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 WO_4 + 1 polyhedra form W₂O₈ dimers through edge-sharing. Together with the WO₄ and NdO₈ units, the three-dimensional set-up is accomplished.

Related literature

The crystal structure determination of scheelite was reported by Dickinson (1920). For a previous investigation of the structure of Nd₂(WO₄)₃, see: Nassau et al. (1969). Isotypic (RE)₂(WO₄)₃ structures have been reported by Templeton & Zalkin (1963) for RE = Eu; Gärtner et al. (1994) for La; Rong et al. (2003) for Dy, Gressling & Müller-Buschbaum (1995) for Ce. For the role of the crystal structure on the thermal expansion of (RE)₂(WO₄)₃ compounds, see: Sumithra & Umarji (2004). For data standardization, see: Gelato & Parthé (1987).

Experimental

Crystal data

• Nd₂(WO₄)₃

• M_r = 1032.03

• Monoclinic, C 2/c

• a = 7.7589 (12) Å

• b = 11.597 (2) Å

• c = 11.516 (2) Å

• β = 109.561 (14)°

• V = 976.4 (3) ų

• Z = 4

• Mo Kα radiation

• μ = 45.72 mm⁻¹

• T = 293 K

• 0.12 × 0.10 × 0.08 mm

Data collection

• Enraf–Nonius CAD-4 diffractometer

• Absorption correction: numerical (HABITUS; Herrendorf, 1997) T_min = 0.033, T_max = 0.119

• 8392 measured reflections

• 2149 independent reflections

• 1830 reflections with I > 2σ(I)

• R_int = 0.082

• 3 standard reflections frequency: 120 min intensity decay: none

Refinement

• R[F² > 2σ(F²)] = 0.025

• wR(F²) = 0.063

• S = 1.09

• 2149 reflections

• 79 parameters

• Δρ_max = 2.79 e Å⁻³

• Δρ_min = −5.24 e Å⁻³

Table 1
Selected bond lengths (Å)

Nd—O4^i	2.387 (4)
Nd—O2^i	2.391 (4)
Nd—O3^i	2.427 (5)
Nd—O6^ii	2.433 (4)
Nd—O1^iii	2.487 (4)
Nd—O5^iv	2.488 (4)
Nd—O1	2.495 (4)
Nd—O2	2.497 (4)
W1—O5^v	1.741 (4)
W1—O4	1.771 (4)
W1—O2^vi	1.838 (4)
W1—O6	1.881 (4)
W2—O3	1.754 (4)
W2—O1	1.808 (4)
Symmetry codes: (i) ; (ii) ; (iii) -x, -y, -z+1; (iv) ; (v) ; (vi) .

Data collection: CAD-4 Software (Enraf–Nonius, 1989); cell refinement: CAD-4 Software; data reduction: HELENA implemented in PLATON (Spek, 2009); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ATOMS (Dowty, 2006); software used to prepare material for publication: SHELXL97.

Supporting information

S1. Comment

Rare earth tungstates with formula (RE)₂(WO₄)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(WO₄)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 Nd₂(WO₄)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-Nd₂(WO₄)₃ structure.

The above mentioned (RE)₂(WO₄)₃ compounds are isotypic with Nd₂(WO₄)₃ and can be considered as a derivative of the scheelite (CaWO₄) structure (Dickinson, 1920). The (RE)₂(WO₄)₃ structure is an ordered defect variant with a threefold scheelite supercell. The matrix that relates the scheelite structure with the (RE)₂(WO₄)₃ structure is (110; 001; 210) (Fig. 1). In the "Ca₃(WO₄)₃" supercell structure one rare earth site is unoccupied, thus leading to the formula (RE)₂(WO₄)₃ and distortions of the coordination polyhedra as a consequence.

The Nd³⁺ 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. These W(1)O_{4 + 1} polyhedra share edges, thus forming W(1)₂O₈ dimers. Together with the W(2)O₄ and NdO₈ units, the three-dimensional structure is assembled (Fig. 1).

