High Thermoelectric Performance in n‑Type Degenerate ZrNiSn-Based Half-Heusler Alloys Driven by Enhanced Weighted Mobility and Lattice Anharmonicity

ZrNiSn-based half-Heusler (HH) alloys are well-established alloys for n-type mid-temperature thermoelectric applications because of their chemical and thermal stability and narrow band gap driven semiconducting behavior. Here, we observed a high figure of merit, ZT ∼ 1, at 873 K in aliovalent Nb-doped composition Zr_1–xNb_xNiSn (x = 0–0.035) for x = 0.03 HH alloy prepared by employing a combination of arc-melting and spark plasma sintering. The high figure of merit is attributed to synergistic enhancement of the power factor with thermal conductivity reduction. Further, the enhancement of the power factor is correlated with the improved weighted mobility and reduction in thermal conductivity is analyzed in the framework of the model based on the theories of Callaway, Slack, and Abeles. The results are corroborated theoretically to establish the efficacy of Nb doping in ZrNiSn within semimetallic regimes for ZT enhancement. The synthesized Hf-free HH alloys establish Nb as an effective dopant for attaining high-ZT and cost-efficient HH-based n-type TE material in a mid-temperature regime for power generation application.