Crystal structure of europium tungstate

Abstract

The structure of europium tungstate, Eu2(WO4)8, has been determined by single-crystal X-ray diffraction data. The crystals are monoclinic, space group, C2/c, with a = 7.676 _+ 0.003, b = 11.463 +_ 0-003, c = 11.396 _+ 0.005/~, fl = 109.63 + 0-04 °, Z = 4, dz = 7.37 g.cm -3 .The atomic arrangement may be described as a scheelite superlattice with ordered vacancies in the cation positions, but atoms are displaced by considerable distances from the scheelite locations. Each W atom has four oxygen neighbors at average distances of 1.74 or 1.78 A. Some ~7 atoms have a fifth neighbor at 2.19 A. Each Eu atom has eight oxygen neighbors at an average distance of 2.43 A. IntroductionThe rare earth elements form a series of isomorphou,~ tungstates with the generic formula R~(WO4)8, where R stands for a rare earth element, and some of these compounds have interesting fluorescence properties (MacDonald, Vogel & Brookman, 1962). We have investigated single crystals of the europium compound by X-ray diffraction and have determined the structure. The atomic arrangement is closely related to that of CaWO4 (scheelite), and it may be described as a scheelite superlattice with ordered vacancies in the cation positions. Considerable distortions of the structure occur around these vacancies. These results may give an indication of the structure at defects in scheelite solid solutions.Single crystals of the isomorphous compound Ce2(WO4)8 were studied by Nelson & McKee (1946) who determined the unit cell and probable space group in agreement with our results. ExperimentalThe synthesis of europium tungstate from europium oxide and tungstic acid and the growth of single crystals at temperatures near the melting point (1130_+15 °C) has been described by MacDonald, Vogel & Brookman (1962 was measured with the General Electric XP~D-5 X-ray goniostat and MoK~ radiation. Unit cell dimensions are based on /~=0.70929 ~ for Mo Kaz. Intensities were measured with a scintillation counter equipped with a pulse height discriminator for the 839 independent reflections permitted by the space group in the sphere with sin 0//~ less than 0.596 (20 <50°); of these, 36 were assigned zero intensity. The data were corrected for absorption assuming that the crystal was a sphere ~dth/~r--2-3, according to the table given by Bond (1959) Unit cell and space groupThe dimensions of the monoclinic unit cell of Eu2(WO4)a are:a=7.676 ± 0.003, b= 11.463 +_ 0-003, c = 11-396 ± 0.005 J~, fl--109.63 ± 0-04 °. The lattice is pseudo-orthorhombic. A face-centered cell with double the volume of the one given above and which is called the 'second setting' (Fig. 1) It is possible that the deviation of fl from 90 ° is less than the experimental error, and this cell is convenient for the indexing of powder patterns. While the spot positions on the single crystal photographs fit an orthorhombic lattice, the monoclinic symmetry is clearly evident in the intensities. Twinning might be expected in this lattice with the result that the intensities would mimic orthorhombic symm...

A structural study in the system Al2O3–WO3

“…Thus, the trivalent rare-earth tungstates crystallize in three different types (Nassau, Levinstein & Loiacono, 1965). In the Euz(WO4)3-type structure, which is characteristic of the larger rare earths, the metal ion is 8-coordinated; in Euz(WO4)3, the average Eu-O distance is 2.425 A (Templeton & Zalkin, 1963).…”

The crystal structure of Bi2(MoO4)3

“…The relation between the structures of tetragonal CaWO4, scheelite, I4t/a and monoclinic Euz(WO4)3, C2/c, was revealed by Templeton & Zalkin (1963). Scheelite can be considered as a stacking up of Ca z+ and WO]-ions.…”

“…These values were obtained by subtracting ¼.¼.0 from the positions of Eu calculated by Templeton & Zalkin (1963). Alternatives (ii) and (iii) differ only in the choice of the origin.…”

Validating the model of a (3 + 1)-dimensional incommensurately modulated structure as generator of a family of compounds for the Eu2(MoO4)3scheelite structure

“…The structure was solved in C2/c SG (monoclinic b axis) taking the Eu 2 (WoO 4 ) 3 crystallographic data [19] as a starting point. In this section, we will focus on the comparison between the results obtained from our refinement on the basis of synchrotron powder diffraction data and superspace model generated from KNd(MoO 4 ) 2 and those reported in [18].…”