Structural diversity, surface composition, and redox behavior in the LA_0.6SR_0.4CoO₃-PrO_2-δ system

Phase composition, crystal structure of components, surface composition, and redox behavior were explored in the (100-x) La_0.6Sr_0.4CoO₃-xPrO_2-δ (LSCPx) system under air. At high temperatures, a fraction of La0.6Sr0.4CoO3 perovskite phase interacts with praseodymium oxide, forming a new layered perovskite-like phase relating to the Ruddlesden-Popper family of A_n+1BnO_3n+1 compounds with n=1. The Ruddlesden-Popper phase with K₂NiF₄-type structure (I4/mmm, no.139) exists in a wide compositional range. At room temperature the LSCPx (2 ≤ x ≤ 40) are two-phase compositions, comprising of the perovskite phase with rhombohedral distortion ( R3c, no. 167) and Ruddlesden-Popper phase. Evolution of lattice parameters for both perovskite and Ruddlesden-Popper phases with the raise in the Pr content was discussed. La0.93Sr0.62Pr0.45CoO4 compound with Ruddlesden-Popper structure was synthesized. Both single phase La0.93Sr0.62Pr0.45CoO4 and newly formed Ruddlesden-Popper phase in the two-phase LSCPx (2 ≤ x ≤ 40) exhibit similar behavior with the temperature variation (25-820^oC): the expansion of the unit cell along the c-axis is noticeably stronger than within the a-b plain. X-ray photoelectron spectroscopy study indicates stronger accommodation of Sr cations within the Ruddlesden-Popper structure. Surface depletion in Co decreases in the two-phase LSCPx compositions with the raise in the fraction of the phase with the Ruddlesden-Popper structure, assuming that the presence of a second phase can facilitate the re-arrangement of cations between the volume and surface. The total oxygen exchange between the gas phase and the two-phase LSCPx (2 ≤ x ≤ 20) compositions decreases gradually with the increase in Pr content in the composites and it becomes almost negligible in the LSCPx (20 ≤ x ≤ 40). This result is unexpected and could be associated with the simultaneous involvement of Pr and Co cations in the redox process.