Phonon Raman scattering of Bi₂Sr₂CaCu₂O_8+d under hydrostatic pressure

Phonon Raman scattering from slightly overdoped Bi₂Sr₂CaCu₂O_8+d samples have been investigated under hydrostatic pressure up to 7.2 GPa. Although most observed phonons harden linearly with increasing pressure, the A_lg-type Cu mode at 140 cm^-1 exhibits a nonlinear hardening with an inflection point around 2 GPa, similar to the dependence of T_c versus pressure. We also find that the mode-specific Grüneisen parameters scale inversely with the corresponding vibrational frequencies, a trend similar to what is observed for molecular solids. The data are interpreted in terms of a nonuniform structural compressibility that results in a redistribution of charge carriers within the Bi₂Sr₂CaCu₂O_8+d unit cell.