Raman scattering in MBa sub 2 Cu sub 3 O sub 7 with M = Y, Eu and GD: Effect of sup (18) O substitution and oxygen vacancies on the Cu-O vibrational modes.

We have studied the Cu-O vibrations in the MBa sub 2 Cu sub 3 O sub 7 systems with M = Y, Eu and Gd by Raman spectroscopy. The observed Raman features are similar in the three systems. The four Raman bands in the 300-700 cm sup (-1) frequency region exhibit isotopic shifts upon sup (18) O substitution implying that these modes involve oxygen vibrations. In oxygen deficient YBa sub 2 Cu sub 3 O sub (7-delta) samples prepared in the range 0 delta = 0.7 within the orthorhombic structure, the A sub (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 cm sup (-1) region shows and 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 cm sup (-1) in oxygen deficient samples suggest that Raman spectroscopy could be used for characterizing oxygen stoichiometry of high T sub c superconductors.