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{{cs1 config|name-list-style=vanc}}
{| class="toccolours" border="1" style="float: right; clear: right; margin: 0 0 1em 1em; border-collapse: collapse;"
! {{chembox header}} | {{PAGENAME}} {{chembox
| Watchedfields = changed
|-
| verifiedrevid = 452010868
|colspan=2 align=center|]
| Name = Uranium trioxide
|-
|align=center|] | ImageFile = UO3 gamma lattice.png
| IUPACName = Uranium trioxide<br />Uranium(VI) oxide
|solid γ-UO<sub>3</sub><br>(gamma ])<br>oxygen diameters sharply<br>reduced for visibility
| OtherNames = Uranyl oxide<br />Uranic oxide
|-
|Section1={{Chembox Identifiers
! {{chembox header}} | General
| CASNo_Ref = {{cascite|correct|CAS}}
|-
| CASNo = 1344-58-7
| ]
| ChemSpiderID = 66635
| Uranium trioxide<br/>Uranium(VI) oxide
| EINECS = 215-701-9
|-
| PubChem = 74013
| Other names
| UNII_Ref = {{fdacite|correct|FDA}}
| ] oxide<br/>Uranic oxide
| UNII = Q47SFG7DIQ
|-
| StdInChI=1S/3O.U
| ]
| StdInChIKey = JCMLRUNDSXARRW-UHFFFAOYSA-N
| UO<sub>3</sub> )
| SMILES = O=(=O)=O
|-
}}
| ]
|Section2={{Chembox Properties
|
| Formula = UO<sub>3</sub>
|-
| MolarMass = 286.29 g/mol
! {{chembox header}} | Properties
| Density = 5.5–8.7 g/cm<sup>3</sup>
|-
| Appearance = yellow-orange powder
| ]
| Solubility = insoluble
| 286.2873 g/mol<br>Commercial samples may<br>have undergone isotope<br>fractionation, and their<br> molecular mass may<br>be significantly different
| MeltingPt = ~200–650&nbsp;°C (decomposes)
|-
}}
| ] and ]
|Section3={{Chembox Structure
| 5.5 &ndash; 8.7 g/cm<sup>3</sup>
| CrystalStruct = ''see text''
|-
| SpaceGroup = ''I''4<sub>1</sub>/amd (''γ''-UO<sub>3</sub>)
| ] (])
| Coordination =
| Partially soluble
}}
|-
|Section4={{Chembox Thermochemistry
| ] (dog lung fluid)
| DeltaHf = −1230&nbsp;kJ·mol<sup>−1</sup><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>
| &lt; 5 days (Morrow, 1972)
| Entropy = 99&nbsp;J·mol<sup>−1</sup>·K<sup>−1</sup><ref name="b1" />
|-
}}
| ]
|Section7={{Chembox Hazards
| ~ 200 &ndash; 650 °C ''decomp.'' (s)
| ExternalSDS =
|-
| GHSPictograms = {{GHS06}}{{GHS08}}{{GHS09}}
! {{chembox header}} | Structure
| GHSSignalWord = Danger
|-
| HPhrases = {{H-phrases|300|330|373|411}}
| ]
| PPhrases = {{P-phrases}}
| T-shape
| NFPA-H = 4
|-
| NFPA-F = 0
| ]
| NFPA-R = 1
| ''γ''-UO<sub>3</sub>: <sup>2+</sup><sup>2&nbsp;</sup>
| NFPA-S =OX
|-
| FlashPt = Non-flammable
| ]
| LD50 =
| I4<sub>1</sub>/amd (''γ''-UO<sub>3)
| PEL =
|-
}}
! {{chembox header}} | Hazards
|Section8={{Chembox Related
|-
| OtherAnions =
| ]
| OtherFunction = ]<br />]
|
| OtherFunction_label = ] ]s
|-
| OtherCompounds =
| Main ]s
}}
| highly toxic: teratogen,<br>immunotoxin, neurotoxin,<br>genotoxin, nephrotoxin
}}
|-
| ]
| inflamable
|-
! {{chembox header}} | Related compounds
|-
| Other ]s
| ]
|-
| Other ]s
| ]
|-
| Related compounds
| ]<br/>]
|-
| {{chembox header}} | <small>Except where noted otherwise, data are given for<br> materials in their ]<br/>]</small>
|}

'''Uranium trioxide (UO<sub>3</sub>),''' also called '''] oxide''', '''uranium(VI) oxide''', and '''uranic oxide''', is the hexavalent ] of ]. The toxic, ], and radioactive solid may be obtained by heating ] to 400&nbsp;°C.
Its most commonly encountered ], γ-UO<sub>3</sub>, is a yellow-orange powder.


'''Uranium trioxide (UO<sub>3</sub>)''', also called '''] oxide''', '''uranium(VI) oxide''', and '''uranic oxide''', is the hexavalent ] of ]. The solid may be obtained by heating ] to 400&nbsp;°C. Its most commonly encountered ] is amorphous UO<sub>3</sub>.
UO<sub>3</sub> gas is produced at high temperatures from ], which comprises 75% of the particulate combustion product of uranium burning in air. The partial pressure of UO<sub>3</sub>(g) is several dozen mbar well below uranium's burning temperature. Health impact assessments for ] munitions should take into account the presence of respiratory UO<sub>3</sub>.


