Emergent Atomic Ordering in Vacancy-Filling Heusler Alloys

Vacancy-filling Heusler alloys serve as an effective structural bridge between conventional half-Heusler and full-Heusler alloys, expanding the compositional and structural diversity of the Heusler family. However, the partial vacancy filling, combined with multiatomic components, naturally induces occupational disorder, posing challenges to the formation of highly crystalline Heusler compounds. In this study, we experimentally observe the atomic occupation ordering in Slater-Pauling semiconductors MRu_1.5Sb (M = Ti, Zr, Hf), where the insufficient Ru atoms adopt a modulated distribution over an excess of available crystallographic sites. Specifically, Ru selectively occupies the 4c and 4d Wyckoff positions, which not only disrupts the local symmetry of these sites but also gives rise to an ordered supercell structure composed of eight conventional unit cells, i.e., a superstructure. Interestingly, the lighter TiRu_1.5Sb exhibits a less pronounced occupational ordering compared to the heavier ZrRu_1.5Sb and HfRu_1.5Sb, resulting in anomalously low lattice thermal conductivity in TiRu_1.5Sb. This distinctive interplay between structural ordering and thermal transport offers additional flexibility for vacancy-filling Heuslers in thermoelectric applications.