Solubility and formation of ternary Widmanstätten precipitates in PbTe in the pseudo-binary PbTe–Bi₂Te₃ system

摘要

A unidirectional solidification experiment by Bridgman method has been performed for the Pb₁₄Bi_28.8Te_57.2 composition, which lies on the pseudo-binary PbTe–Bi₂Te₃ system, resulting in the formation of Widmanstätten precipitates of a ternary compound, most likely with the structure of PbBi₂Te₄ in the PbTe matrix. The formation of the precipitates is caused by the decrease of bismuth solubility in the PbTe phase with decreasing temperature. The PbTe-rich part of the PbTe–Bi₂Te₃ phase diagram was investigated from the compositional variations in the unidirectionally solidified sample and the diffusion couples. This proved that the solubility decreases with decreasing temperature: 15.6 ± 0.9 (583 °C) to 6. 2 _{- 1.7} + 2.1 6. 2_{ - 1.7}^{ + 2.1} (450 °C) at.% Bi. The orientation relationship between the matrix and precipitates has been examined by electron backscatter diffraction technique; precipitation occurs on {111} habit planes in PbTe with orientation relationship (0001)_precipitate//{111}_PbTe and _precipitate//_PbTe. The thermoelectric properties in PbTe with Widmanstätten precipitates as examined by the scanning Seebeck probe method is –46 ± 2 μVK−1.