Less-expensive and abundantly available Hffree half-Heusler (HH) alloys are promising candidates for mid-temperature thermoelectric (TE). In the present work, we combine experimental outcomes with theoretical estimates to understand, design, and synthesize, Hf-free ZrNiSn 1−x Ge x based HH alloys with enhanced TE performance. A state-ofthe-art TE figure-of-merit (ZT) ∼ 0.92 at around 873 K was achieved for the optimal ZrNiSn 0.97 Ge 0.03 HH composition, wherein Ge atoms substitute Sn interstitial sites, as confirmed and understood by X-ray analysis and first-principles calculations, respectively. The isoelectronic Ge-doping improves electronic transport due to enhancement in carrier mobility. Concurrently, the reduction in thermal conductivity is attained by enhanced phonon scattering owing to mass fluctuation and strain field effects. The present work exhibits the efficacy of Ge as an effective dopant for HH alloys and strengthens the possibility of developing Hf-free cost-effective HH materials with high TE performance.

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