Fluctuation of surface composition and chemical states at the hetero-interface in composites comprised of a phase with perovskite structure and a phase related to the Ruddlesden–Popper family of compounds

Surfaces and interfaces in composite solids can possess particular compositions and properties, thereby governing the functions of materials. X-ray photoelectron spectroscopy (XPS) has been used for the first time to explore the cation rearrangement between the surface and bulk material of the crystallites in the two-phase composites formed in the La-Sr-Pr-Co-O system. The two-phase composites contain a perovskite phase (major fraction) and a layered perovskite-like phase (La,Sr,Pr) sub(2)CoO sub(4 plus or minus ) delta related to the Ruddlesden-Popper family of compounds. The difference between the surface and nominal composition was revealed for all the composites explored. The surface concentrations of Pr and La cations are lower compared to the nominal stoichiometry, suggesting their preferable dissolution into the volume of the crystallites. Both Pr super(3+) and Pr super(4+) cations coexist at the surface. The surface of all the composites is enriched in Sr cations that could exist as SrCO sub(3), SrO sub(2), and Sr(OH) sub(2) individual surface phases as well as SrO on the surface of the perovskite phase. The second phase plays an important role in balancing the surface composition at the hetero-interface in the composites. The rise in the fraction of (La,Sr,Pr) sub(2)CoO sub(4 plus or minus ) delta in the composites changes the "surface Sr : bulk Sr" ratio, increasing the later contribution due to the stronger accommodation of Sr cations within its crystal lattice and decreasing the surface depletion of Co cations. In addition to Co super(3+) and Co super(4+) cations, a small fraction of Co super(2+) cations exists in the near-surface region of the composites. The surface of all the composites is strongly enriched in oxygen (by 11.8-13.3 at %). The O1s spectra of the composites contain contributions from the lattice oxygen related to the perovskite phase and layered (La,Sr,Pr) sub(2)CoO sub(4 plus or minus ) delta phase as well as different surface oxygen states. The effect of surface treatment on the evolution of the surface composition has been discussed.