Establishing the carrier scattering phase diagram for ZrNiSn-based half-Heusler thermoelectric materials
Since the thermoelectric materials enable direct conversion between heat and electricity, they are considered an important part of solutions to the energy crisis. Half-Heusler compound is a promising thermoelectric material with excellent electrical power factor and has been developed for the power generation . Although the electron-phonon interaction was found to suppress lattice thermal conductivity  and the acoustic phonons could carry the dominant lattice thermal conductivity, the theoretical analysis suggested this system with a weak acoustic phonon scattering, even smaller than the optical phonon scattering , which contradicts the general expectation. We regulated the carrier concentration of ZrNiSn by doping Sb and obtained the phonon density-of-states by the inelastic neutron scattering spectrometer, HRC. The measurements demonstrated a weak electron-acoustic phonon interaction in ZrNiSn-based compounds, and the electron doping did not make obvious change in the lattice thermal conductivity, while the optical phonons split with different carrier concentration. Hence, we revealed important scattering beyond the expected acoustic phonon scattering: specifically, ionized impurity, grain boundary, and polar optical phonon scatterings can be well screened by mobile electron gas, and this screening effect causes a non-monotonous dependence of the mobility on carrier concentration. This work will help us understand the carrier scattering mechanisms and electron-phonon interactions in thermoelectric semiconductors. Moreover, we established a carrier scattering ‘phase diagram’, which would not only provide guides for the optimization of TE performance, but also refresh the method of designing a new TE system. This work is published on Nature Communications . The neutron-scattering experiment at HRC was performed with the J-PARC under user proposal programs (2017A0071 and 2018B0281).
図1. (a) S(Q,E) of ZrNiSn by inelastic neutron scattering; (b) Carrier concentration dependence of phonon density-of-state; (c) the simulated phonon dispersions of ZrNiSn by first-principles calculations; (d) the scattering effect between LO and TO, rTF is Thomas-Fermi screen radius; (e) Phonon density-of-state vs LO-TO splitting; (f) Carrier concentration vs mobility for half-Heusler compounds.
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