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-delta and (La,Sr)(2)CoO4+delta 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-delta/(La,Sr)(2)CoO4+delta/O-2 triple phase boundary density of 1.1-2.6.10(5) cm/cm(2). Sr is accumulated in (La,Sr)(2)CoO4+delta at the expense of (La,Sr)CoO3-delta, 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-delta 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. (C) 2017 Elsevier B.V. All rights reserved.