Scheelite-type NaDy(WO4)2

The title compound sodium dysprosium(III) bis[tungstate(VI)], NaDy(WO4)2, has been synthesized under high temperature solution growth (HTSG) conditions in air. The compound crystallizes with the scheelite structure and is composed of isolated WO4 tetrahedra ( symmetry) with one set of bond lengths and distorted [(Na/Dy)O8] dodecahedra ( symmetry; occupancy ratio Na:Dy = 1:1) with two sets of bond lengths.

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: WM2287).

Comment
In the past years an increasing interest in the synthesis and characterization of rare-earth double tungstate(VI) crystals with general formula ARE(WO 4 ) 2 (A = alkali metal, RE = rare-earth metal) has been observed due to their interesting magnetic, electric and optical properties (Perets et al., 2007;Huang et al., 2006;Li et al., 1990;Han et al., 2002). These compounds are attractive solid-state laser host materials because of their large rare-earth ion admittance. Most of these crystals have tetragonal symmetry and crystallize with the scheelite structure (CaWO 4 ) in space group I4 1 /a (Sillen & Nylander, 1943).
In the title structure, the Ca 2+ position of the original CaWO 4 structure is statistically occupied by Na + and Dy 3+ ions in an 1:1 ratio. The crystal structure of NaDy(WO 4 ) 2 is composed of a two-direction packing of isolated WO 4 tetrahedra interconnected by distorted [(Na/Dy)O 8 ] dodecahedra, as shown in Fig. 2.

Experimental
Single crystal of NaDy(WO 4 ) 2 were prepared by a high temperature solution reaction, using analytical reagents of Dy 2 O 3 , Na 2 CO 3 and WO 3 in the molar ratio of Na: Dy: W = 8:1:10. The starting mixture was finely ground in an agate mortar to ensure the best homogeneity and reactivity, and then transferred to a platinum crucible to be heated at a temperature of 773 K for 8 h. The sintered product was reground and continuously heated at 1273 K for 20 h, cooled to 673 K at a rate of 4 K/h, and then quenched to room temperature. A few light yellow and prismatically shaped crystals of the title compound were obtained.

Refinement
The Na1 and Dy1 atoms are in a substitutional-type disorder in the structure. Therefore the atomic position and anisotropic displacement parameters of Na1 and Dy1 atoms were constrained to be identical. In the initial least-squares refinement, the occupancy factors of Na1 and Dy1 atoms were set to be free. The results show that the occupancy factors were close to 1:1, viz Na1: Dy1 = 0.50273: 0.49727, and were eventually fixed in a 1:1 ratio. The highest peak in the final difference electron density map is 1.55 e/Å 3 from the W1 site, and the deepest hole is -1.40 e/Å 3 from the Na1/Dy1 site.
supplementary materials sup-2 Figures   Fig. 1. Part of the structure of NaDy(WO 4 ) 2 showing the labelling of the atoms (displacement ellipsoids are drawn at the 50% probability level). Fig. 2. View of the crystal structure of NaDy(WO 4 ) 2 (WO 4 tetrahedra are shaded in seagreen; Na/Dy atoms are drawn as yellow balls).