Raman scattering inMBa2Cu3O7withM=Y,Eu,andGd: Effect ofO18substitution and oxygen vacancies on the Cu-O vibrational modes

We have studied the Cu-O vibrations in the $M{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ systems with $M=\mathrm{Y},\mathrm{E}\mathrm{u},\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{G}\mathrm{d}$ by Raman spectroscopy. The observed Raman features are similar in the three systems. The four Raman bands in the 300-700 ${\mathrm{cm}}^{-1\mathrm{}}$ frequency region exhibit isotopic shifts upon ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ substitution implying that these modes involve oxygen vibrations. In oxygen-deficient $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ samples prepared in the range $0<\mathrm{}\delta <0.7\mathrm{}$ within the orthorhombic structure, the $A_{1g}$ axial stretch mode is found to undergo both a softening and broadening with increasing oxygen deficiency. Furthermore, the Raman feature in the 550-600 ${\mathrm{cm}}^{-1\mathrm{}}$ region shows an increase in intensity. The latter is attributed to disorder activation of the Cu-O chain mode, normally forbidden in the stoichiometric compound. These features and the emergence of a distinct Raman peak near 390 ${\mathrm{cm}}^{-1\mathrm{}}$ in oxygen-deficient samples suggest that Raman spectroscopy could be used for characterizing oxygen stoichiometry of high-$T_c$ superconductors.