Thermodynamique des porteurs de charges dans les verres à conduction ionique

Abstract

Ionic conductivity in glasses is mainly cationic, singly charged cation leading to the highest conductivities. Despite a large variety of chemical compositions (oxide or chalcogenide glasses) and a very large spread for room temperature conductivity (over 6 orders of magnitude) all glasses present important similarities. Below their vitreous transition temperature Tg, ionic conductivity is an activated process [MATH] and isothermal conductivity variations with composition are mainly the result of the activation energy variations ([MATH]) since dl the extrapolated values of conductivity when T tends to infinity ([MATH]) are within only one or two orders of magnitude. This behaviour is explained assuming an ionic displacement due to interstitial pairs in very low concentration. Interstitial pairs formation and migration are activated mechanisms and both contribute to the measured activation energy for ionic conduction. The usual approach developed by physicists for defect formation in ionic crystals explains very simply the similar values obtained for [MATH] but not the [MATH] variations with composition. The dissociation theory, more familiar to chemists, completes this approach and links [MATH] and [MATH] to partial entropy and enthalpy of the alkali salts dissolved in the glassy matrix. From these relationships conductivity variations with composition or quenching rate are interpreted.