  Rare-earth doped form, ceria (CeO2) is of interest as a potential candidate
for solid oxide fuel cells (SOFCs) because of its relatively high oxygen ion
conductivity at temperatures below 600 {\deg}C. At the present time,
computational chemistry has reached a certain maturity which allows prediction
of materials properties that are difficult to observe experimentally. However,
understanding of the roles of dopants on the oxygen ion conduction in CeO2 is
still incomplete for quantitatively reliable analysis due to strong electron
correlation of 4f electrons. In this study, density functional theory
calculations with Hubbard U corrections are conducted to discuss ionic/covalent
interactions in rare-earth-doped CeO2 and their consequences to oxygen ion
conduction. The study suggests that the variable occupancy of empty 4f orbitals
is important typically for early Ln elements to produce the covalent
interactions that essentially affect formation and migration of oxygen
vacancies. This finding is important in understanding the factors responsible
for oxygen ion diffusion in doped CeO2.
