Solubility and microstructure in the pseudo-binary PbTe–Ag2Te system

The solvus lines of the PbTe and Ag2Te phases in the pseudo-binary PbTe–Ag2Te system have been determined using diffusion couples and unidirectional solidification by the Bridgman method. The solubilities of both Ag2Te in PbTe and PbTe in Ag2Te decrease with decrease in temperature. For the former, this change is from 14.9 at% Ag (694 °C) to 0.5 at% Ag (375 °C), while for the latter it is from 12.4 at% Pb (650 °C) to 3.1 at% Pb (375 °C). The decrease in solubilities leads to the formation of precipitates of Ag2Te in PbTe and PbTe in Ag2Te. In particular, fast atomic diffusion in Ag2Te results in the precipitation of PbTe even in quenched samples. From the temperature dependence of these solubilities, heats of solution have been determined. In the diffusion couple, the phase boundary moves toward PbTe. In the region between the phase boundary and the initial interface, PbTe transforms to β-Ag2Te (cubic) retaining the cube-on-cube orientation relationship.

The precipitation in the diffusion couple in fast cooling suggests high chemical diffusivity. The large heat of solution for the Ag2 Te dissolution in PbTe leads to fine microstructure.Figure optionsDownload as PowerPoint slideHighlights
► The solvus lines were determined by diffusion couples and unidirectional solidification.
► The solubilities in both phases decrease with decrease in temperature.
► Chemical diffusivity in Ag2Te phase is high.
► Heat of solution is one of the key parameters to control the number density of precipitates.
► The Ag2Te phase transformed from PbTe in diffusion couples holds the cube-on-cube relationship.