A STUDY OF THE MIXED ALKALI EFFECT BY FREQUENCY-DEPENDENT CONDUCTIVITY IN LI2O-NA2O-P2O5 GLASSES

0.5[(I-x)Li2O-x Na2O]-0.5 P2O5 (0 less-than-or-equal-to x less-than-or-equal-to 1) glasses were investigated by DSC and impedance spectroscopy. A classical mixed alkali effect was shown. This effect was further explored by frequency-dependent conductivity. The temperature dependence of the real part of conductivity sigma(ac) was used to determine an activation energy [E(ac)] independent of frequency. The dependence of conductivity sigma(ac) and of activation energy [E(ac)] on the composition x was investigated. In both cases, a remarkable mixed alkali effect was shown. The coupling model was used to determine a beta = [E(ac)]/Ed. parameter which appears to be independent of the composition.