Scheelite-type NaEr(MoO4)2

Explorations of the A 1+–RE 3+–Mo6+–O2− (A 1+ is an alkali metal cation, RE 3+ is a rare-earth metal cation) quaternary systems prepared by the high-temperature solution growth method led to the title structure, sodium erbium bis(molybdate), NaEr(MoO4)2. It is isostructural to the scheelite structure (CaWO4) and is composed of [MoO4]2− tetrahedra with symmetry and [(Na/Er)O8]14− polyhedra. The [(Na/Er)O8]14− polyhedron is a distorted tetragonal antiprism, also with symmetry, with statistically mixed Na/Er atoms at its centre. There are two sets of Na/Er—O bond lengths [2.420 (4) and 2.435 (3) Å], but just one set of Mo—O bond lengths [1.774 (4) Å].

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: FB2187).
The X-ray diffraction analysis has shown that the title compound NaEr(MoO 4 ) 2 is isostructural with the scheelite. In the title structure, Na and Er atoms are disordered over the same 4a site while Mo atoms reside on 4b site. The structure of NaEr(MoO 4 ) 2 may be regarded as composed of [MoO 4 ] 2tetrahedra and of [(Na/Er)O 8 ] 14polyhedra (each in the form of a distorted tetragonal antiprism) that share the oxygens (Fig. 2). Each oxygen of the [MoO 4 ] 2tetrahedron is shared by the different Na/Er polyhedron and each oxygen of the [(Na/Er)O 8 ] 14polyhedron is shared by the different [MoO 4 ] 2tetrahedron.

Experimental
Single crystals of NaEr(MoO 4 ) 2 have been prepared by the high temperature solution growth (HTSG) method in air. A powder mixture of Na 2 CO 3 (0.4418 g), Er 2 O 3 (0.2657 g) and MoO 3 (2.000 g) at the molar ratio of Na:Er:Mo = 6:1:10 was first ground in an agate mortar and then transferred to a platinum crucible. The sample was gradually heated in air at 1173 K for 24 h. In this stage, the reagents were completely melted. After that, the intermediate product was slowly cooled to 673 K at the rate of 2 Kh -1 . It was kept at 673 for another 10 h and then quenched to room temperature. The obtained crystals were light-red and of the prismatical shape. The dimensions of the used sample were typical for the grown crystals in this batch.

Refinement
The Na and Er atoms are in substitutional disorder in the crystal structure. The tentative refinement that included the corresponding occupancy factors for the disordered Na/Er yielded Na1 : Er1 = 0.501 (2) : 0.499 (2). (The atomic positional and anisotropic displacement parameters of Na1 and Er1 atoms were constrained to be identical by using EADP and EXYZ constraint instructions (SHELXL-97;Sheldrick, 2008).) Therefore the ratio of Na and Er was fixed to 1:1 in the final model with the constrained positional and the displacement parameters of na and Er as given above. The highest peak in the dif-supplementary materials sup-2 ference electron density map equals to 1.12 e/Å 3 at the distance of 0.83 Å from Na1/Er1 site while the deepest hole equals to -1.15 e/Å 3 at the distance of 1.39 Å from Na1/Er1 site, too. Fig. 1. Section of the structure of NaEr(MoO 4 ) 2 with the atom labelling scheme. The displacement ellipsoids are drawn at the 50% probability level.

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 > σ(F 2 ) is used only for calculating Rfactors(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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )