Raman-active phonons in Bi₂Sr₂Ca_n-1Cu_nO_2n+4d (n = 1,2,3) - Phonon assignment and charge redistribution effects

The phonon Raman spectra of Bi₂Sr₂Ca_n-1Cu_nO_2n+4d (n = 1,2,3), high-T_c superconductors have been investigated in a number of single-crystal and polycrystalline samples. From the polarization and doping dependence, and from a comparison with previous reports on other high-T_c cuprates, we identify the A_1g and B_1g symmetry modes that are Raman allowed within the ideal body-centered-tetragonal unit cell. On the basis of the assignments proposed here, we discuss the influence of aliovalent substitutions on the phonon modes in Bi₂Sr_2-xLa_xCuO_6+d and Bi₂Sr₂Ca_1-YY_yCu₂O _8+d. We interpret the observed changes in phonon frequencies and intensities as a combination of internal pressure and charge redistribution effects induced by doping.