RARE-EARTH TUNGSTATES AND MOLYBDATES

 

Several different types of tungstates and molybdates were studied. The most interesting of them are the rare-earth tungstates Ln2WO6 and molybdates Ln2MoO6, crystals A2(WO4)3 (A = Al, Sc) and Dy2(MoO4)3.

The flux method was used to grow for the first time the series of Ln2WO6 and Ln2MoO6 single crystals and to refine their crystallographic characteristics, polymorphism, and physical properties. The structure of "La2WO6" (real composition is La18W10O57) crystals is significantly different from that of other compounds of this group and is characterized by polytypism with ordered alternations of two types of layers of densely packed atoms stacked in the sequences of cacac (layer A) or cbcbc (layer B). The value of the c lattice parameter of these polytypes varies from 22.1 to 179.7 Å.

A novel structure type was found also for BiLaWO6 which can be considered as a transitional one between those of Ln2WO6 and Bi2WO6.

Single crystals of Al2(WO4)3 and Dy2(MoO4)3 were firstly grown and studied in our laboratory. The first of them undergo ferroelastic phase transition near -6oC, can be readily doped with Cr and used as laser material. The Dy2(MoO4)3 is an improper ferroelectric of the gadolinium molybdate family and has the Curie point of 140oC.

Crystal structure of La2MoO6 consists of La2O2 layers and MoO4 tetrahedra. It is similar in some aspects to that of high-temperature superconductor La2CuO4.

Main publications:

  1. Growth and basic properties of rare-earth and yttrium oxytungstate Ln2WO6 crystals. V.K.Yanovskii and V.I.Voronkova. Neorgan. Mater. , 1975, v.11, p.91.

  2. Growth and basic properties of rare-earth and yttrium oxymolybdate Ln2MoO6 crystals. V.K.Yanovskii and V.I.Voronkova. Neorgan. Mater. , 1976, v.12, p. 140.

  3. Growth of rare-earth oxytungstates Ln2WO6 from fluxed melts. V.I.Voronkova and V.K.Yanovskii. Kristallografiya, 1976, v.21, p.236.

  4. Crystallography and properties of Ln2WO6 single crystals. V.K.Yanovskii and V.I.Voronkova. Kristallografiya, 1976, v.20, p. 579.

  5. Polytypism of La2WO6 crystals. V.K.Yanovskii and V.I.Voronkova. Kristallografiya, 1981, v.26, p. 604.

  6. Growth of beta-Ga2O3 and Al2(WO4)3 crystals from sodium polytungstate flux. V.I.Voronkova, V.K.Yanovskii, and V.A.Koptsik. Vestnik Mosk. Univ., Fiz.,Astron., 1968, v.23, p.109.

  7. Growth and properties of Dy2(MoO4)3 crystals. V.I.Voronkova, T.G.Kozinskaya, and V.K.Yanovskii. Kristallografiya, 1978, v.23, p. 865.

 

Scientific Tasks and Results

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