Study of thermal behaviour of 30Li₂O·70TeO₂ glass by X-ray dynamic observation

The thermal behaviour of a 30Li₂O·70TeO₂ glass was studied by use of an in situ high-temperature X-ray diffractometer equipped with a position-sensitive detector (PSD). The dynamical structural changes were also observed by differential thermal analysis (DTA) and differential scanning calorimetry (DSC) measurements. Differences and contradictions exist in previous studies related to the phase relation at high temperature, including the process of phase change. It is found that in the exothermic process after a glass transition between 553 and 623 K, an unknown phase of lithium tellurite (LT-X) crystallizes at first. At almost the same temperature, a minor component of vitreous TeO₂ appears during decomposition of vitreous 30Li₂O·70TeO₂ of the initial phase. After that, the thermal changes of the lithium tellurite and the TeO₂ components proceed independently. The LT-X phase converts to α-Li₂Te₂O₅, and the vitreous TeO₂ phase is followed by crystallization of an α-TeO₂ phase. In the endothermic process between 673 and 793 K, the α-Li₂Te₂O₅ phase transforms into a β-Li₂Te₂O₅ phase and the minor crystalline α-TeO₂ phase melts. Then, β-Li₂Te₂O₅ incongruently melts to form Li₂TeO₃ and a molten TeO₂ phase. Li₂TeO₃ continuously melts at around 763 K. The results show a possibility of reversible change between the α-Li₂Te₂O₅ and the β-Li₂Te₂O₅ phases.