Change in phonon Raman spectra of Bi₂Sr₂Ca_{1 - x}Y_xCu₂O_{8 + d} induced by hole filling and implications for phonon assignments

The phonon Raman spectra of Bi₂Sr₂Ca_{1 - x}Y_xCu₂O_{8 + d} (x = 0-1) have been investigated in a number of well-defined single crystal and polycrystalline samples. From the polarization and Y doping dependence, and from a comparison with previous reports on Bi-based cuprates, we identify the (6A_1g + 1B_1g) symmetry modes that are Raman allowed within the ideal body-centered tetragonal unit-cell. The vibrational modes that involve atoms in the CuO₅ pyramid exhibit drastic changes with increasing Y doping: the bridging-oxygen phonon softens strongly, whereas the CuO₂-plane phonons both harden and increase in intensity with increasing Y content. Although the intensity increases can be attributed to the reduced metallic screening in the hole-depleted Y doped samples, the phonon frequency variations are mainly due to the structural changes induced by the decrease in the average Ca/Y ion radius.