A tetra­gonal form of dysprosium orthomolybdate at room temperature

In the present tetra­gonal modification of dysprosium orthomolybdate, Dy₂(MoO₄)₃, the Dy, one Mo and one O atom are located on a mirror plane with Wyckoff symbol 4e, while another Mo atom is located on a fourfold inverse axis, Wyckoff symbol 2a. A single crystal was selected from a polycrystalline mixture of the Dy₂O₃-ZrO₂-MoO₃ system and was stable at room temperature for at least three months. The structure refinement does not indicate the presence of Zr on the Dy sites (to within 1% accuracy). Thus, the stabilization of the tetra­gonal form is due to disordered positions for a second O atom and split positions for a third O atom that also maintain the DyO₇ coordination, which is not expected for short Dy-O distances [2.243 (6)-2.393 (5) Å].

S1. Comment

It is known that the molybdates of the rare earth elements show interesting fluorescence, laser, piezoelectric, ferroelectric and ferroelastic properties, and are used as catalysts for the oxidation of organic compounds such as toluene or isobutene (Smet et al., 2001; Wang et al., 2008; Nassau et al., 1971; Wenxing et al., 1999). The crystal chemistry of molybdenum compounds is very rich, because Mo adopts different oxidation states and therefore forms various coordination polyhedra, such as tetrahedra (Nassau et al., 1971), pyramids (Alonso et al., 2004) and octahedra (Gall et al., 2002). For example, Gall et al. (2002) synthesized molybdates R₄Mo₄O₁₁ (R = Gd–Tm) with an average oxidation state for Mo of +2.5 and explained the stabilization of the crystal structures through the distortion of trans-edge-sharing Mo octahedra, based on theoretical calculations. With higher oxidation states Mo forms five-fold oxygen coordination, as in the Dy₂MoO₆ structure (Alonso et al., 2004).

Rare earth molybdates with M₂(MoO₄)₃ stoichiometry exist in several polymorphs depending on the temperature and the specific rare earth element (Nassau et al., 1971). A high-temperature β-form is stable at temperatures above 1023–1153 K. The transition from the β-modification to the room-temperature α-form is kinetically prevented during cooling and takes place via a β' phase, which is metastable at room temperature for a long time. For example, crystals of Gd₂(MoO₄)₃ exist in the metastable Pba2 (Keve et al., 1970) form under ambient conditions for years, although the stable low-temperature form is monoclinic (Nassau et al., 1971). Both tetragonal and orthorhombic polymorphic modifications contain a network of corner-sharing polyhedra, in which Gd and Mo cations are coordinated by seven and four O atoms, respectively.

Borchardt & Bierstedt (1967) showed that Gd₂(MoO₄)₃ and the isostructural molybdates of Sm, Eu, Tb and Dy undergo ferroelectric phase transformations in the temperature range 423 K < T₀ < 463 K. Gadolinium molybdate, Gd₂(MoO₄)₃, with a ferroelectric–ferroelastic transition temperature of about 433 K, crystallizes in the space group Pba2 (metastable form) below the phase transition and in the space group P42₁m above it (Jeitschko, 1972). The paraelectric structure at elevated temperature approaches the average structure of the two ferroelectric–ferroelastic orientations (Jeitschko, 1972). Zou et al. (1999) mentioned that the β-modification of (Nd_0.023Gd_0.977)₂(MoO₄)₃ with tetragonal symmetry can exist at room temperature.

Dysprosium molybdate, Dy₂(MoO₄)₃, shows a ferroelectric–ferroelastic transition below 418 K (Roy et al., 1989). According to X-ray powder diffraction, the ferroelectric phase crystallizes in orthorhombic symmetry (space group Pba2; Roy et al., 1989). Above 1303 K, a reconstructive phase transition into a cubic form of Dy₂(MoO₄)₃ was reported (Roy et al., 1989; Brixner, 1973).

In this work, single crystals of Dy₂(MoO₄)₃ with the high-temperature β-form tetragonal crystal structure have been obtained, and the structure refinement based on single-crystal X-ray diffraction data has been performed at room temperature.

