Strategies to Suppress Thermal Conductivity in Half-Heusler Compounds for Thermoelectric Applications: A Comprehensive Review

Half-Heusler (hH) compounds are promising thermoelectric (TE) materials for medium- to high-temperature applications due to their favorable electronic structure, thermal and mechanical stability, and environmental benignity. Despite their high power factor (S2σ), the inherently high thermal conductivity (κ) remains a key bottleneck in achieving higher thermoelectric performance. This review highlights recent advances in enhancing the TE efficiency of hH materials with a focus on strategies aimed at lowering κ. Approaches such as isoelectronic and aliovalent alloying, nanostructuring, and high-entropy engineering are discussed in terms of their influence on phonon scattering mechanisms across multiple length scales. Additionally, synthesis routes such as high-pressure and high-temperature sintering, ball milling, spark plasma sintering, as well as the incorporation of nanoinclusions are also explored for their role in reducing κ. The presence of intrinsic defects, bonding instability, and lattice softening increases lattice anharmonicity in non-18-electron systems (valence electron counts ≠ 18), which is also addressed as a key contributor to low κ. Furthermore, the recently discovered emerging class of quaternary double, triple, and quadruple hH compounds can suppress thermal conductivity intrinsically, attributed to the strong point defect scattering combined with the Umklapp phonon scattering. The review also highlights recent findings on the presence of rattling atoms in selected hH systems, which further contribute toward reducing thermal conductivity by efficiently scattering heat-carrying phonons.