== Production and use == == 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.


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


# U<sub>3</sub>O<sub>8</sub> 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|bibcode=1950JAChS..72.2172S }}</ref> Note that above 750&nbsp;°C even in 5 atm O<sub>2</sub> UO<sub>3</sub> decomposes into ].<ref name="wheeler">{{cite journal|vauthors=Wheeler VJ, Dell RM, Wait E|title= Uranium trioxide and the UO<sub>3</sub> 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>
* U<sub>3</sub>O<sub>8</sub> can be oxidized at 500°C with oxygen.{{ref|sheft}}
* ], (UO<sub>2</sub>(NO<sub>3</sub>)<sub>2</sub>·6H<sub>2</sub>O) can be heated to yield UO<sub>3</sub>. This occurs during the ]. Fuel rods are dissolved in ] to separate ] from ] and the fission products (the ] method). The pure uranyl nitrate is converted to solid UO<sub>3</sub> by heating at 400 °C. After reduction with hydrogen (with with other inert gas present) to ], the uranium can be used in new ] fuel rods. # ], UO<sub>2</sub>(NO<sub>3</sub>)<sub>2</sub>·6H<sub>2</sub>O can be heated to yield UO<sub>3</sub>. This occurs during the ]. Fuel rods are dissolved in ] to separate ] from ] and the fission products (the ] method). The pure uranyl nitrate is converted to solid UO<sub>3</sub> by heating at 400&nbsp;°C. After reduction with hydrogen (with other inert gas present) to ], the uranium can be used in new ] rods.
* ] or ] (Na<sub>2</sub>U<sub>2</sub>O<sub>7</sub>·6H<sub>2</sub>O) may be decomposed. ], also known as ], is converted to uranium trioxide in the ]. ] and ] are intermediates in the process which ends in ]. {{ref|dell}} # ] or ] (Na<sub>2</sub>U<sub>2</sub>O<sub>7</sub>·6H<sub>2</sub>O) may be decomposed. ], also known as ], is converted to uranium trioxide in the ]. ] and ] are intermediates in the process which ends in ].<ref>{{cite journal|vauthors=Dell RM, Wheeler VJ |title= Chemical Reactivity of Uranium Trioxide Part 1. — Conversion to U<sub>3</sub>O<sub>8</sub>, UO<sub>2</sub> and UF<sub>4</sub>|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. Uranium trioxide is shipped between processing facilities in the form of a gel, most often from ] to conversion plants.


], which operates at the world's largest uranium refinery at ], produces high-purity uranium trioxide. ], which operates at the world's largest uranium refinery at ], produces high-purity uranium trioxide.


It has been reported that the corrosion of uranium in a silica rich aqueous solution forms ], uranium trioxide,<ref>Trueman ER, Black S, Read D, Hodson ME (2003) "Alteration of Depleted Uranium Metal" ''Goldschmidt Conference Abstracts,'' p. A493 </ref> and ].<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, ] (UO<sub>2</sub>)<sub>8</sub>O<sub>2</sub>(OH)<sub>12</sub>·12(H<sub>2</sub>O) is formed<ref>. Webmineral.com. Retrieved on 2011-07-19.</ref> in the first week and then after four months ] (UO<sub>2</sub>)O<sub>2</sub>·4(H<sub>2</sub>O) was produced. This alteration of uranium oxide also leads to the formation of ],<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 ].<ref>Ander L, Smith B (2002) "" ''The health hazards of depleted uranium munitions, part II'' (London: The Royal Society)</ref><ref>Smith B (2002) "" ''The health hazards of depleted uranium munitions, part II'' (London: The Royal Society)</ref>
==Health and safety hazards==
Like all hexavalent uranium compounds (also called uranium(VI) compounds), UO<sub>3</sub> 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 ]s and ] leading to ] if inhaled. {{ref|morrow}}{{ref|sutton}}


== Chemistry and structure == == Health and safety hazards ==
Like all hexavalent uranium compounds, UO<sub>3</sub> 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 ]s and ] leading to ] 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}} </ref><ref>{{cite journal|vauthors=Sutton M, Burastero SR|title=Uranium(VI) solubility and speciation in simulated elemental human biological fluids|journal=]|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 ]s and may severely affect their function.
=== Solid state ===


== Structure ==
]


=== Solid state structure ===
]
The only well characterized binary trioxide of any ] is UO<sub>3</sub>, of which several ] are known. Solid UO<sub>3</sub> loses O<sub>2</sub> on heating to give green-colored ]: reports of the decomposition temperature in air vary from 200 to 650&nbsp;°C. Heating at 700&nbsp;°C under H<sub>2</sub> gives dark brown ] (UO<sub>2</sub>), which is used in ] ] rods.


