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Uranium trioxide (UO₃), also known as uranyl oxide, uranium(VI) oxide, or uranic oxide, is the hexavalent oxide of uranium that typically appears as orange-yellow crystalline solid with a molecular weight of 286.027 and a density of approximately 7.29 g/cm³. The compound is insoluble in water but dissolves in acidic solutions. UO₃ exhibits remarkable structural diversity, existing in seven polycrystalline polymorphic forms plus an amorphous modification, with the uranium atom capable of being coordinated to six, seven, or eight oxygen atoms depending on the polymorph. The most thermodynamically stable form is γ-UO₃, which exhibits orthorhombic structure at 293 K and transforms to a tetragonal structure at 373 K.

Thermal Behavior and Chemical Properties

Uranium trioxide is thermally unstable and decomposes to lower oxides rather than melting or subliming. Upon heating, solid UO₃ loses oxygen to form green-colored triuranium octoxide (U₃O₈), with decomposition temperatures reported between 200°C and 650°C in air. Further heating at 700°C under hydrogen atmosphere produces dark brown uranium dioxide (UO₂), which finds application in MOX nuclear fuel rods. At elevated temperatures, gaseous UO₃ exists in equilibrium with solid U₃O₈ and molecular oxygen according to the reaction:

The compound exhibits a T-shaped molecular structure in the gas phase with U-O bond lengths between 1.76 and 1.79 Å, contrasting with the more symmetrical structures typical of most trioxides.

Production and Polymorphic Forms

UO₃ is commonly produced by thermal decomposition of uranyl compounds such as carbonates, oxalates, or nitrates, with heating of uranyl nitrate to 400°C being a standard production method. Industrial processes, such as Cameco's production method, operate at 280°C to convert uranyl nitrate hexahydrate directly to UO₃. The different polymorphic forms exhibit distinct structural and electronic properties, with phases containing uranyl bonds tending to have smaller band gaps and bulk moduli under 100 GPa, while those without uranyl bonds display larger band gaps and bulk moduli exceeding 150 GPa. Commercial UO₃ samples typically exhibit oxygen-to-uranium ratios ranging from 2.98 to 3.09 due to surface hydration, thermal history, and preparation conditions.

References

• International Bio-Analytical Industries, Inc.. Uranium Trioxide
• WebElements. Uranium trioxide
• Chemical Book. URANIUM(VI) OXIDE
• Wikipedia. Uranium trioxide (last edited: Jan 11, 2026)
• Hill M.A. et al.. Los Alamos National Laboratory. Report LA-UR-12-22920: Cameco UO3 Materials Analysis. U.S. Department of Energy (DOE), Office of Scientific and Technical Information (OSTI)
• Brincat, N. A. et al.. (2014). Ab initio investigation of the UO₃ polymorphs: Structural properties and thermodynamic stability. Inorganic Chemistry, 53(23), 12253–12264. DOI: 10.1021/ic500791m
• Smolecule. Uranium trioxide (last modified: Feb 18, 2024)