Chemical Strain and Point Defect Configurations in Reduced Ceria

New density functional theory (DFT+U) calculations on ceria show that next-nearest-neighbor (NNN) configurations for Ce 3+ and oxygen vacancy are more stable than the nearest-neighbor (NN) configuration usually found in such calculations, thus removing a long-standing discrepancy with classical simulations. The calculated chemical expansion due to reduction is now in excellent agreement with the experimental value, in contrast to that predicted by the NN configuration. The results suggest that the reduction mechanism in ceria involves relaxation from metastable NN to stable NNN configurations through charge transfer or oxygen hopping. The energy barrier for oxygen hopping mechanism was found to be 0.43 eV. It also suggests that DFT+U simulations of mixed-valence materials should be conducted with considerable care.