Surface Simulation Studies of Fluorite Oxides

Abstract

Atomic scale computer simulation was used to predict the energies and structures associated with dry surfaces and surfaces stabilised by the addition of hydroxide ions. The (111), (110) and (100) low index surfaces of cubic zirconia, ceria, urania and plutonia were investigated. The (100) surface is dipolar and therefore it was necessary to incorporate a series of surface defects to neutralise the dipole. An extensive range of planar defect configurations were modelled. In addition the incorporation of a trench on the (100) surface was considered. This represents a partial reconstruction of the surface since it incorporates {111 facets along ledges lying in the [110] direction. The trench can be formed from a planar (100) surface by removing rows of stoichiometric surface O^2- - M⁴⁺ - O^2- columns (i.e. rows, three ions deep). The trench configuration was found to be the most stable of all the dry (100) configurations. Nevertheless, for all the fluorite oxides investigated, it was found that the (111) surface was most stable, followed by the (110) surface and then finally the (100) surfaces }

The equilibrium surface energies of hydroxylated surfaces are predicted in addition to the energy to hydroxylate a previously dry surface. All the surfaces studied with the exception of the (111) surface of ZrO₂, were stabilised by the addition of hydroxide ions. This suggests that water adsorption occurs onto these surfaces. This was especially true for all the (100) surfaces (regardless of whether a trench configuration was present). Additionally, on becoming fully hydroxylated, the (100) surface becomes the most stable and the planar surface configuration was found to be more stable than the trench configuration. Therefore, surface hydroxylation not only changes which surface type is most stable, but also the preferred structure of the (100) surface.