We have prepared and identified as a single phase the high-temperature superconducting compound in the chemical system Y-Ba-Cu-O, an orthorhombic, distorted, oxygen-deficient perovskite of stoichiometry Ba2YCu309q (6=2.1). Samples exhibit zero resistance at 91 K, with a transition width of 1.5 K. The Meissner eAect attains a value of 76% of the independently measured diamagnetic susceptibility. We estimate parameters that characterize this superconductor, e.g. , @=3-5 mJ (mole Cu) K 2. The critical current density at 77 K and H =0 exceeds 1100 A/cm .

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For the first time, as far as we can determine, an infrared (IR)

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