Is there a mixed alkali effect in the low temperature ac conductivity of glasses?

Abstract

Recently, the radio frequency ac conductivities of glasses below150 K is shown to be a phenomenon differing from that observed at higher temperatures. This low temperature ac conductivity has been measured between room temperature and 4.2 K for the Na 2O:(1 −x) Rb 2O:4 GeO 2 glass series to determine the presence of the mixed alkali effect commonly observed in high temperature conductivity. The glasses were prepared by melting appropriate powders at about 1200 °C for 1 h, quenching to room temperature and then annealing at 475 °C for half an hour. The results for temperatures less than 100 K have a power law temperature and frequency dependence typical of the low temperature ac conductivity. The classic mixed alkali effect is essentially absent at low temperatures. From this observation and the model of low temperature conductivity, it is inferred that the effect primarily arises from the independent movement of the alkali ions.