According to the single-crystal experiment, dysprosium molybdate shows tetragonal symmetry at room temperature, space group P42₁m, with unit-cell parameters a = 7.295 (2) Å and c = 10.578 (4) Å. On the one hand, the reason for the existence of the tetragonal high-temperature structure at room temperature is not clear, because a monoclinic form of Dy₂(MoO₄)₃ has been observed below 1043 K (Nassau et al., 1971). On the other hand, the symmetry of the low-temperature forms of related compounds is strongly dependent on the synthesis conditions and cation stoichiometry. For example, for the Nd-substituted phase (Nd_0.023Gd_0.977)₂(MoO₄)₃, obtained by the Czochralsky method, a tetragonal crystal structure was found by Zou et al. (1999) at room temperature. Even a 2% cation substitution of Gd through Nd stabilizes a high-temperature form in space group P42₁m with a = 7.356 (1) Å and c = 10.685 (2) Å. The tetragonal Dy₂(MoO₄)₃ form, obtained in present work, is stable at room temperature for at least three months (the structure investigation was repeated after three months in storage). The stabilization of the tetragonal structure could be due to a small replacement of Dy by Zr atoms, analogous to (Nd_0.023Gd_0.977)₂(MoO₄)₃. However, the refinement of the Dy₂(MoO₄)₃ structure does not indicate the presence of Zr on Dy sites to within 1% inaccuracy.

The field stability regions for the various types of R₂(MoO₄)₃ structures, shown by Nassau et al. (1971), depend on temperature and the radius of the rare earth cation. The present Dy₂(MoO₄)₃ compound is situated between Gd₂(MoO₄)₃, which has the metastable β'-phase at room temperature, and Y₂(MoO₄)₃, which has a tetragonal modification in the extended temperature region down to room temperature. The crystal structure of Dy₂(MoO₄)₃ is formed by corner-sharing MoO₄ tetrahedra and Dy polyhedra coordinated by seven O atoms (Figs. 1 and 2). In this structure there are two types of Mo tetrahedra, a regular one and a distorted one, with average Mo—O distances of 1.743 (7) and 1.76 (5) Å, respectively. The DyO₇ polyhedron is characterized by Dy—O distances in the range 2.243 (6)–2.393 (5) Å (Table 1), slightly shorter than those for the coordination polyhedron of Gd in the previously published (Nd_0.023Gd_0.977)₂(MoO₄)₃ structure mentioned above [2.258 (12)–2.418 (8) Å]. Such compression of the structure can be explained by the slightly smaller size of the Dy³⁺ cation (0.97 Å) compared with the Gd³⁺ cation (1.0 Å), according to the Shannon ionic radii (Shannon, 1976). For another molybdate, Dy₂MoO₆, with the Mo^VI oxidation state, longer Dy—O interatomic distances [2.381 (6), 2.401 (7) and 2.421 (7) Å) lead to an eight-fold oxygen coordination of Dy (Alonso et al., 2004).

Nassau et al. (1971) systematized the crystal structures of R₂(MoO₄)₃ according to their rare earth ionic radius and oxygen coordination polyhedra RO_x. For example, eight-fold oxygen coordination is characteristic for the A family of R₂(MoO₄)₃ with large rare earth cations such as La, Pr or Pm, and scheelite- or pseudoscheelite-type structures. Due to the shorter Dy—O distances in the tetragonal form of Dy₂(MoO₄)₃, one would expect a six-fold oxygen coordination of Dy and an orthorhombic crystal structure like Er₂(MoO₄)₃, but the slightly disordered oxygen positions do not distort the structure symmetry and maintain the DyO₇ coordination. Disordered atoms O3 and O4 make the structure more flexible, atom O3 lying in the vicinity of a mirror plane and atom O4 on a general position. The distance from atom O3 to its mirror image is 0.44 (3) Å, and this seems advantageous for seven-fold oxygen coordination.

The shortest metal···metal distances in the title structure are Dy1···Mo1(-y + 1, x, -z + 1) = 3.7343(15 Å, Dy1···Dy1(-y + 1, x, -z + 1) = 3.8572 (17) Å and Mo1···Mo1(-y + 1, x, -z + 1) = 4.243 (3) Å.

Related literature

For related literature, see: Alonso et al. (2004); Borchardt & Bierstedt (1967); Brixner (1973); Gall et al. (2002); Jeitschko (1972); Keve et al. (1970); Nassau et al. (1971); Roy et al. (1989); Shannon (1976); Smet et al. (2001); Wang et al. (2008); Wenxing et al. (1999); Zou et al. (1999).

