Ca-substitution and O-doping effects in superconducting Cu(Ba0.8Sr0.2)2(Yb1−xCax)Cu2O6+z obtained from neutron diffraction refinements

Distinct calcium and oxygen doping effects were studied in the Cu(Ba0.8Sr0.2)2(Yb1−xCax)Cu2O6+z (Cu−1212:P) system by means of neutron diffraction and superconducting quantum interference device experiments in the wide substitution ranges of 0<~x<~0.35 and 0<z<1. The effectiveness of the two different ways to introduce holes into the CuO2 planes was compared both in respect to the capability to increase Tc and in terms of the hole production as estimated from neutron-diffraction data via bond-valence-sum calculation. Oxygen doping was found to increase the hole concentration less efficiently, and further, at a certain hole concentration value higher Tc values were obtained with calcium substitution than with oxygen doping. The two different hole-doping methods exhibited also different Tc vs Cu-O bond length relations. As a conclusion, the possible roles of the hole distribution in the in-plane Cu-O bond and the flatness of the CuO2 planes in determining the superconducting properties were recognized.