Internal friction and frequency-dependent conductivity in the bivariate site energy model for the mixed alkali effect in glasses

The mixed alkali effect is fundamental to understanding ion transport in glasses. It manifests itself in strong changes of various ionic transport properties when one type of ion is successively replaced by another type. Recently, we have developed a new theory of thermally activated hopping in bivariate random site energy landscapes that takes into account site occupation statistics according to generalized Fermi distributions and mixture kinetics in accordance with linear irreversible thermodynamics. The theory was successfully applied to predict the behavior of partial conductivities and their activation energies at low frequencies corresponding to the dc limit. Here we show that the theory is capable to capture the salient features of the mixed alkali effect observed in ac ion conductivities and in the peculiar behavior of the internal friction.