Related literature

The crystal structure determination of scheelite was reported by Dickinson (1920). For a previous investigation of the structure of Nd₂(WO₄)₃, see: Nassau et al. (1969). Isotypic (RE)₂(WO₄)₃ structures have been reported by Templeton & Zalkin (1963) for RE = Eu; Gärtner et al. (1994) for La; Rong et al. (2003) for Dy, Gressling & Müller-Buschbaum (1995) for Ce. For the role of the crystal structure on the thermal expansion of (RE)₂(WO₄)₃ compounds, see: Sumithra & Umarji (2004). For data standardization, see: Gelato & Parthé (1987).

S2. Experimental

Single crystals of the title compound were obtained from the melt. Analytical grade starting materials Nd₂O₃ (Fluka, 99.9%) and WO₃ (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.

S3. 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).

Computing details

Data collection: CAD-4 Software (Enraf–Nonius, 1989); cell refinement: CAD-4 Software (Enraf–Nonius, 1989); data reduction: HELENA implemented in PLATON (Spek, 2009); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ATOMS (Dowty, 2006); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

Fig. 1. The crystal structure of Nd2(WO4)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. WO4 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.

dineodymium(III) tris[tungstate(VI)]

Crystal data

Nd2(WO4)3	F(000) = 1752
Mr = 1032.03	Dx = 7.021 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 25 reflections
a = 7.7589 (12) Å	θ = 10.5–16.9°
b = 11.597 (2) Å	µ = 45.72 mm^−1
c = 11.516 (2) Å	T = 293 K
β = 109.561 (14)°	Block, light-purple
V = 976.4 (3) Å^3	0.12 × 0.10 × 0.08 mm
Z = 4

Data collection

Enraf–Nonius CAD-4 diffractometer	1830 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.082
Graphite monochromator	θmax = 35.0°, θmin = 3.3°
ω/2θ scans	h = −12→12
Absorption correction: numerical (HABITUS; Herrendorf, 1997)	k = −18→18
Tmin = 0.033, Tmax = 0.119	l = −18→18
8392 measured reflections	3 standard reflections every 120 min
2149 independent reflections	intensity decay: none

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.025	w = 1/[σ^2(Fo^2) + (0.0271P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.063	(Δ/σ)max = 0.001
S = 1.09	Δρmax = 2.79 e Å^−3
2149 reflections	Δρmin = −5.24 e Å^−3
79 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.00516 (13)

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 F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² 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 (Ų)

	x	y	z	Uiso*/Ueq
Nd	0.17830 (3)	0.12676 (2)	0.59450 (2)	0.00552 (7)
W1	0.15461 (2)	0.355585 (18)	0.048983 (19)	0.00616 (7)
W2	0.0000	0.11826 (3)	0.2500	0.00700 (7)
O1	0.0124 (5)	0.0407 (4)	0.3888 (4)	0.0087 (6)
O2	0.0717 (5)	0.3000 (4)	0.4608 (4)	0.0103 (7)
O3	0.1944 (5)	0.2058 (4)	0.2795 (4)	0.0138 (8)
O4	0.2205 (6)	0.4270 (4)	0.1932 (4)	0.0127 (7)
O5	0.3630 (5)	0.0366 (4)	0.0596 (4)	0.0104 (7)
O6	0.3846 (5)	0.2870 (4)	0.0765 (4)	0.0115 (7)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Nd	0.00501 (10)	0.00694 (12)	0.00528 (12)	−0.00035 (7)	0.00262 (8)	0.00007 (8)
W1	0.00522 (9)	0.00682 (10)	0.00802 (10)	−0.00014 (6)	0.00430 (6)	0.00015 (6)
W2	0.00763 (11)	0.01014 (13)	0.00401 (12)	0.000	0.00297 (8)	0.000
O1	0.0106 (15)	0.0089 (16)	0.0069 (15)	−0.0007 (12)	0.0032 (12)	0.0005 (12)
O2	0.0089 (14)	0.0138 (18)	0.0111 (17)	0.0023 (13)	0.0074 (12)	0.0020 (14)
O3	0.0163 (17)	0.0133 (19)	0.0120 (18)	−0.0018 (14)	0.0050 (14)	−0.0037 (15)
O4	0.0142 (15)	0.0180 (19)	0.0057 (15)	0.0012 (15)	0.0031 (12)	−0.0004 (14)
O5	0.0110 (15)	0.0111 (17)	0.0102 (17)	−0.0023 (13)	0.0048 (13)	0.0025 (14)
O6	0.0035 (13)	0.0133 (18)	0.0178 (19)	−0.0002 (12)	0.0036 (13)	−0.0059 (15)

