Synthesis, crystal structure and hydrogenation properties of MgLiB ( = 1.11, = 0.40).

The ternary magnesium/lithium boride, MgLiB ( = 1.11, = 0.40, idealized formula MgLiB), crystallizes as its own structure type in 422, which is closely related to the structural family comprising α-AlB, BeAlB and tetra-gonal β-boron. The asymmetric unit of title structure contains two statistical mixtures Mg/Li in Wyckoff sites 8 with relative occupancies Mg:Li = 0.495 (9):0.505 (9) and 4 with Mg:Li = 0.097 (8):0.903 (8). The boron atoms occupy 23 8 sites and two 4 sites. One of the latter sites has a partial occupancy factor of 0.61 (2). Both unique Mg/Li atoms adopt a twelvefold coordination environment in the form of truncated tetra-hedra (Laves polyhedra). These polyhedra are connected by triangular faces to four [B] icosa-hedra. The boron atoms exhibit four kinds of polyhedra, namely penta-gonal pyramid (coordination number CN = 6), distorted tetra-gonal pyramid (CN = 5), bicapped hexa-gon (CN = 8) and gyrobifastigium (CN = 8). At the gas hydrogenation of MgLiB alloy, formation of the eutectic composite hydride LiBH+Mg(BH) and amorphous boron is observed. In the temperature range 543-623 K, the hydride eutectics decompose, forming MgH, LiH, MgB, B and H.