Uranium trioxide

Uranium trioxide

Names
IUPAC names Uranium trioxide Uranium(VI) oxide
Other names Uranyl oxide Uranic oxide
Identifiers
CAS Number	1344-58-7
Properties
Chemical formula	UO3
Molar mass	286.29 g/mol
Appearance	yellow-orange powder
Density	5.5–8.7 g/cm^3
Melting point	~200–650 °C (decomposes)
Solubility in water	Partially soluble
Structure
Crystal structure	see text
Space group	I41/amd (γ-UO3)
Thermochemistry
Std molar entropy (S^o298)	99 J·mol^−1·K^−1[1]
Std enthalpy of formation (ΔfH^o298)	−1230 kJ·mol^−1[1]
Hazards
Safety data sheet	External MSDS
EU classification (DSD)	Very toxic (T+) Dangerous for the environment (N)
R-phrases	R26/28, R33, R51/53
S-phrases	(S1/2), S20/21, S45, S61
Flash point	Non-flammable
Related compounds
Related uranium oxides	Uranium dioxide Triuranium octoxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Uranium trioxide (UO₃), also called uranyl oxide, uranium(VI) oxide, and uranic oxide, is the hexavalent oxide of uranium. The solid may be obtained by heating uranyl nitrate to 400 °C. Its most commonly encountered polymorph, γ-UO₃, is a yellow-orange powder.

Production and use

There are three methods to generate uranium trioxide. As noted below, two are used industrially in the reprocessing of nuclear fuel and uranium enrichment.

1. U₃O₈ can be oxidized at 500 °C with oxygen.^[2] Note that above 750 °C even in 5 atm O₂ UO₃ decomposes into U₃O₈.^[3]
2. Uranyl nitrate, UO₂(NO₃)₂·6H₂O can be heated to yield UO₃. This occurs during the reprocessing of nuclear fuel. Fuel rods are dissolved in HNO₃ to separate uranyl nitrate from plutonium and the fission products (the PUREX method). The pure uranyl nitrate is converted to solid UO₃ by heating at 400 °C. After reduction with hydrogen (with other inert gas present) to uranium dioxide, the uranium can be used in new MOX fuel rods.
3. Ammonium diuranate or sodium diuranate (Na₂U₂O₇·6H₂O) may be decomposed. Sodium diuranate, also known as yellowcake, is converted to uranium trioxide in the enrichment of uranium. Uranium dioxide and uranium tetrafluoride are intermediates in the process which ends in uranium hexafluoride.^[4]

Uranium trioxide is shipped between processing facilities in the form of a gel, most often from mines to conversion plants. When used for conversion, all uranium oxides are often called reprocessed uranium(RepU).^[5]

Cameco Corporation, which operates at the world's largest uranium refinery at Blind River, Ontario, produces high-purity uranium trioxide.

It has been reported that the corrosion of uranium in a silica rich aqueous solution forms uranium dioxide, uranium trioxide,^[6] and coffinite.^[7] In pure water, schoepite (UO₂)₈O₂(OH)₁₂·12(H₂O) is formed^[8] in the first week and then after four months studtite (UO₂)O₂·4(H₂O) was produced. This alteration of uranium oxide also leads to the formation of metastudtite,^{[9] [10]} a more stable uranyl peroxide, often found in the surface of spent nuclear fuel exposed to water. Reports on the corrosion of uranium metal have been published by the Royal Society.^[11][12]

Health and safety hazards

Like all hexavalent uranium compounds, UO₃ is hazardous by inhalation, ingestion, and through skin contact. It is a poisonous, slightly radioactive substance, which may cause shortness of breath, coughing, acute arterial lesions, and changes in the chromosomes of white blood cells and gonads leading to congenital malformations if inhaled.^[13][14] However, once ingested, uranium is mainly toxic for the kidneys and may severely affect their function.

Structure

Solid state structure

The only well characterized binary trioxide of any actinide is UO₃, of which several polymorphs are known. Solid UO₃ loses O₂ on heating to give green-colored U₃O₈: reports of the decomposition temperature in air vary from 200–650 °C. Heating at 700 °C under H₂ gives dark brown uranium dioxide (UO₂), which is used in MOX nuclear fuel rods.

