The influence of Ca doping on the crystal structure superconductivity of orthorhombic YBa2Cu3O7-delta

The changes of the chemical structure of YBa2Cu3O7-delta (delta < 0.04) and of the superconducting transition temperature T-c associated with Ca doping have been investigated by means of neutron and x-ray powder diffraction, oxygen content determination and magnetic susceptibility measurements. In this work, we were able to show that the depression of superconductivity in Ca-substituted orthorhombic YBa2Cu3O7-delta is related to essential structural changes. Using the cation-oxygen distances obtained by neutron diffraction at 1.5 K we calculated the bond valence sums of the cations. This allows us to estimate the charge distribution within the structure and yields a direct correlation between structural changes induced by Ca doping and T-c. Our results show clearly that Ca doping leads to an overdoping of YBa2Cu3O7-delta which cannot be reached by oxygen non-stoichiometry alone.