Phonon Raman scattering of polycrystallineYBa2Cu4O8under hydrostatic pressure

The Raman spectra of polycrystalline ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$ have been measured under various pseudohydrostatic pressures up to 10.6 GPa using a diamond anvil cell at room temperature. Most of the observed phonon scattering peaks shift linearly with increasing pressure. Only the phonon frequency for the in-phase O(2)-O(3) ${\mathit{A}}_{\mathit{g}}$ mode (∼431 ${\mathrm{cm}}^{\mathrm{-}1}$) exhibits nonlinear pressure dependence, which is in contrast to the behavior of the out-of-phase O(2)-O(3) ${\mathit{A}}_{\mathit{g}}$ mode (∼338 ${\mathrm{cm}}^{\mathrm{-}1}$). The frequency shift saturates above ∼6 GPa, which is similar to the pressure dependence of ${\mathit{T}}_{\mathit{c}}$. This suggests that the degree of hybridization between the atomic vibrations of apical and ${\mathrm{CuO}}_2$ plane oxygens may change under a certain pressure.