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Uranium trioxide
	solid γ-UO3 (gamma polymorph) oxygen diameters sharply reduced for visibility
General
Systematic name	Uranium trioxide Uranium(VI) oxide
Other names	Uranyl oxide Uranic oxide
Molecular formula	UO3 (or O3U)
CAS number
Properties
Molar mass	286.2873 g/mol Commercial samples may have undergone isotope fractionation, and their molecular mass may be significantly different
Density and phase	5.5 – 8.7 g/cm
Solubility (water)	Partially soluble
Solubility (dog lung fluid)	< 5 days (Morrow, 1972)
Melting point	~ 200 – 650 °C decomp. (s)
Structure
Molecular shape	T-shape
Coordination geometry	γ-UO3:
Crystal structure	I41/amd (γ-UO3)
Hazards
MSDS	UO3-MSDS
Main hazards	highly toxic: teratogen, immunotoxin, neurotoxin, genotoxin, nephrotoxin
Flash point	inflamable
Related compounds
Other anions	Uranyl nitrate
Other cations	Chromium trioxide
Related compounds	Uranium dioxide Triuranium octaoxide
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

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

UO₃ gas is produced at high temperatures from U₃O₈, which comprises 75% of the particulate combustion product of uranium burning in air. The partial pressure of UO₃(g) is several dozen mbar well below uranium's burning temperature. Health impact assessments for depleted uranium munitions should take into account the presence of respiratory UO₃.

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 entrichment.

• U₃O₈ can be oxidized at 500°C with oxygen.
• 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 with other inert gas present) to uranium dioxide, the uranium can be used in new MOX fuel rods.
• 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.

Uranium trioxide is shipped between processing facilities in the form of a gel.

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

Health and safety hazards

Like all hexavalent uranium compounds (also called uranium(VI) compounds), UO₃ is hazardous by inhalation, ingestion, and through skin contact. It is a poisonous, 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.

Chemistry and structure

Solid state

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.

Uranium trioxide reacts at 400 °C with freon-12 to form chlorine, phosgene, carbon dioxide and uranium(IV) fluoride. 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.

2 CF_{2Cl2 + UO3 → UF4 + CO2 + COCl2 + Cl2}

4 CF_{2Cl2 + UO3 → UF4 + 3COCl2 + 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.

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 , that is uranyl uranate. .

High presure solid forms exist. Gmelin Handbuch (1982) U-C1, 129-135.

• The γ (gamma) form, with the different uranium environments in green and yellow
• 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 a octahedral environment which are distorted towards square planar by an elongation of the axial oxygen-uranium bonds.
• The delta (δ) form was reported by Weller et al. (1988) Polyhedron, 7, 243-244.

Gas phase

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 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 angstroms (176 to 179 picometers).

At elevated temperatures gaseous UO₃ and O₂ are in equilibrium with solid U₃O₈.

/₃ U₃O₈(s) + /₆ O₂(g) ${\displaystyle {\overrightarrow {\gets }}}$ UO₃(g)

With increasing temperature the equilibrium is shifted to the right. This system has been studied at temperatures between 900 and 1500 °C. The vapor pressure of monomeric UO₃ is low but appreciable, about 10 mbar (1 mPa) at 980 °C, rising to 10 mbar (10 Pa) at 1400 °C, 0.34 mbar (34 Pa) at 1800 K, 19 mbar (1.9 kPa) at 2000 K, and 81 mbar (8.1 kPa) at 2200 K. The burning temperature of uranium in air usually exceeds 2500 K.

Aerial oxidation of any uranium compound eventually results in the formation of a uranyl compound, and uranium trioxide is the only uranyl oxide.

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.

Related anions and cations

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

As an acid:

UO₃ + H₂O → UO₄ + H

Dissolving uranium oxide in a strong base like sodium hydroxide forms the doubly negatively charged uranate anion (UO₄). Uranates tend to agglomerate, forming diuranate, U₂O₇

As a base:

UO₃ + H₂O → UO₂ + 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)₂ˑ6H₂O) is soluble in ethers, alcohols, ketones and esters; for example, tributylphosphate. This solubilty 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.

References

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