Alpha

The α (alpha) form: a layered solid where the 2D layers are linked by oxygen atoms (shown in red)

Hydrated uranyl peroxide formed by the addition of hydrogen peroxide to an aqueous solution of uranyl nitrate when heated to 200–225 °C forms an amorphous uranium trioxide which on heating to 400–450 °C will form alpha-uranium trioxide.[3] It has been stated that the presence of nitrate will lower the temperature at which the exothermic change from the amorphous form to the alpha form occurs.[15]

Beta

β (beta) UO3. This solid has a structure which defeats most attempts to describe it.

This form can be formed by heating ammonium diuranate, while P.C. Debets and B.O. Loopstra, found four solid phases in the UO3-H2O-NH3 system that they could all be considered as being UO2(OH)2.H2O where some of the water has been replaced with ammonia.[16][17] No matter what the exact stoichiometry or structure, it was found that calcination at 500 °C in air forms the beta form of uranium trioxide.[3]

Gamma

The γ (gamma) form, with the different uranium environments in green and yellow

The most frequently encountered polymorph is γ-UO3, whose x-ray structure has been solved from powder diffraction data. The compound crystallizes in the space group I41/amd with two uranium atoms in the asymmetric unit. Both are surrounded by somewhat distorted octahedra of oxygen atoms. One uranium atom has two closer and four more distant oxygen atoms whereas the other has four close and two more distant oxygen atoms as neighbors. Thus it is not incorrect to describe the structure as [UO2]^2+[UO4]^2-, that is uranyl uranate.[18]

The environment of the uranium atoms shown as yellow in the gamma form

The chains of U2O2 rings in the gamma form in layers, alternate layers running at 90 degrees to each other. These chains are shown as containing the yellow uranium atoms, in an octahedral environment which are distorted towards square planar by an elongation of the axial oxygen-uranium bonds.

Delta

The delta (δ) form is a cubic solid where the oxygen atoms are arranged between the uranium atoms.[19]

High pressure form

There is a high-pressure solid form with U₂O₂ and U₃O₃ rings in it.^{[20] [21]}

Hydrates

Hydrous and anhydrous forms of UO₃

Several hydrates of uranium trioxide are known, e.g., UO₃·6H₂O.^[3]

Bond valence parameters

It is possible by bond valence calculations^[22] to estimate how great a contribution a given oxygen atom is making to the assumed valence of uranium.^[23] Bond valence calculations use parameters which are estimated after examining a large number of crystal structures of uranium oxides (and related uranium compounds), note that the oxidation states which this method provides are only a guide which assists in the understanding of a crystal structure.

The formula to use is

$s = e^{-\frac{R-R_O}{B}}$

The sum of the s values is equal to the oxidation state of the metal centre.

For uranium binding to oxygen the constants R_O and B are tabulated in the table below. For each oxidation state use the parameters from the table shown below.

Oxidation state	RO	B
U(VI)	2.08 Å	0.35
U(V)	2.10 Å	0.35
U(IV)	2.13 Å	0.35

It is possible to do these calculations on paper or software.^[24][25]

Molecular forms

While uranium trioxide is encountered as a polymeric solid under ambient conditions, some work has been done on the molecular form in the gas phase, in matrix isolations studies, and computationally.

Gas phase

At elevated temperatures gaseous UO₃ is in equilibrium with solid U₃O₈ and molecular oxygen.

2 U₃O₈(s) + O₂(g) ⇌ 6 UO₃(g)

With increasing temperature the equilibrium is shifted to the right. This system has been studied at temperatures between 900 °C and 2500 °C. The vapor pressure of monomeric UO₃ in equilibrium with air and solid U₃O₈ at ambient pressure, about 10⁻⁵ mbar (1 mPa) at 980 °C, rising to 0.1 mbar (10 Pa) at 1400 °C, 0.34 mbar (34 Pa) at 2100 °C, 1.9 mbar (193 Pa) at 2300 °C, and 8.1 mbar (809 Pa) at 2500 °C.^[26][27]

Matrix isolation

Infrared spectroscopy of molecular UO₃ isolated in an argon matrix indicates a T-shaped structure (point group C_2v) for the molecule. This is in contrast to the commonly encountered D_3h molecular symmetry exhibited by most trioxides. From the force constants the authors deduct the U-O bond lengths to be between 1.76 and 1.79 Å (176 to 179 pm).^[28]

Computational study

The calculated geometry of molecular uranium trioxide has C_2v symmetry.

