Understanding the Interplay of Dopants, Interfaces, and Anionic Conductivity in Doped Ceria/Zirconia Heteroepitaxial Structures

Using density functional theory calculations, a comparative study at a microscopic level of the different factors that influence the anionic conductivity in YSZ/doped-CeO2 heterointerfaces is presented. First oxygen migration paths and barriers are examined, either at the YSZ phase or at the interface as a function of the different cations that may be present. Oxygen migration barriers and, simultaneously, vacancy formation energies are found to be significantly lower at the interface. While these two factors should result in a larger ionic conductivity at the interface, the presence of Ce3+ cations, associated with 0 vacancies, is found to increase the oxygen hopping barrier. This drawback can be avoided by introducing aliovalents M3+ cations in the ceria phase. Indeed, a systematic examination of the effects in the oxygen hopping barrier shows that the presence of trivalent dopants that present an ionic radius lower than that of the Ce3+ decreases the oxygen migration barrier and increases the interfacial conductivity. However, some of these cations result in the formation of vacancy-dopant atom pairs, a situation that should be avoided as the radii of those cations are higher than the Ce4+ cation radius. Dopants that prefer a next-nearest configuration should be selected and an adequate balance between the reduction of the oxygen hopping barrier and the preferred configuration (next or next nearest) should be established when selecting the most appropriate dopant.