Enhanced thermoelectric performance of ZrCoSb-based half-Heusler alloys through multi-scale microstructural modulation

As a promising medium-to-high temperature thermoelectric material, ZrCoSb-based half-Heusler (HH) compounds exhibit excellent thermoelectric performance and mechanical robustness. However, further improving their efficiency remains challenging because strategies that suppress lattice thermal conductivity often deteriorate carrier transport. Here, we demonstrate a dual-doping strategy to engineer multiscale microstructural features in ZrCoSb-based alloys, enabling the synergistic regulation of carrier and phonon transport. Initial substitution of Sn at the Sb site increases the carrier concentration, yielding an enhanced power factor (PF) of 29 μW cm⁻¹ K⁻² at 873 K. Subsequent Bi doping forms ZrCoSb_0.8−xBi_xSn_0.2 (0.05 ≤ x ≤ 0.3), where the combined Sn and Bi incorporation introduces abundant point defects and pronounced grain refinement. In addition, compositionally modulated half-Heusler phases with Sb-rich/Bi-poor domains are observed. These multiscale microstructural features generate strong mass and strain-field fluctuations, reducing the lattice thermal conductivity (κ_L) to 1.88 W m⁻¹ K⁻¹ at 873 K for the x = 0.2 sample, corresponding to a 60% reduction compared with the Sn-only doped alloy. As a result of the simultaneous optimization of PF and κ_L, a peak thermoelectric figure of merit (ZT) of 1.1 at 873 K is achieved for the x = 0.2 sample. This work highlights an effective strategy for coupling compositional and microstructural engineering to simultaneously optimize electron and phonon transport properties in HH thermoelectrics.