Exceptional Rare‐Earth Half‐Heusler Thermoelectrics With Sublattice Softening

ABSTRACT Half‐Heusler (HH) compounds are promising thermoelectric (TE) materials, but their intrinsically high lattice thermal conductivity ( κ L ) limits TE performance. Here, we report sublattice softening‐induced intrinsically low κ L and exceptional thermoelectricity in the previously underexplored rare‐earth (RE) containing HHs. Unlike conventional non‐RE HHs, the softened RE‐based lattice framework in RE‐HHs enables vigorous atom vibration within the 4c sublattice, strengthening lattice anharmonicity and phonon damping. This effect can be further amplified when heavier elements occupy the 4c sublattice, effectively suppressing both acoustic and optical phonon propagation and resulting in a pronounced reduction in κ L . Leveraging the low κ L , we identify four RE‐HHs—DyPtSb, Y 0.7 Lu 0.3 PtSb, Sc 0.6 Lu 0.4 PtSb, and Dy 0.7 Y 0.3 PtSb—with peak zT values exceeding 1.0. Notably, Dy 0.7 Y 0.3 PtSb achieves a maximum zT of 1.33 at 875 K. These findings underscore the promising potential of sublattice‐softened RE‐HHs as highly efficient thermoelectrics with broad compositional tunability.