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Amorphous uranium(VI) oxide

Names
IUPAC name Diuranyl heptoxide
Other names Amorphous UO3
Properties
Chemical formula	Am-U2O7
Molar mass	588 g/mol
Appearance	Orange-brown powder
Density	6.8 g/cm
Solubility in water	Partially soluble
Related compounds
Related uranium oxides	Uranyl peroxide Triuranium octoxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C , 100 kPa). Infobox references

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 150 and 500 °C (300 and 930 °F). 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.

Production

Am-U₂O₇ is produced by the thermal decomposition of uranyl peroxide tetrahydrate at temperatures between 150 and 500 °C (300 and 930 °F), in either an air or nitrogen atmosphere. The resultant powder is tan orange in colour. Further heating results in the formation of alpha uranium trioxide (α-UO₃).

Structure and reactivity

Due to 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. Resembling a regular uranate compound, two uranyl () 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. 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 colour from orange to bright yellow.

References

1. 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
2. ^ 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, U₂O₇, J. Inorg. Chem., 2016, 55, pp.3541-3546. doi:10.1021/acs.inorgchem.6b00017
3. ^ A. Shields, A. Miskowiec, J. Niedziela, M. Kirkegaard, K. Maheshwari, M. Ambrogio, R. Kapsimalis and B. Anderson, Shining a light on amorphous U₂O₇: A computational approach to understanding amorphous uranium materials, Opt. Mater., 2019, 89, 295-298. doi:10.1016/j.optmat.2019.01.040
4. ^ Thompson, Nathan B. A.; Middleburgh, Simon C.; Evitts, Lee J.; Gilbert, Matthew R.; Stennett, Martin C.; Hyatt, Neil C. (2020-12-15). "Short communication on further elucidating the structure of amorphous U₂O₇ by extended X-ray absorption spectroscopy and DFT simulations" (PDF). Journal of Nuclear Materials. 542: 152476. Bibcode:2020JNuM..54252476T. doi:10.1016/j.jnucmat.2020.152476. ISSN 0022-3115. S2CID 225232737.
5. P. Burns and K. Hughes, Studtite, (H₂O)₂: The first structure of a peroxide mineral, Am. Mineral., 2003, 88, 1165-1168. doi:10.2138/am-2003-0725

• Uranium(VI) compounds