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

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 sigmaT/d(1/T)) and the activation volume (V-A = -RT d ln sigma/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.