==== Alpha ====
The only well characterized binary trioxide of any ] is UO<sub>3</sub>, of which several ]s are known. Solid UO<sub>3</sub> loses O<sub>2</sub> on heating to give green-colored ]: reports of the decomposition temperature in air vary from 200&ndash;650&nbsp;&deg;C. Heating at 700&nbsp;°C under H<sub>2</sub> gives dark brown ] (UO<sub>2</sub>), which is used in ] ] rods.
{| border="1" cellpadding="3" cellspacing="0"
| ]
| ''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 ] to an aqueous solution of ] 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 ] change from the ] 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 ====
Uranium trioxide reacts at 400 °C with ] to form ], ], ] and uranium(IV) fluoride. The freon-12 can be replaced with ] which forms ] instead of carbon dioxide. This is a case of a hard perhalogenated ] which is normally considered to be inert being converted chemically at a moderate temperature.{{ref|booth}}
{| border="1" cellpadding="3" cellspacing="0"
| ]
| ''The β (beta) UO<sub>3</sub> 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 UO<sub>3</sub>-H<sub>2</sub>O-NH<sub>3</sub> system that they could all be considered as being UO<sub>2</sub>(OH)<sub>2</sub>·H<sub>2</sub>O 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 NH<sub>3</sub>-UO<sub>3</sub>-H<sub>2</sub>O|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 ] 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-UO<sub>3</sub> 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 ====
2 CF<sub>2<sub>Cl<sub>2<sub> + UO<sub>3<sub> &rarr; UF<sub>4<sub> + CO<sub>2<sub> + COCl<sub>2<sub> + Cl<sub>2<sub>
{| border="1" cellpadding="3" cellspacing="0"
| ]
| ''The γ (gamma) form: with the different uranium environments in green and yellow''
| The most frequently encountered polymorph is γ-UO<sub>3</sub>, whose ] has been solved from powder diffraction data. The compound crystallizes in the space group ''I4<sub>1</sub>/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 <sup>2+</sup><sup>2−&nbsp;</sup>, that is uranyl uranate.<ref>{{cite journal|vauthors=Engmann R, de Wolff PM|title=The Crystal Structure of γ-UO<sub>3</sub>|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"
| ]
| ''The environment of the uranium atoms shown as yellow in the gamma form''
| ]
| ''The chains of U<sub>2</sub>O<sub>2</sub> 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 ] ]-] bonds.''
|}


==== Delta ====
4 CF<sub>2<sub>Cl<sub>2<sub> + UO<sub>3<sub> &rarr; UF<sub>4<sub> + 3COCl<sub>2<sub> + CCl<sub>4<sub> + Cl<sub>2<sub>
{| border="1" cellpadding="3" cellspacing="0"
| ]
| ''The delta (δ) form is a ] 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<sub>3></sub> |journal=Polyhedron|volume=7|issue=3|pages=243–244|doi=10.1016/S0277-5387(00)80559-8}}</ref>
|}


==== Epsilon ====
Uranium trioxide can be dissolved in a mixture of ] and ] in ], ultrasound was employed during the dissolution.{{ref|trofimov}}
{| border="1" cellpadding="3" cellspacing="0"

|
{{CleanBr}}
]
| ''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 ] ''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 ====
The most frequently encountered polymorph is γ-UO<sub>3</sub>, whose ] has been solved from powder diffraction data. The compound crystallizes in the space group ''I4<sub>1</sub>/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 <sup>2+</sup><sup>2-&nbsp;</sup>, that is uranyl uranate. {{ref|engmann}}.
There is a high-pressure solid form with U<sub>2</sub>O<sub>2</sub> and U<sub>3</sub>O<sub>3</sub> rings in it.<ref>{{cite journal|vauthors=Siegel S, Hoekstra HR, Sherry E |year=1966 |title=The crystal structure of high-pressure UO<sub>3</sub>|journal=Acta Crystallographica|volume=20|issue=2|pages=292–295 |doi=10.1107/S0365110X66000562|bibcode=1966AcCry..20..292S }}</ref>

High presure solid forms exist. ''Gmelin Handbuch'' (1982) '''U-C1,''' 129-135. <ref>''Gmelin Handbuch'' (1982) '''U-C1,''' 129–135.</ref>


==== Hydrates ====
<gallery> <gallery>
Uranium Trioxides.jpg|Hydrous and anhydrous forms of UO<sub>3</sub>
Image:UO3 gamma lattice.jpg|The γ (gamma) form, with the different uranium environments in green and yellow
UO3 Anhydrous.jpg|Anhydrous forms of UO<sub>3</sub>
Image:UO3 gamma env1.jpg|The environment of the uranium atoms shown as yellow in the gamma form
Image:UO3 gamma rings.jpg|The chains of U<sub>2</sub>O<sub>2</sub> 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 ] ]-] bonds.
Image:UO3lattice.jpg|The delta (δ) form was reported by Weller ''et al.'' (1988) ''Polyhedron,'' '''7,''' 243-244.
</gallery> </gallery>


Several ] of uranium trioxide are known, e.g., UO<sub>3</sub>·6H<sub>2</sub>O, which are commonly known as "uranic acid" in older literature due to their similarity in formula to various metal ], although they are not in fact particularly acidic.<ref name="wheeler" />
{{CleanBr}}


==== bond valence parameters ==== === 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 ====
It is possible by ] calculations it is possible to estimate how great a contribution a given oxygen atom is making to the assumed valence of uranium. Zachariasen, ''J. Less Common Met.'', 1978, '''62''', 1-7. Lists the parameters to allow such calculations to be done for many of the actinides.
At elevated temperatures gaseous UO<sub>3</sub> is in ] with solid ] and molecular ].


