Anharmonic effective pair potentials for the I-Ag bond in α- and β-phase material were investigated by using the EXAFS technique. The spectra near to the I K-edge were measured from 7 to 800K. Phonon energies were estimated by using the potential parameter, via calculation of the dynamic matrix. The extent of anharmonicity was greater in the β-phase than in the α-phase, although the effective pair potentials did not change appreciably during the phase transition. The statistical distribution of Ag in the α-phase, and the superionic conduction mechanism, were considered on the basis of the effective pair potential and the distribution of I-Ag distances. The time spent on a lattice site was greater than the time spent between the potential-energy minima. However, the probability of finding mobile ions at a saddle-point position could be regarded as being significant, and amounted to several percent at higher temperatures in the superionic AgI. The latter exhibited a broad interatomic potential, which was similar to that of octahedrally coordinated compounds, and a strongly correlated displacement in thermal vibration between cation and anion. The strongly correlated displacement was attributed to covalent bonds which interacted locally. It was concluded that covalency was important in achieving large mean-square displacements.

A.Yoshiasa, H.Maeda: Solid State Ionics, 1999, 121[1-4], 175-82