S2. Experimental

Single crystals of Dy₂(MoO₄)₃ were obtained during an investigation of the Dy₂O₃–ZrO₂–MoO₃ phase diagram in an evacuated sealed silica tube from a Dy₂Zr(MoO₄)₅ composition by heating up to 1273 K, followed by cooling at a rate of 5 K h^-1 down to 1073 K, after which the tube was cooled to room temperature by switching off the heating. From the resulting multiphase polycrystalline mixture, colourless single crystals of Dy₂(MoO₄)₃ were selected using an optical microscope.

S3. Refinement

Due to the large atomic displacement parameters, a split position from the original position on a mirror plane for atom O3 was introduced with an occupancy of 0.5. Atom O4 was disordered over two sites (O4A and O4B) separated by 0.63 (2) Å. Atoms O4A and O4B were refined with a common displacement parameter and their occupancy factor was constrained to sum to 1. These positions (O3, O4A and O4B) were then refined with isotropic displacement parameters.

Computing details

Data collection: APEX2 (Bruker, 2004); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT (Bruker, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2008); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

	Fig. 1. The Dy2(MoO4)3 tetragonal unit cell, showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 70% probability level.
	Fig. 2. The structural model of Dy2(MoO4)3, viewed along the b axis. Dark-grey tetrahedra are formed by Mo atoms and light-grey polyhedra by Dy atoms. The positions for atoms O3, O4A and O4B were taken as the average between two close values.

Dysprosium orthomolybdate

Crystal data

Dy2Mo3O12	Dx = 4.748 Mg m^−3
Mr = 804.82	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, P421m	Cell parameters from 727 reflections
Hall symbol: P -4 2ab	θ = 1.9–27.5°
a = 7.295 (2) Å	µ = 16.41 mm^−1
c = 10.578 (4) Å	T = 296 K
V = 562.9 (3) Å^3	Prism, colourless
Z = 2	0.08 × 0.06 × 0.06 mm
F(000) = 708

Data collection

Bruker Kappa APEXII CCD area-detector diffractometer	723 independent reflections
Radiation source: fine-focus sealed tube	645 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.054
Detector resolution: 25 pixels mm^-1	θmax = 27.5°, θmin = 1.9°
ϕ scans	h = −8→9
Absorption correction: multi-scan (SADABS; Bruker, 2004)	k = −9→7
Tmin = 0.319, Tmax = 0.374	l = −13→8
2641 measured reflections

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0063P)^2] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.029	(Δ/σ)max = 0.001
wR(F^2) = 0.045	Δρmax = 1.04 e Å^−3
S = 0.97	Δρmin = −0.88 e Å^−3
723 reflections	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
44 parameters	Extinction coefficient: 0.00073 (19)
0 restraints	Absolute structure: Flack (1983), with 287 Friedel pairs
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.02 (2)

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	Occ. (<1)
Dy1	0.81307 (5)	0.31307 (5)	0.26269 (5)	0.00987 (17)
Mo1	0.79435 (11)	0.29435 (11)	0.64262 (10)	0.0107 (3)
Mo2	1.0000	0.0000	0.0000	0.0127 (4)
O1	0.6285 (8)	0.1285 (8)	0.6903 (9)	0.014 (2)
O2	0.9586 (8)	0.1890 (9)	0.0967 (6)	0.0226 (15)
O3	0.8279 (18)	0.2853 (18)	0.4794 (8)	0.014 (3)*	0.50
O4A	0.683 (3)	0.505 (2)	0.6869 (16)	0.020 (2)*	0.48 (2)
O4B	0.762 (3)	0.5017 (18)	0.7117 (14)	0.020 (2)*	0.52 (2)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Dy1	0.0104 (2)	0.0104 (2)	0.0087 (3)	−0.0025 (3)	−0.00039 (19)	−0.00039 (19)
Mo1	0.0109 (3)	0.0109 (3)	0.0102 (6)	−0.0027 (5)	−0.0001 (3)	−0.0001 (3)
Mo2	0.0165 (5)	0.0165 (5)	0.0052 (7)	0.000	0.000	0.000
O1	0.010 (3)	0.010 (3)	0.022 (5)	0.001 (3)	0.001 (3)	0.001 (3)
O2	0.026 (4)	0.027 (4)	0.015 (3)	−0.002 (4)	0.006 (3)	−0.008 (4)

Geometric parameters (Å, º)

