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.
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).
It has been reported that the corrosion of uranium in a silica rich aqueous solution forms uranium dioxide, uranium trioxide, and coffinite. In pure water, schoepite (UO2)8O2(OH)12·12(H2O) is formed 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, 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.
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. However, once ingested, uranium is mainly toxic for the kidneys and may severely affect their function.
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 MOXnuclear fuel rods.
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. 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.
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. 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.
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.
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 axialoxygen-uranium bonds.
The delta (δ) form is a cubic solid where the oxygen atoms are arranged between the uranium atoms.
High pressure form
There is a high-pressure solid form with U2O2 and U3O3 rings in it.
Hydrous and anhydrous forms of UO3
Several hydrates of uranium trioxide are known, e.g., UO3·6H2O.
Bond valence parameters
It is possible by bond valence calculations to estimate how great a contribution a given oxygen atom is making to the assumed valence of uranium. 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
The sum of the s values is equal to the oxidation state of the metal centre.
For uranium binding to oxygen, the constants RO and B are tabulated in the table below. For each oxidation state use the parameters from the table shown below.
It is possible to do these calculations on paper or software.
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.
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.
Infrared spectroscopy of molecular UO3 isolated in an argon matrix indicates a T-shaped structure (point groupC2v) for the molecule. This is in contrast to the commonly encountered D3hmolecular 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).
The calculated geometry of molecular uranium trioxide has C2v symmetry.
Calculations predict that the point group of molecular UO3 is C2v, 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 D3h species is a saddle point, 49 kJ/mol above the C2v minimum. The authors invoke a second-order Jahn–Teller effect as explanation.
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.
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.
It has been the case that it is possible to insert lithium into the uranium trioxide lattice by electrochemical means, this is similar to the way that some rechargeablelithium 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.
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.
^ abZumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A23. ISBN978-0-618-94690-7.
^Sheft I, Fried S, Davidson N (1950). "Preparation of Uranium Trioxide". Journal of the American Chemical Society. 72 (5): 2172–2173. doi:10.1021/ja01161a082.
^ abcdWheeler VJ, Dell RM, Wait E (1964). "Uranium trioxide and the UO3 hydrates". Journal of Inorganic and Nuclear Chemistry. 26 (11): 1829–1845. doi:10.1016/0022-1902(64)80007-5.
^Dell RM, Wheeler VJ (1962). "Chemical Reactivity of Uranium Trioxide Part 1. — Conversion to U3O8, UO2 and UF4". Transactions of the Faraday Society. 58: 1590–1607. doi:10.1039/TF9625801590.
^Schoepite. Webmineral.com. Retrieved on 2011-07-19.
^Weck P. F.; Kim E.; Jove-Colon C. F. & Sassani D. C (2012). "Structures of uranyl peroxide hydrates: a first-principles study of studtite and metastudtite". Dalton Trans. 111 (41): 9748–52. doi:10.1039/C2DT31242E. PMID22763414.
^Sato T (1963). "Preparation of uranium peroxide hydrates". Journal of Applied Chemistry. 13 (8): 361–365. doi:10.1002/jctb.5010130807.
^Debets PC, Loopstra BO (1963). "On the Uranates of Ammonium II: X-Ray Investigation of the Compounds in the system NH3-UO3-H2O". Journal of Inorganic and Nuclear Chemistry. 25 (8): 945–953. doi:10.1016/0022-1902(63)80027-5.
^Ackermann RJ, Gilles PW, Thorn RJ (1956). "High-Temperature Thermodynamic Properties of Uranium Dioxide". Journal of Chemical Physics. 25 (6): 1089. doi:10.1063/1.1743156.
^Alexander CA (2005). "Volatilization of urania under strongly oxidizing conditions". Journal of Nuclear Materials. 346 (2–3): 312–318. doi:10.1016/j.jnucmat.2005.07.013.
^Gabelnick SD, Reedy GT, Chasanov MG (1973). "Infrared spectra of matrix-isolated uranium oxide species. II: Spectral interpretation and structure of UO3". Journal of Chemical Physics. 59 (12): 6397–6404. doi:10.1063/1.1680018.
^Pyykkö P, Li J (1994). "Quasirelativistic pseudopotential study of species isoelectronic to uranyl and the equatorial coordination of uranyl". Journal of Physical Chemistry. 98 (18): 4809–4813. doi:10.1021/j100069a007.
^Trofimov TI, Samsonov MD, Lee SC, Myasoedov BF, Wai CM (2001). "Dissolution of uranium oxides in supercritical carbon dioxide containing tri-n-butyl phosphate and thenoyltrifluoroacetone". Mendeleev Communications. 11 (4): 129–130. doi:10.1070/MC2001v011n04ABEH001468.
^Dueber, R. E. (1992). "Investigation of the Mechanism of Formation of Insertion Compounds of Uranium Oxides by Voltammetric Reduction of the Solid Phase after Mechanical Transfer to a Carbon Electrode". Journal of the Electrochemical Society. 139 (9): 2363. doi:10.1149/1.2221232.
^Dickens PG, Lawrence SD, Penny DJ, Powell AV (1989). "Insertion compounds of uranium oxides". Solid State Ionics. 32–33: 77–83. doi:10.1016/0167-2738(89)90205-1.
^Dickens, P.G. Hawke, S.V. Weller, M.T. (1985). "Lithium insertion into αUO3 and U3O8". Materials Research Bulletin. 20 (6): 635–641. doi:10.1016/0025-5408(85)90141-2.CS1 maint: multiple names: authors list (link)
^Dickens, P.G. Hawke, S.V. Weller, M.T. (1984). "Hydrogen insertion compounds of UO3". Materials Research Bulletin. 19 (5): 543–547. doi:10.1016/0025-5408(84)90120-X.CS1 maint: multiple names: authors list (link)
^Cotton, Simon (1991). Lanthanides and Actinides. New York: Oxford University Press. p. 128. ISBN978-0-19-507366-9.