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

Abstract

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)₂CoO_4±δ 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 Pr³⁺ and Pr⁴⁺ cations coexist at the surface. The surface of all the composites is enriched in Sr cations that could exist as SrCO₃, SrO₂, and Sr(OH)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)₂CoO_4±δ 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 Co³⁺ and Co⁴⁺ cations, a small fraction of Co²⁺ 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)₂CoO_4±δ phase as well as different surface oxygen states. The effect of surface treatment on the evolution of the surface composition has been discussed.