Carbon contents in HgBa2CuO4+δ and starting materials (HgO, BaO, and CuO) for preparing it were determined by infrared spectroscopy to examine influences of CO32- ions substituted for [HgO2]2- groups on its superconducting properties. Measurements of DC magnetic susceptibilities and resistivities revealed that superconducting transition in an oxygen-annealed sample with a minimum C content of 0.88 mol% was very sharp with Tc 's of 98.0 K (negative magnetization) and 96.5 K (zero resistance), respectively. They are the highest Tc 's that have ever been attained in HgBa2CuO4+δ including single crystals. Structural data obtained by neutron powder diffraction showed our sample to have high quality and a δ value of 0.122(5).
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