Symmetry-guided crystal structure design enhances average zT and mechanical properties in rhombohedral GeTe

As a distinguished thermoelectric (TE) material, GeTe has attracted considerable focus owing to its multiple valence band edges and distinctive phase transition. However, achieving the tradeoff between electrical transport and thermal transport remains a major obstacle to improving its TE performance. To overcome this limitation, we propose regulating the rhombohedral distortion of the crystal lattice. The approach maintains the high band degeneracy characteristic of high-symmetry structures and concurrently benefits from the reduced thermal conductivity of low-symmetry structures, resulting in enhanced TE performance and mechanical properties in GeTe. Specifically, the incorporation of Sb and Bi tailors crystal structure symmetry, thereby optimizing carrier concentration and driving band convergence. Additionally, grain boundaries, dislocations, planar vacancies, and nanoprecipitates enhance multi-frequency phonon scattering. Consequently, Ge_0.92Sb_0.02Bi_0.06Te achieves a maximum zT of ∼1.8 at 723 K, and an excellent average zT (zT_ave) of ∼1.1 between 323 and 723 K, representing an impressive 124 % enhancement compared to pristine GeTe. Meanwhile, the hardness and compressive strength of Ge_0.92Sb_0.02Bi_0.06Te are enhanced to ∼216 Kgf mm⁻² and ∼175 MPa, respectively. This work illuminates the pivotal role of symmetry-driven band structure and multi-scale defects in advancing TE materials.