Chemistry

Corresponding Member of the Academy of Sciences of the USSR A. I. Brodskii and I. F. Franchuk

STUDY OF HIGHER OXIDES AND PEROXIDES OF URANIUM BY THE ISOTOPIC METHOD

The complex uranium—oxygen system has been the subject of many studies \((^{1,2})\) with contradictory results, owing to the formation of solid solutions of variable composition among different modifications of several oxides and to the slow establishment of equilibrium between the components of the solid phases and oxygen. Apparently, the existence of the stoichiometric oxides \( \mathrm{UO} \), \( \mathrm{UO_2} \), \( \mathrm{U_3O_8} \), and \( \mathrm{UO_3} \) may be regarded as proven. In addition to these, a peroxide of composition \( \mathrm{UO_4 \cdot 2H_2O} \) has long been known, and a second peroxide, \( \mathrm{U_2O_7} \) \((^3)\), has recently been discovered; its existence has been confirmed by the thermal decomposition of the first \((^4)\), as well as in the present work. Ideas about the structure of \( \mathrm{UO_4 \cdot 2H_2O} \) still remain contradictory. Most authors consider it to be a true hydrated peroxide \((^5)\) or peruranic acid \((^6)\), but Götte and Schröder \((^7)\) regard it as a product of the coordinative addition of variable amounts of \( \mathrm{H_2O_2} \) and \( \mathrm{H_2O} \) to nonperoxidic \( \mathrm{UO_3} \). The experimental data on which these authors based such a structure proved to be erroneous \((^1)\), but Duval \((^8)\), having studied the thermal decomposition of this substance, again proposed for it the structure \( \mathrm{UO_3 \cdot H_2O_2 \cdot H_2O} \).

The U—O system has been studied in great detail in the high-temperature region with O : U ratios from 1 to 3 in solid phases. Far fewer data are available for the region below approximately \(400^\circ\), where O : U ranges from 2.67 to 4. We studied this region using the isotopic label \( \mathrm{O^{18}} \), introduced into different positions of the initial \( \mathrm{UO_4 \cdot 2H_2O} \), which was subjected to slow thermal decomposition in vacuum at different temperatures up to \(700^\circ\), with investigation of the composition of intermediate and final products.

\( \mathrm{UO_4^{18} \cdot 2H_2O} \), with \( \mathrm{O^{18}} \) only in the peroxide oxygen, was obtained by precipitation from a solution of \( \mathrm{UO_2(NO_3)_2} \) in ordinary water with heavy \( \mathrm{H_2O_2^{18}} \) at room temperature \((^7)\) or with heating to \(90^\circ\) \((^9)\). \( \mathrm{UO_4 \cdot xH_2O^{18}} \) was obtained by exchange of freshly precipitated \( \mathrm{UO_4 \cdot xH_2O} \) with \( \mathrm{H_2O^{18}} \). After drying over \( \mathrm{CaCl_2} \) and heating in vacuum at \(100^\circ\) to constant weight, all samples corresponded to the composition \( \mathrm{UO_4 \cdot 2H_2O} \). The absence of oxygen exchange between \( \mathrm{UO_4} \) and the water of hydration was verified both during preparation of the compound and during its drying.

Preliminary experiments established, in agreement with \((^{3,4})\), that upon gradual heating of \( \mathrm{UO_4 \cdot 2H_2O} \) to \(195^\circ\) an orange compound \( \mathrm{U_2O_7} \) is formed, which reacts vigorously with water or sulfuric acid solution, with liberation of oxygen and formation of \( \mathrm{UO_3} \) or, respectively, uranyl salt. When heated from 200 to \(400^\circ\), \( \mathrm{U_2O_7} \) slowly decomposes with liberation of oxygen and formation of red \( \mathrm{UO_3} \). The content of \( \mathrm{U_2O_7} \) in the solid phase was determined from the volume of oxygen evolved upon treatment with water. On decomposition of \( \mathrm{UO_4 \cdot 2H_2O} \) to \(195^\circ\), it increased as the oxygen pressure in the reaction vessel decreased and reached 50% at 15 mm Hg and below. This dependence is evidently due to the fact that, along with an increase in the oxygen pressure, the pressure of water vapor also increased, partially decomposing \( \mathrm{U_2O_7} \).

