Linear ordering of oxygen clusters in hyperstoicheiometric uranium dioxide

The formation of hyperstoicheiometric oxides of UO2 by the incorporation of additional oxygen atoms in the parent fluorite structure is described in terms of 2 : 2 : 2 oxygen clusters. A 2 : 2 : 2 cluster between two uranium atoms is treated as a co-ordination defect and consideration of electrostatic and polarisation effects together with X-ray photoelectron spectral data suggests that defect ordering is likely along 〈110〉 directions. The effect of chain-like linear ordering of oxygen clusters on structures in the oxide system UO2–U3O8 is assessed and a crystal structure for U4O9 is postulated.