Oxygen migration energy in La and Y Co-doped CeO₂: Effect of lattice constant and grain boundary segregation

The transfer energy of oxygen vacancy of La and Y co-doped CeO₂ is investigated in which the total amount of La and Y is fixed at 20 at%. Although both the La³⁺ (1.16 Å) and Y³⁺ (1.032 Å) ions are larger than the Ce⁴⁺ ions (0.97 Å), individual single doping of La and Y results in an increase and decrease in the CeO₂ lattice constant, respectively, due to the effect of the critical ionic radius (r_c). As the La/Y ratio varies, the lattice constant increases as the La content increases, which leads to a decrease in the migration energy of the oxygen vacancy in the grain interior. When La and Y are co-doped at a ratio of 2:18, its lattice constant is almost the same as that of pure CeO₂ in which the dissociation energy of the oxygen vacancy achieves the lowest value due to the minimum lattice-strain energy. The migration energy at the grain boundary also decreases with increasing La content. This result is explained by the degree of grain boundary segregation of the dopant.