Solubility and formation of ternary Widmanstätten precipitates in PbTe in the pseudo-binary PbTe-Bi_2Te_3 system

Abstract

A unidirectional solidification experiment by Bridgman method has been performed for the Pb_(14)Bi_(28.8)Te_(57.2) composition, which lies on the pseudo-binary PbTe–Bi_2Te_3 system, resulting in the formation of Widmanstätten precipitates of a ternary compound, most likely with the structure of PbBi_2Te_4 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–Bi_2Te_3 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 62^(+21)_(−17) (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 <1120>_(precipitate)//<110>_(PbTe). The thermoelectric properties in PbTe with Widmanstätten precipitates as examined by the scanning Seebeck probe method is –46 ± 2 μVK^(−1).

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