Heavy doping driven ultra-low lattice thermal conductivity and mechanical enhancement in Mg₃(Bi, Sb)₂ for efficient power generation

Thermoelectric (TE) materials always require precise doping to pursue better TE performance. Heavy doping induces complex self-compensation defects and then decreases lattice thermal conductivity, however, which is barely reported in low-temperature TE materials. To evaluate the impact of heavy doping in low-temperature TE materials, the thermoelectric and mechanical properties of NiTe₂ heavy doped Mg₃Bi_1.5Sb_0.5 are systematically studied. Heavy doping introduces substitutional defects and also leads to the formation of self-compensation Mg vacancies, dislocations, and nanoprecipitates, causing significant enhancement in phonon scattering and resulting in an ultra-low lattice thermal conductivity of 0.42 W m⁻¹K⁻¹ at 420 K. In addition, the synergistic effect between the doping element and the self-compensation Mg vacancies also optimizes the carrier concentration, leading to an average ZT of 1 within 300–550 K in _[sbnd]Mg_3.2Bi_1.5Sb_0.5–NiTe₂ 5 % sample. The hierarchical defects provide additional obstacles for dislocation slip and crack extension, enabling an above 100 % and 50 % increase in the yield strength and fracture strength, respectively. With simultaneously elevated thermoelectric and mechanical performance, a high efficiency of 7.25 % under a temperature difference of 300 K is achieved in _[sbnd]Mg₃(Sb, Bi)₂–NiTe₂/MgAgSb power generation module, demonstrating the effectiveness of aliovalent heavy doping strategies in low-temperature thermoelectric materials.