::2 U<sub>3</sub>O<sub>8</sub>(s) + O<sub>2</sub>(g) {{eqm}} 6 UO<sub>3</sub>(g)
The formula to use is


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 UO<sub>3</sub> in equilibrium with air and solid U<sub>3</sub>O<sub>8</sub> at ambient pressure, about 10<sup>−5</sup>&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>
<center><math>s = e^{-(R-Ro)/B}</math></center>


==== Matrix isolation ====
The sum of the ''s'' values is equal to the oxidation state of the metal centre.
Infrared spectroscopy of molecular UO<sub>3</sub> isolated in an argon matrix indicates a T-shaped structure (] ''C<sub>2v</sub>'') for the molecule. This is in contrast to the commonly encountered ''D<sub>3h</sub>'' ] exhibited by most trioxides. From the force constants the authors deduct the U-O bond lengths to be between 1.76 and 1.79 ] (176 to 179 ]).<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 UO<sub>3</sub>|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 ====
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.
]
Calculations predict that the point group of molecular UO<sub>3</sub> is ''C<sub>2v</sub>'', 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<sub>3h</sub>'' species is a saddle point, 49 kJ/mol above the ''C<sub>2v</sub>'' minimum. The authors invoke a second-order ] 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 ===
{|align=center
The crystal structure of a uranium trioxide phase of composition UO<sub>2·82</sub> 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 ReO<sub>3</sub>. 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>
!Oxidation state!!Ro!!B
|-
|U(VI)||2.08Å||0.35
|-
|U(V)||2.10Å||0.35
|-
|U(IV)||2.13Å||0.35
|}


== Reactivity ==
It is possible to do these calculations on paper or software which does it can be obtained free of charge.
Uranium trioxide reacts at 400&nbsp;°C with ] to form ], ], ] and ]. The freon-12 can be replaced with ] which forms ] instead of carbon dioxide. This is a case of a hard perhalogenated ] 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=]|year=1946|volume=68|issue=10|pages=1969–1970|doi=10.1021/ja01214a028|bibcode=1946JAChS..68.1969B }}</ref>


:2 CF<sub>2</sub>Cl<sub>2</sub> + UO<sub>3</sub> → UF<sub>4</sub> + CO<sub>2</sub> + COCl<sub>2</sub> + Cl<sub>2</sub>
=== Gas phase ===
Infrared spectroscopy of molecular UO<sub>3</sub> isolated in an argon matrix indicates a T-shaped structure (] ''C<sub>2v</sub>'') for the molecule. This is in contrast to the commonly encountered ''D<sub>3h</sub>'' 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 ]s (176 to 179 ]s). {{ref|gabelnick}}


At elevated temperatures gaseous UO<sub>3</sub> and ] are in ] with solid ]. {{ref|ackermann-1}} :4 CFCl<sub>3</sub> + UO<sub>3</sub> UF<sub>4</sub> + 3 COCl<sub>2</sub> + CCl<sub>4</sub> + Cl<sub>2</sub>


Uranium trioxide can be dissolved in a mixture of ] and ] in ], 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>
::<sup>1</sup>/<sub>3</sub> U<sub>3</sub>O<sub>8</sub>(s) + <sup>1</sup>/<sub>6</sub> O<sub>2</sub>(g) <math>\overrightarrow{\gets}</math> UO<sub>3</sub>(g)


=== Electrochemical modification ===
With increasing temperature the equilibrium is shifted to the right. This system has been studied at temperatures between 900 and 1500&nbsp;°C. The vapor pressure of monomeric UO<sub>3</sub> is low but appreciable, about 10<sup>&minus;5</sup>&nbsp;mbar (1 mPa) at 980&nbsp;°C, rising to 10<sup>&minus;1</sup>&nbsp;mbar (10 Pa) at 1400&nbsp;°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. {{ref|ackermann}} {{ref|alexander}} The burning temperature of uranium in air usually exceeds 2500 K. {{ref|mouradian}}
The reversible insertion of ] cations into the ] of uranium trioxide by ] using a ] electrode modified with microscopic particles of the uranium oxide has been investigated. This experiment has also been done for U<sub>3</sub>O<sub>8</sub>. This is an example of ] of a solid modified ], the experiment which used for uranium trioxide is related to a ] experiment. It is also possible to reduce uranium trioxide with ] 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 ]<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 αUO<sub>3</sub> and U<sub>3</sub>O<sub>8</sub> |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 UO<sub>3</sub>|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 ] ] work. In these rechargeable cells one of the electrodes is a metal oxide which contains a metal such as ] 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.
Aerial oxidation of any uranium compound eventually results in the formation of a uranyl compound, and uranium trioxide is the only uranyl oxide. {{ref|cotton}}


=== Amphoterism and reactivity to form related uranium(VI) anions and cations ===
==Uranium oxides in ceramics==
Uranium oxide is ] and reacts as ] and as a ], depending on the conditions.
UO<sub>3</sub>-based ceramics become green or black when fired in a reducing atmosphere and yellow to orange when fired with oxygen. Orange-coloured ] is a well-known example of a product with a uranium-based glaze. UO<sub>3</sub>-has also been used in formulations of ], ], and ].


