RELATIONS BETWEEN STRUCTURE AND CONDUCTIVITY IN FAST-ION CONDUCTING GLASSES

The structures of a wide range of fast ion conducting glasses have bee n modelled using the reverse Monte Carlo (RMC) method, based on neutro n diffraction, X-ray diffraction and EXAFS data. The glass structures typically consist of a 'network' component, e.g., phosphate chains, an d a 'salt' component. These two components are strongly interlinked. T here is no evidence that the salt component forms, to any significant extent; nanocrystalline clusters similar to the equivalent fast ion co nducting crystals. The main effect of salt doping is to expand the net work, leading to an increase in the accessible free volume and hence t he number of conduction pathways. There is a strong empirical correlat ion between network expansion and increase in conductivity. However sa lt doping also increases the number of charge carriers and changes the average cation environment, thus decreasing the activation energy and increasing the cation mobility, and these effects cannot be separated from the network expansion. Increased salt doping reduces the number and strength of network cross links and weakens the glass structure, e ventually leading to crystallisation.