Uranium trioxide: Difference between revisions

{{cs1 config|name-list-style=vanc}}

| Watchedfields = changed

| verifiedrevid = 452010868

| ImageFile = UO3 gamma lattice.png

| IUPACName = Uranium trioxide<br />Uranium(VI) oxide

| OtherNames = Uranyl oxide<br />Uranic oxide

|Section1={{Chembox Identifiers

| CASNo_Ref = {{cascite|correct|CAS}}

| ChemSpiderID = 66635

| EINECS = 215-701-9

| PubChem = 74013

| UNII_Ref = {{fdacite|correct|FDA}}

| UNII = Q47SFG7DIQ

| StdInChI=1S/3O.U

| StdInChIKey = JCMLRUNDSXARRW-UHFFFAOYSA-N

| SMILES = O=[U](=O)=O

|Section2={{Chembox Properties

| Solubility = insoluble

| MeltingPt = ~200–650&nbsp;°C (decomposes)

|Section3={{Chembox Structure

| SpaceGroup = ''I''4₁/amd (''γ''-UO₃)

|Section4={{Chembox Thermochemistry

| DeltaHf = −1230&nbsp;kJ·mol⁻¹<ref name=b1>{{cite book| vauthors = Zumdahl SS |title =Chemical Principles 6th Ed.| publisher = Houghton Mifflin Company| year = 2009| isbn = 978-0-618-94690-7|page=A23}}</ref>

| Entropy = 99&nbsp;J·mol⁻¹·K⁻¹<ref name="b1" />

}}

|Section7={{Chembox Hazards

| ExternalSDS = [http://www.ibilabs.com/UO3-MSDS.htm External MSDS]

| GHSPictograms = {{GHS06}}{{GHS08}}{{GHS09}}

| GHSSignalWord = Danger

| HPhrases = {{H-phrases|300|330|373|411}}

| PPhrases = {{P-phrases}}

| NFPA-H = 4

| NFPA-F = 0

| NFPA-R = 1

| NFPA-S =OX

|Section8={{Chembox Related

| OtherFunction = [[Uranium dioxide]]<br />[[Triuranium octoxide]]

| OtherFunction_label = [[uranium]] [[oxide]]s

| OtherCompounds =

== Production and use ==

[[File:Uranium-trioxide-formation.png|500px|Methods of forming uranium trioxide]]

# U₃O₈ can be oxidized at 500&nbsp;°C with oxygen.<ref>{{cite journal|vauthors=Sheft I, Fried S, Davidson N|title=Preparation of Uranium Trioxide|journal=Journal of the American Chemical Society|year= 1950|volume= 72|issue=5|pages=2172–2173|doi=10.1021/ja01161a082}}</ref> Note that above 750&nbsp;°C even in 5 atm O₂ UO₃ decomposes into [[Triuranium octoxide|U₃O₈]].<ref name="wheeler">{{cite journal|vauthors=Wheeler VJ, Dell RM, Wait E|title= Uranium trioxide and the UO₃ hydrates|journal=Journal of Inorganic and Nuclear Chemistry|year=1964|volume=26|issue=11|pages= 1829–1845|doi=10.1016/0022-1902(64)80007-5}}</ref>

# [[Uranyl nitrate]], UO₂(NO₃)₂·6H₂O can be heated to yield UO₃. This occurs during the [[Nuclear reprocessing|reprocessing of nuclear fuel]]. Fuel rods are dissolved in [[nitric acid|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&nbsp;°C. After reduction with hydrogen (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 [[enriched uranium|enrichment of uranium]]. [[Uranium dioxide]] and [[uranium tetrafluoride]] are intermediates in the process which ends in [[uranium hexafluoride]].<ref>{{cite journal|vauthors=Dell RM, Wheeler VJ |title= Chemical Reactivity of Uranium Trioxide Part 1. — Conversion to U₃O₈, UO₂ and UF₄|journal=Transactions of the Faraday Society|year=1962|volume= 58|pages= 1590–1607|doi=10.1039/TF9625801590}}</ref>

