RAMAN-SPECTRA OF BISMUTH CUPRATE HIGH-TC SUPERCONDUCTORS AND 3D-METAL-SUBSTITUTED PHASES

Raman spectra are presented for Bi2Sr2CuO6+y, Bi2Sr2CaCu2O8+y, and compounds formed by cationic substitution of Cu by Fe, Mn, and Co. For n = 1 materials, the spectra show a greater number of modes than those predicted by group theory using the undistorted pseudotetragonal unit cell. These extra modes, however, are consistent with group-theory predictions based on the space group of the supercell of Bi2Sr2CoO6+y, which is caused by commensurate modulation over several periods of the smaller unit cell. Similar analysis is mode of the supercell of Bi2Sr2SrFe2O8+y; predictions are consistent with the rearrangement of low-frequency modes as compared with Bi2Sr2CaCu2O8+y. Accompanying changes in oxygen stoichiometry cause strong shifts in the corresponding high-frequency oxygen stretch modes in both n = 1 and 2 materials. Additional substitution of strontium by calcium in n = 2 compounds leads to a more disordered structure, with consequent broadening of all modes in the spectrum. Overall, consideration of the supercell structure where it exists is essential in interpreting Raman spectra from the superconducting cupric oxides and their substituted compounds.