Effects of Carbonate Defects on the Superconductivity of HgBa₂CuO_4+δ

Carbon contents in HgBa₂CuO_4+δ and starting materials (HgO, BaO, and CuO) for preparing it were determined by infrared spectroscopy to examine influences of CO₃^2- 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 T_c 's of 98.0 K (negative magnetization) and 96.5 K (zero resistance), respectively. They are the highest T_c 's that have ever been attained in HgBa₂CuO_4+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).