Raman-active phonons in Bi₂Sr₂Ca_1-xY_xCu₂O _8+d (x=0-1): Effects of hole filling and internal pressure induced by Y doping for Ca, and implications for phonon assignments

The phonon Raman spectra of Bi₂Sr₂Ca_1-xY_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. A large number of extra "disorder-induced" phonon bands are observed in the ab-plane polarized spectra. In contrast to most previous reports, we argue that the c-axis polarized phonon band around 629 cm^-1 is due to the O(2)_Sr A_1g vibration, while the exclusively ab-plane polarized band around 463 cm^-1 is induced by the O(3)_BiA_1g vibration. With increasing Y doping we find that the vibrational modes involving atoms in the CuO₂ planes rapidly increase in intensity as a result of the reduced metallic screening in the hole-depleted Y-doped samples. We also find that Y substitution gives rise to a substantial hardening of the O(1)_CuA_1g and B_1g phonons by ∼40 cm^-1, whereas the O(2)_SrA_1g phonon is found to soften by ∼20 cm^-1, when x increases from 0 to 1. The phonon frequency changes can be explained by the "internal pressure" induced by the decrease in the average Ca/Y ion size and an additional "charge-transfer" induced by the change in the Cu and Bi valences with Y doping.