Superconducting and structural properties of nearly carbonate-free HgBa₂CuO_4+δ

Nearly carbonate-free HgBa₂CuO_4+δ (Hg-1201) was prepared from high-purity starting materials BaO, HgO and CuO. We heated BaCO₃ at different temperatures to optimize conditions of producing BaO. The residual carbon content in the purified BaO is less than 0.1 at.%. With the high-purity BaO, we could raise the temperature of sintering Hg-1201, which led to a higher T_c, a sharper superconducting transition and good crystallinity. An optimally doped sample showed a T_c of 98.0 K in susceptibility and 97.0 K in resistivity (zero-resistance). By changing the annealing condition carefully, samples with a wide range of doping levels were obtained without degradation, and their superconducting properties were studied. Rietveld refinement of neutron powder diffraction data for an optimally doped sample revealed a slight deficiency in [O-Hg-O]^2- groups, giving a chemical formula of Hg_0.970Ba₂CuO_4.059 (CO₃)_0.0088. This defect structure model provides a hole concentration, p, of 0.20 which is a more reasonable value than p=0.25 obtained in our previous model on the assumption of a stoichiometric cation composition.