Full Text
UDC 548.735.2
CRYSTALLOGRAPHY
T. I. TIMCHENKO, L. V. PETUSHKOVA, E. A. POBEDIMSKAYA,
A. V. PASHKOVA
NEW LANTHANUM OXYTUNGSTATE \(2\mathrm{La}_2\mathrm{O}_3\cdot3\mathrm{WO}_3\)
(Presented by Academician N. V. Belov, 25 VII 1968)
Oxy tungstates of rare-earth elements have been obtained mainly in the form of sinters in the study of systems of the type \(\mathrm{TR}_2\mathrm{O}_3—\mathrm{WO}_3\) \((^{1-6})\). Only recently have some of them, for Ce, Nd, Pr with \(\mathrm{TR}_2\mathrm{O}_3/\mathrm{WO}_3\) ratios equal to \(1:2\), \(1:1\), \(2:3\), become known also in the form of single crystals \((^7)\). From the crystallochemical standpoint, oxy tungstates have been almost unstudied, and only for some of them have the unit-cell parameters and Fedorov groups been established \((^{2,7})\).
Oxy tungstates with a \(\mathrm{La}_2\mathrm{O}_3/\mathrm{WO}_3\) ratio equal to \(2:3\) were not found in the study of the \(\mathrm{La}_2\mathrm{O}_3—\mathrm{WO}_3\) system, but this compound was obtained by us in the ternary system \(\mathrm{La}_2\mathrm{O}_3—\mathrm{WO}_3—\mathrm{NaCl}\) over a wide range of temperatures and concentrations of sodium chloride. The starting charge was a mixture of \(\mathrm{La}_2\mathrm{O}_3\) and \(\mathrm{WO}_3\) oxides in the ratio \(1:3\). Crystals of lanthanum oxy tungstate grow under isothermal conditions due to evaporation of sodium chloride and, in part, \(\mathrm{WO}_3\); the largest ones (up to 3–4 mm in elongation) were obtained at 1000–1400° and solvent concentrations from 50 to 80 wt. %. The yield of crystals and their size depend on the rate and magnitude of evaporation; their habit varies from short prismatic to strongly elongated, almost needle-like. The crystals are colorless, water-transparent, optically uniaxial, positive, and have high refractive indices \((>2.0)\).
Chemical analysis of crystals from several experiments, on recalculation, gives the stable formula \(2\mathrm{La}_2\mathrm{O}_3\cdot3\mathrm{WO}_3\) (\(\mathrm{La}_2\mathrm{O}_3\) 48.90%, \(\mathrm{WO}_3\) 51.52%; theoretical composition: \(\mathrm{La}_2\mathrm{O}_3\) 48.37%, \(\mathrm{WO}_3\) 51.63%). The melting temperature is \(\sim 1560^\circ\), but with decomposition.
Goniometric measurements revealed symmetry \(4/mmm\). The morphology of the crystals is simple: a combination of the tetragonal prism \(\{110\}\) and the tetragonal dipyramid \(\{101\}\) (Table 1). The relative parameters from the goniometric data are \(a=1\), \(c=1.26\). The interplanar spacings (Table 2), calculated from diffraction patterns (URS 50-IM, Cu \(K_\alpha\) radiation, Ni filter), differ from those for previously known compounds of similar chemical composition.
