Defect-Calculation-Guided Performance Optimization of n-Type LuNiSb Half-Heusler Thermoelectrics

Rare-earth (RE) half-Heusler (HH) compounds are potential thermoelectric (TE) materials, which, however, have attracted relatively less attention compared to the non-RE HHs. In this work, the intrinsic defects of LuNiSb and five potential n-type substitutional defects, i.e., TiLu, ZrLu, HfLu, CuNi, and TeSb, were investigated by first-principles defect calculations. The results indicate that undoped LuNiSb exhibits p-type transport properties, with the cationic vacancy as the dominant intrinsic defect, and Ti, Zr, and Hf are potentially effective n-type dopants when substituting at the Lu site as they exhibit low extrinsic defect formation energies. Further experimental studies on Lu0.95M0.05NiSb (M = Ti, Zr, Hf), LuNi0.95Cu0.05Sb, and LuNiSb0.95Te0.05 validate the defect calculation results, manifesting that Ti, Zr, and Hf are effective n-type dopants that greatly improve electrical conductivity. Among them, Lu0.95Zr0.05NiSb exhibits decent TE properties, as the dopant Zr simultaneously optimizes the electrical properties and reduces lattice thermal conductivity. Moreover, a series of Zr-doped Lu1–xZrxNiSb (x = 0, 0.02, 0.03, 0.04, 0.05) was synthesized, and their TE properties were investigated. A maximum figure of merit zT value of 0.47 was obtained at 759 K for Lu0.96Zr0.04NiSb, which demonstrates an increase of ∼175% compared with the undoped LuNiSb and marks an important improvement compared with previously studied n-type RE-HHs.