Synthesis and crystal structure of NaMgFe(MoO4)3

The iron molybdate NaMgFe(MoO4)3 is isostructural with α-NaFe2(MoO4)3 and its structure is built up from [Mg,Fe]2O10 units of edge-sharing [Mg,Fe]O6 octahedra which are linked to each other through the common corners of [MoO4] tetrahedra. The resulting anionic three-dimensional framework leads to the formation of channels along the [101] direction, where the Na+ cations are located.


Chemical context
Iron molybdates have been subject to very intensive research as a result of their numerous applications including as catalysts (Tian et al., 2011), multiferroic properties and more recently as a possible positive electrode in rechargeable batteries (Sinyakov et al., 1978;Mą czka et al., 2011;Devi & Varadaraju, 2012). In these materials, the anionic framework is constructed from MoO 4 tetrahedra linked to the iron coordination polyhedra, leading to a large variety of crystal structures with a high capacity for cationic and anionic substitutions.

Structural commentary
The title NaMgFe(MoO 4 ) 3 structure is based on a threedimensional framework of [Mg,Fe] (Fig. 1). In this structure, two types of layers (A and B), similar to those observed in -NaFe 2 (MoO 4 ) 3 , are aligned parallel to (110) with the sequence -A-B-B 0 -A-B-B 0 -and stacked along [001]. B 0 layers are obtained from B by an inversion centre located on the A planes (Fig. 2). The resulting anionic three-dimensional framework leads to the formation of channels along [101] in which the sodium ions are located (Fig. 3).       The environment of the Na + cation showing displacement ellipsoids drawn at the 50% probability level.

Synthesis and crystallization
Crystals of the title compound were grown in a flux of sodium dimolybdate Na 2 Mo 2 O 7 with an atomic ratio Na:Mg:Fe:Mo = 5:1:1:7. Appropriate amounts of the starting reactants NaNO 3 , were dissolved in nitric acid and the resulting solution was evaporated to dryness. The dry residue was then placed in a platinum crucible and slowly heated in air up to 673 K for 24 h to remove H 2 O and NH 3 . The mixture was ground in an agate mortar, melted for 2 h at 1123 K and then cooled to room temperature at a rate of 5 K h À1 . Crystals without regular shape were separated from the flux by washing in boiling water.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1

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. 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 > 2sigma(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.  (14 0.0189 (13) 0.0086 (11) 0.0179 (12) −0.0027 (9) −0.0027 (10) 0.0008 (9) Geometric parameters (Å, º)