Superstructure of the superconductor Bi₂Sr₂CaCu₂O₈ by high-resolution electron microscopy

Abstract

The structure of the 80-K superconductor Bi₂Sr₂CaCu₂O₈ (ref. 1) determined by X-ray and neutron diffraction^2–4 is an Aurivillius-related phase as suggested by Michel et al.⁵, with approximate unit-cell dimensions a = √2a_p, b = 5√2a_p and c = 8a_p, where the perovskite lattice parameter a_p = 3.8 Å. It consists of layers of perovskite sandwiched between BiO layers. Although electron microscopy does not have sufficient resolution to unambiguously determine the basic structural unit of such a compound, it is an invaluable tool in determining the superstructure⁶. Here we present a high-resolution electron microscope study which reveals waves of distortion along the b-axis, giving a superlattice slightly larger or smaller than 5√2a_p. The superlattice is composed of building blocks four, five, and six times √2a_p/2 in approximately periodic combinations. The very low 'twin' density in Bi₂Sr₂CaCu₂O₈ and the nature of the 'twins' (successive planes with the a and b axes interchanged, also called twist boundaries) implies that twinning does not play an essential role in the superconducting phenomenon.