Structure of Bi_2.1(Ca,Sr)_n+1Cu_nO _2n+4+δ superconductors by electron diffraction

The discovery of a new class of Bi-based oxides with high T_c has given fresh impetus to the research in this area. This family of oxides can be described by the more general formula Bi_2.1(Sr,Ca)Cu _nO_{2n+4+ delta}. Electron diffraction has shown that these compounds have a very clear superstructure along the b axis. Although several models have been proposed, no agreement has been reached on the type of superstructure. In the present work the authors report a theoretical and experimental study of the electron diffraction patterns from these compounds. It is concluded that the superstructure is consistent with a long period structure based on planar defects such as antiphase boundaries. Calculated diffraction patterns using this model are in full agreement with the experimental results. They therefore conclude that the incommensurate structure along the b axis observed in these materials can be explained in terms of a simple defect structure.