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

Abstract

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 TiFe0.5Ni0.5Sb 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 TiFe0.5Ni0.5Sb 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, TiFe0.5Ni0.5Sb 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.

Enhanced Thermoelectric Properties in p‐Type Double Half‐Heusler Ti2−yHfyFeNiSb2−xSnx Compounds

“…The negative Seebeck coefficient of TiCoSb indicates an n‐type conduction behavior (Figure b) . However, Ti 2 FeNiSb 2 is a p‐type semiconductor due to the intrinsic vacancy defects formed by the substitution of Co atoms with Fe and Ni atoms, which is consistent with the calculations and experimental results in the previous reports . As the temperature increases, the Seebeck coefficient of the Sn‐doped samples increases first and then slowly decreases.…”

“…The power factor of Ti 2 FeNiSb 1.7 Sn 0.3 reaches a maximum value of 1.67 mW m −1 K −2 at 823 K. To further understand the electrical transport properties after Sn doping, we tested the room‐temperature Hall carrier concentration and Hall mobility for samples with different doping levels, as shown in Figure d. As the Sn content increases, the carrier concentration increases dramatically from 0.46 × 10 21 to 4.8 × 10 21 cm −3 , which is close to that of the best reported double half‐Heusler TiFe 0.6 Ni 0.4 Sb . In addition, the mobility decreases as the Sn content increases due to the increased scattering centers for electrons.…”

“…The standard diffraction peaks of Ti(Fe 0.5 Ni 0.5 )Sb slightly deviate from that of TiCoSb, which may be due to the lattice distortion caused by Fe and Ni atoms in the Co site. In addition, no observation of additional superlattice diffraction lines related to the Fe and Ni ordering indicates that Fe and Ni atoms are randomly distributed . The diffraction peaks of Ti 2− y Hf y FeNiSb 1.7 Sn 0.3 gradually shift to lower angles as the substituted Hf content increases because of the larger atomic radius of Hf than that of Ti .…”

“…The difference is that Fe atoms and Ni atoms occupy the position of Co atoms in a ratio of 1:1 in Ti 2 FeNiSb 2 . The Fe/Ni disordered scattering makes Ti 2 FeNiSb 2 having much lower intrinsic lattice thermal conductivity than that of TiCoSb …”

“…Although double half‐Heusler compounds have lower intrinsic lattice thermal conductivity, the electrical properties are extremely poor. By tuning the ratio of Fe and Ni, enhanced room‐temperature carrier concentration of ≈4.9 × 10 21 and 6.0 × 10 21 cm −3 were obtained, resulting in decent ZTs ≈1.0 and 0.7 at 973 K in p‐type Ti 0.8 Hf 0.2 Fe 0.6 Ni 0.4 Sb and n‐type Ti 0.9 Hf 0.1 Fe 0.4 Ni 0.6 Sb, respectively . In this work, we focused on the p‐type double half‐Heusler by tuning the hole concentration with Sn doping on the Sb site.…”