Tunable Vacancy Order and Emergent Functionalities in Half‐Heusler Crystals

Abstract Extra chemical order within periodic crystalline lattices offers a promising approach for designing materials with emergent functionalities. However, achieving tunable extra chemical order in crystalline materials remains challenging. Here, it is found that the vacancy order in cation‐deficient half‐Heusler crystals V 1‐ δ CoSb can be tuned from long‐range order (LRO) to short‐range order (SRO), or vice versa. The vacancy LRO and SRO configurations are uncovered by scanning transmission electron microscopy analysis and Monte Carlo simulations. Remarkably, the evolution of vacancy order induces profound changes in electrical, magnetic, and thermal properties, as well as hydrogen storage characteristics. In particular, the electronic density of state effective mass exhibits a nearly threefold increase, while ferromagnetism emerges from infancy when tuning the vacancy order from LRO to SRO. These results elucidate the local chemical order‐property relationship and highlight the great potential of achieving desirable functionalities by designing extra chemical order in crystalline solids.