Overview of Electrons and Orbitals in a Nearly One-Dimensional Co3+/Co4+ System

Matthieu Kauffmann (1) , Olivier Mentré (1) , Alexandre Legris (2) , Sylvie Hébert (3) , Alain Pautrat (3) , Pascal Roussel (1)

1 UCCS - Unité de Catalyse et Chimie du Solide - UMR 8181
2 LMPGM - Laboratoire de Métallurgie Physique et Génie des Matériaux
3 CRISMAT - Laboratoire de cristallographie et sciences des matériaux

Ba6Co6ClO15.5 has been analyzed from the point of view of local electric/magnetic interactions and orbital overlapping, step by step following the different subunits of the crystal structure. Our investigation is based on structural data (X-ray and neutron diffraction, hereafter XRD and ND), experimental magnetic/electric/transport measurements, magnetic structure, and density functional theory (DFT) calculations. Its crystal structure is related to the 12H-BaCoO3-δ form and contains corner sharing pairs of Co4+ tetrahedra, with strong antiferromagnetic (AF) exchanges (estimation of J ∼ 0.07 eV). In the title compound, Co4+ are assigned to “intermediate spin” IS with electronic configuration eg3t2g2, S = 3/2. This rather rare configuration for a d5 cation is explained from the splitting of the t2g manifold because of an important off-centering of the shared corner. The dimers are connected to linear Co3+4O15 subunits, and both tetrahedral−tetrahedral and tetrahedral−octahedral magnetic junctions have been explored by the analysis of the correlation and delocalization Co−O−Co superexchanges, in good agreement with the experimental results. The tetrameric units have been assigned to an ordered mixture of HS and LS Co3+ while below TN, and the localized magnetic moments vanished because of electron transfer toward covalent oxygen ligands and by direct exchanges in the intermediate Co−Co region. Then, the electrons are confined inside these units, which act as strong ferromagnetic connectors between the terminal Co4+ tetrahedra. The overview of the transport properties (conductivity, thermoelectric power, and positive magnetoresistance) are in favor of a variable range hoping (VRH) regime below TN produced by an Anderson-like localization, in which tetrahedral Co4+ play the role of disordered centers with respect to a 1D system.

Domaines: Chimie inorganique

Matthieu Kauffmann, Olivier Mentré, Alexandre Legris, Sylvie Hébert, Alain Pautrat, et al.. Overview of Electrons and Orbitals in a Nearly One-Dimensional Co3+/Co4+ System. Chemistry of Materials, 2008, 20 (5), pp.1741-1749. ⟨10.1021/cm702678v⟩. ⟨hal-00187566⟩