Reactions and phases within the TeO₂-rich part of the Bi₂O₃-TeO₂ system

Abstract

The chemical reactions between Bi₂O₃ and TeO₂ oxide powder and pellets have been investigated by differential scanning calorimetry (DSC) and electron microprobe analyses applying 10°C min^-1 heating and cooling rate, as well as isothermal heat treatment, respectively. The reaction pathway was identified in which the Bi₂O₃ is the stationary phase and the TeO₂ is the moving phase. The reaction starts by grain boundary diffusion. But at the same time, from 450°C the TeO₂ evaporates onto the Bi₂O₃ grains producing TeO₂ layer. Melts participation in the mechanism was proved. The reaction sequence is independent of the sample composition, always the TeO₂-rich phases form first. Bi₂TeO₅ could not be detected until the TeO₂ is fully reacted. The 10°C min^-1 cooling rate applied is relatively high and permits the formation of metastable phases (b-Bi₂Te₄O₁₁, Bi₂Te₇O₁₇) with cubic symmetry between 54 and 100% composition range. Though stable phases form during the subsequent heating the phase diagram has been determined under non-equilibrium conditions. In the contrast with the literature data the 54% composition is considered as the border of solid solution, from which the homogeneous and high-quality single crystal can be grown.