A new interpretation of the dynamic structure model of ion transport in molten and solid glasses

Abstract

We explore progress in understanding the behaviour of cation conducting glasses, within the context of an evolving ''dynamic structure model'' (DSM). This behaviour includes: in single cation glasses a strong dependence of ion mobility on concentration, and in mixed cation glasses a range of anomalies known collectively as the mixed alkali effect. We argue that this rich phenomenology arises from the emergence during cooling of a well-defined structure in glass melts resulting from the interplay of chemical interactions and thermally driven ionic motions. The new DSM proposes the existence of a new site relaxation process, involving the shrinkage of empty $\bar A$ sites (thus tailored to the needs of $A^+$ ions), and the concurrent emergence of empty $C'$’sites, which interrupt the conduction pathways. This reduction of $\bar A$ sites is responsible in the molten glass for the sharp fall in conductivity as temperature drops towards $T_g$. The $C'$ sites play an important role also in the mixed alkali effect, especially in regard to the pronounced asymmetries in diffusion behaviour of dissimilar cations.