The superconducting phase HgBa2CaCu2O6+δ with a sharp transition has been prepared and studied. Through studies of X-ray diffraction patterns, transmission electron microscopy, magnetization, and resistivity, we found that it has a tetragonal symmetry with lattice parameters a=3.862(1) A ̊ and c=12.707(5) A ̊, with a main superconducting transition at ≈ 112 K prior to oxygenation and ≈ 117 K after prolonged oxygenation, and has a rather low irreversibility line. No superstructures of the type previously reported by Schilling et al. and Gao et al. in the >130 K phase were detected in these samples.
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