====As an acid====
Prior to 1960, UO<sub>3</sub> was used as an agent of crystallization in crystalline coloured glazes. It is possible to determine with a ] if a glaze or glass was made from UO<sub>3</sub>.

== Related anions and cations ==
Uranium oxide is ] and reacts as ] and as a ], depending on the conditions.


:UO<sub>3</sub> + H<sub>2</sub>O → {{chem|UO|4|2−}} + 2 H<sup>+</sup>
As an acid:


Dissolving uranium oxide in a strong ] like ] forms the doubly negatively charged ] ] ({{chem|UO|4|2−}}). Uranates tend to concatenate, forming ], {{chem|U|2|O|7|2−}}, or other poly-uranates.
:UO<sub>3</sub> + H<sub>2</sub>O &rarr; UO<sub>4</sub><sup>2−</sup> + H<sup>+</sup>
Important diuranates include ] ((NH<sub>4</sub>)<sub>2</sub>U<sub>2</sub>O<sub>7</sub>), ] (Na<sub>2</sub>U<sub>2</sub>O<sub>7</sub>) and
] (MgU<sub>2</sub>O<sub>7</sub>), which forms part of some ]s. It is worth noting that uranates of the form M<sub>2</sub>UO<sub>4</sub> do ''not'' contain {{chem|UO|4|2−}} ions, but rather flattened UO<sub>6</sub> 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 ] like ] forms the doubly negatively charged
] ] (UO<sub>4</sub><sup>2−</sup>). Uranates tend to agglomerate, forming ],
U<sub>2</sub>O<sub>7</sub><sup>2−</sup,> or other poly-uranates.
Important diuranates include ] ((NH<sub>4<sub>)<sub>2</sub>U<sub>2<sub>O<sub>7<sub>), ] (Na<sub>2</sub>U<sub>2</sub>O<sub>7</sub>) and
] (MgU<sub>2</sub>O<sub>7</sub>), which forms part of some ]s.


:UO<sub>3</sub> + H<sub>2</sub>O → {{chem|UO|2|2+}} + 2 OH<sup>−</sup>
As a base:
Dissolving uranium oxide in a strong acid like ] or ] forms the double positive charged ] ]. The ] formed (UO<sub>2</sub>(NO<sub>3</sub>)<sub>2</sub>·6H<sub>2</sub>O) is soluble in ]s, ], ]s and ]s; for example, ]. This solubility is used to separate uranium from other elements in ], which begins with the dissolution of ] rods in ] to form this salt. The ] is then converted to uranium trioxide by heating.

:UO<sub>3</sub> + H<sub>2</sub>O &rarr; UO<sub>2</sub><sup>2+</sup> + OH<sup>−</sup>
Dissolving uranium oxide in a strong acid like ] or ] forms the double positive charged ] ]. The ] formed (UO<sub>2</sub>(NO<sub>3</sub>)<sub>2</sub>ˑ6H<sub>2</sub>O) is soluble in ]s, ]s, ]s and ]s; for example, ]. This solubilty is used to separate uranium from other elements in ], which begins with the dissolution of ] rods in ]. The ] is then converted to ] by heating.


From ] one obtains ], ''trans''-UO<sub>2</sub>(NO<sub>3</sub>)<sub>2</sub>·2H<sub>2</sub>O, consisting of eight-coordinated uranium with two ] nitrato ligands and two water ligands as well as the familiar O=U=O core. From ] one obtains ], ''trans''-UO<sub>2</sub>(NO<sub>3</sub>)<sub>2</sub>·2H<sub>2</sub>O, consisting of eight-coordinated uranium with two ] nitrato ligands and two water ligands as well as the familiar O=U=O core.


== Uranium oxides in ceramics ==
UO<sub>3</sub>-based ceramics become green or black when fired in a reducing atmosphere and yellow to orange when fired with oxygen. Orange-coloured ] is a well-known example of a product with a uranium-based glaze. UO<sub>3</sub>-has also been used in formulations of ], ], and ].


Prior to 1960, UO<sub>3</sub> was used as an agent of crystallization in crystalline coloured glazes. It is possible to determine with a ] if a glaze or glass was made from UO<sub>3</sub>.


