Raman scattering and X-ray diffraction in Ni and Zn substituted YBa2Cu3O6+δ

A systematic Raman scattering and X-ray diffraction study has been performed on a series of YBa₂Cu_3-xM_xO_6+δ (M=Ni, Zn) superconductors of different doping concentrations x (x=0, 0.05, 0.1, 0.2 and 0.3). We have investigated changes of the structure and of the oxygen vibrational modes induced by substitution of the divalent transition elements Ni and Zn. The structure remains orthorhombic throughout the investigated concentration range for both Ni and Zn substituted samples, without any significant change of the lattice parameters. The Raman spectrum, and especially the O(4) axial stretching vibration, is unaffected by Ni and Zn substitution indicating that Ni and Zn occupy the Cu(2) site in the CuO₂ plane and that the charge balance between the chains and the planes is unaltered. It is argued that the decrease of T_c for increasing M concentration is caused by a mechanism different from that of trivalent substitutions on the Cu(1) site.