Possible Model of the Modulated Structure in High-T_c Superconductor in a Bi-Sr-Ca-Cu-O System Revealed by High-Resolution Electron Microscopy

Abstract

A model for the modulated crystal structure (Matsui et al. : Jpn. J. Appl. Phys. in press) in high- T c superconductor in a Bi-Sr-Ca-Cu-O system is proposed based on the high-resolution electron microscope images. Ideally, it is body-centered orthorhombic with a =0.54, b =5 a =2.7 and c =3.08 nm. The Bi 2 O 2 layers do not form complete sheets, but contain occupational and/or positional fluctuation of Bi-sites to form “Bi-concentrated bands” which extend in the a -direction. Contraction of lattice planes along the b -axis is induced in the Bi-concentrated bands, causing compensating expansion in the remaining Bi-deficient ones.