== References == == References ==
{{reflist|30em}}

{{Uranium compounds}}
{{Oxides}}


]
*{{note|sheft }}{{cite journal | author= Sheft I, Fried S, Davidson N | title= Preparation of Uranium Trioxide | journal= ] | year= 1950 | volume= 72 | pages= 2172-2173 }} ]
]
*{{note|dell}}{{cite journal | author= Dell RM, Wheeler V J | title= Chemical Reactivity of Uranium Trioxide Part 1.conversion to U<sub>3</sub>O<sub>8</sub>, U0<sub>2</sub> and UF<sub>4</sub>| journal= Transaction Faraday Society | year= 1962 | volume= | pages= 1590-1607}}
]
*{{note|morrow}}{{cite journal | author= Morrow, PE, Gibb FR, Beiter HD| title= Inhalation studies of uranium trioxide | journal=Health Physics | year= 1972 | volume=23 | pages=273-280}}
*{{note|sutton}}{{cite journal | author= 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}}
*{{note|trofimov}}{{cite journal | author= 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| pages= 129-130}}
*{{note|booth}}{{cite journal | author= Booth HS, Krasny-Ergen W,Heath RE| title= Uranium Tetrafluoride | journal= ] | year= 1946| volume= 68| pages= 1969-1970}}
*{{note|engmann}}{{cite journal | author= Engmann R, de Wolff PM| title= The Crystal Structure of γ-UO<sub>3</sub> | journal=Acta Crystallographica | year= 1963 | volume=16 | pages=993}}
*{{note|wilson}}{{cite journal | author= Wilson WB| title= High-Pressure High-Temperature Investigation of the Uranium-Oxygen System | journal=Journal Inorganic Nuclear Chemistry | year= 1961 | volume=19 | pages=212-222}}
*{{note|gabelnick}} {{cite journal | author= Gabelnick SD, Reedy GT, Chasanov MG| title= Infrared spectra of matrix-isolated uranium oxide species. II: Spectral interpretation and structure of UO<sub>3</sub>| journal= | year= 1973 | volume= 59 | pages=6397}}
*{{note|ackermann-1}}{{cite journal | author= Ackermann RJ, Thorn RJ, Alexander C, Tetenbaum M | title= Free Energies of Formation of Gaseous Uranium, Molybdenum, and Tungsten Trioxides | journal=Journal of Physical Chemistry | year= 1960 | volume=64 | pages=350-355}}
*{{note|ackemann}}{{cite journal | author= Ackermann RJ, Gilles PW, Thorn RJ | title= | journal= Journal of Chemical Physics | year= 1956 | volume= 25| pages= 1089 }}
*{{note|alexander }}{{cite journal | author= Alexander CA | title= Volatilization of urania under strongly oxidizing conditions | journal= Journal of Nuclear Materials | year= 2005 | volume= 346 | pages= 312-318 }}
*{{note|mouradian}}{{cite journal | author=Mouradian and Baker | title= Burning Temperatures of Uranium and Zirconium in Air| journal= Nuclear Science and Engineering | year= 1963 | volume= 15| pages=388-394}}
*{{note|gilchrist}}{{cite journal | author= Gilchrist RL, Glissmyer JA, Mishima J| title= Characterization of Airborne Uranium from Test Firings of XM774 Ammunition," ''Technical report no. PNL-2944 Richland, WA: Battelle Pacific Northwest Laboratory | journal= | year= 1979| volume= | pages= }}
*{{note|cotton}}{{cite book | author= Cotton, S| year = 1991| title = Lanthanides and Actinides | edition = | publisher = | location = New York| id = | page 128}}
*{{note|stuart}}{{cite journal | author= Stuart | title= Solubility and Hemolytic Activity of Uranium Trioxide | journal= Environmental Research | year= 1979| volume= 18| pages= 385-396}}
*{{note|römpps}}{{cite book | author= Neumüller O-A | year = 1988| title = Römpps Chemie-Lexikon| edition = 6| publisher = Frankh'sche Verlagshandlung | location = Stuttgart| id = ISBN 3-440-04516-1 }}
*{{note|gmelin-en}}{{cite book | author= | year = 1977| title = Gmelin Handbook of Inorganic Chemistry | edition = 8| publisher = | location = | id = | page 98}}
*{{note|gmelin}}{{cite book | author= | year = 1977| title = Gmelin Handbuch der anorganischen Chemie | edition = 8| publisher = | location = | id = |page 118-120}}
*{{note|green-2 }}{{cite journal | author= Green, DW | title= Relationship between spectroscopic data and thermodynamic functions; application to uranium, plutonium, and thorium oxide vapor species | journal= Journal of Nuclear Materials | year= 1980 | volume= 88 | pages= 51-63 }}
* Salbu, B. ''et al.'' (2005) ''Journal of Environmental Radioactivity,'' '''78,''' 125–135.
*{{cite journal | author= Nakajima K, Arai Y | title = Mass-spectrometric investigation of) UO<sub>3</sub> (g) | journal=J Nuclear Mater | year= 2001 | volume=294 | pages=250-255}}
*{{cite journal | author= Ackermann RJ, Gilles PW, Thorn RJ | title= High-Temperature Thermodynamic Properties of Uranium Dioxide| journal= J Chem Phys | year= 1956 | volume= 25| pages= 1089-1097 }}
*{{cite journal | author= Berkowitz J, Inghram MG, Chupka WA . | title= Polymeric Gaseous Species in the Sublimation of Molybdenum Trioxide | journal= J Chem Phys | year= 1957 | volume= 26| pages= 842 }}
*{{cite book | author= Ackermann RJ, Chang AT | year = 1973| title = Thermodynamic Characterization of U<sub>3</sub>O<sub>8-z</sub> Phase | edition = 5| publisher = | location = | id = | page 873-890}}
