Synthesis, crystal structure and hydrogenation properties of MgLi_3 - B_48 -  (x = 1.11, y = 0.40)

The ternary magnesium/lithium boride, MgLi_3 - B_48 -  (x = 1.11, y = 0.40, idealized formula MgLi₂B₄₈), crystallizes as its own structure type in P4₃2₁2, which is closely related to the structural family comprising α-AlB₁₂, Be_0.7Al_1.1B₂₂ and tetra-gonal β-boron. The asymmetric unit of title structure contains two statistical mixtures Mg/Li in Wyckoff sites 8b with relative occupancies Mg:Li = 0.495 (9):0.505 (9) and 4a with Mg:Li = 0.097 (8):0.903 (8). The boron atoms occupy 23 8b sites and two 4a 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 [B₁₂] 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 MgLi₂B₄₈ alloy, formation of the eutectic composite hydride LiBH₄+Mg(BH₄)₂ and amorphous boron is observed. In the temperature range 543-623 K, the hydride eutectics decompose, forming MgH₂, LiH, MgB₄, B and H₂.