Synthesis, crystal structure and hydrogenation properties of Mg_xLi_{3 - x}B_{48 - y} (x = 1.11, y = 0.40)

The ternary magnesium/lithium boride, MgxLi3 - xB48 - y (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 alpha-AlB12, Be0.7Al1.1B22 and tetragonal beta-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 twelve-fold coordination environment in the form of truncated tetrahedra (Laves polyhedra). These polyhedra are connected by triangular faces to four [B-12] icosahedra. The boron atoms exhibit four kinds of polyhedra, namely pentagonal pyramid (coordination number CN = 6), distorted tetragonal pyramid (CN = 5), bicapped hexagon (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 H-2.