Raman spectra of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathrm{x}}$ superconductors with different oxygen content

The Raman spectra of high-purity samples of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathrm{x}}$ were studied as a function of oxygen content 6<x<7. The most important changes occurred in the spectral range corresponding to the stretching vibrations of Cu-O bonds. At the transition from the superconducting orthorhombic to the semiconducting tetragonal phase, x=6.5, the intensity of the highest-frequency mode at 632 ${\mathrm{cm}}^{\mathrm{-}1}$ drops drastically as oxygen is removed from the Cu(1)-O(1) chains. The two Raman modes at 440 and 500 ${\mathrm{cm}}^{\mathrm{-}1}$ merge into a single mode at 472 ${\mathrm{cm}}^{\mathrm{-}1}$. The correlation between the phase transition and changes in the spectra is discussed.