Multiphonon interaction and thermal conductivity in half-Heusler LuNiBi

Half-Heusler compounds are promising candidates for thermoelectrics. The exploration of multiphonon interaction, including four-phonon interaction, in half-Heusler compounds contributes to the deep understanding of thermal transport, which is helpful to optimize thermoelectric properties. In this work, LuNiBi is taken as the typical half-Heusler compound to investigate multiphonon interaction and thermal conductivity. By employing inelastic x-ray scattering and first-principles calculations, we confirm that the low thermal conductivity {\fontencoding{LELA}\selectfont\char91} of LuNiBi is closely related to its small group velocity compared with many other half-Heusler compounds. The noteworthy four-phonon scattering between parallel flat acoustic phonon bands is validated through a combination of experiment and calculation, which may be related to its rattlinglike characteristic. Additionally, the calculation confirms the four-phonon scattering further effectively decreases the thermal conductivity at high temperature. The significant four-phonon scattering processes are associated with weakened three-phonon scattering processes due to critical selection rules; this occurs both around parallel flat acoustic phonon bands and in the 13--15 meV energy region. Our work highlights the significance of multiphonon scattering processes, including four-phonon interaction, which plays an important role in the thermal properties of half-Heusler materials.