Abstract
A series of single-phase samples with the composition of Y1-xCaxBa2Cu3-yCoyO6.5+z (0<x<0.25; 0<y<0.25) have been synthesized and studied by X-ray powder diffraction analyses, and measurements of the resistivity, AC susceptibility, oxygen content and X-ray photoelectron spectroscopy (XPS). It is found that the reduction in the superconductivity induced by the substitution of Co for Cu(1) can be counterbalanced, to an extent, by the replacement of Ca for Y. This phenomenon has been explained with the opposite effect of Co doping for Cu(1) from Ca doping for Y, on the mobile hole concentration of the system. The substitution of Ca for Y induces a delocalization effect which releases the holes in the Cu-O chain and transfers them to the CuO2 sheets, leading to an increase in the mobile hole concentration while the substitution of Co for Cu(1) reduces the mobile hole concentration because of its localization effect on the holes. The XPS measurement for the core level of Cu 2p and O 1s reveals that the oxidation state of the copper changes with the dopant content, and a peak corresponding to the Cu3+ oxidation state is observed in some co-doped samples.
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