CRYSTAL DEFECTS IN BI2SR2CAN-1CUNO4+2DELTA CERAMICS

Extended crystal defects in the high-Tc superconducting phases Bi2Sr2Can-1CunO4+2n+δ (n=2 and n=3) were investigated by high-resolution transmission electron microscopy. A high density of stacking faults corresponding to lamellae of the n=1 phase are observed as a consequence of an incomplete solid state reaction. In this reaction a heterogeneous multiphase sample consisting of the n=1 phase and other Sr-Ca-Cu-O compounds reacts to form the n=2 phase. Small stacking faults not bound to certain crystallographic planes are important for the diffusion of the extended chemical defects in this material. In material which contains the n=3 phase as the majority phase, intergrown lamellae consisting of n≠3 phases are observed with high density. Nano-crystallites of the n=1 phase are coherently intergrown with the matrix on (001) planes. At the interfaces various extended crystal defects were observed. Among these were partial edge dislocations which transform two lamellae of phase n into one lamella of phase 2n+2. The climb of these dislocations might be an important mechanism for the formation of phases with n > 3. © 1990.