Amorphous uranium(VI) oxide

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.^[2][3]

Amorphous uranium(VI) oxide^[1]

Names
IUPAC name Diuranyl heptoxide
Other names Amorphous UO3 Amorphous uranyl peroxide
Properties
Chemical formula	Am-U2O7
Molar mass	588 g/mol
Appearance	Orange-brown powder
Density	6.8 g/cm3
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 [77 °F], 100 kPa). Infobox references

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 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.^[2][4] Resembling a regular uranate compound, two uranyl (UO²⁺
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.^[5] Developments on this structure propose a two-site metastudtite and UO₃-like bonding environment, including the bond types already mentioned.^[4] 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.^[3] 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.