Contributions of dynamic heterogeneity to non-exponential electrical relaxation in ionically conducting glasses and melts

Abstract

The dynamic heterogeneity model for structural relaxation in glassforming liquids assumes that the non-exponential structural relaxation kinetics are due to a distribution of independently relaxing nanoregions which relax at different rates. A simple expression based on this model for the kinetics of the structural relaxation process has been extended to give an estimate of the contribution of microscopic heterogeneity to the non-exponentiality of the electric field relaxation in ionically conducting glasses and melts. Analysis of data near the glass transition temperature T g indicates that for the majority of inorganic glasses, which have very large values of the logarithm of the decoupling index log R τ(T g ) near T g, microscopic heterogeneity makes at best a very minor contribution to the non-exponentiality of the electrical relaxation and, hence, that the source of this non-exponentiality is primarily correlations among the diffusive motions of mobile ions. The opposite result is obtained, however, for glasses such as 0.4Ca(NO 3) 2–0.6KNO 3 with smaller values of log R τ(T g ) . The model also indicates that microscopic heterogeneity should make increasing contributions to non-exponential electrical relaxation in melts as the temperature is increased above T g. This is in accord with the decrease with increasing temperature of the KWW non-exponentiality parameter β σ for electrical relaxation observed for melts above T g.