Uranium trioxide is shipped between processing facilities in the form of a gel, most often from [[Mining|mines]] to conversion plants. When used for conversion, all uranium oxides are often called [[reprocessed uranium]] (RepU).<ref>{{cite web|url=http://www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Transport/Transport-of-Radioactive-Materials/|title=Transport of Radioactive Materials – World Nuclear Association|website=www.world-nuclear.org|access-date=12 April 2018|archive-date=5 February 2015|archive-url=https://web.archive.org/web/20150205234207/http://www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Transport/Transport-of-Radioactive-Materials/|url-status=dead}}</ref>

It has been reported that the corrosion of uranium in a silica rich aqueous solution forms [[uranium dioxide]], uranium trioxide,<ref>Trueman ER, Black S, Read D, Hodson ME (2003) "Alteration of Depleted Uranium Metal" ''Goldschmidt Conference Abstracts,'' p. A493 [http://www.the-conference.com/2003/Gold2003/abstracts/A493.pdf abstract]</ref> and [[coffinite]].<ref>{{cite journal|vauthors= Guo X, Szenknect S, Mesbah A, Labs S, Clavier N, Poinssot C, Ushakov SV, Curtius H, Bosbach D, Rodney RC, Burns P, Navrotsky A|title=Thermodynamics of Formation of Coffinite, USiO4|journal=Proc. Natl. Acad. Sci. USA|year=2015|volume=112|issue= 21|pages= 6551–6555|doi=10.1073/pnas.1507441112|pmid=25964321|pmc=4450415|bibcode=2015PNAS..112.6551G|doi-access=free}}</ref> In pure water, [[schoepite]] (UO₂)₈O₂(OH)₁₂·12(H₂O) is formed<ref>[http://webmineral.com/data/Schoepite.shtml Schoepite]. Webmineral.com. Retrieved on 2011-07-19.</ref> 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]],<ref>{{cite journal|author1=Weck P. F.|author2=Kim E. |author3=Jove-Colon C. F. |author4=Sassani D. C |title=Structures of uranyl peroxide hydrates: a first-principles study of studtite and metastudtite|journal=Dalton Trans|year= 2012|volume= 111|issue= 41|pages= 9748–52| doi= 10.1039/C2DT31242E|pmid=22763414|url=https://zenodo.org/record/1230018 }}</ref><ref>{{cite journal|vauthors= Guo X, Ushakov SV, Labs S, Curtius H, Bosbach D, Navrotsky A |title= Energetics of Metastudtite and Implications for Nuclear Waste Alteration|journal=Proc. Natl. Acad. Sci. USA|year=2015|volume=111|issue=20|pages=17737–17742|doi=10.1073/pnas.1421144111|pmid=25422465|pmc=4273415|doi-access= free}}</ref> 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]].<ref>Ander L, Smith B (2002) "[http://www.royalsoc.ac.uk/downloaddoc.asp?id=1182 Annexe F: Groundwater transport modelling]" ''The health hazards of depleted uranium munitions, part II'' (London: The Royal Society)</ref><ref>Smith B (2002) "[http://www.royalsoc.ac.uk/downloaddoc.asp?id=1183 Annexe G: Corrosion of DU and DU alloys: a brief discussion and review]" ''The health hazards of depleted uranium munitions, part II'' (London: The Royal Society)</ref>

== 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 cell]]s and [[gonads]] leading to [[Congenital disorder|congenital malformations]] if inhaled.<ref name="morrow">{{cite journal|vauthors=Morrow PE, Gibb FR, Beiter HD|title= Inhalation studies of uranium trioxide|journal= Health Physics|year= 1972|volume= 23|pages= 273–280|doi= 10.1097/00004032-197209000-00001|pmid= 4642950|issue= 3|s2cid= 39514654}} [http://www.health-physics.com/pt/re/healthphys/abstract.00004032-197209000-00001.htm abstract]</ref><ref>{{cite journal|vauthors=Sutton M, Burastero SR|title=Uranium(VI) solubility and speciation in simulated elemental human biological fluids|journal=[[Chemical Research in Toxicology]]|year= 2004| volume= 17|pages=1468–1480| doi=10.1021/tx049878k|pmid=15540945|issue=11}}</ref> However, once ingested, uranium is mainly toxic for the [[kidney]]s and may severely affect their function.

