracted beam (𝛹𝑖 𝑎𝑛𝑑 𝛹𝑑 ). In addition, in some experiments presence of cryostat windows impose further constraints. As show in Figure 2.9, 2α is the DAC opening angle. 2𝜃 is the diffraction angle. While the DAC is rotated, the angle 𝛹𝑖 is satisfied by −(𝛼 + 𝑛𝜋) ≤ 𝛹𝑖 ≪ (𝛼 + 𝑛𝜋) (2.7) 35 and 2θ ≤ (𝛼 + 𝛹𝑖 ) (2.8) For example, for the experiments done in sector IDB-16 of APS, we used a symmetric DAC with an opening angle of DAC~70˚. However, we only rotated the DAC by 40˚, since we were constrained by the geometry of the cryostat. 2.2.3.2. Low Temperature Studies in a DAC in Beamlines Many interesting phenomena occurs at low temperatures, which relate with quantum effects. Cryogenic techniques can combine high pressure and low temperature to investigate materials’ properties under such conditions. At low temperature, most materials become brittle,74 so it is crucial that DACs used in low temperature techniques are made of material that would not to become brittle. Many materials have been used successfully in low temperature and high pressure studies including copper-beryllium alloy cells and NiCrAl alloy cells.75 Various types of cryostat can be used for different temperature ranges.76-77 The cryostats we used in our lab are shown Figure 2.14. DAC is placed in the cold finger in the cryostats. Compact cold finger cryostat can provide large access openings, but with a limited lower bound of temperature, which is typically about 7 K. Large optical opening access (> 60˚) allows rotation of the cryostat during exposure for X-ray or neutron studies.78 Cryostat we used in HPCAT, which was designed for DAC, uses windows that are made of Kapton film on the X-ray side and Polyethylene film on the collection side to accommodate optical and X-ray diffraction measurements. 36 During low temperature measurements, pressure in a DAC can be increased by mechanical methods using clamping screws or gear boxes at room temperature, and then the sample can be cooled to low temperature. However, this would not allow examining the properties of samples on different thermal paths and is also limiting our data collection capability since we have limited time during each beamtime. Therefore, going up and down and increasing pressure at room temperature is not feasible timewise. However, DAC can be pressurized using helium gas membrane (Figure 2.15) at low temperature in a liquid helium cryostat. We remotely control the pressure by altering helium gas pressure in a membrane acting on the DAC. Additionally, the ring-like membrane ensures the force acting on the DAC is homogeneous; in this case, diamonds can be maintained aligned and even at very high pressure, so higher pressure can be reached. Double membrane design is a unique design that is developed in HPCAT that allows both increase and decrease of pressure at low temperature. The pressure is measured with online ruby systems76 or using the equation of state of pressure makers such as NaCl or Au by X-ray diffraction. Temperature is controlled by a proportionalintegral-derivative (PID) controller using a balance of the flow of cool helium gas in the cryostat and the heat generated by the heaters, which are attached to the PID controller of a Lake Shore temperature control system. As shown in Figure 2.15(b), double membrane design provides significant flexibility in pressure paths by allowing multiple compression and decompression cycles with controlled amplitude when certain thermal path is intended to reach to perform comparative studies.77 37 2.3 Absorption, Photoluminescence and Photoconductivity Measurements in a DAC Absorption spectra can provide the information on the wavelengths of radiation that the sample can absorb. Experimentally, what we do is to record the intensity of the transmitted light by varying the wavelength. The fundamental absorption involves the transition of electrons from the valence band to the conduction band (Figure 2.16). An electron absorbs a photon and transitions from the valence band to the conduction band. In this transition process, the total energy and momentum of the system must be conserved. In the absorption process that occurs in direct-gap semiconductors, the bottom of the conduction band and top of the valence band lie at k=0, whereas in indirect-gap semiconductors, the bottom of conduction band does not lie at the origin (Figure 2.16). In addition to the fundamental absorption processes, there is also exciton absorption, freecarrier absorption, and absorption processes involving impurities. Once electrons have been excited, the distribution of electrons is no longer in equilibrium; they will decay into lower energy states and emit radiation, a process known as luminescence. Studying the luminescence properties of materials, which is the inverse of the absorption process, is one the main tools in semiconductor research. Depending on the excitation mechanism, luminescence can be classified into different types; for example, when the material is excited by electric field, electroluminescence will be produced, and when the optical absorption excites the sample, the process is called photoluminescence. If luminescent emission takes place during the time of excitation, the phenomenon is known as fluorescence. If emission continues after the excitation is completed, the phenomenon is 38 known as phosphorescence. Phosphorescence is also called long time emission. There are three different types of intrinsic luminescence: band to band luminescence, exciton luminescence, and cross-luminescence. When luminescence is caused by doping impurities, the luminescence is called extrinsic luminescence. The most prominent luminescence process is the band to band transition. The photoluminescence spectrum of this type of transition is typically a broad peak (Figure 2.17)79 that shows band to band photoluminescence spectrum in 𝑀𝑂 𝑆𝑒2. When an incident beam hits on a semiconductor, the valence electrons absorb the incident photons, and transition to the conduction states above the energy gap occurs, which increases the number of free carriers, and causes an increase in the materials electrical conductivity. This optical and electrical phenomenon is called photoconductivity. When a bias voltage and a resistor are used in series with this semiconductor, the current across the resistor can be measured when electrical conductivity of this semiconductor is changed. The overall number of free carriers depends on interplay between the increase of number of carriers created by the incident beam vs. the recombination processes in which the number of free carriers is decreased. 2.3.1 Transport Measurements in a DAC Measurements of the electrical properties of the material are very important in material science and condensed matter physics. The most straight forward method for transport studies under pressure is by making direct electrical contacts with the pressurized sample. While the principles are very simple, the process is extremely 39 delicate. For small samples of small resistance, conventional 2-wire resistance measurements are not applicable as the total resistance will be dominated by the wire resistance, and changes in the sample behavior cannot be detected. In this case, 4-probe (Kelvin probe) or quasi four probe method is used. The principle of Kelvin probe method is shown in Figure 2.18 (right image). The current passing in the sample is direct current (DC) current instead of alternating (AC) current. This is because when AC current is flowing in the sample, it will generate magnetic fields that will cause opposite currents through the sample. And also the AC current will not distribute uniformly throughout the cross section of sample due to the skin effect80 and the higher the frequency of the current, and the higher resistance it will measure.81 In our measurements, when the resistance of the sample under ambient conduction is small (metal samples) and is close to the resistance from connections, in this case, we will add a 1000 Ω resistor in a series circuit with the sample so we can monitor small changes of voltage across the sample. Since the sample in DAC sample chamber is very small, the doable way to set up four probes is quasi-four probe method, which is the right side of the Figure 2.18. In quasi four probe method, only two of them are in contact with the sample, and the other two probes should be put as close as possible to the sample and contact each of them with the other two probes. The nearly DC current passing through the sample through the Pt leads that connected with the lock-in amplifier by copper wires. Since we detect small signals, lock-in amplifiers are needed at low frequency to create nearly DC current. The way we use lock-in amplifier is used as constant voltage sources instead of sending constant current. This is because with temperature changing, all resistances in circuit are 40 changing, and thus the resistance on connections is not detectable. Therefore, we chose to use lock-in amplifier as constant voltage source. The process of how to build the probes will be described in the following paragraph. As shown in Figure 2.18, four platinum wires, which were cut from a 4 microns thick Pt foil, are arranged on the gasket. Suppose the total voltage we applied is Vt, if the resistance of sample is small (few ohms or less), an extra resistor Rex (we usually use 1000 ohms resistor) will be added in the circuit. The resistance of sample (Rs) will be calculated according to the following equation 2.9 𝑉=𝑅 𝑉𝑡 𝑠 +𝑅𝑒𝑥 𝑅𝑠 (2.9) Preparation of a gasket for transport measurements requires special considerations. These include proper insulation of the electrodes from metallic parts of the gasket and the pressure cell and properly arranging them to contact the sample. Starting from a metallic gasket, we first drill a proper size hole in the preindented gasket. We then clean the gasket in acetone, and apply a thin layer of epoxy with good electrical insulating properties such as 2850KT epoxy on the surface of the gasket where the probes are going to be made. It usually takes a few hours to allow the epoxy to set. We usually heat the gasket in the oven at about 70°C to accelerate the process and enhance the strength of the epoxy. Once the epoxy is set, we place the gasket in the diamond cell and pressurize it slowly under microscope until the hole in the gasket starts to shrink. We then take the gasket out of the DAC, and if some epoxy has flowed into the hole we gently remove the extra material. We then examine the gasket carefully, and if needed more epoxy can be added where parts of metal surface of gasket are exposed. One common issue is when the 41 edge of the hole is not totally covered by epoxy, it will cause an electrical short between the sample and the gasket during experiments. To avoid this, we add some alumina powder on top of the hole and pressurize it. We then poke another hole using acupuncture needles through the alumina powder to place the sample. In our experiments, we used Pt wires from a platinum foil of 5-micron thickness as probes. The probes are attached by a tiny amount of super glue to the gasket. Figure 2.18 shows the initial arrangement of the probes on the gasket. We then close the DAC to make sure there is no connection between the probes on opposite sides. A few aspects need to be considered during probes preparation: as shown in Figure 2.18, in (quasi) 4-probe arrangement, (one probe on each side) all the probes will have to contact the sample, so the proper length of them should be cut to avoid shorts between the probes. During the compression, the probes will be expanding, so it is important to make sure there is enough space among probes. Very nice composite gaskets can be made if the insulating parts of the gasket can be drilled mechanically or by laser. During our visits to HPCAT, we developed optimized methods in which an insulating ring from alumina was made using a micro laser machining system.82 This type of gasket preparation allows a good insulating gasket configuration and a large sample area. We are in the process of building a similar system in our group, and I have been working on the initial stages of this system as outlined below. The principle of this system is removing the gasket target material from the gasket by irradiating it with laser beams. The schematic layout of the optical system is shown in Figure 2.19. Green laser is for alignment. Waveplate and Polarizer are for attenuating the 42 beam. LED light is used to illuminate the sample. The most common application of this system is to cut circular holes in preindented DAC gaskets. However, it can be used to cut all kinds of shapes as needed. 2.3.2 New Design DAC for Electrical Measurements Electrical experiments play an important role in investigating many phenomena in high pressure studies, including pressure induced superconductivity, metal to liquid transitions, and so on. As discussed in previous section, the resistivity measurements are kind of difficult since small pieces of electrodes are needed to be made on the gasket, and those electrodes are going to be deformed by compression, which will cause failure of the measurements by shorting among those electrodes. Therefore, many new designs are emerging to simplify those procedures. In collaboration with Almax. Easylab Ltd, we have been provided with a set of anvils with imprinted electrical leads on their culet, as shown in Figure 2.20. Gold patterns are deposited on the diamond. Sample preparation time is significantly reduced using these patterned diamonds. However, unfortunately we had limited supply of these diamonds and after several runs of experiments, the conducting patterns were damaged under the pressurized gasket and also during the cleaning process. A new design of diamond anvil cell using metallic boron-doped diamond electrodes was developed recently, as shown in Figure 2.21.83 It shows that there are no cracks in the metallic diamond electrodes even after over 20 times of compressions above 10 GPa. Although we did not have this kind of DAC in our lab, this design can in the 43 future allow us to improve our experimental capability. Various designs of boron doped diamonds electrodes can be made beside four-terminal electrodes. 