Low-Energy Electron Diffraction Studies of Uranium Dioxide

Abstract

The (110), (111), and (100) surfaces of uranium dioxide have been studied by low-energy electron diffraction. Below 600°C the (110) face displays persistent fractional-order beams characteristic of contamination, and at higher temperatures in vacuum thermal faceting occurs. The (111) surface interacts with O2 at room temperature to produce rings around the integral-order beams, and if this surface is mechanically disrupted, fractional-order arrays are formed in the interval 200°–390°C. For UO2 + x (111) at all temperatures the most characteristic diffraction features are also rings around the integral-order reflections; fractional-order beams are observed from UO2 + x (111) only if the surface is disrupted. It is postulated that rings display a disordered U4O9 substructure which consists of clusters of two or three adjacent atoms occupying regular lattice sites, but separated from neighboring clusters by approximately three lattice spacings; it is further postulated that surface discontinuities act as sources of mobile species which migrate across the surface and in the presence of oxygen condense out as higher oxides in fractional arrays. The (100) surface is stable with respect to faceting up to at least 1600°C SBT, and apparently does not react with O2 to produce rings or fractional-order beams in the diffraction pattern.