Discordant Distortion in Cubic GeMnTe₂ and High Thermoelectric Properties of GeMnTe₂-x%SbTe

GeMnTe₂ adopts a cubic rock salt structure and is a promising mid-temperature thermoelectric material. The pair distribution function analysis of neutron total scattering data, however, indicates that GeMnTe₂ is locally distorted from the ideal rock salt structure with Ge²⁺ cations being discordant and displaced ∼0.3 Å off the octahedron center. By alloying GeMnTe₂ with SbTe, the carrier concentration can be tuned in GeMnTe₂-x%SbTe (x = 15.1), leading to converged multiple broad valence bands and a high Seebeck coefficient of >200 μV K^-1 from 300 to 823 K. The system exhibits a large density-of-state effective mass of >10 m_e and a high weighted mobility of 80 cm² V^-1 s^-1, leading to a power factor of 15 μWcm^-1 K^-2 at 823 K. The composition GeMnTe₂-15.1%SbTe exhibits very low lattice thermal conductivity of ∼0.5 Wm^-1 K^-1 at 823 K, attributed to the combination of off-centering cations in the rock salt structure, Ge/Mn positional disorder, dislocations, and abundant Ge-rich and Mn-rich nanoparticles. A ZT value of ∼1.5 can be achieved for GeMnTe₂-15.1%SbTe with a ZT_ave of 0.96 in the temperature range of 400-823 K.