Enhanced Thermoelectric Performance of ZrCoSb Half-Heusler Compounds by Sn–Bi Codoping

ZrCoSb-based half-Heusler compounds have shown outstanding thermoelectric and mechanical properties, making them highly promising for practical applications. However, most optimization strategies have focused on doping costly Hf at the Zr sites. Here, the structural evolution, thermoelectric properties, and transport mechanisms of ZrCoSb0.8–xSn0.2Bix compounds co-doped with less-costly Sn and Bi at Sb sites were investigated. Specifically, grains were refined via Bi doping, which introduced grain boundary scattering at low temperatures. The effect of grain-boundary scattering on electron transport weakened and became less pronounced at higher temperatures. Thus, the discrepancy in the weighted mobility of ZrCoSb0.8–xSn0.2Bix compounds was reduced. Furthermore, increasing Bi doping level significantly reduced the lattice thermal conductivity due to the introduction of large mass and strain field fluctuations. As a result, the trade-off between the weighted mobility and the lattice thermal conductivity at high temperatures enabled the dimensionless figure of merit (ZT) for ZrCoSb0.71Sn0.2Bi0.09 to be 0.68 at 973 K, an enhancement of 44.6% relative to the single Sn-doped ZrCoSb compound. Overall, this work demonstrates the feasibility of enhancing ZT via co-doping only at the Sb site, and it highlights microstructure effects on thermoelectric properties.