Intrinsically high lattice thermal conductivity has remained a major bottleneck for achieving a high thermoelectric figure of merit (zT) in state-of-the-art ternary half-Heusler (HH) alloys. In this work, we report a stable n-type biphasic-quaternary (Ti,V)CoSb HH alloy with a low lattice thermal conductivity κL ≈ 2 W m-1 K-1 within a wide temperature range (300-873 K), which is comparable to the reported nanostructured HH alloys. A solid-state transformation driven by spinodal decomposition upon annealing is observed in Ti0.5V0.5CoSb HH alloy, which remarkably enhances phonon scattering, while electrical properties correlate well with the altering electronic band structure and valence electron count (VEC). A maximum zT ≈ 0.4 (±0.05) at 873 K was attained by substantial lowering of κL and synergistic enhancement of the power factor. We perform first-principles density functional theory calculations to investigate the structure, stability, electronic structure, and transport properties of the synthesized alloy, which rationalize the reduction in the lattice thermal conductivity to the increase in anharmonicity due to the alloying. This study upholds the new possibilities of finding biphasic-quaternary HH compositions with intrinsically reduced κL for prospective thermoelectric applications.
Keywords: Seebeck coefficient; microstructure; power factor; spark plasma sintering; thermoelectrics; transport phenomena.