Ionic transport mechanisms from conductivity dependence on temperature and pressure

Conductivity measurements, as a function of temperature (30–120 °C) and hydrostatic pressure (1 × 10−4 to 5 × 10−1 GPa), have been performed for ionic conductive sodium-phosphate glasses formulated as xP2O5–(1 − x)Na2O with x = 0.4 and x = 0.5. Isobaric variations of the ionic conductivity with temperature are interpreted on the basis of an activated mechanism for charge carrier formation and migration. The associated activation enthalpy, Δ H i ∗ (around 0.7 eV), is found to be pressure-independent. A low activation entropy, Δ S i ∗ (less than 20 J K−1 mol−1), is found to linearly decrease with increasing pressure. A positive ‘activation volume’, Δ V i ∗ (averaged 5 cm3 mol−1), is calculated from experimental data. Its increase with temperature can be thermodynamically and quantitatively correlated to the diminution of the activation entropy with pressure.