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 sigma T/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.