44 Figure 2.1. A chamber for pressurization of the sample formed by a diamond anvil and a gasket (left). Ruby serves as a pressure gauge. Right image of a Merrill–Bassett DAC, which is from https://journals.iucr.org/j/issues/2008/02/00/aj5098/aj5098fig2.html. 45 Figure 2.2. Image of Boehler-Almax conical support seat/anvil and conventional support seat/anvil from https://www.almaxeasylab.com/TypeIIacBoehlerAlmaxdesign.aspx. 46 Figure 2.3. Image of aligned diamonds and a single beveled anvil. 47 Figure 2.4. Oxford Plasmalab 80 as a PECVD tool and Cambridge Fiji F200 from https://coral.nanofab.utah.edu/lab/lims/equipment. 48 Figure 2.5. Images of a symmetric DAC and a plate DAC. 49 Figure 2.6. Image of the EDM setup in Prof. Shanti Deemyad’s Lab. 50 Figure 2.7. Electron energy levels of ruby (left) and basic setup for ruby pressure measurement in a DAC (right). DAC image from Guoyin Shen and Ho Kwang Mao 2017 Rep. Prog. Phys. 80 016101. 51 Figure 2.8. Image plates of 6Li together with pressure monometers at different pressure and temperatures. The diffraction lines of NaCl (bcc) (and solid helium at low temperature (fcc)) together with ruby fluorescence used to determine the pressure in the vicinity of the sample. Unlabeled patterns are cryostat and DAC background that has no pressure dependence. Diamond reflections are masked with yellow dots. (cite Ackland, Graeme J., Mihindra Dunuwille, Miguel Martinez-Canales, Ingo Loa, Rong Zhang, Stanislav Sinogeikin, Weizhao Cai, and Shanti Deemyad. "Quantum and isotope effects in lithium metal." Science 356, no. 6344 (2017): 1254-1259.). 52 Figure 2.9. Schematic image of electrons cattering by two lattice points and X-ray crystallogaphy in a DAC. 53 Figure 2.10. Schematic drawing of synchrotron facility. A is the electron gun. B is the electron accelerator. C is the particle accumulator ring. D is the booster synchrotron. E is the storage ring. E1 is the insertion device, and E2 is the bending magnet. 54 Figure 2.11. A schematic image of high pressure synchrotron for Angle-dispersive X-ray diffraction. The beam stop is used to avoid a damge to the CCD dector by X-ray beams. 55 Figure 2.12. Single crystal scheme and Ewals construction. 56 Figure 2.13. Integrated diffraction image plate of single crystalline sample Mercury(II) bromide at 2.5 GPa at room temperature using helium as pressure medium (𝜆=0.434 Å, 2𝜃 is from 2 to 22 degree). Few large spots at larger angles indicate the diamond reflections. XRD image from Dr. Weizhao Cai’s sample. 57 Figure 2.14. Cryostat system with low temperature ruby system in Prof. Deemyad’s lab. 58 Figure 2.15. Schematic presentation of gas membrane pressurization method. (a) Cap-can assembly for symmetric DAC and membrane.77 (b) Dual double-membrane setup for compression-decompression experiments at room temperature. A–DAC decompression attachment, B–DAC, and C–double-diaphragm cap-can.77 59 Figure 2.16. The fundamental absorption processes (an direct and indirect gap) in semiconductors. 60 Figure 2.17. Photoluminescence of different thicknesses of molybdenum diselenide.80 61 Figure 2.18. Kelvin probe or 4-probe arrangement for resistivity meaurements. Schematics dragrams (left) and picture of quasi-probe arrangment (right) built on a gasket with 4 platinum probes (5 micron thick). The probes will be pressed using the opposite diamond to sit on the gasket hole on the background and touch the sample. 1, 2, 3, and 4 electrodes are connected to current or voltage source with copper wires. 62 Figure 2.19. The schematic layout of the optical system for laser micro-machining system for diamond anvil cells. 63 Figure 2.20. Patterned diamond by Almax easylab from https://www.almaxeasylab.com/WebSitePatternedAnvils01.aspx. 64 Figure 2.21. DAC assembly with metallic boron doped diamond electrode for electrical transport measurement. Cite: Ryo Matsumoto et al; Japanese Journal of Applied Physics (2017). 65 Table 2.1. Relation between the adjacent planes distance with the Miller indices and lattice parameters for different crystal systems. Cubic 1 ℎ2 + 𝑘 2 +𝑙 2 = 𝑑2 𝑎2 2 Tetragonal 1 ℎ + 𝑘 2 𝑙2 = + 2 𝑑2 𝑎2 𝑐 Hexagonal 1 4 ℎ2 + ℎ𝑘 + 𝑘 2 𝑙2 = ( ) + 𝑑2 3 𝑎2 𝑐2 1 Trigonal 𝑑2 (ℎ2 + 𝑘 2 +𝑙 2 ) (sin 𝛼)2 + 2(ℎ𝑘 + 𝑘𝑙 + ℎ𝑙)((cos 𝛼)2 − 𝑐𝑜𝑠𝛼) = 𝑎2 (1 − 3(cos 𝛼)2 + 2(cos 𝛼)3 ) Orthorhombic 1 ℎ2 𝑘 2 𝑙 2 = + + 𝑑2 𝑎2 𝑏2 𝑐 2 Monoclinic 1 1 ℎ2 𝑘 2 (sin 𝛽)2 𝑙 2 2ℎ𝑙𝑐𝑜𝑠𝛽 = ( + + 2− ) 𝑑2 (sin 𝛽)2 𝑎2 𝑏2 𝑐 𝑎𝑐 1 1 Triclinic = (𝑆 ℎ2 + 𝑆22 𝑘 2 + 𝑆23 𝑘 2 + 2𝑆12 ℎ𝑘 + 2𝑆23 𝑘𝑙 2 (𝑉)2 11 𝑑 + 2𝑆13 ℎ𝑙) 2 2 𝑆11 = 𝑏 𝑐 (sin 𝛼)2, 𝑆12 = 𝑎𝑏𝑐 2 (𝑐𝑜𝑠𝛼𝑐𝑜𝑠𝛽 − 𝑐𝑜𝑠𝛾), 𝑆22 = 𝑎2 𝑐 2 (sin 𝛽)2, 𝑆23 = 𝑎2 𝑏𝑐(𝑐𝑜𝑠𝛾𝑐𝑜𝑠𝛽 − 𝑐𝑜𝑠𝛼), 𝑆33 = 𝑎2 𝑏 2 (sin 𝛾) 2 , 𝑆13 = 𝑎𝑏 2 𝑐(𝑐𝑜𝑠𝛾𝑐𝑜𝑠𝛼 − 𝑐𝑜𝑠𝛽), 66 2.4 Reference 1. Yoneda, A.; Yamamoto, S.; Kato, M.; Sawamoto, H.; Kumazawa, M. Use of Composite Metal Gaskets to Improve Pressure Generation in Multiple Anvil Devices. High Temp. High Press. 1984, 16 (6), 637-656. 2. Akahama, Y.; Kawamura, H. Diamond Anvil Raman Gauge in Multimegabar Pressure Range. High Press. Res. 2007, 27 (4), 473-482. 3. 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Hrubiak, R.; Sinogeikin, S.; Rod, E.; Shen, G. The Laser Micro-machining System for Diamond Anvil Cell Experiments and General Precision Machining Applications at the High Pressure Collaborative Access Team. Rev. Sci. Instrum. 2015, 86 (7), 072202. 84. Matsumoto, R.; Irifune, T.; Tanaka, M.; Takeya, H.; Takano, Y. Diamond Anvil Cell Using Metallic Diamond Electrodes. Jpn. J. Appl. Phys. 2017, 56 (5S3), 05FC01. CHAPTER 3 STUDIES OF MAPbBr3 UNDER PRESSURE MAPbBr3 (Methyl Ammonium Lead Bromide) belongs to the family of semiconducting hybrid-organic –inorganic perovskites with superior properties for solar cell applications. These materials have attracted a very large body of research for their promising applications in the future of solar energy. Perovskites encompass a large class of materials with the general ABX3 formula, where the B cation is 6-fold coordinated surrounded by anions in an octahedral arrangement (BX6), whereas the A cation is 12fold coordinated. Many technologically important compounds that exhibit high temperature superconductivity, colossal magnetoresistance, charge ordering, and spindependent-transport belong to this family. As such, perovskites have been the subject of intensive studies in the past years.1-3 Recently, synthetic hybrid organic-inorganic perovskites (OIPs) based on methylammonium lead halide (MAPbX3, MA = CH3NH3, X = Cl, Br and I) have shown superior properties for applications in photovoltaic technology due to their wide absorption spectra and high absorption coefficients. These materials can be synthesized at a low cost, have strong solar absorption, and excellent power conversion efficiency that currently reaches ~22%. The unique optical and electronic properties of these compounds can be achieved through tuning the electronic 74 structure by chemical substitutions of both organic and inorganic components.4-8 However, the structural instability of these compounds for relatively small variations in temperature, as well as their chemical instability in the presence of moisture, poses a challenge for their application. In addition, due to the presence of lead in their structures, these compounds are not environmentally friendly. Therefore, understanding the factors that contribute to the chemical and structural instabilities of MAPbX3, as well as designing a lead-free equivalent of these compounds, is currently a milestone in photovoltaics. In designing an OIP with optimum properties, stabilization of a cubic structure is one of the important factors. The stability of structure can be deduced by the tolerance factor t and octahedral factor μ:9 𝑡= (𝑟𝐴 + 𝑟𝑋 ) √2 (𝑟𝐵 + 𝑟𝑋 ) 𝜇= 𝑟𝐴 𝑟𝑋 (3.1) (3.2) where 𝑟𝐴 , 𝑟𝐵 , and 𝑟𝑋 are the ionic radii of corresponding ions. Generally, cubic structure in halide perovskite occurs at 0.85 < 𝑡 < 1.11 and 0.44 < 𝜇 < 0.90. 𝑡 < 0.85, and 𝜇 < 0.44 give less symmetric tetragonal or orthorhombic structures and tilting of BX6 octahedral, as shown in Figure 3.1. MAPbBr3 undergoes phase transformation with temperature changing, which is already known. If temperature is less than 144 K, the structure of the sample MAPbBr3 is orthorhombic. When the temperature is in the range between 144 K to 237 K, the structure is tetragonal. Otherwise, it is cubic when temperature is higher than 237 K. The high pressure response of structural and band gap 75 changes of hybrid perovskites with substitution of organic cations and metal atom have been reported recently.10-23 These results manifest that similar to the temperature-induced distortions, the OIP framework that is built from PbX6 octahedra tilts considerably under pressure, leading to a sequence of structural phase transitions. Crystalline samples of methylammonium lead bromide that we investigated (CH3NH3PbBr3, abbreviated here as MAPbBr3) are orange in color since their band gap is ~2.3 eV at ambient conditions.24 The room-temperature structure of MAPbBr3 is cubic with space group Pm3̅m and Z = 1 and shows a diversity of structures during cooling.25-27 The tilt of the PbBr6 octahedra can be described by Glazer notation as a0a0a0, which donates the nontilting system (Figure 3.2a).28 The framework of all the structures of MAPbBr3 consist of vertex-sharing PbBr6 octahedra, in which the disordered/ordered MA cations occupy the three-dimensional channels and hydrogen-bond to the host framework of the PbBr6 octahedra. Because of the flexibility of the OIP structures, there can be a large difference in relative compressibility of the octahedral and the framework of the cation sites. Hydrostatic compression of the lattice, therefore, provides means to systematically manipulate the system and find the optimum lattice parameters ratios. In addition, the effect of stress can be translated to the organic and inorganic part differently and cause the movement of ions. The pressure dependence of the various MAPbBr3 structural phases and the related optoelectronic properties have been studied by several groups. Deuterated MAPbBr3 samples pressurized using isopropanol as the PTM have been shown to undergo a structural phase transition from Pm3̅m phase to Im3̅ phase at ~0.9 GPa, and 76 become amorphous above ~2.8 GPa.29 High pressure studies without a PTM by Wang et al. showed that the Pm3̅m phase transforms to Im3̅ phase at ~0.4 GPa, followed by an orthorhombic Pnma phase at ~1.8 GPa.30 Recently, Jaffe et al. used He as a PTM and observed that the Pm3̅m to Im3̅ and Im3̅ to amorphous phase transitions at ~0.9 and 2.8 GPa, respectively.31 Other studies show that when silicone oil was employed as a pressure medium, the Pm3̅m phase converts to the Im3̅ phase at ~0.5 GPa.30, 32 While several reports assume that the MAPbBr3 phase transitions are not sensitive to the hydrostaticity of the PTM, a comparison between the results of the aforementioned studies questions this assumption. Although the pressure dependence of methylammonium-lead-halides structures has been previously reported, the effect of nonuniform stress is not known. In this section, a systematic study of MAPbBr3 crystal under high pressure is presented, and the interplay between the various phases and photo-physical properties under hydrostatic conditions exerted by PTM of helium and argon, nitrogen and without a PTM, respectively, is also investigated. Synchrotron X-ray diffraction, photoluminescence (PL), and photoconductivity spectroscopies in a diamond anvil cell are used, supported by DFT calculations to elucidate the mechanism of the band gap changes and structural phase transitions under pressure. In addition, first-principles molecular dynamics (FPMD) calculations have also been performed to illustrate the influence of the MA molecules dynamics on the instantaneous and time averaged band gaps of the various structural phases. I am not going to give details about the theoretical calculations parts in this dissertation and will focus on the experimental sections instead. 77 3.1 XRD and Photoluminescence Measurements Single crystals of MAPbBr3 were prepared (Figure 3.3) by the solvent vapor exchange method by Dr. Zhang in Prof. Vardeny’s group. PbBr2 white powder and MABr (Sigma-Aldrich, molar ratio 1:1.2) were dissolved in dimethylformamide (SigmaAldrich, 5 mL, 0.5 M) and stirred for 3 hours. The vial containing the as-prepared solution was placed in a beaker with 2-propanol (20 mL) as the antisolvent. The beaker was then covered and placed in a dark place for 3-7 days. The crystal size can vary from 0.1 mm to 5 mm depending on the growth time. Cubic-shape crystals were then obtained and used as seeds to repeat the same procedure. Bulk crystals with larger size and better quality could be obtained after several iterations. A symmetrical diamond anvil cell with piston cylinder design (DAC) and a triangle Merill-Bassett design DAC were used to generate high pressure. Pressure was applied using four different pressure transmitting conditions under helium, argon, nitrogen, and without a pressure medium. A stainless steel gasket was preindented to ~70 μm in thickness, and a ~150 μm diameter hole served as the sample chamber for all the X-ray measurements. The pressures were determined by the ruby fluorescence method.33 A single crystal of MAPbBr3 together with few ruby chips for pressure calibration were loaded into a DAC chamber. For loading the pressure medium, the gasket was sealed using high pressure loading of dense helium, nitrogen, and argon. In case of argon, some of the pressure medium loading was performed using a cryogenic loading method. While the initial structure of the sample is different at low temperature where the cryogenic loading is done, no differences were observed between the results of the two pressure loading methods. High pressure powder X-ray diffraction 78 data were collected at the 16 ID-B beamline of the High Pressure Collaborative Access Team (HPCAT) at the Advanced Phonon Source (APS), Argonne National Laboratory (λ = 0.4066 Å). The diffraction data were analyzed by the Le Bail fitting method using the GSAS-EXPGUI package.34 High pressure single X-ray diffraction data using He and Ar as pressure media were collected at beamline 13-BM-C of the Advanced Photon Source under the help of Dr. Zhang, Argonne National Laboratory with the X-ray wavelength of 0.434 