The structure and defect structure of high-T c superconducting materials in the system Tl-Ba-Ca-Cu-O

Electron diffraction and high resolution electron microscopy have been applied to study Tl₂Ba₂CaCu₂O₈ and Tl₂Ba₂Ca₂Cu₃O₁₀. The structures only differ in the thickness of the perovskite lamellae and have tetragonal basic unit cells witha=0.385,c=2.93 nm anda=0.385,c=3.58 nm, respectively. A modulation with wave vectors in (100)^* and (010)^* planes occurs in both compounds. This modulation of which the intensity is somewhat sample dependent is much weaker than in the Bi compounds and disappears irreversibly upon heating of the specimen. Intergrowth of lamellae of different phases does occur, although not as frequently as in the Bi compounds.Work performed in the framework of The Institute for Materials Science, I.M.S. (Antwerp) with financial support of I.U.A.P.