Drastic Modification in the Thermal Conductivity of the TiCoSb Half-Heusler Alloy: Phonon Engineering by Lattice Softening and Ionic Polarization

A drastic variation (∼47%) in thermal conductivity (κ) for synthesized samples (TiCoSb 1+ x, x = 0.0, 0.01, 0.02, 0.03, 0.04, and 0.06) is observed. The lowest κ value is reported for the TiCoSb 1.02 sample. Thermal variation of κ is estimated from the temperature- and power-dependent Raman spectroscopy data. Embedded phases and Co vacancy are analyzed, employing scanning electron microscopy and transmission electron microscopy data. X-ray absorption fine structure (XAFS) spectroscopy reveals the Co vacancy in the synthesized samples, and the most ordered phase is TiCoSb 1.02 among the synthesized samples. X-ray photoelectron spectroscopy measurement of the synthesized samples provides direct evidence of Co vacancies and their increase with Sb concentration ( x ). Lattice dynamics are revealed using Raman Spectroscopy (RS) measurements. RS data show that the variation in κ as a function of Sb concentration is observed owing to an alteration in phonon group velocity, related to lattice softening. The polar nature of the TiCoSb half-Heusler (HH) sample is revealed. Longitudinal optical and transverse optical phonon (LO–TO) splitting in RS is observed due to the polar nature of TiCoSb 1+ x synthesized samples. Tailoring in LO–TO splitting due to the screening effect, correlated with Co vacancies, is reported for TiCoSb 1+ x synthesized samples. Lattice softening and LO–TO splitting led to a minimum κ for the TiCoSb 1.02 synthesized sample.