Å. Diffraction data were analyzed using the ATREX IDL software package.35 Polarization, Lorentz, and absorption corrections were applied to the fit peaks. The unit cell and orientation matrix were determined in RSV for each dataset. Lattice parameters were refined in RSV using a least square fitting procedure. Due to the low completeness of the diffraction data at high pressure, only the positions of Pb and Br atoms were determined for some pressure points. The data were refined by using the ambientpressure structure as the starting model or solved by direct methods with the aid of SHELXL-97.36 High pressure photoluminescence measurements of MAPbBr3 were carried out also under helium, argon, nitrogen, and without a PTM. The 488 nm excitation laser line was used in those experiments. Stainless steel gaskets were used with ~70 µm in thickness, and ~250 µm hole served as sample chamber. When MAPbBr3 loaded using He as a PTM, at room temperature, helium sustains hydrostatic conditions up to 20 GPa.37 As shown in Figure 3.4, the ambient-pressure cubic phase I (𝑃𝑚3̅𝑚) of the MAPbBr3 persists during monotonic compression up to 0.85 GPa. Above this pressure, a discontinuous phase transition to phase II (Im3̅) with a small unit volume collapse (V/Z) of ~1.0 Å3 was observed. In addition to the I→II phase 79 transition, another distinct phase transition to phase III was observed at 2.7 GPa, which was accompanied by a large volume drop of ~4.4 Å3 and a discontinuous change of the lattice parameter (Figure 3.4 and 3.5). This structure has not been observed in previous studies.29, 31 Phase II-III transformation is isostructural; both phases have the same space group symmetry and similar lattice parameter, a. Moreover, the color of the single crystal changes from orange to light yellow (Figure 3.4a). When the pressure was released to ambient pressure, the phase transitions and color of the crystal sample are completely recoverable (Figure 3.5). The lattice parameter of phase II is about twice that of phase I, hence the unit-cell volume becomes 8 times larger (Z = 8), and the tilt character changes to a+a+a+ (Figure 3.2). In phase I region, we find a to be isotropically reduced by ~1.8%, whereas in phase II a shrinks by ~3.0% (Figure 3.4, Table 3.1 and Table 3.2). The second-order Birch–Murnaghan equation of state was used to fit the V(P) data of the three obtained phases. The fit yields the zero-pressure bulk modulus B0 of 12.2(8) GPa for phase I, and 13.5(6) and 16.1(9) GPa for phases II and III, respectively; this indicates that MAPbBr3 becomes increasingly harder under compression (Figure 3.4a). In addition, as reported in a recent study, the slow kinetics emerge in the phase transformations of MAPbBr3.19 Therefore, herein we have kept the sample below 1.0 GPa for 24 hours; however, no other phase transitions were observed. The external pressure induces an intriguing structural response of the observed phases. For example, in phase I, the Pb−Br distance reduces by ~1.8% at 0.75 GPa and the PbBr6 octahedra are vertex-shared so that the Pb−Br−Pb angle is equal to 180°. 80 The Pb−Br−Pb bending angle suddenly drops to 165.8(4) ° when the phase I-II transition occurs, and it reduces considerably through the whole phase II region (by ~6.7° up to 2.4 GPa). Meanwhile, the Pb−Br distances are shortened due to the isotropic compression of the crystal, i.e., its length decreases by ~1.5% upon compression up to 2.4 GPa (Figure 3.4b). An additional abrupt increase in the bending angle was observed after the phase II-III transformation occurs. We postulate that the guest MA cations in the inflating voids play a key role in the distortion of the PbBr6 octahedra within the main framework, as illustrated in recent cases of MAPbI3 and a 3D metal-organic framework.11, 38 High pressure enhances the H-bonds and affects the interacting PbBr6 octahedral framework. The rotation of the PbBr6 octahedra within phases II and III can also be directly reflected in the dihedral angle (φ) of Pb−Br−Pb relative to its perpendicular plane, e.g., ab plane (Figure 3.4b) The dihedral angle φ is equal to 90° in phase I, and it is abruptly reduced by ~17.6° at 0.9 GPa and continues to decrease gradually under compression (phase II). On the other hand, in phase III, all the dihedral angles increase above 2.7 GPa (Figure 3.4b). In a second series of experiments, we used argon as a PTM and the structural phase diagram was determined using both high pressure single crystal and powder X-ray measurements. The results of powder and single crystal studies were in excellent agreement for all pressure points. At room temperature argon crystallizes at 1.4 GPa, but below this critical pressure it maintains perfect hydrostatic conditions. We found that under these conditions the cubic phase I is stable up to ~1.0 GPa. At 1.1 GPa a structural transformation to a mixture of phase II and phase IV (space group Pnma, Z = 4) takes 81 place (Figure 3.6 and 3.7). This mixed phase persists at least to our experimental limit of 11.9 GPa (Figure 3.7, Table 3.3 and Table 3.4). The single crystal diffraction data yield the lattice parameters a = 11.5628(10) Å for phase II, and a = 8.1744(11) Å, b = 11.545(12) Å, c = 8.1773(9) Å for phase IV at 1.0 GPa (Table 3.2). In phase IV, the Pb−Br−Pb bending angles appreciably deviate from 180°, i.e., 158.8° and 170.1° at 1.0 GPa. For proper comparison with previous studies, we performed X-ray measurements of MAPbBr3 in the absence of a PTM. Our observations here are consistent with the previous studies under similar conditions.30 We observed that the first phase transition from phase I→II occurs at ~0.4 GPa with a clear volume collapse (Figure 3.8 and 3.9).32 The second phase transition from the cubic phase II to the orthorhombic phase IV took place when the pressure increased above ~1.5 GPa (the peak splitting at ~4.3° and 8.8° in Figures 3.6c and 3.8). In agreement with previous nonhydrostatic studies beyond 2.8 GPa, we observed a gradual amorphization and the MAPbBr3 sample loses its crystalline character (the diffraction image and broad peaks in Figure 3.8). The compression evolution of the unit-cell volume is initially consistent with the single crystal data in He, but then gradually deviates from the trend observed in He (Figure 3.9). The discrepancy is caused by the stress components from nonhydrostatic conditions exerted on the sample grains. The distinct difference between the pressure induced phase transitions of MAPbBr3 under various environments demonstrates that the inhomogeneous strain has a strong effect on its structural stability. The phase stability of MAPbBr3 in helium is quite 82 different as compared to the other media: for example, no phase IV was observed at elevated pressures. A mixed phase II and IV occurs in Ar PTM when the pressure is above ~1.0 GPa. Since Ar solidifies at 1.4 GPa at 300 K, the solid environment (pseudohydrostatic) may hinder the complete transformation from II→IV.37, 39 Amorphization, on the other hand is the consequence of severe nonhydrostatic conditions caused by uniaxial stress components. In an effort to better evaluate this effect, we have used highly uniaxial pressure conditions on a single crystal sample that was bridged between the walls of the gasket and otherwise immersed in helium as a pressure medium (Figure 3.10). In this case, the sample undergoes a phase transition to phase IV at 0.4 GPa, which was the lowest pressure measured; and it transformed to an amorphous phase at 1.7 GPa, which strongly suggests that both transitions to noncubic structures and amorphization are consequences of uniaxial stress. The color of the sample under pressure changes from orange to light yellow (Figure 3.11a). The piezochromism of the MAPbBr3 sample reveals discrete band gap changes under pressure, which can be estimated from the PL spectrum. The PL spectrum was previously investigated under different hydrostatic conditions, e.g., using paraffin as a PTM40 and also without a PTM.30 The pressure-induced shift in the PL band directly depends on the structural phases of MAPbBr3. We measured high pressure PL spectra from MAPbBr3 single crystals in argon (488 nm) up to 3.1 GPa. The PL band shows a red-shift in phase I region (0−0.9 GPa), which most likely originates from the contraction of the Pb−Br bond lengths under compression. Whereas further compression in the mixed phase of II and IV region (above 0.9 GPa) leads to a distinct PL blue-shift from 83 2.33→2.39 eV during compression from 1.2→2.0 Gpa (Figure 3.11a). Such an energy change illustrates that the rotation of the PbBr6 octahedra in different phases may strongly affect the MAPbBr3 electronic structure. In addition, we measured the PL spectrum of MAPbBr3 in helium as PTM. Below 1.0 GPa, when the sample is in the cubic phase I, the PL band red-shifts upon compression. Further compression causes an abrupt PL blue-shift. Also, the sample color exhibited similar trends as when Ar was used as a PTM (Figure 3.11b). The MA cations become ordered and the PbBr6 octahedra largely tilt within phase IV compared to the cubic phase II.26 The MA cations interact with the main framework with strong N−H∙∙∙Br interactions leading to the tilt of the PbBr6 octahedra, which is most likely responsible for the electronic structures of MAPbBr3.41 We also measured the PL spectrum of a MAPbBr3 powder sample under pressure without a PTM. At 0.2 GPa, the PL spectrum shows a broad peak centered at 543 nm, from which we estimate a band gap of 2.28 eV; very close to the band gap value at ambient pressure (2.3 eV).24 In phase I region (0−0.9 GPa), a red shift of the PL band was observed, which likely originates from the decrease of the Pb−Br bond length. An apparent blue-shifted PL spectrum is present in phase IV region when the pressure exceeds ~1.5 GPa and the PL band weakens with further increasing pressure, making it difficult to detect due to the compression-induced amorphization. We also measured the PL spectrum of a MAPbBr3 powder sample under pressure using nitrogen as PTM. All nitrogen data were listed in Figure 3.12 and 3.13. We thought that below 0.4 GPa, the sample is in phase I, and around 1.4 GPa, the sample undergoes 84 phase transition, which is phase II to phase IV. In the phase II region (0.4-1.4 GPa), a red shift of the PL band was observed and followed by an apparent blue-shifted PL spectrum is present in phase IV region. As we discussed before, nitrogen is a better hydrostatic pressure transmitting medium than argon; we are expecting that the phase I and II last longer in pressure range than when argon used as PTM, and the PL spectrum shows that in Phase II, the band gap is increasing, which is inconsistent with results shown in other there PTM conditions. However, sample images in Figure 3.13 made us suspect that the sample was crushed by the gasket hole, causing uniaxial stress acting on sample. 3.2 Photoconductivity Measurements For Photoconductivity (PC) measurements, diamonds with the culet size of 500μm were used. A stainless steel gasket was preindented to 70 micron, and the central part was completely drilled out. The hole was filled with a mixture of alumina and LOCTITE STYCAST 2850KT epoxy powder (weight ratio 10:1) and pressurized till the powder mixture became transparent. The pressure chamber of 150 micrometer diameter was drilled out using laser ablation technique.42 Four Pt leads were initially built on top of one of the diamonds. A flat face of a cleaved single crystal sample was placed directly on top of the electrodes and fixed to the diamond using small dots of glue. The insulated gasket was then placed on the diamond carefully to have the sample inside its chamber and pressed gently with the opposing diamond to close any gap between the gasket and the electrodes without collapsing the hole around the sample. The cell then was opened, and the pressure chamber was filled with mineral oil surrounding the sample. 85 Typically transport measurements in the diamond anvil cell are done without PTM or solid PTM. Presence of large stress under these conditions, however, can cause breakage of the fragile MAPbBr3 samples, which would not allow proper conductivity measurements. This problem would not affect the PL measurements in which various PTM can be used. Although techniques for using liquid medium in transport measurements are developed,43 these studies remain very difficult. For PC measurements, the pressure medium itself needs to be transparent. Here we used mineral oil as PTM, which is transparent, relatively viscous liquid at ambient conditions and is also very inert. Therefore, it can be relatively easily loaded into the pressure chamber without moving the sample from the Pt leads, flowing between the sample and the leads or dissolving the glue dots. A Keithley 2400 Sourcemeter was used to measure the conductivity of the sample with and without illumination. The DAC was placed under high magnification metallurgical microscope where a focused laser beam with wavelength of 532 nm with ~0.6 nm width and 4 mW power was illuminating the sample. The dark and photo currents then were measured as a function of applied voltage between 0-100V. In this voltage range, we did not see any damage to the sample. At each pressure point, the PC data were collected multiple times and simultaneously, collecting the PL spectrum showing perfect internal consistency. Figure 3.14 shows the photocurrent increase up to 0.7 GPa prior to the phase transition (i.e., phase I→II). With increasing pressure, the photocurrent abruptly decreased above 1.0 GPa and became hardly visible at 2.0 GPa. We postulate that the 86 initial increase of the photocurrent originates from the enhancement of the charge carrier mobility under pressure due to the band gap narrowing (Figure 3.14b). Pressure-induced structural changes are responsible for the reduction of photocurrent for pressures above 1.3 GPa. This phenomenon is consistent with the PL blue-shift under pressure (i.e., band gap widening). Typically, under applied pressure, the intermolecular distance decreases, causing the overlap of the electron distributions associated with neighboring molecular to increase. Then it will increase the intermolecular transfer integrals between the highest occupied molecular orbital and the lowest unoccupied molecular orbital states. In the end, the band gap will decrease. As long as band gap is decreasing, more electrons can be excited to the conduction band; therefore, we can expect larger current at the same applied voltage. Both photoconductivity and photoluminescence of MAPbBr3 give us similar results. The photocurrent and photoluminescence recover during the decompression, but they are not enormous. DFT calculations were carried out in Prof. Zurek’s group to develop an understanding of the band gap evolution of the MAPbBr3 phases as a function of pressure. Because the Pb-Br distances, which are influenced by the cation dynamics, are key in determining the electronic structure, the bandgap fluctuates with time. Therefore, to obtain realistic trends, the calculated band gaps were averaged over a number of structures at the experimental pressure and temperature conditions without a PTM. The average gaps of the 𝑃𝑚3̅𝑚 and 𝐼𝑚3̅ phases remained relatively constant as a function of pressure, the gap of Pnma phase increased, which is unexpected as discussed before. The Pnma phase is the only one that can retain perfect symmetry when CH3NH3+ cation is 87 added to the inorganic lattice, and indeed our calculations illustrate that the band gap and Pb-Br distances at 0 K decrease with increasing pressure as seen in Figure 3.15 and 3.16. On the other hand, when the finite temperature lattice dynamics is taken into account, the band gap in this phase increases (Figure 3.17), which is in agreement with our experimental results. 