High ZT and Low Lattice Thermal Conductivity in Defective Half-Heusler Zr0.75 PtSb1−x Bix Alloys: Promising for Mid- and High-Temperature Thermoelectric Applications

In this study, employing first-principles DFT calculation, we explore defective half-Heusler (HH) compounds to achieve a superior figure of merit (ZT ). The defective HH compound Zr0.75 PtSb is engineered by introducing vacancies into the non-18 valence electron count (VEC) HH compound, ZrP tSb. Our analysis encompasses the crystalline and electronic structure, Seebeck coefficient, electrical and thermal conductivity, power factor (PF ), and ZT. The findings reveal p-type semiconducting behavior in the defective HH Zr0.75 P tSb compound, while ZrP tSb 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 ther-mal conductivity (κL ∼ 9.5 W/mK) in the defective HH Zr0.75 P tSb compound, which is further reduced by alloying the Sb site with Bi atoms. The resultant κL for Zr0.75 P tSb is three to four times lower than the prototype XYZ HH compounds. The TE properties—including Seebeck coef- ficient (S), electrical conductivity (σ), electronic thermal conductivity (κe ), ZT of the defective HH Zr0.75 PtSb1−x Bix system are systematically investigated. For the parent compound Zr0.75 PtSb, a peak ZT of approximately 0.97 is predicted at 1000 K within a carrier concentration range of 1020 to 1022 cm−3 . Bi substitution at the Sb site notably tune the TE performance, primarily owing to modifications in the transport properties. In particular, Zr0.75 PtSb0.75 Bi0.25 exhibits a significantly enhanced ZT ≈ 1.41 at 1000 K, marking a 43% increase over the undoped system. These results highlight the potential of defective HH compounds for mid- to high-temperature TE applications and offer guidance for experimental efforts toward optimizing ZT .