Symmetry-breaking origins of the high thermoelectric performance of Ge _1-xMn_xTe alloys

GeTe is a well-known thermoelectric material that undergoes a phase transition from rhombohedral to rock salt phase and shows improved thermoelectric performance when alloyed with Mn. To understand the root causes of this high performance in Ge_1-xMn_xTe alloys, computational modeling can provide an in-depth view of structural and electronic properties. In this work, we use first-principles calculations and alloy modeling techniques to analyze fully disordered alloys in both the rhombohedral and rock salt phases to understand the structural transition, redistribution of bond lengths, and effective electronic structure. Our analysis indicates the thermodynamic stability of the rock salt phase over the rhombohedral phase with increased Mn content due to the absence of lone pair electrons in Mn. We observe band convergence and the emergence of new valence band maxima (VBM) with moderate Mn content around 12.5% near the phase transition, that may be related to the improved thermoelectric properties. However, additional Mn content creates dispersion-less flat bands and strong smearing near VBM. Experimental measurements show increase in effective mass, reduction in mobility with increasing Mn content, and minimum thermal conductivity near the phase transition. We conclude that good thermoelectric properties are achievable in Ge_1-xMn_xTe alloys at compositions close to the phase transition with moderate Mn content due to band convergence, moderate effective mass, and favorable transport properties.