Raman study of Bi2-xPbxSr2Can-1CunO4+2n+delta (n=2,3) superconductors

We report a Raman study of the n=2 and n=3 Bi2-xPbxSr2Can-1CunO4+2n+delta h igh-temperature super conducting cuprates. We find that the frequency of th e O(2)(Sr) A(1g) mode systematically decreases with additional oxygen in th e weakly coupled BiO layers. We speculate that this could be due to an expa nsion of the Cu-O-Bi bond length caused by an oxygen-induced compression of the weakly coupled BiO layers. It is shown that approximately one half of the decrease in the frequency of the O(2)(Sr) A(1g) mode in Bi2Sr2Ca1-zYzCu 2O8+delta reported by Kahihana et al. can be attributed to the effect of ad ditional oxygen and hence charge transfer effects are negligible for this R aman mode. The effect of Pb on the frequency of the O(2)(Sr) A(18) mode, wh ile being systematic, can not be explained. We attribute the small increase in hole concentration with increasing Pb concentration to a partial charge compensation by a reduction in oxygen content. We provide evidence that th e 655 cm(-1) peak can be attributed to epical oxygen sites at the boundarie s of the supercell induced by the incommensurate modulation in the BiO laye rs.