CHEMISTRY

V. K. TRUNOV, L. M. KOVBA, and Academician VIKT. I. SPITSYN

ON DOUBLE OXIDES

IN THE URANIUM—TUNGSTEN—OXYGEN SYSTEM

The study of double oxides of uranium and tungsten has not hitherto been carried out. In the literature there are only indications of the preparation of aqueous uranyl tungstates and polytungstates ((^{1-3})).

We have studied the oxides formed in the interaction of various oxides of uranium and tungsten, and also in the thermal decomposition and reduction with hydrogen of uranyl tungstate. Tungstic acid and uranyl nitrate of chemically pure grade were used as starting preparations; the other preparations used ((\mathrm{WO_2}, \mathrm{W}, \mathrm{UO_2}, \mathrm{U_3O_8}, \mathrm{UO_3})) were obtained from the above-mentioned compounds. The corresponding procedures are well known. The investigation was carried out by the method of X-ray phase analysis (RKD-57 cameras, Cu (K_\alpha) radiation, Ni filter). A number of samples were recorded in Cu (K_\alpha) radiation monochromatized by reflection from a flat single crystal of germanium (this gives better resolution of lines for cameras of the given diameter) ((^4)). To obtain samples in the system (\mathrm{UO_3})—(\mathrm{WO_3}), aqueous suspensions of (\mathrm{H_2WO_4}) and (\mathrm{UO_2(OH)_2}) were boiled for a long time, then evaporated to dryness and calcined at (900^\circ) for 45 hr. X-ray—

Table 1

Results of X-ray phase analysis

phase analysis of the samples obtained showed that in the system ( \mathrm{UO_3—WO_3} ) only one compound is formed—( \mathrm{UO_2WO_4} ) (see Table 1). For comparison, uranium chromate and molybdate were synthesized. Uranium chromate was obtained by dehydration of ( \mathrm{UO_2CrO_4 \cdot 5.5 H_2O} ) at (400^\circ), and the molybdate by sintering ( \mathrm{U_3O_8} ) in air with an excess of ( \mathrm{MoO_3} ) at (750^\circ).

The X-ray patterns of all three substances are very close to one another (Fig. 1), which indicates their isostructural character.

Oxides of uranium and tungsten of lower valence were obtained by thermal decomposition of ( \mathrm{UO_2WO_4} ) in vacuum at (600—900^\circ), reduction of uranium tungstate with hydrogen at temperatures of (300—800^\circ), and sintering of various oxides of uranium and tungsten in evacuated ampoules at (1250^\circ) and (900^\circ). The results of phase analysis of the reaction products are given in Table 1, from which it is evident that in the case of uranium and tungsten oxides of lower valences, two new phases appear. One of these phases is structurally close to ( \mathrm{U_3O_8} ) (Fig. 2). The composition of the phase structurally close to ( \mathrm{U_3O_8} ) corresponds to the formula ( \mathrm{UWO_{5+x}} ). An analogous phase is formed upon decomposition of ( \mathrm{UO_2MoO_4} ). The X-ray pattern of this phase proved to be identical with the X-ray pattern of the ( \mathrm{UWO_{5+x}} ) phase. In the purest form, the ( \mathrm{UWO_{5+x}} ) phase was obtained by thermal decomposition of ( \mathrm{UO_2WO_4} ) in vacuum at temperatures of 800 and (900^\circ). The gross composition of this phase corresponds to the formula ( \mathrm{UWO_{5.5}} ) for the sample obtained at (800^\circ), and to the formula ( \mathrm{UWO_{5.01}} ) for the sample obtained at (900^\circ). The Debye pattern of the other phase is indexed in cubic symmetry with the parameter of the primitive unit cell

[
a = 3.805 \pm 0.001 \ \text{kX}.
]

Fig. 1. Schemes of Debye patterns of ( \mathrm{UO_2WC_4} ) (1), ( \mathrm{UO_2MoO_4} ) (2) and ( \mathrm{UO_2CrO_4} ) (3)

Fig. 2. Schemes of Debye patterns of ( \mathrm{U_3O_8} ) (3), phases ( \mathrm{UWO_{5+x}} ) (1), ( \mathrm{UMoO_{5+x}} ) (2)

The compound with the cubic unit cell was synthesized by sintering ( \mathrm{UO_2} ) and ( \mathrm{WO_3} ) in ratios of (1:1) and (1:10). At compositions ( \mathrm{UO_2:WO_3} = 1:25 ) and (1:50), a tetragonal distortion of the cubic primitive cell was observed. At the same time, when ( \mathrm{WO_3} ) and ( \mathrm{WO_2} ) were sintered in ratios of (15:1), (25:1), and (50:1), and also ( \mathrm{WO_3} ) and ( \mathrm{ThO_2} ) in a ratio of (10:1), the cubic phase is not formed, and a rhombic distortion of the elementary cubic cell occurs, as in the case of ( \mathrm{WO_3} ). Thus, the cubic phase is formed only in the presence of uranium; structurally it is close to ( \mathrm{WO_3} ) and, apparently, contains a smaller amount of oxygen, i.e., its composition is expressed by the formula ( \mathrm{(W,U)O_{3-x}} ). In the sample ( \mathrm{UO_2 + 10WO_3} ), a phase of the ( \mathrm{UO_2} ) type is present, probably containing some amount of tungsten, since the lattice parameter of this phase ((a = 5.461 \pm 0.001) kX) differs from the periods of the lattices of the ( \mathrm{UO_{2+x}} ) phases, although only slightly. In the sample ( \mathrm{UO_2 + 25WO_3} ), the cubic phase is not yet formed. From this it may be concluded that the ratio between U and W in the cubic phase is close to (1:15—20), and (x) is about (1/15—1/20).

Thus, the ( \mathrm{UWO_5} ) samples have different phase compositions depending on the method of preparation: in the products of reduction and thermal decomposition of uranium tungstate, the ( \mathrm{UWO_{5+x}} ) phase was detected, whereas upon sintering the oxides a mixture of the phases ( \mathrm{(U,W)O_{3-x}} ) and ( \mathrm{UO_2} ) or ( \mathrm{(U,W)O_{2+y}} ) is formed. After annealing ( \mathrm{UWO_{5+x}} ) at (1250^\circ), 3 phases were found in the sample: the initial oxide ( \mathrm{UWO_{5+x}} ), ( \mathrm{UO_{2+x}} ) or ( \mathrm{(U,W)O_{2+y}} ), and ( \mathrm{(U,W)O_{3-x}} ), i.e., at a higher temperature the ( \mathrm{UWO_{5+x}} ) phase disproportionates.

No interaction between $\mathrm{UO_2}$ and $\mathrm{WO_2}$ was detected up to 1250°. The lattice parameters of the initial phases do not change upon sintering; i.e., solid solutions apparently are not formed.

In conclusion, we express our deep gratitude to Yu. P. Simanov for a number of valuable suggestions.

Moscow State University
named after M. V. Lomonosov

Received
12 VI 1961

References

1. J. J. Berzelius, Pogg. Ann., 6, 384 (1826).
2. J. Lefort, C. r., 87, 749 (1878).
3. J. Lefort, Ann. chim. phys., (5), 15, 321 (1878).
4. Yu. P. Simanov, V. K. Trunov, Zav. lab., 27, No. 2 (1961).