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

The crystal structure of the new tetragonal boride Mg x Li3 - x B48 - y (x = 1.11, x = 0.40) has been determined by the single-crystal method. This new structure type is closely related to the structural family comprising tetragonal β-boron, α-AlB12 and Be0.7Al1.1B22.


Chemical context
The main requirements for modern hydrogen-storage materials in automotive applications are high gravimetric uptake (above 6.5 wt.% capacity for hydrogen), absorption/desorption of hydrogen at moderate temperatures and pressures, as well as a relatively low cost of the materials and their environmental safety (Sakintuna et al., 2007).Conventional metal compositions, such as LaNi 5 and its substitutional derivatives, titanium and zirconium alloys that are commonly used as hydrogen-storage systems, have a capacity of nearly 1.5 wt.% and thus they cannot satisfy the current needs.More promising candidates for the development of light-weight hydrogen-storage materials may be found with new formulations based on light metals such as Mg and Li (Herbst & Meyer, 2010;Pavlyuk et al., 2013;Pavlyuk et al., 2018Pavlyuk et al., , 2019a, b), b).Upon hydrogenation with boron, magnesium and lithium form complex hydrides of particularly high capacity.Two paradigmatic compositions of Mg(BH 4 ) 2 and LiBH 4 contain as much as 14.9 and 18.5 wt.% of hydrogen, respectively (Li et al., 2023;Puszkiel et al., 2019).Therefore, new insights into the solid-state chemistry of Mg-Li-B ternary alloys and their hydrogen-sorption properties are of primary importance.While studying boron-rich alloys, we have succeeded in the preparation of a new ternary phase of the approximate composition Mg x Li 3 À x B 48 À y (x = 1.11, y = 0.40) and we report its structure here.

Synthesis and crystallization
The Mg-Li-B samples were prepared from the following reactants: magnesium (powder, �99%) lithium (rod, 99.9%) and boron (powder, 99.99%).Appropriate amounts of the components were mixed and pressed into a tablet at a pressure of 6 bar.The tablet was closed inside a tantalum crucible in a glove-box under an argon atmosphere.The crucible was sealed by arc melting under a dry argon atmosphere.The   reaction between the components was initiated in an induction furnace at 1173 K.After 10 min, the sample was cooled to 670 K and homogenized over 48 h.A prismatic-like singlecrystal of the title compound, metallic dark grey in colour, was isolated by mechanical fragmentation from the alloy, the starting composition of which was Mg 5 The bulk sample was also examined by X-ray powder diffraction (diffractometer Rigaku Miniflex D-600, Cu K� radiation).X-ray phase analysis indicates that the prepared sample is a single phase (Fig. 4a).
Taking into account the fact that the compound consists of elements prone to hydride formation, we have examined its hydrogen-sorption properties.Gas-phase hydrogenation was performed using the IMI-COR Hiden Isochema apparatus in the temperature range from 293 to 773 K and hydrogen pressure up to 200 bar for an alloy with composition

Database survey
The title compound crystallizes in a new structure type, which is closely related to the structural family comprising tetragonal �-boron (Vlasse et al., 1978(Vlasse et al., , 1979)), �-AlB 12 (Kasper et al., 1977) and Be 0.7 Al 1.1 B 22 types (Higashi, 1980).Tetragonal �-boron crystallizes in the tetragonal crystal system with space group P4 1 (a = 10.14 and c = 14.17A ˚). Boron atoms occupy 49 positions of Wykoff 4a, three of which are half-populated.Boride �-AlB 12 crystallizes in the tetragonal crystal system with space group P4 1 2 1 2 (a = 10.158 and c =14.270A ˚).The aluminium atoms partially occupy five 8b sites.The boron atoms occupy 21 8b sites and two 4a sites.The Be 0.7 Al 1.1 B 22 compound (P4 1 2 1 2, a = 10.168 and c = 14.262A ˚) crystallizes in the same crystal system and space group as AlB 12 but it adopts a different site distribution of the atoms.Boron atoms occupy 21 8b sites and two 4a sites.Aluminium atoms partially occupy three 8b sites and the beryllium atoms partially occupy five 8b sites.Al 2 À x B 22 or �-AlB 12 (Kasper et al., 1977) and Mg 5 À x B 44 (Pediaditakis et al., 2010) crystallize in the enantiomorphic space groups P4 1 2 1 2 and P4 3 2 1 2, while adopting different structure types than the title compound.In these cases, all five 8b sites are partially occupied by the Al or Mg atoms and three of them are split.In addition, the coordination polyhedra of Al and Mg atoms are different (CN = 16 and CN = 12) as may be compared to the title structure.In addition to the boron atoms, these coordination environments also include metal atoms, which is a direct consequence of a more metal-rich composition.For example, Fig. 5 shows how the [B 12 ] icosahedra are connected with the selected aluminium polyhedra and this mode differs from the one observed for the [B 12 ] icosahedra and truncated tetrahedra in the title phase.

Refinement
The structure was solved by direct methods after an empirical absorption correction.In the first stage of the refinement, the positions of the boron and Mg/Li atoms were obtained correctly by direct methods.The B19 atom (Wyckoff site 4a) showed displacement parameters considerably different than those of other boron atom sites, suggesting that this position is The observed powder X-ray diffraction patterns (Cu K� radiation) for the sample MgLi 2 B 48 before hydrogenation and after gas-phase hydrogenation.

Figure 5
The connection of two types of Al truncated tetrahedra with [B 12 ] icosahedra in the structure of Al 2 À x B 22 (Kasper et al., 1977).

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes.

sup-2
Acta Cryst.(2024).E80, 10-13 Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å shows the two mixed Mg/Li sites (labelled as M1 and M2 for Mg1/Li1 and Mg2/Li2, respectively) and two kinds of the distinct boride polyhedra, which are [B 12 ] icosahedra and fused [B 21 ] double icosahedra.The latter are situated across twofold axes passing through the B19 atom.The twelvefold coordination environments in the form of truncated tetrahedra (Laves polyhedra) are characteristic for both kinds of Mg/Li statistical mixtures.Every such [M2B 12 ] truncated tetrahedron is connected by triangular faces to four [B 12 ] icosahedra, whereas the connection of [M1B 12 ] comprises face-sharing with two [B 12 ] icosahedra and two fused [B 21 ] double icosahedra.The arrangement of the truncated tetrahedra in the environment of the boride clusters is shown in Fig. 2. The [M1B 12 ] and [

Table 1
Experimental details.