Drastic Lowering of Lattice Thermal Conductivity induced by Aliovalent Substitution of TiCoSb-Based Half-Heusler Compound

The performance of the state-of-the-art ternary half-Heusler (HH) compounds is limited by their intrinsically high lattice thermal conductivity (κ_L). In this study, we present a stable n-type biphasic-quaternary (Ti,V)CoSb HH alloy that is equivalent to previously reported nanostructured HH alloys in terms of lattice thermal conductivity (κ_L~ 2 W/mK) within a temperature range of 300-800 K. Aliovalent substitution of TiCoSb, resulting in an effective 18.5 valence electron count (Ti_0.5V_0.5CoSb), results in a significant reduction in κ_L and synergistic enhancement of the power factor with a peak zT of 0.4 (± 0.05) at 873 K. The lowering of κ_L in Ti_0.5V_0.5CoSb HH alloy is attributed to an enhancement in phonon scattering driven by spinodal decomposition upon annealing. First-principles DFT calculations are performed to investigate the structure, stability, electronic structure, and transport properties of the synthesised alloy, which accredits the decrease in lattice thermal conductivity to the alloy's increased anharmonicity. The changing electronic band-structure upon substitution and an increase in carrier concentration have a good correlation with the observed electrical characteristics. Hence, new prospects for discovering biphasic-quaternary HH compositions with intrinsically lower κ for potential thermoelectric applications are supported by this study.