Fast ionic conduction in Na 2S+B 2S 3 glasses: Compositional contributions to non-exponentiality in conductivity relaxations

Abstract

Wide composition, frequency and temperature range conductivity measurements have been performed on the fast ion conducting glass series, Na 2S+B 2S 3. For xNa 2S=0.001, the relaxation is nearly exponential. As the Na 2S concentration increases, the estimated ion-ion separation distance decreases; and so do both the dc conductivity activation energy and the β parameter in the stretched exponential. It is also found that for nearly all of the glasses studied to date, the composition dependencies of the β parameter can be cast on to a master plot of β versus average ion-ion separation distance. In this way, we provide one of the first universal trends for the compositional dependence of the non-exponentiality in glass. The frequency dependence of the isothermal conductivity was found to be sublinear. The data was found, however, to disagree with the frequency dependence predicted by the stretched exponential function. In some cases the frequency dependence was less by a factor of 3 than that predicted. This failure in the high frequency limit is used to suggest that additional relaxation mechanisms are operative in this region.