Calculations predict that the point group of molecular UO₃ is C_2v, with an axial bond length of 1.75 Å, an equatorial bond length of 1.83 Å and an angle of 161° between the axial oxygens. The more symmetrical D_3h species is a saddle point, 49 kJ/mol above the C_2v minimum. The authors invoke a second-order Jahn–Teller effect as explanation.^[29]

Cubic Form of Uranium trioxide

The crystal structure of a uranium trioxide phase of composition UO2·82 has been determined by X-ray powder diffraction techniques using a Guinier-type focusing camera. The unit cell is cubic with a = 4·138 ± 0·005 kX. A uranium atom is located at (000) and oxygens at (View the MathML source), (View the MathML source), and (View the MathML source) with some anion vacancies. The compound is isostructural with ReO3. The U-O bond distance of 2·073 Å agrees with that predicted by Zachariasen for a bond strength S = 1.^[30]

Reactivity

Uranium trioxide reacts at 400 °C with freon-12 to form chlorine, phosgene, carbon dioxide and uranium tetrafluoride. The freon-12 can be replaced with freon-11 which forms carbon tetrachloride instead of carbon dioxide. This is a case of a hard perhalogenated freon which is normally considered to be inert being converted chemically at a moderate temperature.^[31]

2 CF₂Cl₂ + UO₃ → UF₄ + CO₂ + COCl₂ + Cl₂

4 CFCl₃ + UO₃ → UF₄ + 3 COCl₂ + CCl₄ + Cl₂

Uranium trioxide can be dissolved in a mixture of tributyl phosphate and thenoyltrifluoroacetone in supercritical carbon dioxide, ultrasound was employed during the dissolution.^[32]

Electrochemical modification

The reversible insertion of magnesium cations into the lattice of uranium trioxide by cyclic voltammetry using a graphite electrode modified with microscopic particles of the uranium oxide has been investigated. This experiment has also been done for U₃O₈. This is an example of electrochemistry of a solid modified electrode, the experiment which used for uranium trioxide is related to a carbon paste electrode experiment. It is also possible to reduce uranium trioxide with sodium metal to form sodium uranium oxides.^[33]

It has been the case that it is possible to insert lithium^[34][35][36] into the uranium trioxide lattice by electrochemical means, this is similar to the way that some rechargeable lithium ion batteries work. In these rechargeable cells one of the electrodes is a metal oxide which contains a metal such as cobalt which can be reduced, to maintain the electroneutrality for each electron which is added to the electrode material a lithium ion enters the lattice of this oxide electrode.

Amphoterism and reactivity to form related uranium(VI) anions and cations

Uranium oxide is amphoteric and reacts as acid and as a base, depending on the conditions.

As an acid

UO₃ + H₂O → UO2−
4 + 2 H⁺

Dissolving uranium oxide in a strong base like sodium hydroxide forms the doubly negatively charged uranate anion (UO2−
4). Uranates tend to concatenate, forming diuranate, U
2O2−
7, or other poly-uranates. Important diuranates include ammonium diuranate ((NH₄)₂U₂O₇), sodium diuranate (Na₂U₂O₇) and magnesium diuranate (MgU₂O₇), which forms part of some yellowcakes. It is worth noting that uranates of the form M₂UO₄ do not contain UO2−
4 ions, but rather flattened UO₆ octahedra, containing a uranyl group and bridging oxygens.^[37]

As a base

UO₃ + H₂O → UO2+
2 + 2 OH⁻

Dissolving uranium oxide in a strong acid like sulfuric or nitric acid forms the double positive charged uranyl cation. The uranyl nitrate formed (UO₂(NO₃)₂·6H₂O) is soluble in ethers, alcohols, ketones and esters; for example, tributylphosphate. This solubility is used to separate uranium from other elements in nuclear reprocessing, which begins with the dissolution of nuclear fuel rods in nitric acid. The uranyl nitrate is then converted to uranium trioxide by heating.

From nitric acid one obtains uranyl nitrate, trans-UO₂(NO₃)₂·2H₂O, consisting of eight-coordinated uranium with two bidentate nitrato ligands and two water ligands as well as the familiar O=U=O core.

Uranium oxides in ceramics

UO₃-based ceramics become green or black when fired in a reducing atmosphere and yellow to orange when fired with oxygen. Orange-coloured Fiestaware is a well-known example of a product with a uranium-based glaze. UO₃-has also been used in formulations of enamel, uranium glass, and porcelain.

Prior to 1960, UO₃ was used as an agent of crystallization in crystalline coloured glazes. It is possible to determine with a Geiger counter if a glaze or glass was made from UO₃.

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