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

Abstract

The temperature dependences of the ⁶³Cu and ¹²⁷I NMR spin–lattice relaxation time T₁ and the ⁶³Cu NMR spin–lattice relaxation time in a rotating frame T_1ρ were measured for Ag_xCu_1−xI (x = 0.99–0.80). In the γ phase, T₁ and T_1ρ were dominated by the lattice vibration at low temperatures. The minimum of T_1ρ caused by the diffusion of the Cu ion was observed at ca. 360 K. The temperature dependence of T_1ρ 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 T₁ 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 ⁶³Cu NMR T₁ and T_1ρ values. In the α phase, the relaxation of the ⁶³Cu 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 ⁶³Cu NMR T₁.