Crystallisation of bismuth germanate glasses below their glass transition temperature

Abstract

Dielectric measurements of bismuth germanate oxides reveal significant changes in the amorphous phase, between 200 and 350°C, where the crystallisation processes start below the glass transition temperature. The capacitive and loss measurements versus temperature and frequency suggest an incipient glass transition below 350°C associated with an increase in the clusters, mainly those formed by GeO4 tetrahedra, responsible for the dipolar orientation effects. An increase in the capacitive parameter versus temperature at lower frequencies, especially over 250°C has been associated with the increase in mobility of the clusters. The physical meaning of those processes has been associated with the formation of a highly viscous layer enriched in GeO4 which is formed during crystallisation. This layer acts as a diffusion barrier and hinders further crystal growth. A higher frequency is required in the crystallisation processes to compensate for the thermal disorder in the amorphous materials. The crystallisation process is identified by the decrease in the dielectric constant despite the increase in temperature.