For isotopic analysis, oxygen from H₂O₂ was liberated with permanganate, peroxide oxygen from U₂O₇ by the action of water, and oxygen from U₃O₈ was converted into CO₂ by heating with HgCl₂ + Hg(CN)₂ \(^{(10)}\). In the water, the oxygen was analyzed by the method described earlier \(^{(11)}\).

Table 1 presents the results of several characteristic experiments on the stepwise thermal decomposition of \( \mathrm{UO_4^{18}\cdot 2H_2O} \). The oxygen evolved upon heating to 195° has the same isotopic composition as the initial \( \mathrm{H_2O_2^{18}} \).

Table 1

Thermal decomposition of \( \mathrm{UO_4^{18}\cdot 2H_2O} \), prepared from \( \mathrm{H_2O_2^{18}} \) with 1.00% \( \mathrm{O^{18}} \) above natural abundance

The same composition is possessed by the peroxide oxygen from U₂O₇, liberated upon treatment of the solid phase with acidified water; the \( \mathrm{O^{18}} \) content in this oxygen is much higher than its average content in the solid phase. Thus, both in UO₄ and in U₂O₇ the oxygen atoms are not equivalent to one another. In them, the peroxide oxygen retains a structurally isolated position and is the first to split off during thermal decomposition.

When uranium peroxide is heated to 195°, 1.9 moles of water are evolved from it per 1 mole of UO₄ (Table 2). This water contains only from 13 to 24% \( \mathrm{O^{18}} \) from the initial H₂O₂. If heavy uranium peroxide had the structure of the perhydrate \( \mathrm{UO_3\cdot H_2O_2\cdot H_2O} \), the water from its thermal decomposition would have about 50% peroxide \( \mathrm{O^{18}} \) in the absence of exchange with the solid phase, or, in any case, no less than in the latter if such exchange occurred. Meanwhile, the \( \mathrm{O^{18}} \) content in the oxygen from thermal decomposition is always greater than in the water. In agreement with this, thermal decomposition of light uranium peroxide prepared in \( \mathrm{H_2O^{18}} \) always gives water with a higher \( \mathrm{O^{18}} \) content than in the solid phase (experiments 4 and 5 of Table 2).

The fraction of peroxide oxygen passing into the water evolved up to 195° is the greater, the less U₂O₇ remains undecomposed to the lower oxides. This is seen from Table 2, both for experiments with heavy peroxide oxygen and for experiments with the reverse label. From these data it follows that the incorporation of small amounts of peroxide oxygen into the water is caused by isotopic exchange with the decomposition products of U₂O₇, namely with UO₃. Indications of such exchange were available

earlier \((^{12})\), and we confirmed it by direct experiments. A mixture of 0.012 mol of \( \mathrm{UO_3^{18}} \) with 0.48% \( \mathrm{O^{18}} \)* and 0.005 mol of \( \mathrm{UO_4\cdot 2H_2O} \) of normal isotopic composition, when heated to \(195^\circ\), gave water with 0.21% \( \mathrm{O^{18}} \), and on further heating above \(400^\circ\) (decomposition of the exchange \( \mathrm{UO_3} \))—oxygen with 0.30% \( \mathrm{O^{18}} \). In another experiment with a different composition of the mixture, the water and oxygen contained, respectively, 0.39 and 0.45% \( \mathrm{O^{18}} \). Table 2 gives the \( \mathrm{O^{18}} \) contents in water, calculated on the assumption of complete oxygen exchange between it and \( \mathrm{UO_3} \), contained in the solid phase.

Table 2

Dependence of the isotopic composition of the water evolved on heating \( \mathrm{UO_4\cdot 2H_2O} \) to \(195^\circ\) on the content of \( \mathrm{U_2O_7} \) in the solid phase

* In the calculation it was assumed that, on heating to \(195^\circ\), \( \mathrm{UO_4\cdot 2H_2O} \) decomposes according to the scheme
\(2(\mathrm{UO_4\cdot 2H_2O}) = x\mathrm{U_2O_7} + (2 - 2x)\mathrm{UO_3} + (1 - x/2)\mathrm{O_2} + 4\mathrm{H_2O}\),
and that the water formed exchanges only with \( \mathrm{UO_3} \). Instead of \(x\), the numbers from the second column of Table 2 were substituted. The experimentally found \( \mathrm{O^{18}} \) contents in \( \mathrm{UO_3} \) are the mean between the contents of it in \((2 - 2x)\mathrm{UO_3}\), which has exchanged with water, and \(x\mathrm{UO_3}\), obtained in the thermal decomposition of \(x\mathrm{U_2O_7}\) after removal of all the water.

