Enhanced Oxygen Buffering by Substitutional and Interstitial Ni Point Defects in Ceria: A First-Principles DFT plus U Study

The defect chemistry and electronic structure of NiO/CeO(2) solid solutions are studied by means of DFT+U calculations in the limit of low Ni doping. We consider four representative solid solutions in which the Ni atoms are present as substitutional and interstitial point defects in bulk crystalline CeO(2), both in its stoichiometric form and in the presence of O vacancies. In all cases, Ni-doping significantly enhances the O buffering effect of ceria, controlled by O vacancy formation, but the actual microscopic mechanisms are different depending on the specific type and charge state of the point defects. The oxidation state of the Ni dopant is shown to univocally characterize the type of defect, whether interstitial (Ni(+)) or substitutional (Ni(2+)). Interstitial NO defects result from a charge redistribution between the Ni and Ce cations that leads to the formation of characteristic Ni(+)-Ce(3+) defect complexes. O release via vacancy formation in these interstitial solid solutions proceeds similarly as in pure CeO(2), i.e., is mediated by electron localization processes reducing two Ce(4+) ions to Ce(3+). Quite differently, substitutional Ni(2+) point defects yield unsaturated O 2p valence bands and the appearance of unoccupied gap states spatially localized on the O atoms neighboring the Ni defect. Consequently, O vacancy formation in substitutional solid solutions does not lead to reduction of Ce ions but to quenching of these gap states. Ab initio thermodynamics predict the substitutional solid solutions to be more stable than the interstitial ones by more than 2.4 eV over a wide range of pressures and temperatures. We demonstrate that these conclusions are robust with respect to the specific choice of the Hubbard U parameters accounting for the on-site electron Coulomb interaction on the Ni and Ce sites.