HREM Study of Bi-Oxide Based High T(sub C) Superconductors

摘要

A HREM study of the superstructure and structural defects in Bi/sub 2-x/Pb/sub x/Sr/sub 2/CaCu/sub 2/O/sub 8/ and Bi/sub 2-x/Pb/sub x/(SrCa)/sub 2/CuO/sub 6/ (for x=0 and x=0.4) is presented. The superstructures are shown to involve waves of distortion along the b-axis. These waves are locked on to the lattice positions so that they have a local wave length of 4,5,6,7 or 8 times the basic unit cell. The superstructures are composed of roughly periodic combinations of these basic building blocks (waves). The lead doped superconductors show a less pronounced but more complicated superstructure along b where two distinct periodicities close to 4 and 7 times the basic unit cell predominate. In each of the compounds studied the superstructure, in the better ordered crystals, is shown to be commensurate with a unit cell between 5 and 23 times the basic unit cell. The superstructure in Bi/sub 2-x/Pb/sub x/Sr/sub 2/CaCu/sub 2/O/sub 8/ is orthorhombic where as the superstructure in Bi/sub 2-x/Pb/sub x/(SrCa)/sub 2/CuO/sub 6/ is monoclinic. All these superconductors have a low ''twin'' density (ie density of 90/sup 0/ twist boundaries). They exhibit many dislocations and dislocation arrays associated with the weak bonding between BiO planes. They also form non-stoichiometric stacking faults with local changes in the c-axis spacing between layers. (ERA citation 14:015244)