Raman scattering in MBa₂Cu₃O₇ with M=Y, Eu, and Gd: effect of ¹⁸O substitution and oxygen vacancies on the Cu-O vibrational modes

Abstract

We have studied the Cu-O vibrations in the MBa₂Cu₃O₇ 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^-1 frequency region exhibit isotopic shifts upon ¹⁸O substitution implying that these modes involve oxygen vibrations. In oxygen-deficient YBa₂Cu₃O_7-δ samples prepared in the range 0<δ<0.7 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 cm^-1 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 cm^-1 in oxygen-deficient samples suggest that Raman spectroscopy could be used for characterizing oxygen stoichiometry of high-T_c superconductors.