A neutron diffraction study of anion clusters in nonstoichiometric uranium dioxide

Neutron diffraction data have been collected at 970 K on single crystals of UO2.11 and UO2.13. The data were more extensive than those obtained in an earlier neutron study of UO2.12 and were measured at a shorter wavelength, allowing TDS corrections to be made to the Bragg intensities. The new results support the concept that the oxidation of UO2 proceeds by the formation of anion vacancies and of two types of oxygen interstitial. One type is displaced along (110) from the anion sites in UO2 and the other is displaced along (111). In the first crystal the occupation numbers of the three kinds of oxygen defect are close to those expected from the formation of 2:2:2 clusters, whereas for the second crystal the occupation numbers correspond to those calculated for cuboctahedral clusters