3.3 Discussion In summary, we have shown that the structural and optoelectronic properties of the photovoltaic material MAPbBr3 are highly sensitive to the presence of nonuniform stress summarized in Figure 3.18. High-pressure single crystal X-ray diffraction measurements using He as a PTM demonstrate that the I → II and II → III phase transitions occur at ∼0.85 and ∼2.7 GPa, respectively, and no amorphization is observed up to 4.8 GPa. If Ar is employed as a PTM, phase I converts to a mixture of phase II and IV. Similar to helium studies and in contrast to nonhydrostatic measurements, the compression of MAPbBr3 in Ar does not lead to amorphization up to the experimental limits of 12.0 GPa. The PL study under pressure demonstrates that the pressure-induced changes of the PL band are consistent with the structural changes under compression. A PL redshift (band gap narrowing) was observed for MAPbBr3 in phase I followed by an abrupt change to a PL blueshift upon transition to the orthorhombic phase IV. These pressure-induced optoelectronic changes are associated with the H-bonds and the distortion of the PbBr6 octahedra within the MAPbBr3 structures. The stimuli responsive character of MAPbBr3 in different environments under pressure sheds light on optimizing 88 their performance and assisting in the design of perovskites-based optoelectronic devices. The presence of stress even at ambient pressure is common due to mismatch between the thermal expansions coefficients of the films and substrate in devices or just bending flexible devices, and that is an important consideration in designing photovoltaic devices that operate in a broad temperature range. 89 Figure 3.1. Ideal perovskite structure ABX3 at ambient condition. 90 Figure 3.2. Structures of various MAPbBr3 phases observed under high pressure. (a) Phase I, cubic phase having space group Pm3̅m viewed approximately along the [001] direction. (b) Phase II, cubic structure having space group Im3̅ viewed approximately along the [001] direction. (c) Phase III, which is observed here when He was used as the PTM and is isostructural to phase II, viewed approximately along the [001] direction. (d) Projection of phase IV orthorhombic structure with space group Pnma along the [010] direction. The Glazer symbols are added for the corresponding phases. Color code: green Pb atoms, orange Br atoms. For clarity, the MA molecules are not shown. 91 Figure 3.3. Sample preparation process. 92 Figure 3.4. Lattice parameters of MAPbBr3 compressed in He obtained from single crystal diffraction data. (a) Formula-unit volume (V/Z) as a function of pressure is shown in the bottom panel. The lines through the data points are second-order Birch–Murnaghan equation-of-states fit to the volume (V/Z) data in phases I-III. The unit-cell dimensions of phases I, II, and IV obtained from Ar, and phase I at ambient pressure from the literature are added for comparison. The insets show a prominent piezochromism behavior of a single crystal in a DAC chamber at 0.6 and 3.0 GPa compressed in He. (b) Evolution of Pb–Br coordination bond length and Pb–Br–Pb angle in MAPbBr3 as a function of pressure. The insets show the dihedral angle of Pb–Br–Pb relative to the ab plane and a distortion of the PbBr6 octhedra under pressure during the phase transition. Vertical dashed lines in both (a) and (b) indicate two phase transitions at ~0.85 and ~2.7 GPa of MAPbBr3 pressurized in He. 93 Figure 3.5. Integrated XRD patterns of MAPbBr3 compressed in helium (He) up to 4.8 GPa. Phase I ((𝑃𝑚3̅𝑚), phase II, and phase III (Im3̅). 94 Figure 3.6. Le Bail fit of the X-ray data of MAPbBr3 compressed in (a) Ar at 1.6 GPa, (b) He at 1.6 GPa, and (c) without PTM at room temperature. The black circles are the measured scattering intensity, and the red solid line represents the fit to the data. The vertical bars indicate Bragg reflection positions of phase II and IV together with difference profiles (blue lines) shown at the bottom. 95 Figure 3.7. Integrated XRD patterns of MAPbBr3 compressed in argon (Ar) up to 11.9 GPa and released to 3.6 GPa measured at room temperature (λ = 0.4066 Å). (a) Le Bail fit of X-ray data at 1.6 and 2.8 GPa. The black circles are the measured scattering intensity, and the red solid line represents the fit to the data. The vertical bars indicate Bragg reflection positions of the phases Im3̅ and Pnma together with difference profiles (blue lines) shown at the bottom. The apparent asymmetry in the peaks in 7-12 degrees at 0.8 GPa resulted from presence of large diffusive scattering from liquid pressure medium in this region. (b) Wide angle integrated XRD pattern of MAPbBr3 compressed in argon at room temperature (λ = 0.434 Å). 96 Figure 3.8. X-ray powder diffraction data without PTM. (a) Diffraction images and (b) integrated XRD patterns of MAPbBr3 without PTM up to 4.0 GPa. The asterisks indicate the reflections from the gasket. (c) Data refinements of MAPbBr3 sample at 0, 1.1 and 1.6 GPa for phase I, II, and IV, respectively. The diamond reflections are close to the edge of diffraction images in (a). 97 Figure 3.9. Formula-unit volume (V/Z) of phase I (P𝑚3̅𝑚), II (𝐼𝑚3̅), III (𝐼𝑚3̅), and IV (Pnma) of MAPbBr3 as a function of pressure measured by single crystal X-ray diffraction (SXRD) and powder X-ray diffraction (PXRD) using He, Ar, and without PTM. The transition pressures are marked with dashed lines. 98 Figure 3.10. Selected XRD patterns of MAPbBr3 compressed in helium (He) at RT (λ = 0.4066 Å). The insets show the color changes of the crystal sample under compression. 99 Figure 3.11. Proposed phase boundaries of MAPbBr3 compressed in various PTM using λ= 488 nm excitation laser: (a) in argon (Ar), (b) in He, and (c) without a PTM at room temperature. The black squares indicate the excitonic transition energy obtained from the PL band (right panels). The insets in the left panels of (a) and (b) show the optical macrographs of a MAPbBr3 single crystal compressed in Ar and helium, together with the polycrystalline sample without PTM in (c). 100 Figure 3.12. Integrated XRD patterns of MAPbBr3 compressed in nitrogen (N2) up to 6 GPa and released to 3.8 GPa measured at room temperature (λ = 0.4066 Å), and Le Bail fit of X-ray data at 1.2 and 2.7 GPa. The black circles are the measured scattering intensity, and the red solid line represents the fit to the data. The vertical bars indicate Bragg reflection positions of the phases Im3̅ and Pnma together with difference profiles (blue lines) shown at the bottom. 101 Figure 3.13. Proposed phase boundaries of MAPbBr3 compressed in nitrogen PTM using λ= 488 nm excitation laser at room temperature. The black squares indicate the excitonic transition energy obtained from the PL band (right panels). The insets in the left panels show the optical macrographs of a MAPbBr3 crystal compressed in nitrogen. 102 Figure 3.14. Photoconductivity measurements. (a) Photocurrent of MAPbBr3 single crystal in mineral oil as a function of voltage under pressure. (b) Evolution of PL energy as a function pressure (excitation wavelength: 532 nm). 103 Figure 3.15. The band gap (eV) vs. pressure (GPa) for the perfect symmetry Pnma structure at 0 K. 104 Figure 3.16. The minimum and average Pb-Br distance (Å) vs. pressure (GPa) for the perfect symmetry Pnma structure at 0 K. 105 Figure 3.17. The band gap (eV) vs. pressure (GPa) for the perfect symmetry Pnma structure at 300 K. 106 Figure 3.18. Phase transformations in different pressure transmitting media. Phase I (P𝑚3̅𝑚), phase II ((𝐼𝑚3̅), phase III ((𝐼𝑚3̅), and phase IV (Pnma). 107 Table 3.1. Lattice parameters of MAPbBr3 for all measured pressure points compressed in helium. P (GPa) a (Å) b (Å) c (Å) V (Å3) Phase I (Z = 1) 0.10 5.9157(6) 5.9157(6) 5.9157(6) 207.02(4) 0.30 5.8841(6) 5.8841(6) 5.8841(6) 203.72(4) 0.45 5.8600(6) 5.8600(6) 5.8600(6) 201.23(4) 0.60 5.8430(6) 5.8430(6) 5.8430(6) 199.48(4) 0.75 5.8236(7) 5.8236(7) 5.8236(7) 197.50(4) Phase II (Z = 8) 0.90 11.5844(4) 11.5844(4) 11.5844(4) 1554.60(9) 1.10 11.5382(4) 11.5382(4) 11.5382(4) 1534.88(13) 1.40 11.4817(4) 11.4817(4) 11.4817(4) 1513.60(9) 1.60 11.4526(4) 11.4526(4) 11.4526(4) 1502.09(9) 1.80 11.4008(4) 11.4008(4) 11.4008(4) 1481.90(9) 1.90 11.3945(4) 11.3945(4) 11.3945(4) 1479.40(9) 2.20 11.3219(4) 11.3219(4) 11.3219(4) 1451.30(9) 2.40 11.2965(9) 11.2965(9) 11.2965(9) 1441.56(18) Phase III (Z = 8) 3.00 11.1086(18) 11.1086(18) 11.1086(18) 1370.8(4) 3.35 11.069(2) 11.069(2) 11.069(2) 1356.2(4) 4.80 10.8939(6) 10.8939(6) 10.8939(6) 1292.86(12) 108 Table 3.2. Selected crystallographic and experimental parameters of MAPbBr3 compressed in helium. Pressure (GPa) 0.10 0.60 Phase Phase I Crystal system Cubic Space group Pm3̅m 0.75 a = b = c /Å 5.9157(6) 5.8430(6) 5.8236(7) α= β = γ /° 90 90 90 V/Å3 207.02(4) 199.48(4) 197.50(4) Z 1 1 1 Dcal (g/cm3) 3.842 3.987 4.027 Rint 0.1013 0.1636 0.1178 R1/wR2 [I>2σ(I)] a 0.0557/0.1603 0.0976/0.2324 0.0632/0.1379 Goodness of fit on F2 1.098 1.231 1.294 109 Table 3.2. Continued. Pressure (GPa) 0.9 1.4 Phase Phase II Crystal system Cubic Space group Pm3̅m 1.9 2.40 a = b = c /Å 11.5844 11.4817 11.3945 11.2965 α= β = γ /° 90 90 90 90 V/Å3 1554.6 1513.6 1479.4 1441.56 Z 8 8 8 8 Dcal (g/cm3) 4.093 4.204 4.301 4.414 Rint 0.0933 0.0944 0.1185 0.1244 R1/wR2 [I>2σ(I)] a Goodness of fit on F2 0.0452/0.1 0.0563/0.1 0.0758/0.2 0.1142/0.3 1.198 1.331 1.111 1.288 110 Table 3.3. Lattice parameters of MAPbBr3 for all measured pressure points compressed in argon. P (GPa) a (Å) b (Å) c (Å) V (Å3) 5.8534(3) 200.56(3) Phase I (Z = 1) 0.57 5.8534(3) 5.8534(3) Phase II (Z = 8) 1.0 11.5628(17) 11.5628(17) 11.5628(17) 1545.9(2) 2.5 11.2759(4) 11.2759(4) 11.2759(4) 1433.68(9) Phase IV (Z = 4) 1.0 8.1744(11) 11.545(12) 8.1773(9) 771.7(8) 2.5 7.958(2) 11.2738(15) 7.9878(18) 716.6(3) 111 Table 3.4. Selected crystallographic and experimental parameters of MAPbBr3 compressed in argon. Pressure (GPa) 1.0 2.5 1.0 2.50 Phase Phase II Phase IV Crystal system Cubic Orthorhombic Space group Im3̅ Pnma a/Å 11.5628(10) 11.2759(4) 8.1744(11) 7.958(2) b/Å 11.5628(10) 11.2759(4) 11.545(12) 11.2738(15) c/Å 11.5628(10) 11.2759(4) 8.1773(9) 7.9875(18) α= β = γ /° 90 90 90 90 V/Å3 1545.9(2) 1433.68(9) 771.7(8) 716.6(3) Z 8 8 4 4 Dcal (g/cm3) 4 4.204 4.301 4.414 Rint 0.1336 0.1242 0.1191 0.1104 R1/wR2 [I>2σ(I)] a 0.0713/0.17 0.0937/0.23 0.0990/0.25 0.0821/0.22 Goodness of fit on F2 1.171 1.148 1.091 1.077 112 3.4 Reference 1. Cava, R. J.; Batlogg, B.; Krajewski, J. 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Mechanism of Pressure-Induced Phase Transitions, Amorphization, and Absorption-Edge Shift in Photovoltaic Methylammonium Lead Iodide. J. Phys. Chem. Lett. 2016, 7 (17), 3458-3466. 113 12. Lee, Y.; Mitzi, D. B.; Barnes, P. W.; Vogt, T. Pressure-Induced Phase Transitions and Templating Effect in Three-Dimensional Organic-Inorganic Hybrid Perovskites. Phys. Rev. B 2003, 68 (2), 020103. 13. Ou, T.; Yan, J.; Xiao, C.; Shen, W.; Liu, C.; Liu, X.; Han, Y.; Ma, Y.; Gao, C. Visible Light Response, Electrical Transport, and Amorphization in Compressed Organic Lead Iodine Perovskites. Nanoscale 2016, 8 (22), 11426-11431. 14. Wang, L.; Wang, K.; Zou, B. Pressure-Induced Structural and Optical Properties of Organometal Halide Perovskite-Based Formamidinium Lead Bromide. J. Phys. Chem. Lett. 2016, 2556-2562. 15. Jiang, S.; Fang, Y.; Li, R.; Xiao, H.; Crowley, J.; Wang, C.; White, T. J.; Goddard, W. A.; Wang, Z.; Baikie, T.; Fang, J. Pressure-Dependent Polymorphism and Band-Gap Tuning of Methylammonium Lead Iodide Perovskite. Angew. Chem., Int. Ed. 2016, 55 (22), 6540-6544. 16. Capitani, F.; Marini, C.; Caramazza, S.; Postorino, P.; Garbarino, G.; Hanfland, M.; Pisanu, A.; Quadrelli, P.; Malavasi, L. High-Pressure Behavior of Methylammonium Lead Iodide (MAPbI3) Hybrid Perovskite. J. Appl. Phys. 2016, 119 (18), 185901. 17. Wang, K.; Liu, R.; Qiao, Y.; Cui, J.; Song, B.; Liu, B.; Zou, B. Pressure-Induced Reversible Phase Transition and Amorphization of CH3NH3PbI3. 2015, arXiv:1509.03717. arXiv.org e-Print archive. https://arxiv.org/abs/1509.03717 (accessed May 7, 2018). 18. Li, Q.; Li, S.; Wang, K.; Quan, Z.; Meng, Y.; Zou, B. High-Pressure Study of Perovskite-Like Organometal Halide: Band-Gap Narrowing and Structural Evolution of [NH3-(CH2)4-NH3]CuCl4. J. Phys. Chem. Lett. 2017, 500-506. 19. Szafrański, M.; Katrusiak, A. Photovoltaic Hybrid Perovskites under Pressure. J. Phys. Chem. Lett. 2017, 2496-2506. 20. Wang, P.; Guan, J.; Galeschuk, D. T. K.; Yao, Y.; He, C. F.; Jiang, S.; Zhang, S.; Liu, Y.; Jin, M.; Jin, C.; Song, Y. Pressure-Induced Polymorphic, Optical, and Electronic Transitions of Formamidinium Lead Iodide Perovskite. J. Phys. Chem. Lett. 2017, 8 (10), 2119-2125. 