The atomic structure of the UO2 (111) surface and the effects of additional surface oxygen studied by elevated temperature STM

The structure of the UO2+x (111) surface has been investigated by elevated temperature STM. Images of atomic terraces reveal two different types of surface termination. One of them corresponds to the stoichiometric UO2 (111) surface and can be resolved atomically in empty state images above similar to 1.6 V sample bias. The observed (1 x 1) ordering is thought to be due to uranium states because its occurrence corresponds to the bottom of the empty uranium 5f band. On these terraces mobile oxygen forms a local (root 3 x root 3)R30 degrees superstructure. The other terrace type observed on top of the UO2+x (111) surface is thought to be a phase of a higher uranium oxide which has grown epitaxially.