What variable-pressure variable-temperature measurements are telling us about ion transport in glass

Abstract

Variable-pressure, variable-temperature (VPVT) measurements of ionic conductivity in typical inorganic (sodium aluminoborate and lithium–sodium phosphate) glasses provide information concerning two independent activation parameters: the activation energy (E_A = −R d ln σT/d(1/T)) and the activation volume (V_A = −RT d ln σ/dP). In single-cation glasses there is no simple link between E_A and V_A. The strong increase in E_A seen with decreasing alkali content is not paralleled by an increase in V_A. However, in mixed cation glasses, maxima are observed in both E_A and V_A. To account for these observations, a new model is being developed where ion transport is limited by a shortage of available volume. In this model, the measured V_A values represent the volumes of opened up ‘target sites’, the larger values observed in mixed-cation glasses providing evidence for the coupled motion of unlike ions. The extent of this pairwise coupling increases with increasing temperature.