Formation of Co3+ octahedra and tetrahedra in YBaCo4O8.1

High-resolution neutron and synchrotron X-ray powder diffraction experiments were performed, at 300 and 10 K, for the determination of the structure of YBaCo4O8.1, which was prepared by controlled oxidation of the Kagomé lattice compound YBaCo4O7. Our diffraction data demonstrate that YBaCo4O8.1 crystallizes in the orthorhombic Pbc21 space group with the formation of a large superstructure (a=12.790 Å, b=10.845 Å, c=10.149 Å), with respect to the parent trigonal YBaCo4O7 material. The Co ions occupy both corner-sharing tetrahedral and edge-sharing octahedral sites, in contrast to YBaCo4O7, which has only corner-sharing tetrahedra. The octahedral sites form by the addition of two extra oxygen atoms and the drastic displacements of some of the original O atoms relative to the parent. The edge-sharing octahedra form isolated zigzag chains parallel to the c-axis linked to one another via tetrahedra. While found in a few phosphates, silicates and germanates, this motif appears unique to YBaCo4O8.1 among mixed-metal oxides. No structural phase transition or long range antiferromagnetic ordering are observed at 10 K.