RAMAN-ACTIVE PHONONS IN BI2SR2-XLAXCUO6- PHONON ASSIGNMENT AND CHARGE-REDISTRIBUTION EFFECTS(D )

Phonon Raman spectra of Bi2Sr2-xLaxCuO6+d single crystals with x value s close to 0, 0.4, and 0.8 have been investigated. Room-temperature me asurements were performed in the five main polarization geometries, in cluding the c-axis polarized configuration. We identify the 4A(1g) sym metry modes that are Raman allowed within the ideal body-centered-tetr agonal unit cell, as well as several disorder activated phonon bands f rom 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 fr om the overdoped (x = 0) to the underdoped (x = 0.8) regime.