Glass transition, kinetics of crystallization and anomalous phase transformations in tellurite melts

A lead-tellurite (0.3PbO:0.7TeO2) eutectic glass was investigated through a combination of thermal, structural and vibrational spectroscopic studies to examine the kinetics of crystallization and phase transformations during the heating of the glass and the cooling of the liquid. A linear relation was found to correlate the glass transition and crystallization temperatures, from which the ideal glass transition was determined. Among the several kinetic models that were analyzed, the primary crystallization was found to agree most to the Johnson–Mehl–Avrami model, suggesting the microstructural evolution as a two-dimensional crystallization and growth. The complexity of the transformation process was evidenced from the dependence of activation energy (E) on the crystallized fraction (α) using model-free isoconversional methods. The decreasing trend of E(α) was found to be characteristic of a reversible process followed by an irreversible one. Furthermore, nucleation was found to maximize at a temperature much lower than the experimentally observed crystallization onset. The structural evolution of the devitrified phases depicted the coexistence of phases (PbTeO3 and α-TeO2) during the two-stage crystallization. Anomalous crystallization into the Pb2Te3O8 phase was observed when the devitrified melt was cooled. Such an anomaly is explained using amorphous phase separation, which is inherent to the tellurite glass system owing to the presence of unique asymmetric structural units in the corresponding melts.