Chemical and electronic characterization of cobalt in a lanthanum perovskite. Effects of strontium substitution

• Published in: Journal of solid state chemistry 2010-01, Vol.183 (1), p.27-32
• Main Authors: Hueso, Jose L., Holgado, Juan P., Pereñíguez, Rosa, Mun, Simon, Salmeron, Miquel, Caballero, Alfonso
• Format: Article
• Language: English
• Subjects: ABSORPTION SPECTROSCOPY; ALKALINE EARTH METAL COMPOUNDS; ANGULAR MOMENTUM; CATIONS; CHALCOGENIDES; CHARGED PARTICLES; COBALT COMPOUNDS; Cobaltites; Condensed matter: structure, mechanical and thermal properties; CRYSTAL DEFECTS; Crystal field splitting; CRYSTAL STRUCTURE; Crystalline state (including molecular motions in solids); Exact sciences and technology; EXAFS; FINE STRUCTURE; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; IONS; LANTHANUM COMPOUNDS; MATERIALS SCIENCE; MINERALS; NEXAFS; OXIDE MINERALS; OXIDES; OXYGEN COMPOUNDS; PARTICLE PROPERTIES; PEROVSKITE; PEROVSKITES; Physics; POINT DEFECTS; RARE EARTH COMPOUNDS; SPECTROSCOPY; SPIN; STRONTIUM COMPOUNDS; STRONTIUM IONS; Structure of solids and liquids; crystallography; Theory of crystal structure, crystal symmetry; calculations and modeling; TRANSITION ELEMENT COMPOUNDS; VACANCIES; X-RAY SPECTROSCOPY; XANES; XAS
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1016/j.jssc.2009.10.008

Two different cobaltites, LaCoO 3 and La 0.5Sr 0.5CoO 3− δ , have been prepared and characterized by means of high energy Co K-edge and low energy O K-edge X-ray absorption spectroscopy (XAS). Even though half of the La(III) is substituted by Sr(II), little or no changes can be detected in the formal oxidation state of cobalt atoms. The presence of strontium cations induces two main effects in the chemical and electronic state of the perovskite. The charge balance with Sr(II) species is reached by the formation of oxygen vacancies throughout the network, which explains the well-known increase in the reactivity of this substituted perovskite. O K-edge XAS experiments show that the Sr(II) species induce the transitions of d electrons of cobalt cations from low to high spin configuration. We propose that this change in spin multiplicity is induced by two cooperative effects: the oxygen vacancies, creating five coordinated cobalt atoms, and the bigger size of Sr(II) cations, aligning the Co–O–Co atoms, and favoring the overlapping of π-symmetry cobalt and oxygen orbitals, reducing the splitting energy of e g and t 2g levels. Change in spin multiplicity induced by the bigger size of Sr(II) cations, aligning the Co–O–Co atoms, and favoring the overlapping of π-symmetry cobalt and oxygen orbitals.