HIGH-RESOLUTION ELECTRON-MICROSCOPY OF TRICLINIC MODULATIONS IN BISMUTH BARIUM CUPRATES

In order to investigate the role of incommensurate modulations in high -T(c) superconductor oxide structures, a compound with bulk compositio n Bi2Ba2NdCu2O8+delta has been investigated by high-resolution transmi ssion electron microscopy (HRTEM) as well as selected area electron di ffraction (SAD) techniques. The SAD patterns reveal a tetragonal unit cell of this material, with parameters a = 5.60 angstrom and c = 31.4 angstrom; these are significantly larger than those of the bismuth-str ontium analogue, Bi2Sr2CaCu2O8. The HRTEM images and the satellites (i .e. their positions as well as their intensities) observed in the SAD patterns suggest that the basic structure is modulated by two differen t types of modulation. These two superlattices both have triclinic sym metry; however, they differ in their cell parameters.