== Structure ==

=== Solid state structure ===

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

==== Alpha ====

{| border="1" cellpadding="3" cellspacing="0"

| [[File:UO3alphalattice.jpg|center|100px]]

| ''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&nbsp;°C forms an amorphous uranium trioxide which on heating to 400–450&nbsp;°C will form alpha-uranium trioxide.<ref name="wheeler" /> 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.<ref>{{cite journal|doi= 10.1002/jctb.5010130807|author=Sato T|title=Preparation of uranium peroxide hydrates|journal=Journal of Applied Chemistry|year=1963|volume=13|issue=8|pages=361–365}}</ref>

|}

==== Beta ====

{| border="1" cellpadding="3" cellspacing="0"

| [[File:UO3betalattice.jpg|center|150px]]

| ''The β (beta) UO₃ form: This solid contains multiple unique uranium sites and distorted polyhedra.''

| 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.<ref>{{cite journal|vauthors=Debets PC, Loopstra BO |title=On the Uranates of Ammonium II: X-Ray Investigation of the Compounds in the system NH₃-UO₃-H₂O|journal=Journal of Inorganic and Nuclear Chemistry|year=1963|volume=25|issue=8|pages=945–953|doi=10.1016/0022-1902(63)80027-5}}</ref><ref>{{cite journal|author=Debets PC|title=The Structure of β-UO3|journal=Acta Crystallographica|year=1966|volume=21|issue=4|pages=589–593 |doi =10.1107/S0365110X66003505|bibcode=1966AcCry..21..589D }}</ref> It was found that [[calcination]] at 500&nbsp;°C in air forms the beta form of uranium trioxide.<ref name="wheeler" /> Later experiments found the most reliable method for synthesizing pure β-UO₃ was to calcinate uranyl nitrate hexahydrate at 450&nbsp;°C for 6 days and cool slowly over 24 hours.<ref>{{cite journal |last1=Spano |first1=Tyler |last2=Shields |first2=Ashley |last3=Barth |first3=Brodie |last4=Gruidl |first4=Jeremiah |last5=Niedziela |first5=Jennifer |last6=Kapsimalis |first6=Roger |last7=Miskowiec |first7=Andrew |title=Computationally Guided Investigation of the Optical Spectra of Pure β-UO3 |journal=Inorganic Chemistry |date=2020 |volume=59 |issue=16 |pages=11481–11492 |doi=10.1021/acs.inorgchem.0c01279 |pmid=32706579 |osti=1649257 |s2cid=220746556 |url=https://www.osti.gov/biblio/1649257}}</ref>

|}

==== Gamma ====

{| border="1" cellpadding="3" cellspacing="0"

| [[File:UO3 gamma lattice.jpg|center|150px]]

| ''The γ (gamma) form: with the different uranium environments in green and yellow''

| The most frequently encountered polymorph is γ-UO₃, whose [[x-ray crystallography|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−&nbsp;, that is uranyl uranate.<ref>{{cite journal|vauthors=Engmann R, de Wolff PM|title=The Crystal Structure of γ-UO₃|journal=Acta Crystallographica|year= 1963|volume=16|issue=10|pages=993–996|doi=10.1107/S0365110X63002656|url=http://journals.iucr.org/q/issues/1963/10/00/a03972/a03972.pdf|doi-access=free}}</ref>

|}

{| border="1" cellpadding="3" cellspacing="0"

| [[File:UO3 gamma env1.jpg|center|150px]]

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

| [[File:UO3 gamma rings.jpg|center|150px]]

| ''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 [[Axis of rotation|axial]] [[oxygen]]-[[uranium]] bonds.''

|}

==== Delta ====

{| border="1" cellpadding="3" cellspacing="0"

| [[File:UO3lattice.jpg|center|150px]]

| ''The delta (δ) form is a [[Cubic (crystal system)|cubic]] solid where the oxygen atoms are arranged between the uranium atoms.''<ref>{{cite journal|author1=M. T. Weller|author2=P. G. Dickens|author3=D. J. Penny|year=1988|title=The structure of δ-UO_3> |journal=Polyhedron|volume=7|issue=3|pages=243–244|doi=10.1016/S0277-5387(00)80559-8}}</ref>

|}

==== Epsilon ====

{| border="1" cellpadding="3" cellspacing="0"

|

[[File:EpsilonUO3.jpg|center|150px]]

