A new modelling approach to superconductor layered structures

The analysis of the structural data of high Tcsuperconducting cuprates (HTSCs) shows that large differences exist in X-ray diffraction (XRD) patterns collected from powders and thin films with respect to the single crystal case, depending in many cases on the synthesis routes and material treatments. These differences are often disregarded and the assessment on the purity of the crystalline phase is made on the basis of XRD pattern indexing only. However, when relative intensities of XRD reflections belonging to the same family (e.g. (001)) differ from those obtained from the single crystal structure, the simple pattern indexing is not sufficient for the structural characterization and important structural features are missed. These might be helpful for discussing magnetic or electrical behaviour of specimens, as the related properties may depend on the structural correlation along the caxis. In this article a structural modelling of cuprates of the Bi–Sr–Ca–Cu–O family is proposed by considering the electronic density in the projection perpendicular to the layers of the structure. In particular, the structures of Bi-2201, Bi-2212 and Bi-2223 HTSCs were reduced to monomers and intergrowth effects are simulated by generating polymers with different percentages of these phases. By using this model, much information in terms of intergrowth and site occupancy can be gained from a careful analysis of intensities and widths of 001 reflections.