Phase separation and thermoelectric properties of the Pb _0.25Sn _0.25Ge _0.5Te compound

The Pb _0.25Sn _0.25Ge _0.5Te compound is a promising p-type thermoelectric material. It is single phased at elevated temperature and undergoes upon cooling an allotropic transformation from the cubic Ge _β, to the rhombohedral Ge _α structure. In addition, a phase separation takes place in the course of aging treatments within the miscibility gap at lower temperature. The phase separation in this pseudo-ternary system takes place by a discontinuous precipitation process, giving rise to a heterogeneously nucleated coupled structure of Pb- and Ge-rich lamellae, with 10 ² nm to 2 μm wide spacing. The presence of Sn atoms in the structure tends to suppress the spinodal decomposition and is characteristic of the low Sn content compounds. The phase separation by discontinuous precipitation follows the Johnson-Mehl-Avrami kinetics. In the course of further lengthy aging treatments at 390 °C, the microstructure consists of spheroidized Ge-rich precipitates in a Pb-rich telluride matrix. The thermoelectric transport properties undergo significant changes in the initial stages of the phase separation process, leading to a stable and relatively elevated figure of merit, ZT = 0.95 ± 0.07 at 400 °C.