Polytypic arrangements of cuboctahedral oxygen clusters in U3O7

The presence of special oxygen aggregates called cuboctahedral clusters (de fects) is considered to be the manner ill which oxygen is incorporated into the fluorite-type UO2 structure during its oxidation. The structural princ iples of the enrichment of UO2 with oxygen were determined by analysis of t he spatial ordering of clusters in beta U4O9-y. These rules were applied to the family of U3O7 phases, i.e. to the deformed crystalline UO2-oxidation products reported usually as being tetragonal. The geometrical constraints imposed on the cluster locations by the UO2 lattice were considered in deta il. Consequently, a set of cluster ordering schemes, characterized by a low energy of the mutual cluster interactions, and lowering of primary cubic s tructural symmetry. was specified. The structures were compared with the av ailable data on the superstructure and on the axial ratio (c/a) of the know n U3O7 phases. This procedure resulted in: (i) the description of the U3O7 oxide as a family of polytypes with different stackings of identical cluste r layers, (ii) the determination of the cluster ordering scheme in alpha U3 O7 and beta U3O7. It was shown that the variability of the cluster layer st ackings is a decisive factor which affects the cia ratio. (C) 2000 Elsevier Science Ltd. All rights reserved.