Received 15 June 2011
A tetragonal form of dysprosium orthomolybdate at room temperature
Sesegma Dorzhieva,a Ihor Chumak,b Angelina Sarapulova,b* Daria Mikhailova,b Jibzema Bazarovaa and Helmut Ehrenbergc
aBaikal Institute of Nature Management, Siberian Branch of the Russian Academy of Sciences, Sakhyanova Street 6, Ulan-Ude, Russian Federation,bInstitute for Complex Materials, IFW Dresden, Helmholtzstrasse 20, 01069 Dresden, Germany, and cKarlsruhe Institute of Technology (KIT), Institute for Applied Materials (IAM), Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany
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].
It is known that the molybdates of the rare earth elements show interesting fluorescence, laser, piezoelectric, ferroelectric and ferroelastic properties, and they are used as catalysts for the oxidation of organic compounds such as toluene and 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 fivefold 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 -form is stable at temperatures above 1023-1153 K. The transition from the -modification to the room-temperature -form is kinetically prevented during cooling and takes place via a ' 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 the 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 P21m 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 -modification of (Nd0.023Gd0.977)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 have been obtained with the high-temperature -form tetragonal crystal structure, and the structure refinement based on room-temperature single-crystal X-ray diffraction data has been performed.
According to the single-crystal experiment, dysprosium molybdate shows tetragonal symmetry at room temperature, space group P21m, 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 (Nd0.023Gd0.977)2(MoO4)3, obtained by the Czochralsky method, a tetragonal crystal structure was found at room temperature by Zou et al. (1999). Even a 2% cation substitution of Gd through Nd stabilizes a high-temperature form in the space group P21m with a = 7.356 (1) Å and c = 10.685 (2) Å. The tetragonal Dy2(MoO4)3 form, obtained in the 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 (Nd0.023Gd0.977)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% accuracy.
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 '-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 (Nd0.023Gd0.977)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 MoVI oxidation state, longer Dy-O interatomic distances [2.381 (6), 2.401 (7) and 2.421 (7) Å] lead to an eightfold 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 radii and oxygen coordination polyhedra ROx. For example, eightfold 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 sixfold 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 sevenfold oxygen coordination.
The shortest metal-metal distances in the title structure are Dy1Mo1ii = 3.7343 (15) Å, Dy1Dy1ii = 3.8572 (17) Å and Mo1Mo1ii = 4.243 (3) Å [symmetry code: (ii) -y + 1, x, -z + 1].
| || Figure 1 |
The Dy2(MoO4)3 tetragonal unit cell, showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 70% probability level.
| || Figure 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.
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 to 1273 K, followed by cooling at a rate of 5 K h-1 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.
Due to the large atomic displacement parameters, a split position from the original position on a mirror plane was introduced for atom O3, 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.
Data collection: APEX2 (Bruker, 2004); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT; 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.
Supplementary data for this paper are available from the IUCr electronic archives (Reference: LG3065 ). Services for accessing these data are described at the back of the journal.
This work was supported by the Deutscher Akademischer Austauschdienst (DAAD) (postdoctoral fellowship grant to SD).
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