Thermal and electrical properties of (B2O3-TeO2-Li2O-CoO) glasses

Borotellurite glasses in the composition, (B2O3)0.2-(TeO2)0.3-(CoO)x-(Li2O)0.5-x; x = 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45 and 0.50 have been synthesized and their non-crystalline nature confirmed by XRD. Density of the glasses increased and molar volume decreased up to x = 0.3 and reversed their trends for x > 0.3. The glass transition temperature is increased up to x = 0.30 and decreased for x > 0.3. Changes in density, molar volume and glass transition temperature around x = 0.3 may be ascribed to structural/topological modifications occurring in the glasses. Polaron parameters have been determined. Thermal stability is found to increase with increasing x. Activation energy for dc conduction at high temperature has been determined by applying Mott's small polaron hopping model. Conductivity and activation energy were found to pass through minimum and maximum respectively for x around 0.30, which may be due to a changeover of conduction mechanism from predominantly ionic regime to electronic regime. This transition can be attributed to the interaction between electrons and alkali ions. This result is of significance both scientifically and technologically. Mott's variable range hopping model has been employed to understand conductivity at low temperature for the present glasses.