Network forming units in alkali borate and borophosphate glasses and the mixed glass former effect

Abstract

A theoretical approach is presented for relating structural information to transport properties in ion conducting borophosphate glasses. It relies on the consideration of the different types of glass forming units and the charges associated with them. First it is shown how changes of the unit concentration with the overall glass composition can be understood. Then it is demonstrated how the changes in the unit concentrations upon borate–phosphate mixing lead to a re-distribution of Coulomb traps for the mobile ions and a subsequent change in long-range ionic mobilities. The theories are tested against experiments and yield good agreement with the measured data both for the unit concentrations and the variation of the activation energy.