Raman-active phonons in Phonon assignment and charge-redistribution effects

Phonon Raman spectra of (Formula presented) single crystals with (Formula presented) 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 (Formula presented)-axis polarized configuration. We identify the (Formula presented) 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 (Formula presented) mode at (Formula presented) softens by (Formula presented) between (Formula presented) and (Formula presented) We attribute this softening to a weakening of the (Formula presented) bond due to a reduction in Cu and Bi valences when going from the overdoped (Formula presented) to the underdoped (Formula presented) regime.