RAMAN INVESTIGATION OF PB-SUBSTITUTED BI2SR2CU6-CRYSTALS(DELTA SINGLE)

Raman scattering experiments have been carried out to investigate the vibrational spectrum of Pb-doped Bi2Sr2CuO6+delta single crystals with molar Pb concentrations x in the range 0 less than or equal to x less than or equal to 0.4. The intensities and frequencies of several of t he vibrational modes, with frequencies omega in the range 15 less than or equal to omega less than or equal to 650 cm(-1), are found to depe nd sensitively on the degree of Pb substitution. This information has been used to clarify, and in some cases confirm, the vibrational origi n of the modes. The introduction of Pb into the single crystals also r educes the structural modulation that is present in the undoped crysta ls. The degree of modulation was monitored using electron diffraction techniques and it was found that the modulation vanished at a Pb dopin g level of x approximate to 0.35. The influence of the modulation on t he Raman spectra was investigated and it was found that two modes at 3 0 and 52 cm(-1), which were previously associated with the structural modulation, soften with Pb doping but are still present in spectra obt ained from unmodulated samples. It is suggested that these modes arise from rigid layer vibrations, similar to those observed in layered dic halcagonides.