| Summary: | Compositional tuning is one of the important approaches to enhance the electronic and thermal transport properties of thermoelectric materials since it can generate point defects as well as control the phase evolution behavior. Herein, we investigated the Ti addition effect on the grain growth during melt spinning and thermoelectric transport properties of Hf<sub>0</sub><sub>.5</sub>Zr<sub>0.5</sub>NiSn<sub>0.98</sub>Sb<sub>0.02</sub> half-Heusler compound. The characteristic grain size of melt-spun ribbons was reduced by Ti addition, and very low lattice thermal conductivity lower than 0.27 W m<sup>−1</sup> K<sup>−1</sup> was obtained within the whole measured temperature range (300–800 K) due to the intensified point defect (substituted Ti) and grain boundary (reduced grain size) phonon scattering. Due to this synergetic effect on the thermal transport properties, a maximum thermoelectric figure of merit, <i>zT</i>, of 0.47 was obtained at 800 K in (Hf<sub>0</sub><sub>.5</sub>Zr<sub>0.5</sub>)<sub>0.8</sub>Ti<sub>0.2</sub>NiSn<sub>0.98</sub>Sb<sub>0.02</sub>.
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