First-principles study on defect chemistry and migration of oxide ions in ceria doped with rare-earth cations

Abstract

Oxygen transport in rare-earth oxide (RE₂O₃) doped CeO₂ with fluorite structure has attracted considerable attention owing to both the range of practical usage (e.g., fuel cells, sensors, etc.) and the fundamental fascination of fast oxide ion transport in crystalline solids. Using density-functional theory, we have calculated the formation energies of point defects and their migration properties in RE₂O₃ doped CeO₂(RE = Sc, Y, La, Nd, Sm, Gd, Dy, and Lu). The calculated results show that oxygen vacancies are the dominant defect species obtained by RE³⁺ doping. They form associates with the RE³⁺ ions, and the corresponding defect association energy is a strong function of the ionic radii of the RE³⁺ dopants. The migration of an oxygen vacancy was investigated using the nudged elastic band method. The lowest activation energy for oxygen vacancy hopping is obtained for a straightforward migration path between two adjacent oxygen sites. The migration energy of an oxygen vacancy also strongly depends on the ionic radii of the neighbouring dopant cations. Accordingly, we have identified two factors that affect the oxygen vacancy migration; (1) trapping (or repelling) of an oxygen vacancy at the NN site of the RE³⁺ dopant, and (2) reduction (or enlargement) of the migration barrier by RE³⁺ doping. These findings provide insight for atomistic level understanding of ionic conductivity in doped ceria and would be beneficial for optimizing ionic conductivity.