Dy1—O2^i	2.243 (6)	Mo1—O1	1.784 (8)
Dy1—O2	2.243 (6)	Mo1—O4A	1.803 (16)
Dy1—O3^i	2.304 (9)	Mo1—O4A^i	1.803 (16)
Dy1—O3	2.304 (9)	Mo2—O2^vi	1.743 (7)
Dy1—O4A^ii	2.307 (15)	Mo2—O2	1.743 (7)
Dy1—O4A^iii	2.307 (15)	Mo2—O2^vii	1.743 (7)
Dy1—O4B^ii	2.351 (14)	Mo2—O2^viii	1.743 (7)
Dy1—O4B^iii	2.351 (14)	O1—Dy1^ii	2.393 (5)
Dy1—O1^iv	2.393 (5)	O1—Dy1^ix	2.393 (5)
Dy1—O1^v	2.393 (5)	O3—O3^i	0.44 (3)
Mo1—O4B^i	1.697 (13)	O4A—O4B	0.632 (17)
Mo1—O4B	1.697 (13)	O4A—Dy1^iv	2.307 (15)
Mo1—O3	1.745 (9)	O4B—Dy1^iv	2.351 (14)
Mo1—O3^i	1.745 (9)
O2^i—Dy1—O2	76.6 (3)	O4B^iii—Dy1—O4A^x	62.2 (7)
O2^i—Dy1—O3^i	136.2 (4)	O1^iv—Dy1—O4A^x	117.2 (3)
O2—Dy1—O3^i	147.1 (4)	O1^v—Dy1—O4A^x	50.5 (3)
O2^i—Dy1—O3	147.1 (4)	O4A^iv—Dy1—O4A^x	161.0 (5)
O2—Dy1—O3	136.2 (4)	O4B^i—Mo1—O4B	93.7 (13)
O3^i—Dy1—O3	11.0 (8)	O4B^i—Mo1—O3	107.2 (7)
O2^i—Dy1—O4A^ii	77.4 (4)	O4B—Mo1—O3	118.7 (7)
O2—Dy1—O4A^ii	130.3 (5)	O4B^i—Mo1—O3^i	118.7 (7)
O3^i—Dy1—O4A^ii	71.6 (5)	O4B—Mo1—O3^i	107.2 (7)
O3—Dy1—O4A^ii	79.4 (5)	O3—Mo1—O3^i	14.5 (11)
O2^i—Dy1—O4A^iii	130.3 (5)	O4B^i—Mo1—O1	112.8 (6)
O2—Dy1—O4A^iii	77.4 (4)	O4B—Mo1—O1	112.8 (6)
O3^i—Dy1—O4A^iii	79.4 (5)	O3—Mo1—O1	110.4 (4)
O3—Dy1—O4A^iii	71.6 (5)	O3^i—Mo1—O1	110.4 (4)
O4A^ii—Dy1—O4A^iii	88.3 (10)	O4B^i—Mo1—O4A	114.2 (13)
O2^i—Dy1—O4B^ii	79.1 (4)	O4B—Mo1—O4A	20.5 (6)
O2—Dy1—O4B^ii	117.0 (5)	O3—Mo1—O4A	110.7 (7)
O3^i—Dy1—O4B^ii	79.1 (5)	O3^i—Mo1—O4A	96.9 (8)
O3—Dy1—O4B^ii	84.9 (4)	O1—Mo1—O4A	101.5 (6)
O4A^ii—Dy1—O4B^ii	15.6 (4)	O4B^i—Mo1—O4A^i	20.5 (6)
O4A^iii—Dy1—O4B^ii	76.1 (8)	O4B—Mo1—O4A^i	114.2 (13)
O2^i—Dy1—O4B^iii	117.0 (5)	O3—Mo1—O4A^i	96.9 (8)
O2—Dy1—O4B^iii	79.1 (4)	O3^i—Mo1—O4A^i	110.7 (7)
O3^i—Dy1—O4B^iii	84.9 (4)	O1—Mo1—O4A^i	101.5 (6)
O3—Dy1—O4B^iii	79.1 (5)	O4A—Mo1—O4A^i	134.6 (14)
O4A^ii—Dy1—O4B^iii	76.1 (8)	O4B^i—Mo1—O4B^xi	71.0 (8)
O4A^iii—Dy1—O4B^iii	15.6 (4)	O4B—Mo1—O4B^xi	34.6 (6)
O4B^ii—Dy1—O4B^iii	62.5 (9)	O3—Mo1—O4B^xi	100.1 (4)
O2^i—Dy1—O1^iv	77.2 (3)	O3^i—Mo1—O4B^xi	95.1 (4)
