Phase Boundary Engineering Enables Multivalley Activation for High-Performance ZrCoSb Half-Heuslers

P-type ZrCoSb-based half-Heusler compounds are promising thermoelectric materials for high-temperature power generation because of their excellent thermal stability and mechanical robustness. However, their performance still lags behind that of state-of-the-art p-type ZrCoBi systems, mainly due to the limited doping efficiency of conventional aliovalent doping strategies, which hinders the realization of highly degenerate carrier transport and consequently restricts power factor optimization. This study highlights the pivotal role of phase diagram engineering in maximizing the thermoelectric performance of ZrCoSb-based half-Heusler materials. Through precisely tuning the Co content guided by experimental phase diagrams, simultaneous enhancement of electronic transport properties and suppression of lattice thermal conductivity have been realized. Under Co-deficient conditions, the Sn doping efficiency approaches 100%, increasing the hole concentration to 3.8 × 1021 cm-3 and activating the secondary Γ valence-band valley in addition to the primary L valleys. The resulting multivalley transport increases the effective valley degeneracy from 8 to 10 and yields a peak power factor of 40.4 μW cm-1 K-2 and an average power factor of 37.6 μW cm-1 K-2 over 300-973 K, representing the high reported value to date among p-type ZrCoSb-based half-Heuslers. The synergistic effect of Co vacancy engineering and Hf alloying suppresses thermal transport, culminating in a peak zT of 1.2 at 973 K. Finally, a single-stage thermoelectric module was fabricated using the optimized material, demonstrating a conversion efficiency of ∼11% at a temperature difference of 673 K, overperforming all previously reported ZrCoSb-based single-stage thermoelectric devices. This work establishes phase diagram engineering as a powerful and universal strategy for developing high-performance thermoelectric materials and devices.