DISORDER IN THE BIO-SUBLATTICE OF BI2SR2CAN-1CUNO2N+4+Z PHASES

Defects in the crystal structure of Bi2Sr2Can-1CunO2n+4+z were characterized by high-resolution transmission electron microscopy (HREM) and optical diffraction techniques. It was found that in addition to the well-known intergrowth defects of the n = 3 phase, another type of defect concerning mainly the BiO sublattice of the n = 1, 2 and 3 phases is present. These phases consist of a sequence of lamellae (on a nanometer scale) parallel to the c-axis with two different types of crystal structure, the B-centered intrinsic structure and the A-centered faulted structure. Besides these two centered crystal structures, a shear in the b-direction between the BiO layers was observed in the b-c plane. Because of this, non-periodic shear reflections appear, which are compatible only with a primitive Bravais lattice. In these grains regions of different size form domains of incommensurate structure, separated by (001) boundaries. The phase of the displacement field in these domains changes from one domain to another by pi/4.7. In addition, the results presented in this paper fully explain the discrepancies observed in the previous determinations of the crystal structure of Bi2Sr2CaCu2O8+z.