Raman scattering from Ba2YCu3O7 above and below the 90-K superconducting transition: Evidence for oxygen vacancies in the Cu-O chains

Raman scattering data from different types of samples of the normal and superconducting phases of Ba2YCu3O7 (Tc90 K) and Ba2Y0.75Sc0.25 Cu2.4Ni0.6+2O7 (Tc 65 K) are reported and assigned. A Raman line observed at 632 cm-1 is assigned to the stretching mode of the Cu-O chains. This mode is Raman inactive in the ordered O-deficient lattice but appears to be activated by oxygen vacancies along the chains. Another sample-dependent line is observed at 365 cm-1 which gains intensity below 150 K but disappears in the superconducting phase. In the Ni-alloyed material the Raman results suggest that the Ni2+ ions primarily occupy Cu sites in the Cu-O chains.