31P NMR spin-lattice relaxation: structural glass dynamics in Rb1-x(ND4)xD2PO4

We performed 3 1P NMR measurements of the temperature-dependent spin-lattice relaxation time in several mixed single crystals of Rbi_x(ND4 ) xD2P04 [DRADP] having different ammonium concentration x (x =0.22, 0.44, 0.78) as well as in three pure single crystals (ND4)D2P04 [DADP], (NH4)H2P04 [ADP], and RbH2P04 [RDP]. The 3 1P NMR spin-lattice relaxation-time measurements in mixed crystals show that the phosphorus nuclei are strongly influenced by the gradual slowing down of the motions of protons in the nearby O-D • • • O acid bonds in the structural spin-glass states. In addition to a common Tl minimum observed in all ammonium-containing systems, a second Tl minimum is observed in the temperature region of the glass transition in mixed spin-glass systems but not in pure ADP, DADP, or RDP. We attribute this lower temperature Tx minimum to the extreme slowing down of the O-D • • • O intrabond motion, which is unique to the glass system. In addition, the correlation times and activation energies for the Tx minima in all samples were determined. A comparison between the pure and mixed systems reveals that the mixed system 7^ minimum must be fit to a distribution of correlation functions but that each pure system requires only a single correlation function. Furthermore, measurements on systems having x =0.78 and 0.22 show that 3 1P NMR can be used to determine the threshold concentrations that characterize the glass phase.