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Practical applications require further improvement on the performance of TE materials.
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The energy conversion efficiency for TE materials depends heavily on the TE material performance. Many thermoelectric materials are being explored for power generation applications, such as half-Heusler 2, PbTe 3, 4, 5, silicides 6, CoSb 3 7, and Mg 3Sb 2 8. They can be used to generate electricity based on the Seebeck effect when a thermal gradient exists or to transfer heat against temperature gradient based on the Peltier effect when an electric current is applied 1. Thermoelectric (TE) materials enable direct conversion between heat and electricity. Furthermore, it is possible to establish a carrier scattering phase diagram, which can be used to select reasonable strategies for optimization of the thermoelectric performance. We find that chemical doping brings strong screening effects to ionized impurities, grain boundary, and polar optical phonon scattering, but has negligible influence on lattice thermal conductivity. With ZrNiSn-based half-Heusler materials as an example, we use high-quality single and polycrystalline crystals, various probes, including electrical transport measurements, inelastic neutron scattering measurement, and first-principles calculations, to investigate the underlying electron-phonon interaction. Here, we show that chemical doping plays multiple roles for both electron and phonon transport properties in half-Heusler thermoelectric materials. Generally, the main role of chemical doping lies in optimizing the carrier concentration, but there can potentially be other important effects. Chemical doping is one of the most important strategies for tuning electrical properties of semiconductors, particularly thermoelectric materials.