A new mechanism of ionic conduction in glass

Abstract

A new cluster bypass (CB) model is proposed as a basis for developing theories of ionic conduction in glass which recognize the essential continuity of glass/ melt behaviour. The continuous random network model is modified in order to provide pathways for ion migration which are located within the residual liquid (or connective tissue) surrounding ordered microdomains (or clusters) within the structure of the glass. The existence of this residual liquid at T g accounts for the high conductivities and the correspondingly high values of Angell's decoupling index, R r, which are found in many inorganic (network) glasses. The CB model differs from some current viewpoints, where the presence of clusters enhances the ionic conductivity. The model, however, is consistent with the weak electrolyte hypothesis, since the cations which are trapped (even temporarily) within the clusters do not contribute to the d.c. conductivity. The application of a simple percolation theory to conductivity data in mixed cation systems permits an estimation of the degree of connectivity in the conduction pathways and further refines the model. A critical evaluation of the CB model in relation to other concepts which are amenable to quantitative treatment, such as the defect hypothesis, is still in progress.