Anion-Vacancy-Induced Magneto−Crystalline Anisotropy in Fluorine-Doped Hexagonal Cobaltites

The two cobalt hexagonal perovskites 6H-Ba6Co6F0.93O16 and 10H-Ba5Co5F0.77O12.88 were prepared, and their structures were examined by X-ray and neutron diffraction and by 19F solid state NMR spectroscopy. The magnetic and transport properties of these compounds were probed by magnetic susceptibility and electrical resistivity measurements, and their electronic structures by density functional and tight-binding calculations. The [BaOF1−x] layers of these compounds create corner-sharing tetrahedral Co2O7 dimers at the interface between their face-sharing octahedral oligomers. Our density functional calculations leads to an unambiguous charge distribution model, which assigns high-spin Co3+ ions for the tetrahedral sites and low-spin Co3+/Co4+ ions for the octahedral sites, and this model should be valid for the parent BaCoO3−δ and the related oxychlorides and oxybromides as well. The F− vacancies in the [BaOF1−x] layers cause a strong distortion in the tetrahedral dimer Co2O7, which in turn affects the spin orientation of the high-spin Co3+ ions of the CoO4 tetrahedra, i.e., parallel to the c-direction in Ba6Co6F1−xO16−δ but perpendicular to the c-direction in Ba5Co5F1−xO13−δ. This difference in the spin orientations is related to the d-states of the distorted CoO4 tetrahedra with high-spin Co3+ (d6) ion on the basis of tight binding calculations and spin−orbit coupling as perturbation.