Abstract
Theoretical investigation of the cluster-distortion coupling model is carried out in the low-temperature phases of two typical superionic conductors AgI and PbF2, which are characterized by two types of phase transitions, 1st-order order-disorder type and Faraday type, respectively. It is found that the static local distortion induced therefrom by clustering of mobile ions is quite consistent with the negative local distortion experimentally obtained from EXAFS measurement in β-AgI, and with the anomalous temperature dependence of the lattice constant of β-PbF2. The effective interaction energy between mobile ions decreases with temperature due to the static and dynamic distortions. The specific heat and ionic conductivity are also in agreement with experimental observations in both materials.
