Leveraging carrier mobility enables high-performance Mg₃(Sb, Bi)₂ thermoelectrics

Carrier transport critically governs the thermoelectric performance of semiconductors, but its optimization remains challenging due to the coexistence of multiple scattering mechanisms. Herein, we construct a mobility diagram for Mg₃(Sb, Bi)₂ by capturing the effects of acoustic-phonon, grain-boundary and polar-optical-phonon scattering to guide targeted optimization. This approach enables a top-tier carrier mobility of 179 cm² V^-1 s^-1 in this material system. The exceptional transport properties yield a peak figure of merit (zT) of ∼2.0 at 723 K and an average zT of 1.4 over the range of 300-723 K. These material-level improvements translate into outstanding device performance: a single-leg module reaches ∼13% conversion efficiency and a fully Mg-based two-pair module achieves ∼8% under a temperature difference of 297 K. These findings highlight not only the high potential of Mg₃(Sb, Bi)₂ for efficient power generation, but also the pivotal role of carrier transport as a design metric in thermoelectric materials.