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

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.