Their satisfactory agreement with the observed values serves as further confirmation of the rapid oxygen exchange between \( \mathrm{UO_3} \) and water, leading to transfer into the latter of peroxide oxygen that had partly entered \( \mathrm{UO_3} \) during the thermal decomposition of uranium peroxides: the vapor of light water from \( \mathrm{UO_4^{18}\cdot 2H_2O} \) decomposes the \( \mathrm{U_2O_7^{18}} \) formed during thermal decomposition and rapidly exchanges oxygen with the \( \mathrm{UO_3^{18}} \) obtained thereby.

The data presented definitively confirm that uranium tetroxide is a true peroxide of the structure \( \mathrm{UO_4\cdot 2H_2O} \). In accordance with this structure, it does not exchange oxygen with an \( \mathrm{H_2O_2} \) solution. Such exchange should have been expected for bound \( \mathrm{H_2O_2} \) in the structure of the perhydrate \( \mathrm{UO_3\cdot H_2O_2^{18}\cdot H_2O} \). We did not find it even after 500 h of residence of heavy-oxygen uranium peroxide in a solution of light \( \mathrm{H_2O_2} \).

From Table 1 it is seen that between \(195^\circ\) and approximately \(400^\circ\) decomposition of \( \mathrm{U_2O_7} \) occurs. Up to \(270^\circ\) it is small, and the change in the \( \mathrm{O^{18}} \) content in the evolved oxygen, as also in the oxygen from hydrolytic decomposition of \( \mathrm{U_2O_7} \), is likewise small. Near \(300^\circ\) the isotopic composition of the oxygen changes sharply, although, as is seen from experiment 1, the \( \mathrm{U_2O_7} \) content remains almost the same. At this temperature, evidently, a phase transformation of the solid solution occurs, after which the oxygen atoms acquire the ability to migrate readily in the lattice, so that the isotopic label is distributed among all oxygen atoms in \( \mathrm{U_2O_7} \) and \( \mathrm{UO_3} \). As a result, the \( \mathrm{O^{18}} \) content in peroxide oxygen from \( \mathrm{U_2O_7} \) above \(300^\circ\) is the same as its average content in the final decomposition product \( \mathrm{U_3O_8} \), obtained on heating above \(700^\circ\). At all temperatures the isotopic composition of the peroxide oxygen from hydrolysis of \( \mathrm{U_2O_7} \) coincides with the composition of the oxygen from its thermal decomposition, and in \( \mathrm{U_2O_7} \) the peroxide oxygen retains a structurally distinct position despite its exchange with the other oxygen atoms. Easy migration of oxygen atoms was previously detected by other me—

* Everywhere the numbers refer to the excess content of \( \mathrm{O^{18}} \) above natural.

by the same methods also in the region of higher temperatures, with the composition of the solid phase between $\mathrm{U_3O_8}$ and $\mathrm{UO_2}$ [2].

At 400–420° the decomposition of $\mathrm{U_2O_7}$ is completed and the evolution of oxygen ceases even under prolonged heating. In this case the color of the solid phase changes from orange to red; it does not give off oxygen upon interaction with water, and its composition approaches $\mathrm{UO_3}$. At 450–500° the evolution of oxygen begins again and continues on heating to 700–800°. In this interval the color of the solid phase becomes dark green and its composition approaches $\mathrm{U_3O_8}$. Between 350 and 700° the isotopic composition of the evolved oxygen does not change and remains the same as in the final $\mathrm{U_3O_8}$. This confirms the equivalence of the oxygen atoms in $\mathrm{UO_3}$ and $\mathrm{U_3O_8}$, which is also consistent with X-ray structural data [1].

Thus, in contrast to $\mathrm{UO_3}$ and $\mathrm{U_3O_8}$, which have the structure of oxides, $\mathrm{UO_4 \cdot 2H_2O}$ and $\mathrm{U_2O_7}$ are true peroxide compounds with structurally distinct atoms of peroxide oxygen.

Institute of Physical Chemistry
named after L. V. Pisarzhevskii
Academy of Sciences of the Ukrainian SSR

Received
6 III 1961

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