Doping of ceria surfaces with lanthanum: a DFT + U study

In this paper we use density functional theory corrected for on-site Coulomb interactions (DFT + U) to study the defects formed in the ceria (111) and (110) surfaces doped with La. To describe consistently the defect formed with substitutional La(3+) doping at a Ce(4+) site we use DFT and DFT + U, with U = 5 eV for Ce 4f states and U = 7 eV for O 2p states. When La(3+) substitutes on a Ce(3+) site, an La'(Ce) + O.(o)defect state, with an oxygen hole, is formed at both surfaces, but only with the DFT + U approach. The formation energy of an oxygen vacancy in a structure with two La dopants in their most stable distribution is reduced over the undoped surfaces but remains positive. Formation of an oxygen vacancy results in the appearance of a reduced Ce(3+) cation and a compensated oxygen hole, instead of compensation of both oxygen holes, which is typical of metal oxides doped with lower valence cations. We tentatively suggest that the key role in the formation of this unusual defect is played by cerium and arises from the ease with which cerium can be reduced, as compared to other metal oxides. Experimental confirmation of these results is suggested.