Suppression of Sr surface segregation in La(1-x)Sr(x)Co(1-y)Fe(y)O(3-δ): a first principles study.

Based on systematic first principles calculations, we investigate Sr surface segregation (SSS) in La(1-x)Sr(x)Co(1-y)Fe(y)O(3-δ) (LSCF) (a typical perovskite ABO(3) compound), a bottleneck causing efficiency degradation of solid oxide fuel cells. We identify two basic thermodynamic driving forces for SSS and suggest two possible ways to suppress SSS: applying compressive strain and reducing surface charge. We show that compressive strain can be applied through doping of larger elements and surface coating; surface charge can be reduced through doping of higher-valence elements in the Sr- and B-site or lower-valence elements in the La-site and introducing surface A-site vacancies. The net effect of oxygen vacancy is to enhance SSS because its effect of increasing surface charge overrides its effect of inducing compressive strain, while Co substitution of Fe always enhances SSS because it induces tensile strain as well as increases surface charge. Our results explain the recent experimental observation of SSS suppression in LSCF by a La(1-x)Sr(x)MnO(3-δ) (LSM) coating.