Chemistry

L. M. Vidavskii, E. G. Lavut, L. M. Kovba, E. A. Ippolitova

On the Study of the Conditions for the Formation of Various Modifications of Uranium Trioxide

(Presented by Academician V. I. Spitsyn, October 7, 1963)

One amorphous and five crystalline modifications of uranium trioxide are known (hexagonal $\alpha$-, monoclinic $\gamma$-, cubic $\delta$-$\mathrm{UO_3}$, as well as $\beta$- and $\varepsilon$-$\mathrm{UO_3}$, whose unit cells have not been established) ($^{1,2}$). Oxidation of uranium oxide-oxide at elevated oxygen pressures in the temperature range $450$–$700^\circ$ yields $\alpha$-, $\beta$-, and $\gamma$-$\mathrm{UO_3}$. The conditions for formation of the $\alpha$- and $\beta$-phases from $\mathrm{U_3O_8}$ have not been sufficiently clarified. The $\delta$-$\mathrm{UO_3}$ phase was obtained as a result of the thermal decomposition of uranyl hydroxide $\beta$-$\mathrm{UO_2(OH)_2}$ ($^3$). Oxidation of $\mathrm{U_3O_8}$ by ozone, nitrogen dioxide, or atomic oxygen at temperatures of $200$–$350^\circ$ gives $\varepsilon$-$\mathrm{UO_3}$. Thermal decomposition of some compounds of hexavalent uranium ($\mathrm{UO_4 \cdot 2H_2O}$, $(\mathrm{NH_4})_2\mathrm{U_2O_7}\cdot n\mathrm{H_2O}$, $\mathrm{UO_3 \cdot 2H_2O}$) usually leads to the formation of amorphous $\mathrm{UO_3}$, but in some cases crystalline $\alpha$-, $\beta$-, or $\gamma$-phases, respectively, may be obtained ($^{4,1}$).

The aim of our work was to determine the conditions of formation and stability of various modifications of $\mathrm{UO_3}$. The oxidation of uranium oxide-oxide was studied over a wide range of temperatures and oxygen pressures; the interaction of various oxidants (ozone, nitrogen dioxide, oxygen under pressure) with the defective hexagonal phase $\alpha$-$\mathrm{UO_{3-x}}$ formed upon heating amorphous uranium trioxide $\mathrm{UO_3}$ (A) to $525^\circ$ in air was also investigated. In addition, the crystallization of amorphous trioxide and the thermal decomposition of uranyl hydroxide $\alpha$-$\mathrm{UO_2(OH)_2}$*, as well as ammonium diuranate $(\mathrm{NH_4})_2\mathrm{U_2O_7}\cdot n\mathrm{H_2O}$, were investigated. Amorphous trioxide was obtained by calcining uranium peroxide dihydrate ($400^\circ$), ammonium diuranate by the action of an excess of ammonia on a solution of uranyl nitrate, and $\alpha$-uranyl hydroxide by heating $\mathrm{UO_3}$ (A) or $\mathrm{UO_3\cdot 2H_2O}$ with water at a temperature of $250^\circ$ in an autoclave.

The data obtained (Table 1) show that oxidation of $\mathrm{U_3O_8}$ to $\alpha$-$\mathrm{UO_{3-x}}$ proceeds much more rapidly than further oxidation of the latter to $\mathrm{UO_3}$. Thus, in experiment No. 6, after 30 hours at a temperature of $500$–$550^\circ$ and an oxygen pressure of 90 atm, all $\mathrm{U_3O_8}$ was oxidized, whereas $\alpha$-$\mathrm{UO_{3-x}}$ was one of the principal phases; in experiment No. 16, after 80 h under the same conditions, the initial $\alpha$-$\mathrm{UO_{3-x}}$ partially remained unchanged. This conclusion agrees with the observation ($^2$) of the low reactivity of $\mathrm{UO_{2.9}}$ oxide.

The phase obtained from $\mathrm{U_3O_8}$ at low temperatures is $\varepsilon$-$\mathrm{UO_3}$. It is formed in experiments with oxygen under pressure below $440$–$450^\circ$, and in experiments with ozone or nitrogen dioxide below $350^\circ$. The formation of $\varepsilon$-$\mathrm{UO_3}$ upon oxidation of $\mathrm{U_3O_8}$ under oxygen pressure had not previously been detected.

The first bright lines of the X-ray diffraction pattern of $\varepsilon$-$\mathrm{UO_3}$ correspond to the 001, 110, and 200 lines of uranium oxide-oxide; the remaining $\mathrm{U_3O_8}$ lines are split in the Debye pattern of $\varepsilon$-$\mathrm{UO_3}$. Using the principle of homology, it was possible to index all lines of $\varepsilon$-$\mathrm{UO_3}$ in a triclinic cell with parameters $a = 4.002$; $b = 3.841$; $c = 4.165$ Å; $\alpha = 98^\circ17'$; $\beta = 90^\circ33'$; $\gamma = 120^\circ28'$; $z = 1$; $\rho_{\text{X-ray}} = 8.73$; $\rho_{\text{pycn}} = 8.54$ ($^1$). The values of the parameters $a$, $b$, and $\gamma$ are very close to their values for $\mathrm{U_3O_8}$ in the corresponding aspect, and the distortion of the cell is expressed only in the deviation of the angles $\alpha$ and $\beta$ from $90^\circ$.

