Abstract
The effects of substitution for copper in the system YBa2Cu3-xMxO7-y, with M=Ni, Zn and Ni12/Zn12/, and 02 planes are not the predominant factors for the suppression of Tc in the case of substitution for Cu(2). Semi-quantitative analyses reveal that the Zn and Ni ions act as impurity scattering centres with a radius of scattering cross larger than the average Cu-O bond length of the CuO2 planes, which directly destroys the superconducting path in CuO2 planes and decreases the superconducting energy gap. This may be the main reason for the suppression of superconductivity.