O2—Dy1—O1^iv	117.8 (3)	O1—Mo1—O4B^xi	145.5 (3)
O3^i—Dy1—O1^iv	76.3 (3)	O4A—Mo1—O4B^xi	51.1 (6)
O3—Dy1—O1^iv	82.5 (4)	O4A^i—Mo1—O4B^xi	90.0 (9)
O4A^ii—Dy1—O1^iv	96.5 (6)	O4B^i—Mo1—O4B^xii	34.6 (6)
O4A^iii—Dy1—O1^iv	152.3 (5)	O4B—Mo1—O4B^xii	71.0 (8)
O4B^ii—Dy1—O1^iv	111.8 (5)	O3—Mo1—O4B^xii	95.1 (4)
O4B^iii—Dy1—O1^iv	161.1 (4)	O3^i—Mo1—O4B^xii	100.1 (4)
O2^i—Dy1—O1^v	117.8 (3)	O1—Mo1—O4B^xii	145.5 (3)
O2—Dy1—O1^v	77.2 (3)	O4A—Mo1—O4B^xii	90.0 (9)
O3^i—Dy1—O1^v	82.5 (4)	O4A^i—Mo1—O4B^xii	51.1 (6)
O3—Dy1—O1^v	76.3 (3)	O4B^xi—Mo1—O4B^xii	39.8 (5)
O4A^ii—Dy1—O1^v	152.3 (5)	O2^vi—Mo2—O2	110.1 (2)
O4A^iii—Dy1—O1^v	96.5 (6)	O2^vi—Mo2—O2^vii	110.1 (2)
O4B^ii—Dy1—O1^v	161.1 (4)	O2—Mo2—O2^vii	108.1 (4)
O4B^iii—Dy1—O1^v	111.8 (5)	O2^vi—Mo2—O2^viii	108.2 (4)
O1^iv—Dy1—O1^v	67.3 (3)	O2—Mo2—O2^viii	110.1 (2)
O2^i—Dy1—O4A^iv	60.1 (3)	O2^vii—Mo2—O2^viii	110.1 (2)
O2—Dy1—O4A^iv	136.4 (3)	O3^i—O3—Mo1	82.8 (5)
O3^i—Dy1—O4A^iv	76.1 (5)	O3^i—O3—Dy1	84.5 (4)
O3—Dy1—O4A^iv	87.1 (5)	Mo1—O3—Dy1	167.0 (9)
O4A^ii—Dy1—O4A^iv	47.8 (8)	O4B—O4A—Mo1	70 (2)
O4A^iii—Dy1—O4A^iv	134.7 (4)	O4B—O4A—Dy1^iv	86 (2)
O4B^ii—Dy1—O4A^iv	62.2 (7)	Mo1—O4A—Dy1^iv	152.3 (11)
O4B^iii—Dy1—O4A^iv	123.9 (4)	O4B—O4A—Dy1^ii	126 (2)
O1^iv—Dy1—O4A^iv	50.5 (3)	Mo1—O4A—Dy1^ii	79.9 (5)
O1^v—Dy1—O4A^iv	117.2 (3)	Dy1^iv—O4A—Dy1^ii	127.1 (7)
O2^i—Dy1—O4A^x	136.4 (3)	O4A—O4B—Mo1	89 (2)
O2—Dy1—O4A^x	60.0 (3)	O4A—O4B—Dy1^iv	78 (2)
O3^i—Dy1—O4A^x	87.1 (5)	Mo1—O4B—Dy1^iv	160.6 (9)
O3—Dy1—O4A^x	76.1 (5)	O4A—O4B—Mo1^xii	135 (2)
O4A^ii—Dy1—O4A^x	134.7 (4)	Mo1—O4B—Mo1^xii	98.8 (7)
O4A^iii—Dy1—O4A^x	47.8 (8)	Dy1^iv—O4B—Mo1^xii	80.5 (4)
O4B^ii—Dy1—O4A^x	123.9 (4)

Symmetry codes: (i) y+1/2, x−1/2, z; (ii) y, −x+1, −z+1; (iii) −x+3/2, y−1/2, −z+1; (iv) −y+1, x, −z+1; (v) y+1, −x+1, −z+1; (vi) −y+1, x−1, −z; (vii) −x+2, −y, z; (viii) y+1, −x+1, −z; (ix) −y+1, x−1, −z+1; (x) x+1/2, −y+1/2, −z+1; (xi) −y+3/2, −x+3/2, z; (xii) −x+2, −y+1, z.