Approaching the minimum lattice thermal conductivity in TiCoSb half-Heusler alloys by intensified point-defect phonon scattering

Half-Heusler (HH) alloys based on TiCoSb are becoming popular semiconducting materials for mid-temperature thermoelectric (TE) applications, due to their superior Seebeck coefficient, moderate electrical conductivity, and excellent mechanical properties. However, their practical applicability is mainly limited by their high lattice thermal conductivity. Here, we show how a multi-alloying approach that involves co-substitution of Zr and Hf on the Ti site and Bi on the Sb site can significantly lower the heat transport of the TiCo0.85Fe0.15Sb alloy due to enhanced mass and strain-field fluctuations in the lattice. The substantial rise in point-defect phonon scattering leads to a sharp reduction in the lattice thermal conductivity from 8 to 2 W m−1 K−1 at ∼300 K and from 5 to 1.74 W m−1 K−1 at ∼843 K in Ti0.5Zr0.2Hf0.3Co0.85Fe0.15Sb0.96Bi0.4. The achieved thermal conductivity is the lowest value reported so far among TiCoSb-based alloys. Importantly, the reduction in thermal conductivity outweighs the concomitant decrease in the power factor, ultimately leading to an improved thermoelectric figure of merit ZT. Our findings show that creating large point defects through heavy isovalent substitution is an effective approach to significantly reduce the phonon transport in HH alloys.