P-31 NMR SPIN-LATTICE RELAXATION - STRUCTURAL GLASS DYNAMICS IN RB1-X(ND4)XD2PO4

We performed P-31 NMR measurements of the temperature-dependent spin-lattice relaxation time in several mixed single crystals of Rb1-x(ND4)xD2PO4 [DRADP] having different ammonium concentration x (x=0.22, 0.44, 0.78) as well as in three pure single crystals (ND4)D2PO4 [DADP], (NH4)H2PO4 [ADP], and RbH2PO4 [RDP]. The P-31 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 T1 minimum observed in all ammonium-containing systems, a second T1 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 T1 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 T1 minima in all samples were determined. A comparison between the pure and mixed systems reveals that the mixed system T1 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 P-31 NMR can be used to determine the threshold concentrations that characterize the glass phase.