Li0.5Al0.5Mg2(MoO4)3

The title compound, lithium/aluminium dimagnesium tetrakis[orthomolybdate(VI)], was prepared by a solid-state reaction route. The crystal structure is built up from MgO6 octahedra and MoO4 tetrahedra sharing corners and edges, forming two types of chains running along [100]. These chains are linked into layers parallel to (010) and finally linked by MoO4 tetrahedra into a three-dimensional framework structure with channels parallel to [001] in which lithium and aluminium cations equally occupy the same position within a distorted trigonal–bipyramidal coordination environment. The title structure is isotypic with LiMgIn(MoO4)3, with the In site becoming an Mg site and the fully occupied Li site a statistically occupied Li/Al site in the title structure.

The title compound, lithium/aluminium dimagnesium tetrakis[orthomolybdate(VI)], was prepared by a solid-state reaction route. The crystal structure is built up from MgO 6 octahedra and MoO 4 tetrahedra sharing corners and edges, forming two types of chains running along [100]. These chains are linked into layers parallel to (010) and finally linked by MoO 4 tetrahedra into a three-dimensional framework structure with channels parallel to [001] in which lithium and aluminium cations equally occupy the same position within a distorted trigonal-bipyramidal coordination environment. The title structure is isotypic with LiMgIn(MoO 4 ) 3 , with the In site becoming an Mg site and the fully occupied Li site a statistically occupied Li/Al site in the title structure.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: WM2760). In recent years, great attention has been devoted to the examination of metal oxides containing mobile lithium ions due to their potential application in the fields of energy and electronics, such as LiMO 2 materials (M= Mn, Fe, Co, Ni) (Whittingham & Silbernagel, 1976;Mizushima et al., 1980;Kanno et al., 1994) which allowed to construct electrochemical generators with a high energy density. In our field of research we are interested especially to doubles molybdates of alkali and divalent metals, and we tried to explore systems like Li 2 O-MO-MoO 3 . For such systems, the compounds Li 3 Fe(MoO 4 ) 3 and Li 2 Fe 2 (MoO 4 ) 3 are already known, the crystal structures of which have been determined by Klevtsova & Magarill (1970) and the structural similarity to NaCo 2.31 (MoO 4 ) 3 and to other framework oxides is noted.
The asymmetric unit of compound (I) is composed of two MgO 6 octahedra and three MoO 4 tetrahedra sharing corners, as well as a (li/Al) site (Fig. 1). The structure of (I) can be described as being composed of two types of infinite chains expanding parallel to [100]. The first chain is built up from Mg2O 6 octahedra and Mo1O 4 tetrahedra sharing corners, forming double chains with composition (Mg 2 Mo 2 O 14 ) n and with a cis arrangement of the MoO 4 tetrahedra relative to the MgO 6 octahedra (Fig. 2a). The second type of chain is formed by Mg1O 6 octahedra and Mo2O 4 tetrahedra, also linked by corners but in a trans arrangement. Single chains of the second type are linked by sharing edges between two adjacent Mg1O 6 octahedra (Fig. 2b). The linkage between the two types of chains leads to a layer-like arrangement parallel to (010) (Fig. 3), whereas the linkage into a three-dimensional framework is provided by Mo3O 4 tetrahedra by sharing corners. In this framework channels are present where the mixed-occupied Li + /Al 3+ sites are located (Fig. 4) (Nord & Kierkegaard, 1984 (Solodovnikov et al., 1997). The (Li/Al) site has a distorted trigonalbipyramidal coordination environment.
The structure of compound (I) is isotypic with LiMgIn(MoO 4 ) 3 (Khazheeva et al., 1985). The In 3+ site becomes Mg2 in the title structure, and the Li site in LiMgIn(MoO 4 ) 3 has full occupation, whereas in the title structure it is a mixedoccupied (Li/Al) site with half-occupancy for each of the metal ions. In fact, charge compensation can only be ensured by insertion of Al 3+ in the same site as Li + ((Mg 2+ + In 3+ + Li + ) = (2Mg 2+ + (Al 3+ /Li + )) (Fig. 5). The unit-cell parameters of triclinic Li 0.5 Al 0.5 Mg 2 (MoO 4 ) 3 indicate some resemblance to the structures of Ag 2 M 2 (MoO 4 ) 3 (M = Zn, Mg, Co) (Tsyrenova et al., 2004(Tsyrenova et al., , 2001Gicquel-Mayer et al., 1981), but a close comparison of the structures reveals some differences. The latter have mixed frameworks of MoO 4 tetrahedra and pairs of MO 6 octahedra sharing common edges, whereas in structure (I) MO 6 octahedra and also Mg 2 O 10 units surrounded by MoO 4 tetrahedra are present. A further comparison of the structure of compound (I) with the Li 2 M 2 (MoO 4 ) 3 family (M=Mg, Mn, Co, Ni, Cu, Zn) (Efremov & Trunov, 1972;Ozima & Zoltai, 1976) reveals that the substitution of some of the lithium ions by aluminium has changed the crystal structure. The latter family adopts the lyonsite structure type, space group Pnma, and their general formula can be written as A 16 B 12 O 48 . Generally, the A site is statistically occupied by Li + and a M 2+ ion.

Experimental
The title compound, Li 0.5 Al 0.5 Mg 2 (MoO 4 ) 3 , was obtained serendipitously by a solid state reaction from appropriate quantities of LiNO 3 (Fluka, 62575), (NH 4 ) 2 Mo 4 O 13 (Fluka, 69858) and Mg(NO 3 ) 2 . 6H 2 O (Fluka, 63079) placed in a porcelain crucible, and slowly annealed in air at 623 K for 12 h, in order to eliminate volatile products. The resulting mixture was then heated to 853 K for 7 days before slowly cooling at 5 K/day to 773 K. Finally, the furnace was cooled at 50 K/day to room temperature. A qualitative EDX analysis of a selected crystal using a FEI Quanta 200 system revealed the presence of Al, Mo, Mg and O (Fig. 6). Aluminium was not present in the employed educts of the reaction mixture, but the incorporation of aluminium from the porcelain crucible is the most likely source of this element.

Refinement
During the first stages of refinement the site in the channels was first attributed solely to Li. However, the refined composition did not satisfy electrical neutrality. After considering the presence of Al (see 'experimental part′) on this site with an occupancy ratio of 1:1 for Li and Al, electrical neutrality was achieved. Both metals were refined with the same coordinates and the same anisotropic displacement parameters.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.