Correlation between far infrared absorption and electrical conductivity of rubidium germanate glasses

The far infrared spectra of xRb(2)O-(1 - x)GeO2 (0 < x less than or equal to 0.27) glasses show that at low alkali content (x less than or equal to 0.075) Rb alkali ions occupy one type of sites (M), while for higher alkali contents two types of sites (L and H) exist, such that the corresponding ion motion frequencies are in the order nu(L) < nu(M) < nu(H), and increase with increasing alkali oxide content. The H band arises from ion motion in sites similar to those in the crystal. The low frequency band (L) represents ion motion in 'secondary' sites having no crystalline analog. According to the free-ion model, the ionic conductivity should be primarily due to the Rb ions in M and H type sites. This model does not predict the composition dependence of the activation energy of dc conductivity correctly because it does not fully account for the strains in the glass network which make up a significant fraction of total activation energy for the Rb germanate system.