Lone-pair engineering: Achieving ultralow lattice thermal conductivity and enhanced thermoelectric performance in Al-doped GeTe-based alloys

Abstract The lone pair in Ⅳ-Ⅵ compounds plays a significant role on tuning the phonon and electron transport for excellent thermoelectric performance. Here, we theoretically reveal a unique lone-pair distortion feature in Al-doped GeTe and further experimentally demonstrate the reduction of lattice thermal conductivity by Al doping. Due to the reduction of phonon velocity and enhanced anharmonicity, together with other phonon scattering mechanisms of point defects and stacking faults, an ultralow lattice thermal conductivity of ∼0.21Wm-1 K−1 at 773 K can be obtained in Al-doped GeTe. Besides, owing to the distorted distribution of Al-3s lone-pair electrons, the Al also show unusual p-type doping behavior in GeTe. By systemically combining the doping effects of Al, Sb and Pb, we can obtain a high figure of merit (ZT) of 2.21 at 773 K and a high average ZT of 1.51 within 300–773 K in the Ge0.74Al0.02Pb0.1Sb0.1Te. This study demonstrates the impact of lone-pair distortion on enhancing the ZT of GeTe, which also paves a new path way for developing other high-ZT thermoelectric materials.