Cuboctahedral oxygen clusters in U3O7

When UO2 is oxidised to U3O7, the positions in the crystal lattice of all the uranium atoms and of about 70% of the oxygen atoms are hardly affected. The remaining oxygen atoms occupy new sites which are located 310 pm along 〈1 1 0〉 vectors from the holes in the fluorite framework of UO2. These results, which are based on the analysis of neutron diffraction powder data, are consistent with the concept that excess oxygen in U3O7 is accommodated in cuboctahedral anionic clusters.