UDC 548.51:546.6578

CRYSTALLOGRAPHY

P. V. KLEVTSOV, L. Yu. KHARCHENKO, R. F. KLEVTSOVA

HYDROTHERMAL SYNTHESIS AND SYMMETRY OF CRYSTALS OF OXYTUNGSTATES $\mathrm{Ln}_2\mathrm{W}_2\mathrm{O}_9$ ($\mathrm{Ln} = \mathrm{Ce}, \mathrm{Pr}, \mathrm{Nd}$)

(Presented by Academician N. V. Belov, December 16, 1966)

In recent years a considerable number of works have appeared on the synthesis and crystallization of medium rare-earth tungstates of composition $\mathrm{Ln}_2(\mathrm{WO}_4)_3$. However, up to now comparatively little information has been given on the investigation of binary oxide systems of the type $\mathrm{Ln}_2\mathrm{O}_3 — \mathrm{WO}_3$ and of new phases in these systems.

Recently Rode and Karpovym ($^1$), in studying the system $\mathrm{Nd}_2\mathrm{O}_3 — \mathrm{WO}_3$ by the pyrosynthesis method, in addition to the previously known phases with molar ratio $\mathrm{Nd}_2\mathrm{O}_3 : \mathrm{WO}_3 = 1 : 3$ and $1 : 1$ ($^2$), obtained a new compound of composition $\mathrm{Nd}_2\mathrm{W}_2\mathrm{O}_9$ with molar ratio $\mathrm{Nd}_2\mathrm{O}_3 : \mathrm{WO}_3 = 1 : 2$. According to the authors’ data, on heating this compound exhibits two polymorphic transformations at 438 and 1248° and melts incongruently at 1354°.

In studying the hydrothermal crystallization of rare-earth tungstates in aqueous solutions of alkali-metal chlorides, we synthesized, for cerium, praseodymium, and neodymium, crystals of compounds with a molar ratio of the oxides of the rare-earth elements and tungsten equal to $1 : 2$.

These compounds proved stable in a wide range of solvents (individual and mixed chlorides of lithium, sodium, potassium, and ammonium) in the temperature range 450–550° and at pressures of the order of 500–1700 atm. The optimum conditions for the synthesis of $\mathrm{Pr}_2\mathrm{W}_2\mathrm{O}_9$: a) mixed solvent $\mathrm{NaCl}$ 20% + $\mathrm{NH}_4\mathrm{Cl}$ 5%, charge $\mathrm{Pr}_2(\mathrm{WO}_4)_3$, temperature 550°, filling coefficient 60%; b) concentrated $\mathrm{LiCl}$, charge $\mathrm{LiPr}(\mathrm{WO}_4)_2$, filling 60%, temperature 550°; for $\mathrm{Nd}_2\mathrm{W}_2\mathrm{O}_9$ and $\mathrm{Ce}_2\mathrm{W}_2\mathrm{O}_9$—concentrated $\mathrm{LiCl}$ solutions (35% and above) at 525–550°; charges $\mathrm{Nd}_2(\mathrm{WO}_4)_3$ and $\mathrm{Ce}_2(\mathrm{WO}_4)_3$, respectively, filling coefficient 50–60%.

The color of the crystals of praseodymium oxytungstate is green, of neodymium oxytungstate—lilac, and of cerium oxytungstate—yellow.

Single crystals of $\mathrm{Pr}_2\mathrm{W}_2\mathrm{O}_9$ were obtained in the form of elongated, flattened prisms up to 0.3–0.4 mm in size. However, in most cases, both for $\mathrm{Pr}_2\mathrm{W}_2\mathrm{O}_9$ and for $\mathrm{Nd}_2\mathrm{W}_2\mathrm{O}_9$, weakly faceted intergrowths were obtained, having the form of “combs,” irregular plates, and isometric pieces. The size of the intergrowths was up to 1 mm.

The symmetry of the obtained $\mathrm{Pr}_2\mathrm{W}_2\mathrm{O}_9$ crystals was investigated by methods of optical and X-ray goniometry. For optical-goniometric measurements, well-formed crystals with all-sided faceting were used. In analysis of the stereographic projection it was found that the crystals belong to the monoclinic system, class $L_2PC$. The symmetry axis $L_2$ passes along the length of the crystal. The most highly developed faces are those of the pinacoid $(10\bar{1})$, giving the crystals a platy habit. In the plane $xz$, in addition to the $z$ axis, the axes $x_1$ and $x_2$ of other, most developed zones of faces lie at angles $\beta_1 = 107^\circ$ and $\beta_2 = 133^\circ$. In this case the plane $yx_2$ coincides with the plane of the plate.

