Hole doping into Co-12 s2 copper oxides with s fluorite-structured layers between CuO 2 planes

Abstract

In this work, the first three members ( s = 1 , 2, 3) of the Co-12 s2 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 CoO 4-tetrahedra chains get broken and the lattice symmetry of the Co-12 s2 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 CuO 2 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 Ca II-for-Y III substitution in CoSr 2YCu 2O 7+ δ ) is found not to be completely accommodated in the CuO 2 planes but be rather effectively trapped at the charge-reservoir Co atoms. Superconductivity appears in the Co-1212 (CoSr 2YCu 2O 7+ δ ) samples with the copper valence of 2.13 or higher, whereas in the Co-1222 (CoSr 2(Ce 0.25Y 0.75) 2Cu 2O 9+ δ ) and Co-1232 (CoSr 2(Ce 0.67Y 0.33) 3Cu 2O 11+ δ ) samples Cu valence does not increase high enough to induce superconductivity.