High ZT and low lattice thermal conductivity in defective half-Heusler Zr0.75PtSb1-xBixalloys: promising for mid- and high-temperature thermoelectric applications.

In this study, employing first-principles DFT calculation, we exploredefective half-Heusler (HH)compounds to achieve a superior figure of merit (ZT). Thedefective HHcompound Zr0.75PtSb is engineered by introducing vacancies into the non-18 valence electron count (VEC) HH compound, ZrPtSb. Our analysis encompasses the crystalline and electronic structure, Seebeck coefficient, electrical and thermal conductivity, power factor (PF), andZT. The findings revealp-typesemiconducting behavior in thedefective HHZr0.75PtSb compound, while ZrPtSb compound exhibits metallic characteristics. Further alloying the Sb site with Bi atoms effectively tunes the band gap, resulting from significant changes in the electronic structure. This work highlights low lattice thermal conductivity (κL∼9.5W mK-1) in thedefective HHZr0.75PtSb compound, which is further reduced by alloying the Sb site with Bi atoms. The resultantκLfor Zr0.75PtSb is three to four times lower than the prototype XYZ HH compounds. The TE properties-including Seebeck coefficient (S), electrical conductivity (σ), electronic thermal conductivity (κe),ZTof thedefective HHZr0.75PtSb1-xBixsystem are systematically investigated. For the parent compound Zr0.75PtSb, a peakZTof approximately 0.97 is predicted at 1000 K within a carrier concentration range of 1020to 1022cm-3. Bi substitution at the Sb site notably tune the TE performance, primarily owing to modifications in the transport properties. In particular, Zr0.75PtSb0.75Bi0.25exhibits a significantly enhancedZT ≈ 1.41 at 1000 K, marking a 43% increase over the undoped system. These results highlight the potential ofdefective HHcompounds for mid- to high-temperature TE applications and offer guidance for experimental efforts toward optimizingZT.