Discovery of Half-Heusler Alloys with Intrinsically Low Lattice Thermal Conductivity via the Equivalent Valence Electron Principle

Owing to the inherently high thermal conductivities of ternary half-Heusler (HH) thermoelectric materials, which limits substantial increase in their thermoelectric figure-of-merit (ZT), there is a pressing need to explore other systems of HH materials with intrinsic low lattice thermal conductivity. In this study, we propose the concept of equivalent valence electron principle to design quinary HH materials, the high-quality quinary Zr₂CoNiSnSb and Nb₂FeCoSnSb HH alloys were fabricated, that show a lower lattice thermal conductivity and higher ZT value compared with those of traditional undoped ternary materials. X-ray diffraction and aberration-corrected scanning transmission electron microscopy analyses indicated that these quinary HH alloys exhibit a typical F4̅3m space group and have good crystallinity. Band calculations further revealed that these quinary HH materials have a relatively moderate band gap and potential to be excellent thermoelectric materials. Specifically, a peak ZT value of 0.58 was obtained at 1123 K for Zr₂CoNiSn_0.8Sb_1.2 by fine-tuning the matrix composition. The equivalent valence electron principle proposed in this paper can not only provide guidance for broadening high-performance HH systems, but also provide ideas for the design of high-entropy alloys and high-throughput screens.