21. Jaffe, A.; Lin, Y.; Karunadasa, H. I. Halide Perovskites Under Pressure: Accessing New Properties Through Lattice Compression. ACS Energy Lett. 2017. 22. Sun, S.; Deng, Z.; Wu, Y.; Wei, F.; Isikgor, F.; Brivio, F.; Gaultois, M.; Ouyang, J.; Bristowe, P.; Cheetham, T. Variable Temperature and High-Pressure Crystal Chemistry of Perovskite Formamidinium Lead Iodide: A Single Crystal X-ray Diffraction and Computational Study. Chem. Comm. 2017. 114 23. Postorino, P.; Malavasi, L. Pressure-Induced Effects in Organic-Inorganic Hybrid Perovskites. J. Phys. Chem. Lett. 2017. 24. Jang, D. M.; Park, K.; Kim, D. H.; Park, J.; Shojaei, F.; Kang, H. S.; Ahn, J.-P.; Lee, J. W.; Song, J. K. Reversible Halide Exchange Reaction of Organometal Trihalide Perovskite Colloidal Nanocrystals for Full-Range Band Gap Tuning. Nano Lett. 2015, 15 (8), 5191-5199. 25. Poglitsch, A.; Weber, D. Dynamic Methylammoniumtrihalogenoplumbates (II) Observed by Spectroscopy. J. Chem. Phys. 1987, 87 (11), 6373-6378. Disorder in Millimeter‐Wave 26. Swainson, I. P.; Hammond, R. P.; Soullière, C.; Knop, O.; Massa, W. Phase Transitions in the Perovskite Methylammonium Lead Bromide, CH3ND3PbBr3. J. Solid State Chem. 2003, 176 (1), 97-104. 27. Knop, O.; Wasylishen, R. E.; White, M. A.; Cameron, T. S.; Oort, M. J. M. V. Alkylammonium Lead Halides. Part 2. CH3NH3PbX3 (X=Cl, Br, I) Perovskites: Cuboctahedral Halide Cages with Isotropic Cation Reorientation. Can. J. Chem. 1990, 68 (3), 412-422. 28. Glazer, A. The Classification of Tilted Octahedra in Perovskites. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1972, 28 (11), 3384-3392. 29. Swainson, I. P.; Tucker, M. G.; Wilson, D. J.; Winkler, B.; Milman, V. Pressure Response of an Organic−Inorganic Perovskite: Methylammonium Lead Bromide. Chem. Mater. 2007, 19 (10), 2401-2405. 30. Wang, Y.; Lü, X.; Yang, W.; Wen, T.; Yang, L.; Ren, X.; Wang, L.; Lin, Z.; Zhao, Y. Pressure-Induced Phase Transformation, Reversible Amorphization, and Anomalous Visible Light Response in Organolead Bromide Perovskite. J. Am. Chem. Soc. 2015, 137 (34), 11144-11149. 31. Jaffe, A.; Lin, Y.; Beavers, C. M.; Voss, J.; Mao, W. L.; Karunadasa, H. I. HighPressure Single-Crystal Structures of 3D Lead-Halide Hybrid Perovskites and Pressure Effects on their Electronic and Optical Properties. ACS Cent. Sci. 2016, 2 (4), 201-209. 32. Kong, L.; Liu, G.; Gong, J.; Hu, Q.; Schaller, R. D.; Dera, P.; Zhang, D.; Liu, Z.; Yang, W.; Zhu, K.; Tang, Y.; Wang, C.; Wei, S.-H.; Xu, T.; Mao, H.-k. Simultaneous Band-Gap Narrowing and Carrier-Lifetime Prolongation of Organic–Inorganic Trihalide Perovskites. Proc. Natl. Acad. Sci. U. S. A. 2016, 113 (32), 8910-8915. 33. Mao, H. K.; Bell, P. M.; Shaner, J. W.; Steinberg, D. J. Specific Volume Measurements of Cu, Mo, Pd, and Ag and Calibration of the Ruby R1 Fluorescence Pressure Gauge from 0.06 to 1 Mbar. J. Appl. Phys. 1978, 49 (6), 3276-3283. 115 34. Toby, B. EXPGUI, a Graphical User Interface for GSAS. J. Appl. Cryst. 2001, 34 (2), 210-213. 35. Dera, P.; Zhuravlev, K.; Prakapenka, V.; Rivers, M. L.; Finkelstein, G. J.; GruborUrosevic, O.; Tschauner, O.; Clark, S. M.; Downs, R. T. High Pressure Single-Crystal Micro X-ray Diffraction Analysis with Gse_Ada/Rsv Software. High Pressure Res. 2013, 33 (3), 466-484. 36. Sheldrick, G. M. A Short History of Shelx. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64 (1), 112-122. 37. Klotz, S.; Chervin, J. C.; Munsch, P.; Marchand, G. L. Hydrostatic Limits of 11 Pressure Transmitting Media. J. Phys. D: Appl. Phys. 2009, 42 (7), 075413. 38. Cai, W.; Katrusiak, A. Giant Negative Linear Compression Positively Coupled to Massive Thermal Expansion in a Metal–Organic Framework. Nat. Commun. 2014, 5, 4337. 39. Angel, R. J.; Bujak, M.; Zhao, J.; Gatta, G. D.; Jacobsen, S. D. Effective Hydrostatic Limits of Pressure Media for High-Pressure Crystallographic Studies. J. Appl. Cryst. 2007, 40 (1), 26-32. 40. Matsuishi, K.; Ishihara, T.; Onari, S.; Chang, Y. H.; Park, C. H. Optical Properties and Structural Phase Transitions of Lead-Halide Based Inorganic–Organic 3D and 2D Perovskite Semiconductors Under High Pressure. Phys. Status Solidi B 2004, 241 (14), 3328-3333. 41. Amat, A.; Mosconi, E.; Ronca, E.; Quarti, C.; Umari, P.; Nazeeruddin, M. K.; Grätzel, M.; De Angelis, F. Cation-Induced Band-Gap Tuning in Organohalide Perovskites: Interplay of Spin–Orbit Coupling and Octahedra Tilting. Nano Lett. 2014, 14 (6), 3608-3616. 42. Hrubiak, R.; Sinogeikin, S.; Rod, E.; Shen, G. The Laser Micro-Machining System for Diamond Anvil Cell Experiments and General Precision Machining Applications at the High Pressure Collaborative Access Team. Rev. Sci. Instrum. 2015, 86 (7), 072202. 43. Jaramillo, R.; Feng, Y.; Rosenbaum, T. Four-Probe Electrical Measurements with a Liquid Pressure Medium in a Diamond Anvil Cell. Rev. Sci. Instrum. 2012, 83 (10), 103902. CHAPTER 4 BENZ[A]ANTHRACENE: PRESSURE RESPONSE Benz[a]anthracene (BaA) is one of polycyclic aromatic hydrocarbons (PAHs) with chemical formula C18H12. Polycyclic aromatic hydrocarbons consist of several aromatic rings, which only have carbon and hydrogen. They are considered as one of the most abundant carbon materials in the solar system.1 Since they can be applied in organic devices,2 the investigations of PAHs have attracted considerable interests. In crystalline PAHs, the packing of the molecules usually adopts a herringbonelike motif, which makes it possible to tune their electronic properties with dopants. For example, alkali or rare-earth metal atoms are reported to occupy the free spaces, which can form novel types of organic superconductors.3-6 Applying external pressure has insightful effects on the structures and properties of PAHs.7-8 A prominent example is pentacene (C22H14), which is an aromatic hydrocarbon with five linearly-fused aromatic rings. This wide-band gap insulator undergoes an insulator-metal transition at high pressure, i.e., at 27 GPa (for single crystal) or 36 GPa (for powder).9 A condition for achieving the metallic state under high pressure is that the band gap of PAHs needs to be closed at pressures before a pressure-induced chemical reaction occurs. However, this is not always the case. The band gap reduction in pentacene 117 (C22H14) only continues up to about 10 GPa, then above that pressure chemical transformation takes place.10-11 In theory, several PAHs were predicted to become metallic in the megabar (100 GPa) regime; however, most of them undergo reactions at much lower pressures. A common reaction in PAHs under certain pressure is polymerization, which happens when the intermolecular distances are reduced to a critical threshold value.12 Polymerization has been even observed in the simplest aromatic hydrocarbon benzene (C6H6). The molecular benzene (with crystal structure P21/c) was predicted to become metallic at 190 GPa.13 However, experimentally it was found later that a polymerization takes place around 16 GPa at room temperature.14-15 The polymeric benzene maintains semiconducting behavior at least up to 209 GPa at room temperature.16 The structural evolution of several PAHs under pressure has recently been investigated using single crystal or powder X-ray diffraction.17-22 Pressure studies of BaA was mainly focused on its fluorescence spectra.23-24 Systematic high pressure studies on this material will be presented in this chapter, and the properties of Benz[a]anthracene (BaA, C18H12) will be fully investigated. Piezochromism and the structural and electronic properties of BaA up to 50.6 GPa at room temperature in a diamond anvil cell (DAC) will be discussed in the following text. I mainly participated in the studies of the structural properties of BaA; however, all results are going to be presented in this section. BaA molecule consists of four fused benzene rings. Solid BaA at ambient conditions has a monoclinic unit cell containing two molecules, with space group P21 and lattice parameters of a = 7.95 Å; b= 6.50 Å; c=12.10 Å; β = 100.5°.25 The structure of 118 BaA belongs to herringbone-type family, with the herringbone angle of 49.8° (Figure 4.1).26 4.1 Piezochromism of Benz[a]anthracene Benz[a]anthracene sample with 99% purity was purchased from the SigmaAldrich. The purified sample was loaded using rhenium or a stainless steel gasket with ~120 μm hole. The thickness of the gasket was ~65 μm, and the sample was pressurized in a DAC. Pressure was determined by using the ruby fluorescence method,27 and the equation of state of sodium chloride (NaCl)28 was also used in some of the X-ray diffraction experiments to calibrate pressure. Angle-dispersive X-ray diffraction (XRD) measurements were conducted in two different beamlines: The first series of experiments were performed at 16-ID-B beamline of the High Pressure Collaborative Access Team (HPCAT) at the Advanced Photon Source (APS), Argonne National Laboratory (λ = 0.4066 Å). In these measurements, nitrogen was used as pressure transmitting medium (PTM). Another series of experiments were performed at 12.2.2 beamline at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory (λ = 0.4966 Å), and for these studies no pressure medium was used. High pressure fluorescence measurements of BaA were performed in a WITEC Alpha SNOM Raman system by using the 488 nm excitation laser. The incident power was set to 1 mW to avoid damaging the samples. Optical visible absorption measurements under high pressure were performed on BaA polycrystalline samples using tungsten-halogen light source and Ocean Optics USB4000 spectrometer. 119 Photoconductivity measurements were performed by the quasi-four probe techniques. Pressure chamber was a 150-micron diameter hole in an insulated stainless steel gasket and was filled with alumina powder. The gasket was totally insulated by 2850KT blue epoxy. Platinum leads were cut from a 5-micron thickness Pt foil and were arranged to measure the resistivity of the sample by quasi four probe technique. A visible 415 nm blue-violet laser was used to illuminate the sample. Details on how our theoretical collaborators calculated the equation of state, band structure, optimized structure, and enthalpy using DFT method will not be presented in this section. Piezochromic materials means the materials can change colors under pressure. They have potential applications in pressure sensing devices. Piezochromism is mainly observed in organics during the grinding process, and the reversion of the color normally requires heat or solvent treatments.29-30 However, direct color conversion for organic molecular crystals using external pressure is an alternative approach that is rarely used at present.31-33 In the first series of experiments, a BaA single crystal was compressed when silicone oil was used as pressure transmitting medium. Results showed that the sample gradually changed from yellow-green to dark orange when the pressure was increased to 8.6 GPa, and the sample reverted to the original color when the pressure was released to 0.3 GPa (Figure 4.2a). However, when the crystal was compressed to ~14.2 GPa, the sample retained some orange tone after full decompression, which indicated a partial but permanent loss of molecular character. Next, another single crystal sample was loaded and pressurized directly up to 40.5 GPa, and then the pressure was released to ~0.1 GPa. 120 This time the sample retained its high-pressure dark brown color at low pressure, which is suggesting that most of the sample had gone through an irreversible chemical reaction (Figure 4.2b). Based on these observations, it was postulated the threshold pressure of chemical reaction of BaA to be ~15.0 GPa. 4.2 Fluorescence and XRD Measurements Fluorescence measurements were conducted on the BaA polycrystalline sample up to 14.3 GPa at room temperature. No pressure transmitting medium was applied for these measurements. The fluorescence spectra of BaA are shown in Figure 4.3b. The band shape of fluorescence spectra at lower pressure is similar to previous studies, except for a clear splitting of the two bands at short wavelengths.28 The fluorescence spectrum at 0.5 GPa can be deconvoluted to four vibronic bands: 0-0 (~500 nm), 0-1 (~508 nm), 0-2 (~531 nm), and 0-3 (~571 nm) from the lowest excited singlet state to the ground state (Figure 4.3c). Further compression causes the bands to be broaden, which are nearly featureless at long wavelengths (Figure 4.3b). This may be caused by the inhomogeneous deformation and nonhydrostaticity as previously observed in other PAHs, such as anthracene.28, 34-35 The broadening of bandwidth at shorter wavelengths (e.g., 0-0 and 0-1 bands) is due to the change of intermolecular interactions under compression. With increasing the pressure, the color of polycrystalline sample changes to orange and gradually darkens as discussed before (Figure 4.3a). Meanwhile, all the bands are red shifted to lower energies. When the pressure was released from 14.3 GPa to 0.2 GPa, a completely reversed change was observed. This 121 behavior agrees well with the piezochromism that was previously described. The effect of pressure on the shift of vibrionic bands is plotted in Figure 4.3c. The pressure dependence is approximately linear; the 0-1 and 0-2 bands exhibit a comparable slope of ~16.1 nm/GPa, which is greater than that of the 0-0 band (13.6 nm/GPa). After releasing the pressure back to 0.2 GPa, the sample was recompressed; however, the fluorescence was lost above 15 GPa. After recovery from 39.5 GPa, the D (disorder) and G peaks appeared at ~1380 and ~1620 cm-1, respectively, and a broad peak centered at 2850 cm-1 from the C-H stretching modes could be observed see Figure 4.4.36-37 D peak is associated with breathing mode of sp2-bonded carbon rings, and G peak is due to the bond stretching of sp2 carbon atoms.36 These peaks are consistent with a transition of BaA molecular phase to hydrogenated amorphous carbon. X-ray diffraction measurements, which are described in the next part, will prove that the sample became amorphous when the pressure was above 15 GPa. In order to probe the structural evolution of BaA under pressure, XRD measurements were carried out with and without pressure transmitting medium at room temperature. In the first run of XRD measurements at HPCAT, liquid nitrogen was used as pressure transmitting medium. The sample was