QUASIEPITAXIAL GROWTH OF A MONOCLINIC PHASE ON UO2 SINGLE-CRYSTALS UPON LEACHING IN H2O

Surface oxidation of (110)-oriented UO2 single crystals leached in dem ineralized water at 180 degrees C for 24 h has been studied by means o f the backscattering/channeling technique. The O sublattice was analyz ed using the O-16(He-4, He-4)O-16 resonance occurring at 3045 keV. Qua siepitaxial growth of an oxidized surface layer was observed and attri buted to the phase transformation from the cubic UO2 to a monoclinic U Or phase with r=2.28+/-0.05. X-ray diffractometry was also used to stu dy the orientation of (110) atomic planes of the transformed layer wit h respect to the bulk crystal and to measure the change of the (110) i nterplane distance. A simple geometrical model of the quasiepitaxial g rowth 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 ind ications about atom location. The observed monoclinic phase is compare d with other phases of uranium oxides.