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 . 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 X-ray diffraction is a generic term for phenomena associated with changes in the direction of X-ray beams due to interactions with the electrons around atoms. It occurs due to elastic scattering, when there is no change in the energy of the waves. ...

. 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 , 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 a''m''-U₂O₇, 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) is a computational quantum mechanical modelling method used in physics, chemistry and materials science to investigate the electronic structure (or nuclear structure) (principally the ground state) of many-body ...

(DFT), have helped to predict a local structure. Resembling a regular uranate compound, two

uranyl The uranyl ion with the chemical formula has a linear structure with short U–O bonds, indicative of the presence of multiple bonds between uranium and oxygen, with uranium in the oxidation state +6. Four or more ligands may be bound to the u ...

() 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 a''m''-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

{{Uranium compounds Uranium(VI) compounds