compressed up to 14.4 GPa. There is no evidence of structural phase transitions over the investigated pressure range (Figure 4.5). All diffraction peaks shift to the higher 2θ angle range (smaller d-spacing) as the crystal is compressed. However, when the pressure was raised up to 14.4 GPa, the intensity of peaks decreased, and most peaks vanished. This together with prior observation of irreversibility of the appearance of the sample after decompression from 122 florescence measurements confirms occurrence of a chemical reaction above this pressure point. The diffraction patterns before the chemical reaction can be refined by the ambient-pressure phase with the space group P21 (Figure 4.5a).25 For example, the Le Bail refinement yields the following lattice parameters at 1.4 GPa: a = 7.5662(12) Å, b = 6.3644(4) Å, c = 11.727(2) Å, and β = 98.54(2) °. Another run without pressure transmitting medium were carried out at beamline 12.2.2 in ALS at room temperature. Pressure range was 0.8-38.8 GPa. Similar to the first run, no sign of first-order phase transitions was observed. When the pressure exceeds 15.4 GPa, almost no peak of crystalline phase can be detected except the peak (001), which stays up to the experimental limit of 38.8 GPa. Meanwhile, a broad peak appears in the range of 4.0-5.5°, and its intensity increases under decompression (see Figure 4.6). The (001) peak is the only existing peak after the pressure was released to 6.7 GPa. This is consistent with the pressure-induced irreversible reaction of BaA shown in the optical observations before. As shown in Figure 4.7, the d spacing of (001) peak decreases under compression in both runs. Above 15.4 GPa, peak (001) can be resolved into two peaks. The newly emerged peak (most likely from polymers) exhibits an anomalous compression (increase of d spacing) initially,38 and then back to normal during decompression. The coexistence of molecular and polymeric phases has been found previously in other hydrocarbons, such as benzene15 and phenanthrene.18 Density-functional equation of state was calculated by Prof. Yao’s group to examine the structural evolution of BaA under pressure. At ambient pressure, the calculated herringbone angle φ in the molecular phase is 46.33°, which decreases by 123 ~2.0° when the pressure increases to 9.0 GPa (see Figure 4.8). The reduction of angle φ reveals that intermolecular interaction under compression is enhancing with the nearest neighbor C-C distances decreasing. The calculated lattice parameters, the molecular volume, and the experimental data extracted from the XRD are shown in Figure 4.9. Remarkably, BaA exhibits an anisotropic response to compression along three crystallographic axes. From the experimental data, lattice parameter a has the highest compressibility. At 9.1 GPa, the value of a is reduced by 15.2% from its ambientpressure value, whereas b and c are reduced by 9.3% and 11.5%, respectively, which altogether yields a volume reduction of ~ 31.1% (Figure 4.9). The monoclinic angle β decreases by 5.5% in this pressure range. Based on the third-order Birch-Murnaghan39 equation of state, the zero-pressure experimental and theoretical bulk moduli B0 are 9.6 GPa and 10.2 GPa, respectively, and the pressure derivative B' are 4.7 and 7.8. Moreover, the molecular phase P21 at different pressures up to 300 GPa at 0 K is optimized by our collaborators. Below 117 GPa, this structure undergoes substantial distortions as the pressure increases but maintains the molecular identity. At 118 GPa, the molecular phase transforms to a polymeric phase (labeled as polymer I) but within the same P21 space group. The enthalpy calculations show that the polymer I phase is thermodynamically more stable than the molecular P21 phase above 30 GPa (Figure 4.10). Thus, there are likely significant kinetic barriers for the polymerization of BaA, which delay the transition. Intermolecular interactions are enhancing under pressure. These interactions eventually promote the molecules to interconnect together forming amorphous hydrocarbon polytypes that ultimately transform to polymers. The direct transition of 124 BaA molecules to crystalline polymeric phase is hindered by activation barriers. 4.3 Absorption and Photoconductivity Measurements To better investigate bandgap of BaA, photoconductivity and absorption measurements were carried out. Absorption spectra were collected at different pressures using visible light for the powder sample at room temperature. The measured energy gap decreases from 2.4 eV at 0.5 GPa down to 1.0 eV at 50.6 GPa (see Figure 4.11). The band gap is also studied by our collaborators using DFT calculations. The results show that the band gap becomes smaller as the pressure increases. BaA does not reach a metallic state after it transforms to the polymeric phase, but it behaves as a narrow-band gap semiconductor at higher pressures. The intermolecular distances in crystalline BaA decrease under compression, which enhances the orbital overlap and reduces the band gap.40 At 118 GPa, the molecular phase converts to polymer I and the band gap jumps to 4.3 eV. The band gap of polymer I only reduces about 5% when the pressure is increased to 300 GPa (Figure 4.11b). The photoconductivity of polycrystalline BaA samples was carried out under pressure at room temperature. The sample was illuminated with the blue violet light. Figure 4.12 shows the steady increase in photocurrent up to 13.4 GPa before onset of the chemical reaction. The increase of the photoconductivity of BaA molecular phase is attributed to an increase of charge carrier mobility under pressure, which is similar with previously phenomenon observed in other PAHs such as pentacene and rubrene.41-42 125 4.4 Discussion BaA displays remarkable piezochromism without any phase transition prior to polymerization. It shows reversible color change from yellow-green to light orange between ambient pressure and ~15 GPa, and irreversible color change when pressure is above 15 GPa. Synchrotron X-ray diffraction measurements with and without pressure transmitting medium reveal that there are no structural phase transitions within this pressure range. At ~15 GPa, an irreversible chemical reaction takes place and above in which the polymeric products of BaA become amorphous. DFT calculations show that the molecular phase becomes less favorable than its polymeric phase at as low as 30 GPa; however, the direct transformation to the latter is hindered by the activation barrier, which means temperature factor will make the threshold happen earlier since measurements were performed at room temperature, whereas DFT calculations were done at 0 K. This amorphous state is a metastable state. When a certain temperature is considered, the critical transition pressure to the amorphous state is lowered as calculations show. This is because of adding energy to the system; the system can pass the kinetic barriers and reach the energetic threshold between molecular phase and the amorphous state. The visible light responses of BaA to pressure, including the reduction of the band gap and increasing of the photocurrent, revealed considerable orbital overlap and increase of charge carrier mobility at high pressure. In a word, our studies on BaA provide new insights on the herringbone-type PAHs with different molecular configurations and their tunable electronic properties 126 under pressure. Majority of the figures shown in this chapters are credit of Dr.Weizhao Cai. 127 Figure 4.1. Structure of BaA. (a) Molecular structure of BaA. (b) Herringbone-type crystal structure of BaA viewed along the [001] direction. 128 Figure 4.2. Optical macrographs of a BaA single crystal compressed in silicone oil to 14.2 GPa and released to 0.2 GPa. Another single crystal was compressed up to 40.5 GPa and decompressed to 0.1 GPa. Few ruby chips for pressure calibration lie on the top side of the chamber for (a) and (b). 129 Figure 4.3. Fluorescence measurements. (a) Microphotographs of a BaA polycrystalline sample upon compression to 14.3 GPa at room temperature. (b) Fluorescence spectra of polycrystalline BaA measured at different pressures. The 0-0 band was partially cut off by the Notch filter below 0.5 GPa. The excitation laser wavelength is 488 nm. (c) Deconvolution of the fluorescence spectra at 0.5 and 8.4 GPa. (d) Vibronic bands shift sas a function of pressure. Decompression data at 0.2 GPa are shown by open symbols. 130 Figure 4.4. Raman spectrum of recovered BaA sample from 39.5 GPa at room temperature. 131 Figure 4.5. XRD patterns of polycrystalline BaA with liquid nitrogen as PTM up to 14.4 GPa measured at room temperature (λ = 0.4066 Å). (a) Asterisks and squares indicate the reflections from solid nitrogen (hexagonal phase) and gasket. (b) Le Bail fit of X-ray data at 1.4 GPa. The black circles are the measured scattering intensity, and the red solid line represents the fit to the data. The vertical bars indicate Bragg reflection positions of the phase P21 together with difference profiles (blue lines) shown at the bottom. The fitted R values are Rp = 0.26% and Rwp = 0.59%. 132 Figure 4.6. Integrated XRD patterns of BaA upon compression up to 38.8 GPa (black) and released to 6.7 GPa (orange) measured at room temperature (λ = 0.4966 Å). The diffraction peaks marked with triangles and circles indicate peaks from NaCl and Re gasket, respectively. 133 Figure 4.7. Changes of 2θ angles of (001) peak as a function of pressure in BaA. The inset indicates the d spacing of (001) peak as a function of pressure. The cycles indicate reflections from newly formed polymers. The decompression data are illustrated as unfilled symbols. 134 Figure 4.8. Changes of herringbone angle φ as a function of pressure. The inset shows the definition of angle φ. 135 Figure 4.9. Lattice parameters of BaA. (a) Evolution of lattice parameters of BaA as a function of pressure. (b) The molecular volume (V/Z) as a function of pressure. The solid line is the third-order Birch–Murnaghan equation of state fitted to the volume data. Experimental and calculated data are indicated as circles and triangles, respectively. 136 Figure 4.10. Calculated pressure-dependent enthalpies of BaA for molecular phase P21 and polymer I from ambient pressure to 300 GPa. The polymer I becomes more thermodynamically stable than the molecular phase at 30 GPa while the kinetic barrier for this transition vanishes at 117 GPa. 137 Figure 4.11. Absorption measurements. (a) Pressure-dependent absorption spectra measured from BaA polycrystalline sample. (b) Band gap energy changes as a function of pressure. Three different experimental runs and DFT data are shown by solid and open symbols, respectively. The inset enhances the calculated band gap around the polymerization pressure. 138 Figure 4.12. Photocurrent as a function of voltage under high pressure in BaA at room temperature. The inset shows the photocurrent change as a function of pressure at 100 V. 139 4.5 Reference 1. Harvey, R. G. Polycyclic Aromatic Hydrocarbons. 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R.; Gavezzotti, A. Crystal Structures of Polynuclear Aromatic Hydrocarbons. Classification, Rationalization and Prediction from Molecular Structure. Acta Crystallogr., Sect. B 1989, 45 (5), 473-482. 27. Mao, H. K.; Bell, P. M.; Shaner, J. W.; Steinberg, D. J. Specific Volume Measurements of Cu, Mo, Pd, and Ag and Calibration of the Ruby R1 Fluorescence Pressure Gauge from 0.06 to 1 Mbar. J. Appl. Phys. 1978, 49 (6), 3276-3283. 28. Decker, D. L. High Pressure Equation of State for NaCl, KCl, and CsCl. J. Appl. Phys. 1971, 42 (8), 3239-3244. 29. Dong, Y.; Zhang, J.; Tan, X.; Wang, L.; Chen, J.; Li, B.; Ye, L.; Xu, B.; Zou, B.; Tian, W. Multi-Stimuli Responsive Fluorescence Switching: the Reversible Piezochromism and Protonation Effect of a Divinylanthracene Derivative. J. Mater. Chem. C 2013, 1 (45), 7554-7559. 30. Ito, H.; Saito, T.; Oshima, N.; Kitamura, N.; Ishizaka, S.; Hinatsu, Y.; Wakeshima, M.; Kato, M.; Tsuge, K.; Sawamura, M. Reversible Mechanochromic Luminescence of [(C6F5Au)2(miu-1,4-Diisocyanobenzene)]. J. Am. Chem. Soc. 2008, 130 (31), 10044-10045. 31. Nagura, K.; Saito, S.; Yusa, H.; Yamawaki, H.; Fujihisa, H.; Sato, H.; Shimoikeda, Y.; Yamaguchi, S. Distinct Responses to Mechanical Grinding and Hydrostatic Pressure in Luminescent Chromism of Tetrathiazolylthiophene. J. Am. Chem. Soc. 2013, 135 (28), 10322-10325. 32. Wang, X.; Liu, Q.; Yan, H.; Liu, Z.; Yao, M.; Zhang, Q.; Gong, S.; He, W. Piezochromic Luminescence Behaviors of Two New Benzothiazole-Enamido Boron Difluoride Complexes: Intra- and Inter-Molecular Effects Induced by Hydrostatic Compression. Chem. Commun. 2015, 51 (35), 7497-7500. 33. Wang, Y.; Tan, X.; Zhang, Y.-M.; Zhu, S.; Zhang, I.; Yu, B.; Wang, K.; Yang, B.; Li, M.; Zou, B.; Zhang, S. X.-A. Dynamic Behavior of Molecular Switches in Crystal under Pressure and Its Reflection on Tactile Sensing. J. Am. Chem. Soc. 2015, 137 (2), 931-939. 142 34. Dreger, Z. A.; Lucas, H.; Gupta, Y. M. High-Pressure Effects on Fluorescence of Anthracene Crystals. J. Phys. Chem. B. 2003, 107 (35), 9268-9274. 35. Dreger, Z. A.; Balasubramaniam, E.; Gupta, Y. M.; Joly, A. G. High-Pressure Effects on the Electronic Structure of Anthracene Single Crystals: Role of Nonhydrostaticity. J. Phys. Chem. A 2009, 113 (8), 1489-1496. 36. Jackson, B. R.; Trout, C. C.; Badding, J. V. UV Raman Analysis of the C:H Network Formed by Compression of Benzene. Chem. Mater. 2003, 15 (9), 1820-1824. 37. Casiraghi, C.; Ferrari, A. C.; Robertson, J. Raman Spectroscopy of Hydrogenated Amorphous Carbons. Phys. Rev. B 2005, 72 (8), 085401. 38. Cai, W.; Katrusiak, A. Giant Negative Linear Compression Positively Coupled to Massive Thermal Expansion in a Metal–Organic Framework. Nat. Commun. 2014, 5, 4337. 39. Birch, F. Finite Elastic Strain of Cubic Crystals. Phys. Rev. 1947, 71 (11), 809−824. 40. Troisi, A.; Orlandi, G. Dynamics of the Intermolecular Transfer Integral in Crystalline Organic Semiconductors. J. Phys. Chem. A. 2006, 110 (11), 4065-4070. 41. Rang, Z.; Haraldsson, A.; Kim, D. M.; Ruden, P. P.; Nathan, M. I.; Chesterfield, R. J.; Frisbie, C. D. Hydrostatic-Pressure Dependence of the Photoconductivity of SingleCrystal Pentacene and Tetracene. Appl. Phys. Lett. 2001, 79 (17), 2731-2733. 