| Description |
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. |
