Pressure dependence of the ionic conductivity of Na- and Na–Rb borate glasses

The ionic conductivity of Na- and Na–Rb borate glasses is investigated as a function of hydrostatic pressure. Activation volumes of the dc conductivity are obtained from pressure-dependent ac conductivity measurements at a constant temperature of 180 °C. The activation volume of Na borate glasses decreases with increasing alkali content. It depends linearly on a separation parameter which is defined as the ratio of the average ionic distance and the average network atom distance. The activation volume of Na–Rb borate glasses increases from both end-members towards the mixed-alkali regime and passes through a maximum near the relative content Na/(Na + Rb) = 0.4. In addition to mobility anomalies, observed earlier in these borate glasses, the maximum in the activation volume is a further important fingerprint of the mixed-alkali effect.