Defect Engineering for Enhancement in Thermoelectric Performance of (Zr, Hf)NiSn based n-type Half-Heusler Alloys

Defect engineering of thermoelectric (TE) materials enables the alteration of their crystal lattice by creating an atomic-scale disorder, which can facilitate a synergistic modulation of the electrical and phonon transport, leading to the enhancement of their TE properties. This work employs a compositional nonstoichiometry strategy for manipulation of Ni-vacancies and Ni-interstitials through Ni-deficient and Ni-excess compositions of (Zr, Hf)Ni1 +/- xSn-based half-Heusler (HH) alloys to realize a state-of-the-art TE figure-of-merit (ZT) of similar to 1.4 at 873 K in 4 atomic % Ni-excess HH composition, which corresponds to a remarkable TE conversion efficiency of similar to 12%, estimated using the cumulative temperature dependence model. These alloys are synthesized employing arc-melting followed by spark plasma sintering and are characterized for their phase, morphology, structure, and composition along with electrical and thermal transport properties to examine the implication of Niexcess and Ni-deficiency on the TE properties of the synthesized Zr0.6Hf0.4NiSn HH alloy. A significant enhancement (similar to 30%) of ZT is observed in the low doping limit of Ni-excess HH compositions over their stoichiometric counterpart due to Ni-interstitials and in situ full-Heusler precipitation, which enable a strong phonon scattering for a drastic reduction in lattice thermal conductivity and lead to an enhancement of ZT. However, Ni-deficient HH compositions exhibit a deterioration in the TE properties owing to the vacancy-induced bipolarity. The defect-mediated optimization of electrical and thermal transport, thus, opens up promising avenues for boosting the TE performance of HH alloys.