Polytypic arrangements of cuboctahedral oxygen clusters in U3O7 Original Research Article

The presence of special oxygen aggregates called cuboctahedral clusters (defects) is considered to be the manner in which oxygen is incorporated into the fluorite-type UO2 structure during its oxidation. The structural principles of the enrichment of UO2 with oxygen were determined by analysis of the spatial ordering of clusters in βU4O9−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 detail. Consequently, a set of cluster ordering schemes, characterized by a low energy of the mutual cluster interactions, and lowering of primary cubic structural symmetry, was specified. The structures were compared with the available data on the superstructure and on the axial ratio (c/a) of the known U3O7 phases. This procedure resulted in: (i) the description of the U3O7 oxide as a family of polytypes with different stackings of identical cluster layers, (ii) the determination of the cluster ordering scheme in αU3O7 and βU3O7. It was shown that the variability of the cluster layer stackings is a decisive factor which affects the c/a ratio.