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. 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, 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 ([UO2 ] 2+) groups are bridged by a μ2-O atom, where both uranium atoms are bonded to a 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, although 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.