Mixed electrical conduction in a hydrous pantellerite glass

Abstract

Electrical conductivity measurements were carried out on pantelleritic trachyte glasses containing up to 3.5wt.% dissolved H2O. At temperatures below about 700K, we find evidence for small polaron conduction due to the presence of both ferrous and ferric iron in the glass. At the higher temperatures of the investigation (up to 973K), a marked change in the conductivity–temperature relation is observed, which suggests that an ionic conduction mechanism becomes the dominant means of charge transport. In the ionic conduction regime, conductivity increases with increasing H2O concentration. Activation energies are similar for both the anhydrous and hydrous glasses, indicating that the conductivity is controlled by sodium diffusion even for the highest H2O concentration examined. A slight variation in activation energy is observed depending on the H2O concentration, which is most likely associated with the depolymerising effect of dissolved water on the silicate network structure. At low temperatures, we find a dramatic effect of fO2 on the conductivity that supports an electronic conduction mechanism based on small polaron hopping between Fe3+ and Fe2+ sites. This electronic pathway controls the overall electrical conductivity in these alkali-rich glasses at temperatures exceeding 500°C if conditions remain anhydrous at an oxygen fugacity of 0.2atm.