Phase separation and thermoelectric properties of the Pb0.25Sn0.25Ge0.5Te compound

Highlights: ► Quasi-ternary (Pb,Sn,Ge)Te compounds with Sn (>7 at%) content are single phased at elevated (>600 °C) temperature. ► On cooling, the compounds undergo a discontinuous precipitation process. ► The discontinuous precipitation gives rise to heterogeneously nucleated coupled Pb- and Ge-rich telluride lamellae. ► After aging (>500 h at 400 °C), the microstructure consists of a Pb-rich matrix with spheroidized Ge-rich telluride precipitates. ► The aged compounds display a stable thermoelectric figure of merit, ZT = 0.95. - Abstract: The Pb0.25Sn0.25Ge0.5Te 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 Geβ, to the rhombohedral Geα 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 102 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.