DEFECT STRUCTURE AND OXYGEN DIFFUSION IN UO2+DELTA

Coherent diffuse neutron scattering techniques have been used to study disorder in single crystal samples of hyperstoichiometric UO2+delta (delta = 0.13, 0.14). Above 850 K the crystals have a single phase and the excess anions give rise to diffuse scattering intensity. At 858 K a pattern of elastic diffuse scattering is observed, which is interpreted in terms of uncorrelated simple clusters of defective anions of the type first postulated by Willis. At 1573 K quasielastic energy broadening of the diffuse scattering is observed, indicating the dynamic nature of the disorder. The characteristic distribution of intensity at this temperature resembles that observed in other fluorite compounds in the fast-ion phase. It is possible to estimate the oxygen self-diffusion coefficient at 1573 K, and it is found to be comparable to that in the pure compound some 1000 K higher.