*{{cite journal | author=Chapman AT, Meadows RE | title=Volatility of UO<sub>2+/-x</sub> and Phase Relations in the System Uranium Oxygen | journal=J Am Ceramic Soc | year=1964| volume=47 | pages= 614-621}}
*{{cite journal | author= Drowart J, Pattoret A, Smoes S| title= Heat of sublimation of uranium and consistency of thermodynamic data for uranium compounds | journal=J Nuclear Materials | year=1964 | volume=12 | pages=319-322 }}
*{{cite journal | author= Hoekstra HR, Siegel | title= Uranium-Oxygen System at high pressure | journal=J Inorg Nuclear Chem | year= 1970 | volume=32 | pages=3237-3248}}
*{{cite journal | author= Hutchings, G. J. | title= Uranium-Oxide-Based Catalysts for the Destruction of Volatile Chloro-Organic compounds | journal= Nature | year= 1996 | volume= 384| pages= 341-343 }}
*{{cite journal | author= Chatillion C, Defoort F, Froment K| title= Mass spectrometric critical assessment of thermodynamic data for UO<sub>3</sub> (g) | journal=J Phys Chem Solid | year= 2005 | volume=66 | pages=379-382}}
*{{cite journal | author= Roberts LEJ, Walter AJ| title= Equilibrium Pressures and Phase Relations in the Uranium Oxide System | journal= Journal of Inorganic and Nuclear Chemistry | year= 1961 | volume=22 | pages=213-229}}
*{{cite journal | author= Rauh EG, Ackermann RJ | title=First ionization potentials of some refractory oxide vapors | journal= J Chem Phys| year= 1974 | volume= 60| pages=1396}}
*{{cite journal | author= Cort B | title= Infrared Characterization of Uranium Oxide Powders Using a Metal Light Pipe| journal= Applied Spectroscopy| year= 1987| volume=41 | pages=493-495}}
*{{cite journal | author= Chazel V | title=Effect of U<sub>3</sub>O<sub>8</sub> specific surface area on in vitro dissolution, biokinetics, and dose coefficients | journal=Radiation Protection Dosimetry | year= 1998 | volume=79 | pages=39-42}}
*{{cite journal | author= Ansoborlo E | title= Exposure implications for uranium aerosols formed at a new laser enrichment facility: application of the ICRP respiratory tract and systemic model| journal= Radiation Protection Dosimetry| year=1998 | volume= 79| pages=23-27}} Abstract: "... urine assay could be useful, provided that measurements are made soon after a known acute intake."
*{{cite journal | author= Stradling GN | title= Treatment for Actinide-bearing Industrial Dusts and Aerosols| journal= Radiation Protection Dosimetry| year= 2000 | volume= 87| pages=41-50}}
*{{cite journal | author= Chazel V | title= Variation of solubility, biokinetics and dose coefficient of industrial uranium oxides according to specific surface area| journal=Radiation Protection Dosimetry | year= 2000 | volume= 88| pages=223-231}}
*{{cite journal | author= Ansoborlo E | title= Determination of the physical and chemical properties, biokinetics, and dose coefficients of uranium compounds handled during nuclear fuel fabrication in France| journal= Health Physics| year= 2002 | volume= 82| pages=279-289}}
*{{cite journal | author= Stradling N | title= Optimising monitoring regimens for inhaled uranium oxides | journal= Radiation Protection Dosimetry| year= 2003 | volume=105 | pages=109-114}}
*{{cite journal | author= Stradling N | title= Anomalies between radiological and chemical limits for uranium after inhalation by workers and the public | journal= Radiation Protection Dosimetry| year= 2003 | volume=105 | pages=175-178}}
*{{cite journal | author= Khaskelis, A.I | title= Uranium oxide weathering: spectroscopy and kinetics| journal=Transactions of the American Nuclear Society | year= 2004 | volume= 91| pages=890-891}} Abstract: "During nuclear fuel fabrication or reprocessing stages of a nuclear fuel cycle, it is possible for small particles of uranium oxides to escape into the environment. This paper reports measurements of rates of oxidation of uranium dioxide particles in controlled gas environments using in-situ phosphorescence spectroscopy. Comparison is made with reduction and reoxidation of uranium trioxide...." From
*{{cite journal | author= Schuenemana RA, Khaskelisa AI, Eastwooda D, van Ooijb WJ, Burggraf LW | title= Uranium oxide weathering: spectroscopy and kinetics| journal=Journal of Nuclear Materials | year= 2003 | volume= 323| pages=8-17}}
*{{cite journal | author= Chatillon, C | title= Mass spectrometric critical assessment of thermodynamic data for UO<sub>3</sub>(g)| journal= Journal of Physics and Chemistry of Solids| year=2005 | volume=66 | pages=379-383}}
*{{cite journal | author= Busby C, Morgan S | title= Did the use of Uranium weapons in Gulf War 2 result in contamination of Europe?| journal= European Biology and Bioelectromagnetics| year= | volume= 1(5)| pages= 650-668}}
*{{cite journal | author= Hoekstra HR, Siegel S| title=Recent Developments in the Chemistry of the Uranium-Oxygen System | journal= Proceedings of the Second International Conference on Peaceful Uses of Atomic Energy (Geneva: UN)| year= 1958 | volume= 7| pages=394-400}}
* H. Wanner and I. Forest, eds. (2004) '''' (Paris: ] and French Nuclear Energy Agency)
* by Edouard Bastarache
* from the ]; note that UO<sub>3</sub> is moderately soluble (Morrow, 1972.)
* from ]

