Ion Dynamics in Mixed Alkali Borate Glasses

Abstract

AbstractFrequency-dependent conductivities are a valuable tool for studying the ion dynamics on different time scales, the latter being determined by the inverse of the experimental frequency. Therefore, wide range conductivity spectra probe the transition from elementary steps of the ionic movement to macroscopic transport. We have studied the ion dynamics in mixed alkali borate glasses where we have systematically varied the total and the relative ion concentration. The glass system under investigation isy[xLi2O · (1−x) Na2O] · (1−y) B2O3withx= 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 andy= 0.1, 0.2, 0.3. The conductivity spectra can be formally divided into a low-frequency and a high-frequency regime. In the low-frequency regime the conductivities of all glasses show a transition from their dc values into a dispersive regime where the conductivity is found to increase continuously with frequency, tending towards a linear frequency dependence at sufficiently low temperatures. The conductivity spectra can be described by the MIGRATION concept developed by Funke. We discuss the spectral shape of the conductivity spectra and its implications for the ion transport. In addition, we report on a new mixed alkali effect occurring in the high-frequency regime of the ac conductivity. From our results we conclude that apart from a low-frequency contribution to the conductivity which is linked to ion hopping sequencess, there is an additional contribution involving localized motions of both the mobile ions and the glassy network.