Effects of Mn substitution on the phases and thermoelectric properties of Ge0.8Pb0.2Te alloy

The series of (Ge0.8Pb0.2)1−xMnxTe alloys with x = 0, 0.03, 0.05, 0.08, 0.10, 0.13 and 0.18 were prepared by melting, quenching and spark plasma sintering (SPS) techniques. The effects of Mn on the phases and thermoelectric properties of the alloys were investigated. Experimental results show that all alloys are composites containing minor NaCl-type structure PbTe-based and major GeTe-based phases with the rhombohedral and the cubic structures without any phase arising from Mn. All Mn atoms were dissolved in these phases and reduce their unit cell. Mn atoms in GeTe-based phase can stabilize its high temperature cubic structure. The layer-like microstructures are formed by the Spinodal decomposition. All the samples show p-type conduction. Although the electrical resistivity for the alloy (Ge0.8Pb0.2)1−xMnxTe increases, its Seebeck coefficient increases while its thermal conductivity reduces significantly as Mn content x increases due to the enhanced phonon scattering from the solute atoms Mn in these three phases and the microstructures. As a result, the figure of merit ZT of the (Ge0.8Pb0.2)1−xMnxTe composites can be enhanced with proper Mn content. The maximum ZT of 1.3 was obtained in the sample (Ge0.8Pb0.2)0.9Mn0.1Te at 723 K, which is much higher than that of Ge0.8Pb0.2Te alloy.