Enhanced Thermoelectric\nPerformance of ZrCoSb Half-Heusler\nCompounds by Sn–Bi Codoping

ZrCoSb-based half-Heusler compounds have shown outstanding\nthermoelectric\nand mechanical properties, making them highly promising for practical\napplications. However, most optimization strategies have focused on\ndoping costly Hf at the Zr sites. Here, the structural evolution,\nthermoelectric properties, and transport mechanisms of ZrCoSb<sub>0.8–<i>x</i></sub>Sn<sub>0.2</sub>Bi<sub><i>x</i></sub> compounds co-doped with less-costly Sn and Bi at\nSb sites were investigated. Specifically, grains were refined via\nBi doping, which introduced grain boundary scattering at low temperatures.\nThe effect of grain-boundary scattering on electron transport weakened\nand became less pronounced at higher temperatures. Thus, the discrepancy\nin the weighted mobility of ZrCoSb<sub>0.8–<i>x</i></sub>Sn<sub>0.2</sub>Bi<sub><i>x</i></sub> compounds was\nreduced. Furthermore, increasing Bi doping level significantly reduced\nthe lattice thermal conductivity due to the introduction of large\nmass and strain field fluctuations. As a result, the trade-off between\nthe weighted mobility and the lattice thermal conductivity at high\ntemperatures enabled the dimensionless figure of merit (<i>ZT</i>) for ZrCoSb<sub>0.71</sub>Sn<sub>0.2</sub>Bi<sub>0.09</sub> to be\n0.68 at 973 K, an enhancement of 44.6% relative to the single Sn-doped\nZrCoSb compound. Overall, this work demonstrates the feasibility of\nenhancing <i>ZT</i> via co-doping only at the Sb site, and\nit highlights microstructure effects on thermoelectric properties.