Design of High-Performance Disordered Half-Heusler Thermoelectric Materials Using 18-Electron Rule

Ternary half-Heusler (HH) alloys display intriguing functionalities ranging from thermoelectric to magnetic and topological properties. For thermoelectric applications, stable HH alloys with a nominal valence electron count (VEC) of 18 per formula or defective HH alloys with a VEC of 17 or 19 are assumed to be promising candidates. Inspired by the pioneering efforts to design a TiFe_0.5Ni_0.5Sb double HH alloy by combining 17-electron TiFeSb and 19-electron TiNiSb HH alloys, both high-performance n-type and p-type materials based on the same parent TiFe_0.5Ni_0.5Sb are developed. First-principles calculation results demonstrate their beneficial band structure having a high band degeneracy that contributes to their large effective mass and thereby maintains their high Seebeck coefficient values. Due to the strong Fe/Ni disorder effect, TiFe_0.5Ni_0.5Sb exhibits a much lower lattice thermal conductivity than does TiCoSb, consistent with very recently reported results. Furthermore, tuning the ratio of Fe and Ni leads to achieving both p- and n-types, and alloying Ti by Hf further enhances the thermoelectric performance significantly. A peak ZT of ≈1 and ≈0.7 at 973 K are achieved in the p-type and n-type based on the same parent, respectively, which are beneficial and promising for real applications.