Locally off-centered Ge atoms contribute to high thermoelectric performance of globally averaged cubic MnGeTe2 alloys

Exploring high-performance and environment-benign thermoelectric materials free of phase change during their service is a long-standing challenge. Herein a maximum ZT of 1.4 at 878 K is reported for non-toxic Sb-doped MnGeTe ₂ alloys with a consistent cubic rock salt structure above room temperature. Pristine MnGeTe ₂ is a heavily self-doped p-type semiconductor with its hole density ( p _H) exceeding 10 ²¹ cm ⁻³. Due to the strong contribution of localized 3 d orbitals from Mn to the valence band maxima lying along the Γ–K direction with a high valley degeneracy of 4, MnGeTe ₂ features a superb density of states (DOS) effective mass ( m* ∼ 10 m _e), endowing it a large Seebeck coefficient of ∼105 μV K ⁻¹ at 300 K despite its over high p _H. Moreover, Ge atoms are locally discordant in MnGeTe ₂ and displaced ∼0.33 Å off the octahedron center, inducing soft polar phonons that effectively scatter the heat carrying acoustic phonons and enable a low lattice thermal conductivity ( κ _lat ∼0.57 W m ⁻¹ K ⁻¹ at 871 K). Sb doping not only efficiently neutralizes the excess p _H of MnGeTe ₂ leading to high Seebeck coefficient (> 250 μV K ⁻¹), but further drops κ _lat to ∼0.49 W m ⁻¹ K ⁻¹ by reinforced point defect scattering. Our work highlights the significance of local off-centering atoms in developing high-performance bulk thermoelectric materials.