A UNIFIED MODEL FOR IONIC TRANSPORT IN ALKALI DISILICATES BELOW AND ABOVE THE GLASS-TRANSITION

Ionic conductivities of sodium and potassium disilicate glasses have been measured below and above their vitreous transition temperatures in a large temperature range (400-1350 K). A microscopic model is proposed, assuming that the ionic displacement results from the migration of interstitial pairs whose concentration is an activated function of the temperature. At low temperatures, the migration of these interstitial pairs only proceeds by an activated jump mechanism. Above the ideal glass transition temperature, a free volume displacement mechanism is superposed on the model, justifying the deviation from an Arrhenius law and the conductivity increase above T(g). Numerical fitting of experimental data to this model enables the determination of characteristic parameters such as the ideal glass transition temperature and the defect formation and migration enthalpies.