Polarized Raman measurements of ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Pr}}_{\mathit{x}}$${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

We report polarized Raman measurements on small single crystals embedded in ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Pr}}_{\mathit{x}}$ ${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ polycrystalline pellets. The measurements show two-mode behavior in the Cu-O bending mode of the ${\mathrm{CuO}}_2$ planes (${\mathit{B}}_{1\mathit{g}}$ mode). The increase in frequency with x of the Ba-Ba stretching mode and the chain copper–apical oxygen Cu(1)-O(4) stretching mode is consistent with hole localization on Ba sites due to Ba 5d–O 2p covalency. This localization may be related to the suppression of superconductivity in this compound, which increases with increasing x.