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Hole doping into Co-12s2 copper oxides with s fluorite-structured layers between CuO2 planes
Authors:H Fjellvåg  Y Morita  J-M Lee  R-S Liu  VPS Awana  Y Matsui  M Karppinen
Institution:a Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
b Centre for Materials Science and Nanotechnology, University of Oslo, N-0315 Oslo, Norway
c National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
d National Synchrotron Radiation Research Center, Hsinchu, Taiwan, ROC
e Department of Chemistry, National Taiwan University, Taipei, Taiwan, ROC
f Institute for Energy Technology, N-2027 Kjeller, Norway
g National Physical Laboratory, Krishnan Matg, New Delhi 12, India
Abstract:In this work, the first three members (s=1, 2, 3) of the Co-12s2 homologous series of multi-layered copper oxides are gradually doped with holes through high-pressure oxygenation (HPO). The phases differ from each other only by thickness of the fluorite-structured layer block, (Ce,Y,Ca)-O2-(Ce,Y)]s−1, between two identical CuO2 planes. High-resolution transmission-electron microscopy (HRTEM) and electron diffraction (ED) analyses together with both synchrotron X-ray and neutron powder diffraction data, reveal that as a consequence of HPO the charge-reservoir CoO4-tetrahedra chains get broken and the lattice symmetry of the Co-12s2 phases changes from orthorhombic to tetragonal. Oxygen contents are analyzed for the samples with wet-chemical and thermogravimetric techniques. The valence state of copper in the CuO2 plane is determined from Cu L-edge X-ray absorption near-edge structure (XANES) spectra to be compared with the values estimated through bond-valence-sum (BVS) calculations from the crystal structure data. The positive charge induced by oxygen loading (or aliovalent CaII-for-YIII substitution in CoSr2YCu2O7+δ) is found not to be completely accommodated in the CuO2 planes but be rather effectively trapped at the charge-reservoir Co atoms. Superconductivity appears in the Co-1212 (CoSr2YCu2O7+δ) samples with the copper valence of 2.13 or higher, whereas in the Co-1222 (CoSr2(Ce0.25Y0.75)2Cu2O9+δ) and Co-1232 (CoSr2(Ce0.67Y0.33)3Cu2O11+δ) samples Cu valence does not increase high enough to induce superconductivity.
Keywords:Multi-layered copper oxides  Fluorite-structured layers  Co-12s2 homologous series  Crystal structure  Oxygen content  Hole-doping  Cu valence  XANES spectroscopy  Superconductivity
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