Transport properties of phosphomolybdate and phosphotungstate glasses

Abstract

The d.c. conductivity of phosphomolybdate and phosphotungstate glasses is discussed. The conductivity of these glasses is due to the hopping of electrons between two valence states (Mo⁵⁺ to Mo⁶⁺ or W⁵⁺ W⁶⁺). In some of the glasses, the activation energy itself is found to be a function of temperature. This appears to be due to thermally activated and variable-range hopping mechanisms operating in different temperature regimes. The relation between conductivity and the [M⁵⁺]/[M_total](M = Mo, W) ratio does not show any systematic variation. This anomaly can be understood using the structural models of these glasses. In contrast, Mott's theory and the Triberis and Friedman model have been used to obtain conductivity parameters such as the percolation distance R_ij and 2αR_ij (α is the tunnelling probability). The conductivity parameter 2αR_ij is quite useful to resolve the controversy regarding the tunnelling term exp(2αR_ij) existing in the literature. For low values of 2αR_ij, it is shown that the exp (2αR_ij) term is very significant.