Latest revision as of 09:57, 30 November 2024

Uranium trioxide
Names
IUPAC names Uranium trioxide
Uranium(VI) oxide
Other names Uranyl oxide
Uranic oxide
Identifiers
CAS Number
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.014.274 Edit this at Wikidata
EC Number
  • 215-701-9
PubChem CID
UNII
CompTox Dashboard (EPA)
InChI
  • InChI=1S/3O.UKey: JCMLRUNDSXARRW-UHFFFAOYSA-N
SMILES
  • O=(=O)=O
Properties
Chemical formula UO3
Molar mass 286.29 g/mol
Appearance yellow-orange powder
Density 5.5–8.7 g/cm
Melting point ~200–650 °C (decomposes)
Solubility in water insoluble
Structure
Crystal structure see text
Space group I41/amd (γ-UO3)
Thermochemistry
Std molar
entropy
(S298)
99 J·mol·K
Std enthalpy of
formation
fH298)
−1230 kJ·mol
Hazards
GHS labelling:
Pictograms GHS06: ToxicGHS08: Health hazardGHS09: Environmental hazard
Signal word Danger
Hazard statements H300, H330, H373, H411
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 4: Very short exposure could cause death or major residual injury. E.g. VX gasFlammability 0: Will not burn. E.g. waterInstability 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g. calciumSpecial hazard OX: Oxidizer. E.g. potassium perchlorate
4 0 1OX
Flash point Non-flammable
Safety data sheet (SDS) External MSDS
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 , 100 kPa). checkverify (what is  ?) Infobox references
Chemical compound

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 is amorphous 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 enrichment.

Methods of forming uranium trioxide

  1. U3O8 can be oxidized at 500 °C with oxygen. Note that above 750 °C even in 5 atm O2 UO3 decomposes into U3O8.
  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.

Uranium trioxide is shipped between processing facilities in the form of a gel, most often from mines to conversion plants.

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, 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.

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 to 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. 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.

Beta

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. It was found that calcination at 500 °C in air forms the beta form of uranium trioxide. Later experiments found the most reliable method for synthesizing pure β-UO3 was to calcinate uranyl nitrate hexahydrate at 450 °C for 6 days and cool slowly over 24 hours.

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 , 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 axial oxygen-uranium bonds.

Delta

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

Epsilon

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 P-1 space group.

High pressure form

There is a high-pressure solid form with U2O2 and U3O3 rings in it.

Hydrates

  • Hydrous and anhydrous forms of UO3 Hydrous and anhydrous forms of UO3
  • Anhydrous forms of UO3 Anhydrous forms of UO3

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.

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 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.

Matrix isolation

Infrared spectroscopy of molecular UO3 isolated in an argon matrix indicates a T-shaped structure (point group C2v) for the molecule. This is in contrast to the commonly encountered D3h 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).

Computational study

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.

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.

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.

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.

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 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 → UO
4 + 2 H

Dissolving uranium oxide in a strong base like sodium hydroxide forms the doubly negatively charged uranate anion (UO
4). Uranates tend to concatenate, forming diuranate, U
2O
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 UO
4 ions, but rather flattened UO6 octahedra, containing a uranyl group and bridging oxygens.

As a base

UO3 + H2O → UO
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 to form this salt. 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.

References

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Uranium compounds
U(II)
U(III)
Organouranium(III) compounds
  • U(C5H5)3
  • U(IV)
    Organouranium(IV) compounds
  • U(C8H8)2
  • U(C5H5)4
  • U(C5H5)3Cl
  • U(IV,V)
    U(IV,VI)
    U(V)
    U(VI)
    U(XII)
    • UO6 (hypothetical)
    Oxides
    Mixed oxidation states
    +1 oxidation state
    +2 oxidation state
    +3 oxidation state
    +4 oxidation state
    +5 oxidation state
    +6 oxidation state
    +7 oxidation state
    +8 oxidation state
    Related
    Oxides are sorted by oxidation state. Category:Oxides
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