New ternary boride MgNiB ₄ : structural and hydriding properties

The crystal structure of magnesium nickel tetraboride, MgNiB₄, was solved and refined based on single-crystal X-ray diffraction data. MgNiB₄ crystallizes in the Pbam space group [a = 5.8791 (2), b = 11.2982 (5) and c = 3.2771 (1) Å] and is isostructural with the YCrB₄ type. The MgNiB₄ and YCrB₄ structures both belong to the AlB₂-type structural family, for which the formation of 6₃-nets by B atoms is typical. In MgNiB₄, B atoms form five- and seven-membered ring nets, which result from a rearrangement of the 6₃-nets. Strong covalent B-B interactions are established according to electronic structure calculations using the tight-binding linear muffin-tin orbital atomic spheres approximation (TB-LMTO-ASA) method. The maximum hydrogen absorption by the MgNiB₄ alloy reached 3.75 wt% H₂.