Incommensurate superlattice and 90° 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 using electron diffraction. The wave vector of the superlattice, q=(0,1/4.7,1), 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 90^∘{}, are commonly observed in this system.