42. Rang, Z.; Nathan, M. I.; Ruden, P. P.; Podzorov, V.; Gershenson, M. E.; Newman, C. R.; Frisbie, C. D. Hydrostatic Pressure Dependence of Charge Carrier Transport in Single-Crystal Rubrene Devices. Appl. Phys. Lett. 2005, 86 (12), 123501. CHAPTER 5 LITHIUM: PRESSURE AND TEMPERATURE RESPONSE Lithium, with only three electrons, is the simplest and lightest metal in the periodic table under ambient condition. Because of this simple electronic structure, properties of lithium are expected to be uncomplicated. On the other hand, due to its small mass, which causes the de Broglie wavelength to reach a distance of about a (a is the interatomic distance) for quantum solids, it may be expected to have complex quantum behavior. It still requires a lot of experimental efforts to fully understand the correlation between the electronic and structural transformations and other properties of alkali metals. Lithium has complicated pressure and temperature induced structural behavior. However, isotope effect in lithium is not fully investigated. Our results will present a comprehensive picture of how the quantum interplay of the nuclei and electrons leads to surprising unsuspected behavior in this simple element. To show the quantum effects in lithium, very low temperatures are needed to lower the system energy to the level that quantum effects are dominant in the system. This is only because, when the temperature is low enough, quantum effects become visible, as opposed to the classical behavior. 144 For light elements (lithium, sodium), the nuclear zero-point vibrations can be orders of magnitude larger than the very small free energy differences between all possibilities of crystal structures.1 Quantum effects are especially noticeable in metallic systems of low mass at high densities, leading to very unusual properties as well as unconventional state of matter.2-5 As pressure increases, the importance of the zero-point energy can either increase or decrease depending on whether the system has Coulomb 1 interaction, which is proportional to the 𝑟 or short-range interaction, which is usually described by means of exponentially decaying potentials. Unfortunately, there is no direct method to measure the zero-point energy, but the differences between isotopes provides some insight into the dependence of the zero-point energy on compression. For example, 4 He is almost twice as dense as 3He at the same pressure and temperature,6 and it has a hexagonal rather than cubic crystal structure. They are not crystallized under ambient pressure regardless of how low the temperature is. Because of their zero point energy, i.e., they remain liquid even at T=0 K. Away from ambient conditions of pressure and temperature, lithium exhibits complicated behavior including a lot of temperature-and pressure-induced phase transitions to low-symmetry structures,7 metal to semiconductor (~80 GPa) to metal transitions (~120 GPa),8-9 and superconductivity with an anomalous isotope effect.10-13 At room temperature and pressures below 7 GPa, 7Li crystallizes in the bcc structure. When cooled below T≈77 K (at P = 0 GPa), bcc 7Li undergoes a martensitic transition to a close-packed structure,14 which is identified as the 9R structure. 9R structure has a nine-layer stacking sequence and has previously been assumed to be the 145 ground-state structure of lithium. It is unexpected that such a simple metal would have a very complicated crystal structure at zero pressure. However, despite many theoretical efforts to understand the small differences between the energies of various close-packed structures, there is still no clear explanation in the literature.15-17 The material packing arrangement in solids depends on the Gibbs energy (G) in thermodynamics, which is defined as 𝐺 = 𝑈 + 𝑃𝑉 − 𝑇𝑆 (5.1) where U is the internal energy, V is volume, P is pressure, and S is the entropy. G is the minimized potential when the system reaches equilibrium at constant pressure and temperature. Entropy difference between phases can drive thermal phase transitions, but the kinetics of the transition can prevent some transformations. Since the free-energy configurations of materials often contains many local minima, the fundamental role of quantum effects in controlling the kinetics of changing between them has only recently been recognized.18 Therefore, to determine the thermodynamic states of materials, the phase diagram needs to be constructed along certain the P-T paths. However, studies on lithium are extremely challenging at high pressure. Lithium reacts with many materials, and these reactions cause embrittlement of the gasket metals and diamonds. The chemical reactivity of lithium gets worse under high pressure. 146 Diffraction experiments are also challenging due to low X-ray and neutron scattering cross sections. The high-pressure and low-temperature structural phase diagram of lithium has previously been constructed based on limited studies and only using 7Li.7, 19-20 The crystal structures of both 6Li and 7Li were mapped out as a function of temperature and pressure using X-ray angle dispersive diffraction by the High Pressure Collaborative Access Team at the Advanced Photon Source. The design and maintenance as well as experimental support were provided by Drs. S. Sinogeikin, J. Smith, and C. Kenney-Benson. In situ diffraction pattern of samples with 99.99% lithium content were collected using a ~30.5 KeV (λ = 0.4066 Å) X-ray beam. The samples were enriched 7Li (99.9% 7Li obtained from Oak Ridge National Labs with metallic trace of Ca (< 195 ppm), K (< 75 ppm) and Na (< 300 ppm)) and 6Li-rich (95.6% 6Li and 4.4% 7Li together with metallic trace elements of Ca (< 280 ppm), K (< 100 ppm), and Na (< 240 ppm), (Sigma-Aldrich). Rhenium or stainless steel was used as gasket material. To prevent contamination and the reaction with nitrogen, all of the sample preparation was performed inside a high-purity argon glovebox (O2 and H2O < 0.1 ppm, see Figure 5.1), and the samples had never been exposed to the atmosphere. The mineral oil on the samples was removed by multiple steps of rinsing in ultra-high purity pentane, and then we used clean razor blades to remove the unfresh surface layers. The samples were loaded in the gasket together with pressure calibrators (ruby and NaCl). Helium gas loading was done by Dr. Tkachey in GSECARS sector of APS. The experiments were conducted during multiple beamtimes in APS beamline 16-ID-B. 147 5.1 6Li and 7Li Under Pressure at Low Temperature With only three electrons, lithium is a very poor scattering material of X-rays, and the low-temperature structures of 7Li have mainly been studied by neutron scattering. Neutron studies have not been performed on 6Li because of its high neutron absorption cross section. Hence, the P-T structural phase diagram of 6Li has not previously been reported. Moreover, the structural boundaries of 7Li below 80 K were uncertain, especially with respect to transformations between the martensite and the fcc phase. While initial studies by our group using neutron scattering revealed the boundaries of hR3 and fcc are different from what has been previously thought but due to limitation in the X-ray experimental capability, we could not show where these boundaries exactly are located. Luckily, advances in synchrotron beam quality allow exploration of the lowtemperature and high-pressure regions of the lithium phase diagram using diamond anvil pressure cells. Moreover, a new background subtraction method was investigated on XRD measurements. First, one measurement was carried out on the sample. The XRD signal was coming from the sample and background. And then, another XRD measurement was performed on the area without the sample, which including just the signal from background. The difference between those signals is just signal from the sample. By using this method, XRD signals are nice and clean to clarify the martensite boundaries. In addition, fine pressure control can be achieved by applying the double membrane. First of all, we observed a large difference between the martensitic transition 148 region in the 6Li and 7Li samples under similar conditions, see Figure 5.2. This contrasts early ambient-pressure studies, which addressed that there was no difference between these two isotopes.21-22 These studies showed that the temperature dependence of specific heats of 6Li and 7Li samples during heating occurred between 90 and 170 K in both cases. These anomalies were ascribed to a reverse transition from the martensite. Although the transformation appeared to be very similar in both isotopes, in 6Li sample, smaller fraction of bcc phase appeared to transform to the martensite and ended at a slightly higher temperature. Our results showed that the 6Li samples within the resolution of the experiment did not show any evidence of a martensitic transition or any other structural phase transitions during isobaric cooling between 0.2 GPa and 2.0 GPa (Figure 5.2A and 5.3A and 5.4). They remained in the pure bcc structure down to the lowest temperature we measured (~16 K), regardless of whether we used helium or mineral oil as the pressure transmitting medium. However, isobaric cooling 7Li at pressures below 3.3 ± 0.3 GPa always resulted in the appearance of martensite peaks mixed with bcc peaks whenever below 75 K. Under hydrostatic conditions during isobaric cooling to low temperature, bcc 7Li transformed to fcc at pressures higher than 3.3 ± 0.3 GPa. This removes the uncertainty in the previous boundaries of the fcc and martensite states.19 For 6Li, we observed the first evidence of a transformation from the bcc phase to the martensite at ~20 K and ~2 GPa during both isobaric cooling and isothermal compression. With further pressurization of the 6Li sample to ≥4.5 GPa at T = 20 K, we 149 observed the appearance of additional fcc peaks (Figure 5.5). For 6Li, the point at which bcc transformed to fcc was above 4.0 GPa (Figure 5.2A). These boundaries based on isobaric cooling do not always match up with the results from isothermal compression. It means the thermal path is an important factor for altering lithium structure. The fcc structure always remained stable during isobaric cooling (Figure 5.2A, B), and unlike the bcc structure, it did not undergo a martensitic transformation. We also found the critical pressure for bcc→fcc at room temperature; it was independent of the isotopic mass. Moreover, we observed that the pressure onset of phase transitions that include low-temperature paths depended strongly on the specific temperature-pressure path taken. For example, reaching ~3.5 GPa and ~20 K during isothermal compression of the 7Li sample led to mixed bcc and martensite structures (data point 3 Figure 5.2B), whereas during isobaric cooling to the same P-T point, no evidence of the martensite was observed, and instead the sample crystallized in fcc plus bcc (data point 7 Figure 5.2B). We also found (Figure 5.2, 5.6 and 5.7) that the martensite phase was only ever obtained from bcc→martensite transformations. Although transitions from martensite to fcc were seen, the reverse process never occurred. In the lowest pressure isobaric cooling path (0.2-0.5 GPa), a twin-chamber design was used (see Figure 5.8). In this design, both 7Li and 6Li were loaded in the same gaskets but in two different chambers using mineral oil as pressure transmitting medium. This kind of design allowed the samples to follow identical thermal paths. We pressurized the bcc 7Li sample at ambient temperature to above 8 GPa to produce fcc and then cycled back across the phase boundary to demonstrate low 150 hysteresis (Figure 5.2D). We then cooled at about 10 GPa to 20 K and depressurized to 2 GPa, reaching the region previously ascribed to the 9R phase. We then warmed the sample up to 120 K, which was above the reported 9R phase boundary.19 Surprisingly, the sample remained fcc throughout this whole thermal path. Further warming to room temperature induced a transition back to bcc. Finally, we cooled back to 100 K and repressurized until the martensitic phase appeared above 3 GPa. A similar result was found for 6Li where we synthesized the fcc ground state by pressurizing to 9 GPa, cooling to 20 K, and then depressurizing to ambient pressure (Figure 5.2C). It showed the sample remained fcc until the last data point. Prior to our experiments, the ground state of lithium remained unsolved and controversial. The 9R structure was believed to be the ground state of lithium based on the consistent diffraction patterns from ambient-pressure isobaric-cooling studies,23-26 therefore, fcc phase was ignored by theorists, even in high pressure work.27-32 However, the observed bcc to 9R transition is not reversible and upon heating, 9R phase transforms to fcc before returning to bcc phase (see Figure 5.9). Our collaborations in the University of Edinburgh theory group led by Prof. Ackland performed well-converged free energy calculations using all-electron density functional theory (DFT), revealing that fcc is the actual ground state, and bcc structure is stable at high temperature and low pressure since it has higher entropy and volume. Absence of the reverse transformation from fcc to bcc or a martensite structure during the isothermal decompression at low temperature as we observed is precisely what theoretical was predicted to occur. Bcc to fcc transition is kinetically hindered, and it will 151 undergo the bcc to martensite phase transition instead. In classical thermodynamics, free-energy differences between phases are independent of the nuclear mass, but at low temperatures, quantum effects can lead to differences in behavior for different isotopes, since both vibrational and zero-point effects are mass-dependent. 