Amorphous uranium(VI) oxide
{{Chembox
| ImageFile = U2O7 Model.jpg
| OtherNames = Amorphous UO3
Amorphous uranyl peroxide
| IUPACName = Diuranyl heptoxide
| SystematicName =
| Section1 = {{Chembox Properties
| Formula = Am-U2O7
| MolarMass = 588 g/mol
| Density = 6.8 g/cm3
| Appearance = Orange-brown powder
| Solubility = Partially soluble
}}
| Section2 = {{Chembox Related
| OtherAnions =
| OtherFunction = Uranyl peroxide
Triuranium octoxide
| OtherFunction_label = uranium oxides
| OtherCompounds =
}}
| Section3 =
| Section4 =
| Section5 =
| Section6 =
}}
Amorphous uranium(VI) oxide (am-U2O7) is an orange diuranyl compound, most commonly obtained from the thermal decomposition of uranyl peroxide tetrahydrate at temperatures between {{convert|150|and|500|C|abbr=on|sigfig=2}}. It exists at room temperature as a powder. Am-U2O7 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.{{cite journal | doi = 10.1021/acs.inorgchem.6b00017 | title = Structure and Reactivity of X-ray Amorphous Uranyl Peroxide, U2O7 | date = 2016 | last1 = Odoh | first1 = Samuel O. | last2 = Shamblin | first2 = Jacob | last3 = Colla | first3 = Christopher A. | last4 = Hickam | first4 = Sarah | last5 = Lobeck | first5 = Haylie L. | last6 = Lopez | first6 = Rachel A. K. | last7 = Olds | first7 = Travis | last8 = Szymanowski | first8 = Jennifer E. S. | last9 = Sigmon | first9 = Ginger E. | last10 = Neuefeind | first10 = Joerg | last11 = Casey | first11 = William H. | last12 = Lang | first12 = Maik | last13 = Gagliardi | first13 = Laura | last14 = Burns | first14 = Peter C. | journal = Inorganic Chemistry | volume = 55 | issue = 7 | pages = 3541–3546 | pmid = 26974702 }}{{cite journal | doi = 10.1016/j.optmat.2019.01.040 | title = Shining a light on amorphous U2O7: A computational approach to understanding amorphous uranium materials | date = 2019 | last1 = Shields | first1 = Ashley E. | last2 = Miskowiec | first2 = Andrew J. | last3 = Niedziela | first3 = J.L. | last4 = Kirkegaard | first4 = Marie C. | last5 = Maheshwari | first5 = Ketan | last6 = Ambrogio | first6 = Michael W. | last7 = Kapsimalis | first7 = Roger J. | last8 = Anderson | first8 = Brian B. | journal = Optical Materials | volume = 89 | pages = 295–298 | doi-access = free }}
Production
Am-U2O7 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 color. Further heating results in the formation of alpha uranium trioxide (α-UO3).
Structure and reactivity
Because of the amorphous nature of am-U2O7, the long-range atomic structure of this compound has not been determined. However, recent computational investigations, chiefly accomplished using density functional theory (DFT), have helped to predict a local structure.{{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 U2O7 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}} Resembling a regular uranate compound, two uranyl ({{chem|UO|2|2+}}) groups are bridged by a μ2-O atom, where both uranium atoms are bonded to an 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.{{cite journal | doi = 10.2138/am-2003-0725| title = Studtite, [(UO2)(O2)(H2O)2](H2O)2: The first structure of a peroxide mineral| date = 2003| last1 = Burns| first1 = Peter C.| last2 = Hughes| first2 = Karrie-Ann| journal = American Mineralogist| volume = 88| issue = 7| pages = 1165–1168}} Developments on this structure propose a two-site metastudtite and UO3-like bonding environment, including the bond types already mentioned. Few other suggestions for the local atomic structure of am-U2O7 have been made. However, a crystalline form of U2O7, calculated as a two-site 6 and 8-coordinate structure, has been reported. In the same study, it was again found that the U2O7 species contained peroxide bonding. Am-U2O7 is known to undergo hydrolysis in the presence of water, to produce a crystalline metaschoepite powder. In addition to a change in crystallinity, this reaction involves a change in color from orange to bright yellow.
References
{{reflist}}{{Uranium compounds}}