Quasiepitaxial growth of a monoclinic phase on UO2 single crystals upon leaching in H2O

Surface oxidation of (110)-oriented UO2 single crystals leached in demineralized water at 180 degrees C for 24 h has been studied by means of the backscattering/channeling technique. The O sublattice was analyzed using the O-16(He-4, He-4)O-16 resonance occurring at 3045 keV. Quasiepitaxial growth of an oxidized surface layer was observed and attributed to the phase transformation from the cubic UO2 to a monoclinic UOr phase with r=2.28+/-0.05. X-ray diffractometry was also used to study the orientation of (110) atomic planes of the transformed layer with respect to the bulk crystal and to measure the change of the (110) interplane distance. A simple geometrical model of the quasiepitaxial growth is proposed and the distortion of the UO2 cubic elementary cell due to the incorporation of surplus oxygen atoms is determined. Monte Carlo simulations of channeling data confirm the model and provide indications about atom location. The observed monoclinic phase is compared with other phases of uranium oxides.