Energetics of Superstructures in the Bi-Series of Superconducting Compounds
The existence of structural patterns in Bi and Tl-containing superconducting compounds has been well established. In this work, we concentrate on Bi-containing compounds whose stoichiometries are described by the homologous series 3AO-nA’BO3 (3A=Bi2Sr and nA’=SrCa n−1, B=Cu). The first three compounds of this series commonly known as 2021 (Bi2Sr2CuO6), 2122 (Bi2CaSr2Cu2O8), and 2223 (Bi2Ca2Sr2Cu3O10) (corresponding to n = 1, 2, and 3 respectively) have well been studied and the Tc of these compounds has been found to increase with n (at least for n ≤ 3) in the general formula. In other words, Tc apparently is correlated with the number of Cu-O planes present in the perovskite layer of the unit cell.
Structural information on higher order members is, however, incomplete. Based on the structural patterns of the homologous series, model structures and stoichiometries of the first five members (n=1 to 5) were setup. We have investigated the energetics of both model and XRD (where known) structures using atomistic simulation techniques. Preliminary results, using a pairwise potential model, revealed large differences between X-ray and model structures which indicate that distortions and buckling in the planes seen by XRD and which have been ignored in the model structures, are associated with large relaxation energies. These may contribute to the limited phase stability seen in this system. Our results also highlight the need for very accurate interatomic potentials, which we are in the process of developing. An explanation of observed buckling in the Cu-O planes of higher order compounds is also suggested.
KeywordsHomologous Series Interatomic Potential Strontium Titanate Ceramic Superconductor Body Potential
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