The results of measurements on the optical goniometer are presented in Table 1.

The indices of the growth forms are given in the setting \(x, z\) with the monoclinic angle \(\beta = 107^\circ\). A unit face was not observed on the crystals. Its coordinates were calculated on the basis of X-ray data.

To determine the parameters of the unit cell and the space group, X-ray photographs were taken on an RKOPe camera (Cu radiation), as well as zero-, first-, and second-layer rotation photographs about the \(b\) axis (Ag radiation). The symmetry of the Laue patterns and rotation photographs, in agreement with the optical goniometry data, corresponds to monoclinic symmetry.

From the Weissenberg photograph \(h0l\), the angles \(\beta\) were determined for different choices of the \(x\) and \(z\) axes, and from oscillation photographs the corresponding parameters \(a\) and \(c\). The above-mentioned monoclinic angles \(\beta = 107^\circ\) and \(133^\circ\) correspond to four possible choices of the base of the unit cell \(ac\), which are shown in Fig. 1 (in this case the parameter \(c\) is constant, while the corresponding \(z\) axis in some variants changes direction to the opposite one). In variants 1 and 2 the cells have similar angles \(\beta\) (\(106.5\) and \(107.5^\circ\), respectively), while in variants 3 and 4 they are \(\sim 133^\circ\). However, cells 2 and 4 are characterized by a body-centered basis \(B\).

Fig. 1. Four possible variants for choosing the unit cell of \(\mathrm{Pr_2W_2O_9}\)

For the intended subsequent X-ray structural investigations and in indexing the powder diffractogram (Table 2), we chose the first setting.

Table 1

Growth forms of \(\mathrm{Pr_2W_2O_9}\) crystals

The parameters of the unit cell, refined by the least-squares method from the diffractogram data \((^3)\), are as follows: \(a = 7.70\ \text{Å}\), \(b = 9.84\ \text{Å}\), \(c = 9.27\ \text{Å}\), and \(\beta = 106.5^\circ\).

Chemical analysis found a content of Pr 36.0%, W 46.6%, which, on recalculation to a formula, gives the compound \(\mathrm{Pr_2W_2O_9}\) (calculated data for this compound: Pr 35.5%, W 46.0%).

From these data the density of an ideal \(\mathrm{Pr_2W_2O_9}\) crystal was calculated as \(d_x = 7.82\ \text{g/cm}^3\) (for 4 formula units in the unit cell). The mean density value measured by the pycnometric method is \(d_{\text{meas}} = 7.69\ \text{g/cm}^3\).

The space group \(C_{2h}^{5} = P2_1/c\) was determined unambiguously from the extinctions: in the \(h0l\) zone reflections are present only with \(l = 2n\), in the zone

\(0k0\)—with \(k = 2n\) (from the \(hk0\) scan); for reflections of general type, no extinctions are observed.

The crystals of neodymium and cerium oxytungstates were identified by comparing powder X-ray diffraction patterns. The compounds \(\mathrm{Nd_2W_2O_9}\) and \(\mathrm{Ce_2W_2O_9}\) are isostructural with \(\mathrm{Pr_2W_2O_9}\). Their powder X-ray diffraction patterns are very close

Table 2

X-ray data for \(\mathrm{Pr_2W_2O_9}\)

to one another. For \(\mathrm{Nd_2W_2O_9}\), the values of \(d_{hkl}\), and consequently also the unit-cell parameters, are smaller than the corresponding values for \(\mathrm{Pr_2W_2O_9}\) only by a few units in the second decimal place.

Differential thermographic analysis up to \(1000^\circ\) for \(\mathrm{Nd_2W_2O_9}\) showed that this compound undergoes a reversible polymorphic transition in the region \(410\)–\(420^\circ\).

Institute of Inorganic Chemistry
Siberian Branch of the Academy of Sciences of the USSR

Received
10 IX 1966

REFERENCES

1. E. Ya. Rode, V. N. Karpov, Izv. AN SSSR, ser. Neorganicheskie materialy, 2, 688 (1966).
2. H. J. Borchardt, J. Chem. Phys., 39, 504 (1963).
3. F. A. Brukentsov, A. N. Rebenko, ZhSKh, 7, 136 (1966).