Hole doping into Co-12s2 copper oxides with s fluorite-structured layers between CuO2 planes

Fjellvåg, H. , Morita, Y. , Nagai, T. , Lee, J.-M. , Chen, J.-M. , Liu, R.-S. , Hauback, B.C. , Awana, V.P.S. , Matsui, Y. , Yamauchi, H. , Karppinen, M.
Journal of solid state chemistry, Vol. 179, no. 3 (March 2006), 632-645
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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)-[O 2-(Ce,Y)]s-1, between two identical CuO 2 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)2Cu 2O9+δ) and Co-1232 (CoSr 2(Ce0.67Y0.33)3Cu2O 11+δ) samples Cu valence does not increase high enough to induce superconductivity. © 2005 Elsevier Inc.
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