Oxygen diffusion at the {111} and {110} surfaces of yttrium doped ceria.

Abstract

Monte Carlo and molecular dynamics simulations have been combined to study the distribution of Y3+ and O2− ions and the diffusion of O2− ions within CeO2 materials. The calculations indicate that at lower temperatures clustering of Y3+ ions in the bulk material inhibits the diffusion of oxygen ions. Y3+ ions and oxygen vacancies have also been shown to be prevalent within the {111} and {110} surface layers although the degree of Y3+ segregation is much greater at the {110} surface. By combining Monte Carlo and molecular dynamics simulations, both the space charge layer and surface structure have been demonstrated to influence oxide ion diffusion. Oxygen ions are able diffuse parallel to the surface within the {111} surface layers and calculated diffusion coefficients are observed to be greater for the {111} surface but reduced for the {110} surface when compared to that of the bulk material. The distribution of ions at the surface is presented and employed to calculate the charge density. Segregation of dopant ions and vacancies to the surface creates a space charge layer that modifies surface diffusion to a depth of approximately 20 Å. These calculations explain, in part, the wide variation in conductivities observed in grain boundaries of polycrystalline materials and the difficulties in interpreting experimental results.