Raman spectra of bismuth cuprate high-Tc superconductors doped with thulium and lead

Raman spectra of Bi4Sr4Ca2-xTmxCu4O16+y and Bi2-xPbxSr2 Ca2Cu3Oy are presented as a function of increasing x. For Bi4Sr4Ca2-xTmxCu4O16+y, the period of superstructure modulation decreases somewhat and oxygen content in the Bi-O planes increases as x increases; for Bi2-xPbxSr2Ca2Cu3Oy, more lead (increasing x) has the opposite effect of increasing the modulation period and decreasing the oxygen content of the Bi-O layers. More modes are seen in these spectra than are predicted from an analysis based on the space group of the undistorted cell; these extra modes are discussed in terms of group theory predictions of a commensurate supercell with dimensions 4 or 5 times the basic lattice spacing. Extra oxygen in Bi4Sr4Ca2-xTmxCu4O16+y causes the high frequency mode, associated with vibration of oxygen in the Bi-O layers, gradually to broaden and then split. At higher lead additions to Bi2Sr2Ca2Cu3Oy, spectra are consistent with formation of a new phase due to limits on lead solubility; the low frequency modes all but disappear while the high frequency mode is strongly shifted down. Other details of the spectra confirm the necessity of considering the superstructure as well as the symmetry of the undistorted unit cell.