Superlattice structures in solid solution of high-T_c YBa₂Cu₃O_7-δ and (Pb,Cu)Sr₂(Ca,Y)Cu₂O_7-δ superconductors

A complete solid solution range exists between the systems YBa₂Cu₃O_7-δ and (Pb,Cu)Sr₂(Ca,Y)Cu₂O_7-δ has been found with general stoichiometry (Pb_0.75xCu_1-0.75x)(Sr_2xBa _2-2x)(Ca_0.5xY_1-0.5x)Cu₂O _7-δ. Energy dispersive spectrometry and X-ray diffraction identified that a true solid solution exists. Superlattice structures observed by electron diffraction across the solid solution range have a modulation range have a modulation periods along a* which can be varied by altering both the compositional parameter x and the overall oxygen content. The existence of these superlattices infers that the solid solution is non-random and therefore thermodynamically non-ideal. The superconducting transition temperatures, T_c, across the solid solution range are also strongly dependent on the composition, x, but no direct relationship wit h the modulation period has been established. From these studies it may be concluded that the solid solution between known superconductors is possible, although involving some partial ordering of the lattice, but ordering of cations in the rock-salt to charge reservoir layer is not a significant factor in determining the superconducting properities, which depend more closely on the overall composition and hence on the ability of the charge reservoir layer to transfer charge to the superconducting layers.