Incommensurate superlattice and 90degrees twist boundaries in the superconducting phase of Bi-Sr-Ca-Cu-O.

We have observed an incommensurate superlattice structure in the Bi-Sr-Ca-Cu-O system by electron diffraction. The wave vector of the superlattice, q = (O, 1 over (4.7), 1), has an incommensurate component along the b-axis and a commensurate component along the c-axis. Twist boundaries perpendicular to the c-axis, across which the crystallographic a and b-axes are rotated 90degrees, are commonly observed in this system.