A 120 K-class superconductor in the Tl-Ba-Ca-Cu-O system was synthesized, and the flux exclusion proved to be greater than 90% of the perfect diamagnetism. X-ray and electron diffraction studies showed a tetragonal unit cell with lattice parameters a=5.4454(7) Å and c=35.586(9) Å, indicating triple perovskite-like layers interposed between Tl2O2layers. A structure model is proposed, in which the layer sequence along the c-axis is Tl2O2-BaO-CuO2-Ca-CuO2-Ca-CuO2-BaO. The chemical composition is Tl2Ba2Ca2Cu3O10+xassuming trivalent thallium. If the same average Cu valence as in YBa2Cu3O7is assumed, this composition requires x=0.5 for oxygen.
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