Table 1
Results of the goniometric study of crystals of \(2\mathrm{La}_2\mathrm{O}_3\cdot3\mathrm{WO}_3\)
| Symbol | \(\rho_{\text{meas}}\), deg. | \(\varphi_{\text{meas}}\), deg. | Symbol | \(\rho_{\text{meas}}\), deg. | \(\varphi_{\text{meas}}\), deg. |
|---|---|---|---|---|---|
| \(\{110\}\) | 90 | 45 | \(\{101\}\) | 51.38 | 0 |
| \(\{110\}\) | 90 | 135 | \(\{101\}\) | 51.38 | 90 |
| \(\{110\}\) | 90 | 225 | \(\{101\}\) | 51.38 | 180 |
| \(\{110\}\) | 90 | 315 | \(\{101\}\) | 51.38 | 270 |
For the X-ray goniometric study, a crystal measuring \(0.4 \times 0.2 \times 0.2\) mm was selected. The symmetry \(4mm\) of the Laue pattern, taken in the camera
RCP along the elongation axis (Mo radiation) confirms the tetragonal symmetry of the crystal (diffraction class \(4mmmP - /n - c\)). The parameters of the tetragonal cell, determined from rotation radiographs in the same camera, are \(a = 10.06 \pm 0.03\) Å, \(c = 12.63 \pm 0.06\) Å. At a density \(d_{\text{meas}} = 7.16\ \text{g/cm}^3\) (hydrostatic weighing in \(\mathrm{CCl}_4\)), there are 4 formula units per cell. To determine the space group, 0- and 1-layer line Weissenberg photographs along the \(b\) axis were used
Table 2
Diffraction data for crystals of \(2\mathrm{La}_2\mathrm{O}_3\cdot3\mathrm{WO}_3\)
| \(d\), Å | \(I/I_1\) | \(hkl\) | \(d\), Å | \(I/I_1\) | \(hkl\) | \(d\), Å | \(I/I_1\) | \(hkl\) |
|---|---|---|---|---|---|---|---|---|
| 5.2958 | 3 | — | 2.4947 | 25 | 040 | 1.1736 | 16 | 440 |
| 4.6707 | 7 | — | 2.4397 | 3 | \(\{\,040<br>041<br>015\) | 1.6980 | 14 | 442 |
| 4.1986 | 2 | 121 | 2.2012 | 3 | 332 | 1.6650 | 8 | 154 |
| 3.8567 | 4 | 013 | 2.0077 | 8 | 035 | 1.5986 | 5 | — |
| 3.2045 | 7 | \(\{\,023<br>031\) | 1.9665 | 8 | — | 1.5861 | 8 | 353 |
| 3.1317 | 27 | 004 | 1.9529 | 27 | 044 | 1.5655 | 5 | \(\{\,336<br>055\) |
| 3.0644 | 100 | 222 | 1.9333 | 6 | \(\{\,026<br>151\) | 1.5418 | 5 | 155 |
| 2.9308 | 5 | 032 | 1.9202 | 4 | 144 | 1.5302 | 11 | \(\{\,262<br>163\) |
| 2.7015 | 3 | 231 | 1.8647 | 9 | — | 1.4479 | 5 | 362 |
| 2.5636 | 6 | 124 | 1.8358 | 4 | 251 | 1.4154 | 5 | — |
| 2.5177 | 8 | 133 | 1.8019 | 19 | \(\{\,343<br>053\) | 1.3367 | 5 | 272 |
(Weissenberg RCP; Mo \(K_{\alpha}\) radiation). Analysis of the extinctions made it possible to determine the space group quite unambiguously as \(D_{4h}^{15} = P4_2/nmc\). The infrared spectrum of lanthanum oxytungstate (I. I. Plyusnina) is characterized by absorption bands in the regions 810—860, 740, 620, and 440 \(\text{cm}^{-1}\).
Moscow State University
named after M. V. Lomonosov
Received
19 VII 1968
CITED LITERATURE
- G. I. Tyushevskaya, N. S. Afonskii, V. I. Spitsyn, DAN, 170, No. 4, 859 (1966).
- V. K. Trunov, G. I. Tyushevskaya, N. S. Afonskii, Neorg. khim., 18, No. 4 (1968).
- E. Ya. Rode, V. N. Karpov, Izv. AN SSSR, Neorg. materialy, 2, No. 4, 638 (1966).
- E. Ya. Rode, G. M. Balagina et al., Neorg. khim., 18, No. 5 (1968).
- L. Y. Chang, M. J. Sorger, B. Phillips, J. Inorg. and Nucl. Chem., 28, No. 5, 1179 (1966).
- H. J. Borchardt, J. Chem. Phys., 39, 504 (1963).
- P. V. Klevtsov, L. Yu. Kharchenko, R. F. Klevtsova, DAN, 176, No. 3 (1967).