Resonant Raman scattering of oxygen-deficient ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$: Evidence for the coexistence of ortho-I, ortho-II, and tetragonal microstructures

The polarized Raman spectra of a single crystal of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (δ∼0.6) at 15 K were studied using the 647.1, 568.2, 530.9, and 482.5-nm ${\mathrm{Kr}}^{+}$ laser lines. The results give clear evidence for a multiphase scattering volume with the main microstructure phases corresponding to ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ (OI), ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.5}$ (OII), and ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_6$ (T). Significant differences in the frequencies of the OI-, T-, and OII-related modes, as well as variations of the mode intensities with ħ${\mathrm{\omega }}_{\mathit{L}}$ and scattering geometry, result in apparent shifts of the Ba, Cu(2), and O(4) lines.