Raman-active phonons in Bi₂Sr_2-xLa_xCuO_6+d: Phonon assignment and charge-redistribution effects

Phonon Raman spectra of Bi₂Sr_2-xLa_xCuO_6+d single crystals with x values close to 0, 0.4, and 0.8 have been investigated. Room-temperature measurements were performed in the five main polarization geometries, including the c-axis polarized configuration. We identify the 4A_1g symmetry modes that are Raman allowed within the ideal body-centered-tetragonal unit cell, as well as several disorder activated phonon bands from the BiO and/or SrO layers. For an increasing degree of La doping, the O(2)_Sr A_1g mode at 624 cm^-1 softens by 12 cm^-1 between x=0 and x=0.8. We attribute this softening to a weakening of the Cu-O(2)_Sr-Bi bond due to a reduction in Cu and Bi valences when going from the overdoped (x=0) to the underdoped (x=0.8) regime.