Synthesis, crystal structure and hydrogenation properties of Mg x Li3 − x B48 − y (x = 1.11, y = 0.40)

The ternary magnesium/lithium boride, Mg x Li 3 − x B 48 − y ( x = 1.11, y = 0.40, idealized formula MgLi 2 B 48 ), crystallizes as its own structure type in P 4 3 2 1 2, which is closely related to the structural family comprising α-AlB 12 , Be 0.7 Al 1.1 B 22 and tetragonal β-boron. The asymmetric unit of title structure contains two statistical mixtures Mg/Li in Wyckoff sites 8 b with relative occupancies Mg:Li = 0.495 (9):0.505 (9) and 4 a with Mg:Li = 0.097 (8):0.903 (8). The boron atoms occupy 23 8 b sites and two 4 a 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 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 MgLi 2 B 48 alloy, formation of the eutectic composite hydride LiBH 4 +Mg(BH 4 ) 2 and amorphous boron is observed. In the temperature range 543–623 K, the hydride eutectics decompose, forming MgH 2 , LiH, MgB 4 , B and H 2 .