Inelastic X-ray scattering study of the phonon dispersions and lattice thermal conductivity in the vacancy-defective half-Heusler thermoelec

Vacancy-deficient half-Heusler compounds exhibit anomalously low lattice thermal conductivity (κlat), challenging conventional defect-phonon scattering models. Our recent inelastic neutron scattering (INS) studies on polycrystalline NbCoSb systems have revealed phonon softening in long/short-range ordered (LRO/SRO) vacancy configurations compared to vacancy-free TiCoSb, but no anharmonic broadening is observed - in contrast to the vacancy-rich Zintl phase. To resolve whether κlat suppression arises from uniform phonon softening or momentum-selective modifications of phonon dispersions, we propose high-resolution inelastic X-ray scattering (IXS) studies on single-crystal Nb₀.₈CoSb (LRO/SRO) and TiCoSb. Millimeter-scale single crystals have been synthesized, and Laue/STEM confirms the unique vacancy order and phase purity. Preliminary INS data confirm phonon softening in defect systems, but lack momentum-resolved dynamics due to polycrystalline averaging effects. IXS on ESRF’s ID28 beaml