Nd2(WO4)3

The title compound, dineodymium(III) tris[tungstate(VI)], is a member of the Eu2(WO4)3 structure family and crystallizes isotypically with other rare earth tungstates and molybdates of this formula type. The structure is a derivative of the scheelite (CaWO4) structure and can be considered as an ordered defect variant with a threefold scheelite supercell and one rare earth (RE) site unoccupied. The Nd3+ cations are coordinated by eight O atoms in form of a distorted bicapped trigonal prism. The two unique W cations are tetrahedrally surrounded by O atoms. One WO4 tetrahedron has 2 symmetry and is relatively undistorted whereas the other tetrahedron differs considerably from an ideal geometry. This is caused by an additional remote O atom at a distance of 2.149 (4) Å. The resulting WO4 + 1 polyhedra form W2O8 dimers through edge-sharing. Together with the WO4 and NdO8 units, the three-dimensional set-up is accomplished.

The title compound, dineodymium(III) tris[tungstate(VI)], is a member of the Eu 2 (WO 4 ) 3 structure family and crystallizes isotypically with other rare earth tungstates and molybdates of this formula type. The structure is a derivative of the scheelite (CaWO 4 ) structure and can be considered as an ordered defect variant with a threefold scheelite supercell and one rare earth (RE) site unoccupied. The Nd 3+ cations are coordinated by eight O atoms in form of a distorted bicapped trigonal prism. The two unique W cations are tetrahedrally surrounded by O atoms. One WO 4 tetrahedron has 2 symmetry and is relatively undistorted whereas the other tetrahedron differs considerably from an ideal geometry. This is caused by an additional remote O atom at a distance of 2.149 (4) Å . The resulting WO 4 + 1 polyhedra form W 2 O 8 dimers through edgesharing. Together with the WO 4 and NdO 8 units, the threedimensional set-up is accomplished.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: SI2175).

Comment
Rare earth tungstates with formula (RE) 2 (WO 4 ) 3 are interesting materials due to their negative thermal expansion behaviour (Sumithra & Umarji, 2004). Therefore detailed structural data are required for a better understanding and a quantification of these effects. For a number of (RE) 2 (WO 4 ) 3 structures single-crystal data were already published: RE = Eu (Templeton & Zalkin, 1963); La (Gärtner et al., 1994); Dy (Rong et al., 2003); Ce (Gressling & Müller-Buschbaum, 1995). Preparation and an investigation of the crystal structure and the thermal behaviour of the Nd member were also reported some time ago by Nassau et al. (1969), including indication of a phase transition at 1318 K. However, the structural characterization of both low-and high-temperature phases remained preliminary. Although we tried to isolate the proposed high-temperature phase of Nd 2 (WO 4 ) 3 by rapid quenching of the sample from above the transition temperature, we could obtain only the low-temperature polymorph. Here we report the details of the corresponding α-Nd 2 (WO 4 ) 3 structure.
The Nd 3+ cations are coordinated by eight oxygen atoms in form of a distorted bi-capped trigonal prism. The Nd-O distances range from 2.387 (4) to 2.497 (4), conform with the RE-O distances in the isotypic compounds. The W atoms are tetrahedrally surrounded by oxygen atoms. The W2 atom lies at Wyckoff site 4e with site symmetry 2 and W-O distances between 1.754 (4) and 1.808 (4) Å. The W1 atom is on a general position with similar distances between 1.741 (4) and 1.881 (4) Å. An additional O atom lies 2.149 (4) Å from W1, resulting in an overall '4 + 1' coordination for the W1 atom.

Experimental
Single crystals of the title compound were obtained from the melt. Analytical grade starting materials Nd 2 O 3 (Fluka, 99.9%) and WO 3 (Aldrich, 99.5%) were mixed and heated under atmospheric conditions in a platinum crucible to 1473 K. Then the furnace was slowly cooled to 1273 K during 24 h, held at that temperature for 5 h and then cooled to 1173 K during 30 h. Then the crucible was taken from the furnace and was quenched in water. Light-purple crystals of the title compound suitable for X-ray diffraction studies were broken from a large chunk by gentle crushing between two glass slides.

Refinement
The highest peak is 0.76 Å from W2 and the deepest hole 0.69 Å from the same atom. Finally, structure data were standardized with the program STRUCTURETIDY (Gelato & Parthé, 1987). Fig. 1. The crystal structure of Nd 2 (WO 4 ) 3 in a projection along the b-axis, drawn with displacement ellipsoids at the 74% probability level. Nd atoms are given in blue, W atoms in red and O atoms in white. WO 4 units are given as red tetrahedra. Nd-O bonds have been omitted for clarity. The unit cell of the (distorted) scheelite substructure is indicated. with blue lines. 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.