Phonon-mode characterization of orthorhombic and tetragonal YBa₂YCu₃O_7−δ single crystals by Raman spectroscopy

Phonon spectra have been studied with the use of Raman-scattering techniques in orthorhombic and tetragonal YBa2Cu3O7-δ (δ ≲ 0.8) single crystals with 16O and 18O isotopes for different light polarizations. The isotopic frequency shifts for O phonon modes corresponding to the substitution of 18O for 16O have been determined. All the O phonon modes as well as the Ag modes of Ba and Cu2 have been identified by using factor-group analysis. Relative intensities and polarizations of phonon lines have been analyzed qualitatively. We argue that the bridges connecting Cu2–O2–O3 planes to the Cu1–O1 chains produce preferential polarization of the Ag lines along the c axis.