A transmission electron microscope and group theoretical study of the new Bi-based high-Tcsuperconductors and some closely related Aurivillius phases

Abstract

The Bi-based superconductors of approximate compositions Bi2Sr2CuO6 and Bi2Sr2CaCu2O8, and the closely related Aurivillius phases, Bi2BaNb2O9 and Bi4Ti3O12, are discussed from a group theoretical point of view. The structural complexities associated with these phases are treated in terms of successive modulation of an appropriately chosen parent structure. The space-group symmetries of the two Aurivillius phases and the average-structure space-group symmetries of the two superconductors are interpreted by such an approach. It is proposed that compositional ordering of the oxygens midway between adjacent Bi layers is responsible for the enlarged square root 2ap* square root 2ap average-structure basal-plane cell of the superconductors. In the case of the incommensurate q= gamma a*+ delta c* modulation, the dimensional inflexibility of the CuO4 square-planar sheets is seen as the underlying structural case.