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 5×10−1GPa), 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, ΔHi∗ (around 0.7eV), is found to be pressure-independent. A low activation entropy, ΔSi∗ (less than 20JK−1mol−1), is found to linearly decrease with increasing pressure. A positive ‘activation volume’, ΔVi∗ (averaged 5cm3mol−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.