Phase constitution, Sr distribution and morphology of self-assembled La-Sr-Co-O composite films prepared by PLD

Thin compact La-Sr-Co-O composite films consisting of both (La,Sr)CoO3‐δ and (La,Sr)2CoO4+δ crystallites and a total La:Sr ratio of 70:30 were prepared by phase separation on a single crystal Y-stabilized zirconia substrate using pulsed laser deposition (PLD) from a single target. The self-assembled composite films have grains of about 50 nm lateral extension with a (La,Sr)CoO3‐δ/(La,Sr)2CoO4+δ/O2 triple phase boundary density of 1.1-2.6·105 cm/cm2. Sr is accumulated in (La,Sr)2CoO4+δ at the expense of (La,Sr)CoO3‐δ, but less than expected for thermodynamic equilibrium. Insufficient cation diffusivity for complete demixing can cause the formation of metastable La-Sr-Co-O phases: (1) a Co-deficient perovskite phase La0.7Sr0.3Co0.9O3‐δ with high oxygen reduction activity, (2) a higher-order Ruddlesden-Popper phase. These results lay the basis for an accurate quantification and understanding of the potentially high oxygen reduction reaction activities of such films.