Raman scattering from ${\mathrm{Ba}}_2$Y${\mathrm{Cu}}_3$${\mathrm{O}}_7$ 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 ${\mathrm{Ba}}_2$Y${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ ($T_c\approx 90$ K) and ${\mathrm{Ba}}_2$ ${\mathrm{Y}}_{0.75}$ ${\mathrm{Sc}}_{0.25}$ ${\mathrm{Cu}}_{2.4}$ ${\mathrm{Ni}}_{0.6}^{+2\mathrm{}}$ ${\mathrm{O}}_7$ ($T_c\approx 65$ K) are reported and assigned. A Raman line observed at 632 ${\mathrm{cm}}^{-1\mathrm{}}$ 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 ${\mathrm{cm}}^{-1\mathrm{}}$ which gains intensity below 150 K but disappears in the superconducting phase. In the Ni-alloyed material the Raman results suggest that the ${\mathrm{Ni}}^{2+}$ ions primarily occupy Cu sites in the Cu-O chains.