CHANGE IN PHONON RAMAN-SPECTRA OF BI2SR2CA1-XYXCU2O8+D INDUCED BY HOLE FILLING AND IMPLICATIONS FOR PHONON ASSIGNMENTS

The phonon Raman spectra of Bi2Sr2Ca1-xYxCu2O8+d (x=0-1) have been inv estigated in a number of well-defined single crystal and polycrystalli ne 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 idea l body-centered tetragonal unit-cell. The vibrational modes that invol ve atoms in the CuO5 pyramid exhibit drastic changes with increasing Y doping: the bridging-oxygen phonon softens strongly, whereas the CuO2 -plane phonons both harden and increase in intensity with increasing Y content. Although the intensity increases can be attributed to the re duced metallic screening in the hole-depleted Y doped samples, the pho non frequency variations are mainly due to the structural changes indu ced by the decrease in the average Ca/Y ion radius.