Geometric parameters (Å, º)

Nd—O4^i	2.387 (4)	W1—O5^v	1.741 (4)
Nd—O2^i	2.391 (4)	W1—O4	1.771 (4)
Nd—O3^i	2.427 (5)	W1—O2^vi	1.838 (4)
Nd—O6^ii	2.433 (4)	W1—O6	1.881 (4)
Nd—O1^iii	2.487 (4)	W1—O6^v	2.149 (4)
Nd—O5^iv	2.488 (4)	W2—O3	1.754 (4)
Nd—O1	2.495 (4)	W2—O3^vi	1.754 (4)
Nd—O2	2.497 (4)	W2—O1^vi	1.808 (4)
Nd—Nd^i	3.9708 (7)	W2—O1	1.808 (4)
O4^i—Nd—O2^i	110.36 (14)	O5^v—W1—O4	105.4 (2)
O4^i—Nd—O3^i	70.63 (16)	O5^v—W1—O2^vi	111.35 (18)
O2^i—Nd—O3^i	70.79 (14)	O4—W1—O2^vi	101.06 (18)
O4^i—Nd—O6^ii	99.92 (14)	O5^v—W1—O6	105.45 (19)
O2^i—Nd—O6^ii	130.65 (14)	O4—W1—O6	94.60 (19)
O3^i—Nd—O6^ii	84.51 (15)	O2^vi—W1—O6	134.0 (2)
O4^i—Nd—O1^iii	73.56 (14)	O5^v—W1—O6^v	96.3 (2)
O2^i—Nd—O1^iii	148.71 (14)	O4—W1—O6^v	157.2 (2)
O3^i—Nd—O1^iii	135.38 (14)	O2^vi—W1—O6^v	76.80 (16)
O6^ii—Nd—O1^iii	76.29 (14)	O6—W1—O6^v	72.57 (17)
O4^i—Nd—O5^iv	87.53 (15)	O3—W2—O3^vi	109.3 (3)
O2^i—Nd—O5^iv	70.42 (14)	O3—W2—O1^vi	104.30 (18)
O3^i—Nd—O5^iv	124.37 (14)	O3^vi—W2—O1^vi	109.20 (19)
O6^ii—Nd—O5^iv	150.80 (14)	O3—W2—O1	109.20 (19)
O1^iii—Nd—O5^iv	78.93 (13)	O3^vi—W2—O1	104.30 (18)
O4^i—Nd—O1	139.13 (15)	O1^vi—W2—O1	120.3 (3)
O2^i—Nd—O1	95.50 (13)	W2—O1—Nd^iii	126.85 (18)
O3^i—Nd—O1	149.90 (14)	W2—O1—Nd	119.87 (19)
O6^ii—Nd—O1	84.99 (14)	Nd^iii—O1—Nd	111.72 (15)
O1^iii—Nd—O1	68.28 (15)	W1^vi—O2—Nd^i	134.6 (2)
O5^iv—Nd—O1	71.57 (13)	W1^vi—O2—Nd	115.63 (18)
O4^i—Nd—O2	140.26 (15)	Nd^i—O2—Nd	108.61 (14)
O2^i—Nd—O2	71.39 (14)	W2—O3—Nd^i	136.9 (2)
O3^i—Nd—O2	73.11 (15)	W1—O4—Nd^i	137.0 (3)
O6^ii—Nd—O2	60.61 (13)	W1^v—O5—Nd^vii	138.8 (2)
O1^iii—Nd—O2	126.35 (12)	W1—O6—W1^v	107.43 (17)
O5^iv—Nd—O2	127.07 (13)	W1—O6—Nd^viii	130.5 (2)
O1—Nd—O2	77.15 (13)	W1^v—O6—Nd^viii	106.94 (16)

Symmetry codes: (i) −x+1/2, −y+1/2, −z+1; (ii) x−1/2, −y+1/2, z+1/2; (iii) −x, −y, −z+1; (iv) x, −y, z+1/2; (v) −x+1/2, −y+1/2, −z; (vi) −x, y, −z+1/2; (vii) x, −y, z−1/2; (viii) x+1/2, −y+1/2, z−1/2.