Nearly carbonate-free HgBa2CuO4+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 Tc, a sharper superconducting transition and good crystallinity. An optimally doped sample showed a Tc 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 Hg0.970Ba2CuO4.059 (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.
ASJC Scopus subject areas