Dynamical structure of AgxCu1-xI (x=0.99-0.80) in ionic and superionic phases studied by solid 63Cu and 127I NMR spin-lattice relaxation time measurements

The temperature dependences of the Cu-63 and I-127 NMR spin-lattice relaxation time T-1 and the Cu-63 NMR spin-lattice relaxation time in a rotating frame T-1 rho were measured for AgxCu1-xI(x = 0.99-0.80). In the gamma phase, T-1 and T-1 rho were dominated by the lattice vibration at low temperatures. The minimum of T-1 rho caused by the diffusion of the Cu ion was observed at ca. 360 K. The temperature dependence of T-1 rho in the range 420-300 K can be explained in terms of a distribution of correlation times which arises from a distribution of activation energies for the ionic diffusion. The mean and the width of the distribution for the activation energies increased with increasing Cu concentration. The rapid T-1 decrease observed above ca. 400 K can be attributed to the motion of the thermally generated defects. The enthalpy for the formation of the defect was estimated as 120 +/- 10 kJ mol(-1) from Cu-63 NMR T-1 and T-1 rho values. In the alpha phase, the relaxation of the Cu-63 nucleus was in the fast motion region and the activation energy of the ionic diffusion was determined as 7 +/- 1 kJ mol(-1) from the temperature dependence of Cu-63 NMR T-i.