Effect of Sr and Co doping on structure and phonon raman spectra of YBa₂Cu₃O_7-d

The effect of cosubstitution of Sr for Ba and Co for Cu in the superconductor YBa_2-ySr_yCu_3-xCo_xO_7-d on Raman spectra and relevant bond distances deduced with neutron diffraction measurements has been investigated. Specifically the 0(4) stretching 500 cm^-1 Raman frequency shifts, on the one hand, upward upon Sr doping, on the other hand downward upon Co doping. The net effect is interpreted by a weakening of the net force constant for the O(4) atom involving both the Cu(1)-O(4) and Cu(2)-O(4) bonds, and the O(4) stretching vibration is found to systematically change with the Cu(1)-Cu(2) bond distance. A brief discussion is made on the relationship between Tc and the structural changes induced by the dopants.