7Li and 6Li have a large relative mass difference: ∆𝑚 𝑚7 ~15%. In lithium, the zero-point energy is large at ~40 meV/atom, equivalent to 500 K (KT=0.02585 eV at room temperature). The zero-point energy is larger in 6Li than 7Li by 7 a factor of √6. Early ambient-pressure measurements showed that at low temperature, 6Li has a slightly larger bcc lattice constant about 0.04% difference.22 Also, isotope effects in the shear modulus in lithium under pressure are evidence of the quantum contribution.3334 Although the large zero-point energy of lithium has a large contribution to the vibrational energies and may influence its equilibrium structures,32-33 the difference in the martensite transition line during isobaric cooling paths of the two isotopes is unexpected. Zero-point effects contribute to the pressure, so that a higher pressure is required to compress 6Li to the same volume as 7Li, since 6Li has a bigger zero point energy. Our collaborations calculated and proved that bcc to fcc transition shows only a very small mass dependence, but the bcc to 9R transition shows a very strong mass dependence. Although 9R is not a stable phase of lithium and can only be obtained starting from bcc, the conditions where this occurs are reproducible and isotope-dependent. Under hydrostatic conditions, the transition line bcc→9R is suppressed to lower temperature and higher pressure in the 6Li samples compressed with 7Li. By contrast, the 152 transition line 9R→fcc appears to be at higher pressure in 6Li: As discussed above, the 9R→fcc crossover pressure for isobaric cooling is 3.3 ± 0.3 GPa for the 7Li and at least 4.0 GPa for the 6Li material. This suggests that the role of quantum effects is to inhibit the transition rather than to destabilize the 9R phase. 5.2 Conclusion Our experimental results are well consistent with theoretical calculations done by our collaborators on properties of lithium; this clarifies the understanding of this simplest of metals. In both 6Li and 7Li, the ground state structure is fcc rather than 9R. A combination of zero-point energy and vibrational entropy stabilizes the bcc structure where it exists. Only the transition from bcc creates a metastable martensite, as it does not arise from the structure’s thermodynamic stability caused by different free energy but from the kinetic effects, the diffusive bcc→fcc phase transition has no low-energy path, so instead bcc undergoes a martensitic transition with large hysteresis on both isobaric and isothermal paths, which is a diffusionless phase transition. In contrast to 7Li, we observed that in the 6Li samples, the bcc structure remained stable compared to the martensite to the lowest measured temperatures for pressures up to ~2 GPa. The difference between 6Li and 7Li indicates that quantum effects may play a crucial role in the transition kinetics. Dr Weizhao Cai contributes a lot for the figures in article: Quantum and isotope effects in lithium metal. 153 5.3 Potassium Under Pressure at Low Temperature The electron density of alkalis increases rapidly with pressure, their electronic structures changes dramatically. While high pressure structural boundaries of Li, Na, K are determined partially, these boundaries at low temperature remain mostly unexplored.7, 35-37 Superconductivity of elements and compounds can be strongly altered by applying pressure.38-42 However, pressure can suppress superconductivity in some materials, but it favors it in others.43-45 Previously, properties of lithium under pressure were investigated and they showed superconductivity above 30 GPa , with a pressure dependent transition temperature of 20 K at 48 GPa.12 Transition temperature Tc increases rapidly with pressure in fcc phase, reaching values around 12-17 K. However, for potassium, the structural and transport properties under pressure and low temperature are not fully explored.46-48 Based on our studies of lithium, we know that the thermal path plays an important role in determining the proper kinetics of phase transitions. The wise way is to perform parallel measurements of multiple properties of materials under pressure at low temperature. Pressure is known to induce superconductivity in many elements or compounds, which are not superconducting at ambient conditions. The aim here was to measure the XRD and resistivity of K under pressure at low temperature, and to investigate its superconductivity characteristics. The resistivity of a meatal can be described by equation 5.2 𝜌= 𝑚 𝑛𝑒 2 𝜏 (5.2) 154 where m is the effective mass of electrons, n is the number density of electrons, and 𝜏 is the collision time. Measurements of the resistance of K under pressure at low temperature were performed. Like lithium, potassium is also reactive, and to prevent contamination, the sample was prepared inside the high purity argon glovebox (H2O and O2 < 0.1 ppm). The sample was pressured using gas membrane, and DAC was cooled in a gas helium cryostat. To measure the resistivity, the platinum probes were arranged according to procedures in Chapter 2. The maximum pressure reached is about ~ 18.4 GPa, and the low temperature is ~ 3 K. We did not see any indication of superconductivity, and the resistance changes with pressure and temperature are shown in Figure 5.10. Resistance changes in lithium at high pressures and low temperatures were reported a few years ago.43 According to our results, the resistances of potassium does change with pressure and temperature. Resistance decreases with lowering the temperature at a certain pressure. This trend is due to the weakening of the scattering with decreasing temperature; therefore, the electrical conductivity of potassium is going to be larger, i.e., its resistance is becoming smaller. This can be seen according to the equation 5.2. At very low temperature, resistance appears to have reached a plateau. It most likely is because of electron-impurity scattering or electron-defect. Additionally, resistance increases with pressure at certain temperature up to 17.5 GPa. Resistance suddenly decreases under the next pressure point 18.4 GPa. The sample got shorted after 18.4 GPa during the measurements (Figure 5.11). Since 18.4 GPa is the last pressure data 155 point, the reliability of this set of data is questionable and needed to be clarified in the future. 156 Figure 5.1. Argon glovebox system in Prof. Shanti Deemyad’s lab. A microscope is used for viewing the sample while loading. 157 Figure 5.2. Observed stable and metastable crystal structures of 6Li and 7Li measured along the identified P-T paths. (A) Isobaric cooling paths are connected by gray lines as guides to eye. Data points we collected during isothermal compression or isobaric warming are labeled in numerical order. We used mineral oil (crossed symbols) or He (dotted and solid symbols) as pressure transmitting media. Blue dotted lines show the onset of the bcc to close-packed transitions on cooling. (B) Isobaric results for 7Li. Open symbols are data from previous studies measured either using mineral oil or no pressure medium during isothermal compression and isobaric cooling. Points 3 and 7 are very close in P and T but were approached via different thermal paths — the resulting structures are 9R + bcc vs. fcc + bcc, respectively. (C) Experimental paths for 6Li in P-T space to examine the possibility of a reverse transformation from fcc→9R during decompression. Dotted lines are the transition lines from (A) and (B). During decompression, we observed the pure fcc structure deep in what was previously identified as the 9R stability region. (D) Experimental paths for 7Li in P-T space with the same observation of the fcc structure in the 9R stability region. Points 12–14 show the martensitic transition of 7Li during isothermal compression, followed by a transition to fcc. 158 Figure 5.3. Synchrotron X-ray diffraction patterns of 6Li at variable pressures and temperatures. The angle-dispersive diffraction measurements were performed using a wavelength of 0.4066 Å. (A) Selected diffraction patterns of 6Li from three different cooling paths. (Points 1→4, 8→10 and 11→13 in Figure 5.4). The reflections from bcc (red) and martensite (blue) phases are labelled by their hkl indices, using the 9R structure for the martensite. Not all 9R peaks are visible because the sample recrystallized to a highly textured quasi-single-crystal. (B) Diffraction patterns of 6Li during cooling to the base temperature 17 K and isothermal decompression to 0.5 GPa (points 18→22 in Figure 5.4). Only pure the fcc phase (green) was observed (Figure 5.3). For clarity, the Compton scattering of the diamonds and reflections from the cryostat window have been removed in both panels A and B. 159 Figure 5.4. Various thermal paths and the structures of the 6Li sample using helium as pressure transmitting medium. Numbers and arrows are guides to the eye for following the thermal history of the sample. Open symbols are used for data acquired during warming and decompression. 160 Figure 5.5. Image plates of 6Li together with pressure monometers at different pressure and temperatures. The diffraction lines of NaCl (bcc) (and solid helium at low temperature (fcc)) together with ruby fluorescence used to determine the pressure. Panel F shows lithium in mixed 9R+bcc phase where diffraction lines of lithium are clearly smeared and split showing highly texture features. Unlabeled patterns are cryostat and DAC background that has no pressure dependence. Diamond reflections are masked. 161 Figure 5.6. Synchrotron X-ray diffraction patterns of 7Li during cooling under nearly isobaric conditions. 162 Figure 5.7. Various thermal paths and the structures of the 6Li sample using mineral oil as the pressure transmitting medium. Numbers and arrows are guides to eye for following the thermal history of the sample. 163 Figure 5.8. Left picture shows a micrograph of the helium loaded gasket with piece of lithium and ruby surrounded by helium. On the right panel showing 2-D X-ray image of the lithium isotope samples loaded inside the twin chamber design gasket (red spots are samples). 164 Figure 5.9. Phase transitions in lithium as function of pressure and temperature. 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Sakata, M.; Nakamoto, Y.; Matsuoka, T.; Ohishi, Y.; Shimizu, K. In Structural Phase Transition of Potassium Under High-Pressure and Low-Temperature Condition, Journal of Physics: Conference Series, IOP Publishing: 2017; p 042020. CHAPTER 6 DISCUSSION In this chapter, I summarize what was presented in Chapters 1 through 5 and draw the conclusions from various experimental tools in brief to give a clearer picture of this dissertation to readers. Chapter 1 is the introduction section in the dissertation. In this part, what pressure can do when applied on the materials and why it can tune and alter the electronic and structural properties of matter were explained. Many phenomena will be produced due to high pressure including conversion from a metallic state to semiconductor, amorphization caused by pressure etc. Therefore, the aim of Chapter 1 is to motivate the high pressure research. In Chapter 2, experimental techniques used in high pressure studies were introduced. First of all, the major tool for generation of high pressure known as diamond anvil cell was described in detail. In order to study the structure of various of materials under pressure, synchrotron X-ray diffraction measurement was introduced and also other measurements such like photoluminescence, absorption, and photoconductivity spectroscopy were also shown to evaluate the correlation between electronic and structural properties. 172 In Chapter 3, 4 and 5, three different materials Methylammonium lead bromide perovskite, Benz[a]anthracene, and Lithium were studied under high pressure. Each chapter showed how the material responds to the pressure and temperature as summarized next. 6.1 Phase Transitions Under Pressure In Chapter 3, we showed that semiconductor organic-inorganic perovskite MAPbBr3 has different phase transitions using different transmitting media (Helium, Argon, nitrogen and no PTM), which tells us that hydrostatic conditions play a significant role in phase transition. When helium used as PTM, phase I → II and II → III transitions occurs at ~ 0.85 and ~2.7 GPa, no indication of amorphization is observed up to 4.8 GPa. In this pressure region, helium creates a nice hydrostatic condition. When using argon as PTM, which is perfect hydrostatic up to ~1.4 GPa, phase I converts to a phase II+IV at ~0.9 GPa and doesn’t turn to be amorphization up to 12 GPa. When nitrogen used as PTM, phase II → IV transition occurs at ~ 1.4 GPa. When no PTM is used, the sample became amorphous at ~ 2.8 GPa. This tells us that nonhydrostatic condition can cause sample to be amorphous. To demonstrate this opinion, we performed another series of measurements, which load the sample using helium as PTM but the sample is bridged between gasket hole walls. The result shows that the sample becomes amorphization. Additionally, PL study under pressure demonstrates that pressure-induced changes of the PL band are consistent with the structural changes. In Chapter 4, we found that BaA exhibits remarkable piezochromism without 173 phase transition prior to polymerization. It shows reversible color change from yellowgreen to light orange between ambient pressure and ~15 GPa and irreversible when pressure is above 15 GPa. Synchrotron X-ray diffraction measurements with and without PTM revealed no structural phase transitions within this pressure range too. At ~15 GPa, an irreversible chemical reaction takes place and above which the polymeric products of BaA become amorphous. The band gap of BaA is reducing under compression but is not becoming metal up to ~50 GPa. In Chapter 5, metal lithium was investigated under pressure at various temperatures. We showed that Li6 and Li7 have strong isotopic effect in low temperature region. In contrast to 7Li, we observed that in the 6Li samples, the bcc structure remained stable compared to the martensite phase to the lowest measured temperatures for pressures up to ~2 GPa. In both 6Li and 7Li, the ground state structure is fcc rather than 9R. At the end of this section, some preliminary resistivity measurements on potassium were presented. In summary, pressure can dramatically change materials’ electronic and crystal structures. Different materials show different properties under high pressure besides different phase transitions due to high pressure, and the high pressure effects often can be complex and counterintuitive. It is significant to study the materials under high pressure to understand the underlying physical phenomena that govern the properties of these materials. Indeed, pressure is a fundamental dimension in science.