View source for Chemistry:Amorphous uranium(VI) oxide

{{Chembox
| Name           = Amorphous uranium(VI) oxide<ref>L. Sweet, C. Henager, S. Hu, T. Johnson, D. Meier, S. Peper and J. Schwantes, PNNL-20951: Investigation of Uranium Polymorphs, PNNL for U.S Department of Energy, 2011. {{doi|10.13140/RG.2.1.3073.0004}}</ref>
| ImageFile      = U2O7 Model.jpg
| OtherNames     = Amorphous UO<sub>3</sub>
| IUPACName      = Diuranyl heptoxide
| SystematicName = 
| Section1       = {{Chembox Properties
| Formula = Am-U<sub>2</sub>O<sub>7</sub>
| MolarMass = 588 g/mol
| Density = 6.8 g/cm<sup>3</sup>
| Appearance = Orange-brown powder
| Solubility = Partially soluble
}}
| Section2       = {{Chembox Related
| OtherAnions = 
| OtherFunction = [[Chemistry:Uranyl peroxide|Uranyl peroxide]]<br/>[[Chemistry:Triuranium octoxide|Triuranium octoxide]]
| OtherFunction_label = [[Chemistry:Uranium|uranium]] [[Chemistry:Oxide|oxide]]s
| OtherCompounds =
}}
| Section3       = 
| Section4       = 
| Section5       = 
| Section6       = 
}}

'''Amorphous uranium(VI) oxide''' '''(''am''-U<sub>2</sub>O<sub>7</sub>)''' is an orange diuranyl compound, most commonly obtained from the thermal decomposition of [[Chemistry:Uranyl peroxide|uranyl peroxide tetrahydrate]] at temperatures between {{convert|150|and|500|C|abbr=on|sigfig=2}}. It exists at room temperature as a powder. ''Am''-U<sub>2</sub>O<sub>7</sub> does not comprise a regular, long-range atomic structure, as demonstrated by its characteristic diffuse scattering pattern obtained by X-ray diffraction. As a result, the molecular structure of this material is little understood, although experimental and computational attempts to elucidate a local atomic environment have yielded some success.<ref name=":0">S. Odoh, J. Shamblin, C. Colla, S. Hickam, H. Lobeck, R. Lopez, T. Olds, J. Szymanowski, G. Sigmon, J. Neuefeind, W. Casey, M. Lang, L. Gagliardi and P. Burns, Structure and Reactivity of X-ray Amorphous Uranyl Peroxide, U<sub>2</sub>O<sub>7</sub>, J. Inorg. Chem., 2016, 55, pp.3541-3546. {{doi|10.1021/acs.inorgchem.6b00017}}</ref><ref name=":1">A. Shields, A. Miskowiec, J. Niedziela, M. Kirkegaard, K. Maheshwari, M. Ambrogio, R. Kapsimalis and B. Anderson, Shining a light on amorphous U<sub>2</sub>O<sub>7</sub>: A computational approach to understanding amorphous uranium materials, Opt. Mater., 2019, 89, 295-298. {{doi|10.1016/j.optmat.2019.01.040}}</ref>

== Production ==
''Am''-U<sub>2</sub>O<sub>7</sub> is produced by the thermal decomposition of uranyl peroxide tetrahydrate at temperatures between {{convert|150|and|500|C|abbr=on|sigfig=2}}, in either an air or nitrogen atmosphere. The resultant powder is tan orange in colour. Further heating results in the formation of [[Chemistry:Uranium trioxide|alpha uranium trioxide (α-UO<sub>3</sub>)]].

== Structure and reactivity ==
Due to the amorphous nature of a''m''-U<sub>2</sub>O<sub>7</sub>, the long-range atomic structure of this compound has not been determined. However, recent computational investigations, chiefly accomplished using [[Density functional theory|density functional theory (DFT)]], have helped to predict a local structure.<ref name=":0" /><ref name=":2">{{Cite journal|last1=Thompson|first1=Nathan B. A.|last2=Middleburgh|first2=Simon C.|last3=Evitts|first3=Lee J.|last4=Gilbert|first4=Matthew R.|last5=Stennett|first5=Martin C.|last6=Hyatt|first6=Neil C.|date=2020-12-15|title=Short communication on further elucidating the structure of amorphous U<sub>2</sub>O<sub>7</sub> by extended X-ray absorption spectroscopy and DFT simulations|url=https://research.bangor.ac.uk/portal/files/36046880/U2O7_EXAFS_DFT_Final_Update.pdf|journal=Journal of Nuclear Materials|language=en|volume=542|pages=152476|doi=10.1016/j.jnucmat.2020.152476|bibcode=2020JNuM..54252476T |s2cid=225232737 |issn=0022-3115}}</ref> Resembling a regular uranate compound, two [[Chemistry:Uranyl|uranyl ([UO<sub>2</sub><nowiki>]</nowiki><sup>2+</sup>)]] groups are bridged by a μ<sub>2</sub>-O atom, where both uranium atoms are bonded to a [[Chemistry:Peroxide|O-O peroxo]] unit. In this case, a tetrameric ring would be the most stable conformation of the compound. The presence of a peroxide bond in species obtained in this temperature range is unusual; uranyl peroxide has previously been considered to be the only peroxide bearing uranium compound.<ref>P. Burns and K. Hughes, Studtite, [(UO<sub>2</sub>)(O<sub>2</sub>)(H<sub>2</sub>O)<sub>2</sub>](H<sub>2</sub>O)<sub>2</sub>: The first structure of a peroxide mineral, Am. Mineral., 2003, 88, 1165-1168. {{doi|10.2138/am-2003-0725}}</ref> Developments on this structure propose a two-site metastudtite and UO<sub>3</sub>-like bonding environment, including the bond types already mentioned.<ref name=":2" /> Few other suggestions for the local atomic structure of a''m''-U<sub>2</sub>O<sub>7</sub> have been made, although a crystalline form of U<sub>2</sub>O<sub>7</sub>, calculated as a two-site 6 and 8-coordinate structure, has been reported.<ref name=":1" /> In the same study, it was again found that the U<sub>2</sub>O<sub>7</sub> species contained peroxide bonding. ''Am''-U<sub>2</sub>O<sub>7</sub> is known to undergo hydrolysis in the presence of water, to produce a crystalline [[Chemistry:Schoepite|metaschoepite]] powder. In addition to a change in crystallinity, this reaction involves a change in colour from orange to bright yellow.

== References ==
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[[Category:Uranium(VI) compounds]]

{{Sourceattribution|Amorphous uranium(VI) oxide}}