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 diffracti on and superconducting quantum interference device experiments in the wide substitution ranges of 0 less than or equal to x less than or equal to 0.35 and 0 < z < 1. The effectiveness of the two different ways to introduce ho les into the CuO2 planes was compared both in respect to the capability to increase T-c 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 cert ain hole concentration value higher T-c values were obtained with calcium s ubstitution than with oxygen doping. The two different hole-doping methods exhibited also different T-c 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 propert ies were recognized. [S0163-1829(99)06929-5].