| ''The proposed crystal structure of the epsilon (ε) form consists of sheets of uranium hexagonal bipyramids connected through edge-sharing polyhedra. These sheets are connected through the axial uranyl oxygen atoms. The proposed structure is in the [[Triclinic crystal system|triclinic]] ''P-1'' space group.''<ref>{{cite journal |last1=Spano |first1=Tyler |last2=Hunt |first2=Rodney |last3=Kapsimalis |first3=Roger |last4=Niedziela |first4=Jennifer |last5=Shields |first5=Ashley |last6=Miskowiec |first6=Andrew |title=Optical vibrational spectra and proposed crystal structure of ε-UO3 |journal=Journal of Nuclear Materials |date=2022 |volume=559 |page=153386 |doi=10.1016/j.jnucmat.2021.153386 |osti=1843704 |s2cid=244423124 |url=https://www.sciencedirect.com/science/article/pii/S0022311521006061#sec0011}}</ref>

|}

==== High pressure form ====

There is a high-pressure solid form with U₂O₂ and U₃O₃ rings in it.<ref>{{cite journal|vauthors=Siegel S, Hoekstra HR, Sherry E |year=1966 |title=The crystal structure of high-pressure UO₃|journal=Acta Crystallographica|volume=20|issue=2|pages=292–295 |doi=10.1107/S0365110X66000562|bibcode=1966AcCry..20..292S }}</ref>

==== Hydrates ====

<gallery>

Uranium Trioxides.jpg|Hydrous and anhydrous forms of UO₃

UO3 Anhydrous.jpg|Anhydrous forms of UO₃

</gallery>

Several [[hydrates]] of uranium trioxide are known, e.g., UO₃·6H₂O, which are commonly known as "uranic acid" in older literature due to their similarity in formula to various metal [[oxyacids]], although they are not in fact particularly acidic.<ref name="wheeler" />

=== Molecular forms ===

==== Gas phase ====

With increasing temperature the equilibrium is shifted to the right. This system has been studied at temperatures between 900&nbsp;°C and 2500&nbsp;°C. The vapor pressure of monomeric UO₃ in equilibrium with air and solid U₃O₈ at ambient pressure, about 10⁻⁵&nbsp;mbar (1 mPa) at 980&nbsp;°C, rising to 0.1&nbsp;mbar (10&nbsp;Pa) at 1400&nbsp;°C, 0.34&nbsp;mbar (34&nbsp;Pa) at 2100&nbsp;°C, 1.9&nbsp;mbar (193&nbsp;Pa) at 2300&nbsp;°C, and 8.1&nbsp;mbar (809&nbsp;Pa) at 2500&nbsp;°C.<ref>{{cite journal|vauthors=Ackermann RJ, Gilles PW, Thorn RJ|title=High-Temperature Thermodynamic Properties of Uranium Dioxide|journal=Journal of Chemical Physics|year= 1956|volume=25|issue=6|page=1089|doi=10.1063/1.1743156|bibcode=1956JChPh..25.1089A}}</ref><ref>{{cite journal|author=Alexander CA|title=Volatilization of urania under strongly oxidizing conditions|journal=Journal of Nuclear Materials|year=2005|volume=346|issue=2–3|pages=312–318|doi=10.1016/j.jnucmat.2005.07.013|bibcode=2005JNuM..346..312A}}</ref>

==== 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 [[ångström|Å]] (176 to 179 [[1 E-12 m|pm]]).<ref>{{cite journal|vauthors=Gabelnick SD, Reedy GT, Chasanov MG | title=Infrared spectra of matrix-isolated uranium oxide species. II: Spectral interpretation and structure of UO₃|journal= Journal of Chemical Physics|year=1973|volume=59|issue=12|pages=6397–6404|doi=10.1063/1.1680018| bibcode=1973JChPh..59.6397G}}</ref>

==== Computational study ====

[[File:Uranium-trioxide-Pyykko-3D-balls-B.png|thumb|200px|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.<ref>{{cite journal|vauthors=Pyykkö P, Li J |title= Quasirelativistic pseudopotential study of species isoelectronic to uranyl and the equatorial coordination of uranyl|journal= Journal of Physical Chemistry|year= 1994|volume= 98|issue= 18|pages= 4809–4813|doi = 10.1021/j100069a007}}</ref>

