Half‐Calcium Substitution of the CaB₆ Surface with Hydrogen: Synthesis, Structural Characterization, and Electronic Structure

Modifying the surface structure of borides to form boron–hydrogen bonds, as observed in hydrogenated borophene, may enhance their catalytic activity, as well as their hydrogen storage and release capabilities. In this study, the formation of boron–hydrogen bonds at the surface of calcium hexaboride (CaB 6 ) is demonstrated. Through an ion‐exchange reaction, approximately half of the calcium ions in CaB 6 are replaced by protons, forming three‐center two‐electron (BHB) bonds according to the reaction: CaB 6 + H + → HCa 0.5 B 6 + 0.5Ca 2+ . Scanning transmission electron microscopy reveals that the resulting HCa 0.5 B 6 consists of alternating Ca‐rich and Ca‐deficient planes, attributed to the propagation of the ion‐exchange reaction from the (2 × 1) reconstructed surface. Owing to the strong sensitivity of catalytic properties to surface states, the surface modification strategy proposed in this study may provide new opportunities for the design of highly efficient functional materials such as boride‐based catalysts.