The defect structure of hyper-stoichiometric uranium dioxide

Earlier neutron-diffraction measurements [Willis (1964), Proc. Br. Ceram. Soc. 1, 9-19] on a single crystal of UO_2.12 at 800°C showed that interstitial oxygen atoms enter the UO₂ lattice at two kinds of site, which are displaced by about 1 Å along the (110) and (111) directions from the cubic-coordinated interstitial sites. It was also shown that the solution of oxygen is accompanied by the formation of vacancies in the normal oxygen sites, but that the uranium sublattice remains undisturbed. A new analysis of the same data has been carried out, which introduces constraints between some of the parameters and allows a more satisfactory treatment of the anharmonic motion of the oxygen atoms. The new analysis leads to a much more precise determination of the occupation numbers of the (110) and (111) oxygen atoms and of the normal oxygen vacancies. The simplest model to account for these new results is the so-called 2:2:2 configuration of oxygen defects.