=== Cubic form of uranium trioxide ===

The crystal structure of a uranium trioxide phase of composition UO_2·82 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.<ref>{{cite journal|doi=10.1016/0022-1902(55)80036-X|volume=1|issue=4–5|title=A cubic form of uranium trioxide|journal=Journal of Inorganic and Nuclear Chemistry|pages=309–312|year=1955|last1=Wait|first1=E.}}</ref>

== Reactivity ==

Uranium trioxide reacts at 400&nbsp;°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.<ref>{{cite journal|vauthors=Booth HS, Krasny-Ergen W, Heath RE |title=Uranium Tetrafluoride|journal=[[Journal of the American Chemical Society]]|year=1946|volume=68|issue=10|pages=1969–1970|doi=10.1021/ja01214a028}}</ref>

Uranium trioxide can be dissolved in a mixture of [[tributyl phosphate]] and [[thenoyltrifluoroacetone]] in [[supercritical carbon dioxide]], ultrasound was employed during the dissolution.<ref>{{cite journal|vauthors=Trofimov TI, Samsonov MD, Lee SC, Myasoedov BF, Wai CM |title=Dissolution of uranium oxides in supercritical carbon dioxide containing tri-''n''-butyl phosphate and thenoyltrifluoroacetone|journal=Mendeleev Communications|year=2001|volume=11|issue=4|pages=129–130|doi=10.1070/MC2001v011n04ABEH001468}}</ref>

=== Electrochemical modification ===

The reversible insertion of [[magnesium]] cations into the [[crystal structure|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.<ref>{{cite journal|doi=10.1149/1.2221232|title=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|year=1992|last1=Dueber|first1=R. E.|journal=Journal of the Electrochemical Society|volume=139|issue=9|pages=2363–2371|bibcode=1992JElS..139.2363D}}</ref>

It has been the case that it is possible to insert [[lithium]]<ref>{{cite journal|title=Insertion compounds of uranium oxides|vauthors=Dickens PG, Lawrence SD, Penny DJ, Powell AV|year=1989| volume=32–33|doi=10.1016/0167-2738(89)90205-1|pages =77–83|journal=Solid State Ionics}}</ref><ref>{{cite journal|title=Lithium insertion into αUO₃ and U₃O₈ |vauthors=Dickens PG, Hawke SV, Weller MT | year=1985 |volume=20 |doi=10.1016/0025-5408(85)90141-2 |issue=6 |pages=635–641 |journal =Materials Research Bulletin }}</ref><ref>{{cite journal|title=Hydrogen insertion compounds of UO₃|vauthors=Dickens PG, Hawke SV, Weller MT|year=1984|volume=19|doi=10.1016/0025-5408(84)90120-X|issue=5|pages=543–547|journal =Materials Research Bulletin}}</ref> 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 (chemistry)|amphoteric]] and reacts as [[acid]] and as a [[base (chemistry)|base]], depending on the conditions.

====As an acid====

:UO₃ + H₂O → {{chem|UO|4|2−}} + 2 H⁺

[[magnesium diuranate]] (MgU₂O₇), which forms part of some [[yellowcake]]s. It is worth noting that uranates of the form M₂UO₄ do ''not'' contain {{chem|UO|4|2−}} ions, but rather flattened UO₆ octahedra, containing a uranyl group and bridging oxygens.<ref>{{cite book|last= Cotton|first= Simon|year=1991|title=Lanthanides and Actinides|publisher=Oxford University Press|location=New York|page=128|isbn=978-0-19-507366-9}}</ref>

====As a base====

Dissolving uranium oxide in a strong acid like [[sulfuric acid|sulfuric]] or [[nitric acid]] forms the double positive charged [[uranyl]] [[cation]]. The [[uranyl nitrate]] formed (UO₂(NO₃)₂·6H₂O) is soluble in [[ether]]s, [[Alcohol (chemistry)|alcohols]], [[ketone]]s and [[ester]]s; 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]] to form this salt. The [[uranyl nitrate]] is then converted to uranium trioxide by heating.

== 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 [[Vitreous enamel|enamel]], [[uranium glass]], and [[porcelain]].

== References ==

{{reflist|30em}}

{{Oxides}}

[[Category:Uranium(VI) compounds]]

[[Category:Amphoteric compounds]]