Half-Heusler compounds with a valence electron count of 18, including ZrNiSn, ZrCoSb, and NbFeSb, are good thermoelectric materials owing to favorable electronic structures. Previous computational studies had predicted a high electrical power factor in another half-Heusler compound NbCoSn, but it has not been extensively investigated experimentally. Herein, the synthesis, structural characterization, and thermoelectric properties of the heavy-element Pt-substituted NbCoSn compounds are reported. Pt is found to be an effective substitute enabling the optimization of electrical power factor and simultaneously leading to a strong point defect scattering of phonons and the suppression of lattice thermal conductivity. Post-annealing significantly improves the carrier mobility, which is ascribed to the decreased grain boundary scattering of electrons. As a result, a maximum power factor of similar to 3.4 mW m(-1)K(-2)is obtained at 600 K. In conjunction with the reduced lattice thermal conductivity, a maximum figure of meritzTof similar to 0.6 is achieved at 773 K for the post-annealed NbCo0.95Pt0.05Sn, an increase of 100% compared to that of NbCoSn. This work highlights the important roles that the dopant element and microstructure play in the thermoelectric properties of half-Heusler compounds.

This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - Projektnummer (392228380). This work was funded by the ERC Advanced Grant No. 742068 "TOPMAT". This work was funded by the National Natural Science Foundation of China (no. 51761135127). F. Serrano acknowledges financial support by the Spanish Ministry of Economy and Competitivity through grant MAT2017-84496-R and the European Union's Horizon 2020 research and innovation program under the Marie Skodowska-Curie grant agreement No 839821. Dr T. Luo is grateful for the financial support from the Alexander von Humboldt Stiftung. We thank ALBA synchrotron (Barcelona, Spain) for providing the SXRD beam time. A. W. and W. X. acknowledge the financial support from Deutsche Forschungsgemeinschaft (Project No. BA 4171/4-1).