* $x \approx 0.1$.

** Rhombic, $a = 10.23$; $b = 6.89$; $c = 4.28$ Å.

Table 1

Results of phase and chemical analysis of uranium trioxide obtained by various methods

* The chemical composition of preparations containing significant impurities of bound nitrogen or water is not given.

Upon oxidation of \(U_3O_8\) or \(\alpha\)-\(UO_{3-x}\) under oxygen pressure in the temperature range \(450—550^\circ\), the principal phase obtained is \(\alpha\)-\(UO_3\). In all our experiments the \(\beta\)-phase was not formed as the principal one, but was present together with the \(\gamma\)-phase as an impurity to the \(\varepsilon\)- or \(\alpha\)-modification (except experiment No. 18). In experiment No. 18 (starting substance—a mixture of \(UO_3\) (A) and \(\alpha\)-\(UO_{3-x}\)), \(\beta\)-\(UO_3\) was not detected; possibly it was present in an amount below the sensitivity limit of X-ray phase analysis, since from \(UO_3\) (A) under analogous conditions pure \(\gamma\)-\(UO_3\) is formed (experiment No. 17). This latter fact is not consistent with the literature data \((^1)\), where formation of \(\alpha\)-\(UO_3\) was noted. In experiments with increased oxygen pressure, formation of the \(\gamma\)-phase was observed over the entire temperature range studied, and not only above \(550^\circ\), as indicated in \((^1)\).

From consideration of the results obtained, it may be concluded that the formation of one or another modification of uranium trioxide is influenced by the structure of the starting substance, the calcination temperature, and, in the case of thermal decomposition, also the rate of heating. Thus, upon slow decomposition of ammonium diuranate (experiments Nos. 21 and 22) an amorphous phase is formed, whereas as a result of comparatively rapid heating (experiment No. 20) \(\beta\)-\(UO_3\) is formed, owing to the closeness of its structure to the structure of the starting uranate.

Most of the lines of the roentgenogram of \(\beta\)-\(UO_3\), including all the strong lines, could be indexed in a rhombic subcell with parameters \(a=\)

$= 6.866 \pm 0.003\ \text{Å},\ b = 3.906 \pm 0.001\ \text{Å};\ c = 7.14 \pm 0.004\ \text{Å}$, which is close to the parameters of ammonium diuranate $a = 4.03 \sqrt{3}\ \text{Å},\ c = 7.25\ \text{Å}$. The parameters of the true unit cell of $\beta$-$\mathrm{UO_3}$ could not be established.

The thermal decomposition of $\alpha$-$\mathrm{UO_2(OH)_2}$ was carried out at a temperature of 370–400°. Under these conditions the formation of $\gamma$-$\mathrm{UO_3}$ was to be expected (³). In our experiment $\alpha$-$\mathrm{UO_3}$ was obtained, which can also be explained by the difference in the rate of heating.

Oxidation of uranium dioxide usually is accompanied by the formation of $\varepsilon$-$\mathrm{UO_3}$ below 450° and $\alpha$-$\mathrm{UO_3}$ in the interval 450–550°. This is probably explained by the phase transformation of uranium dioxide in the interval 400–500° (⁵). $\varepsilon$-$\mathrm{UO_3}$ has a distorted $\mathrm{U_3O_8}$ structure and therefore forms below the temperature of transition of $\mathrm{U_3O_8}$ to the hexagonal modification, whereas from hexagonal $\mathrm{U_3O_8}$ and $\mathrm{UO}_{3-x}$ hexagonal $\alpha$-$\mathrm{UO_3}$ is obtained. Amorphous uranium trioxide crystallizes with formation of $\gamma$-$\mathrm{UO_3}$, if decomposition to $\mathrm{UO}_{3-x}$ does not occur.

Moscow State University
named after M. V. Lomonosov

Received
19 II 1963

REFERENCES

¹ H. R. Hoekstra, S. Siegel, J. Inorg. and Nucl. Chem., 18, 154 (1961).
² H. R. Hoekstra, S. Siegel, Proceedings of the II International Conference on the Peaceful Uses of Atomic Energy, 7, Moscow, 1959, p. 506.
³ J. K. Dawson, E. Waite et al., J. Chem. Soc., No. 9, 3531 (1956).
⁴ E. N. P. Corduenke, J. Inorg. and Nucl. Chem., 23, No. 3–4, 285 (1961).
⁵ F. Grønvold, J. Inorg. and Nucl. Chem., 1, 357 (1955).