The replacement of [HgO2]2- by the tetrahedral sulfate anion [SO4]2- in the Hg-1201 superconductor

The possibility of existence of a Hg1-xSxBa2CuO4+2x+δ solid solution [(Hg, S)-1201] was studied at 18 kbar and 920 degrees C. Monophasic samples were obtained in the range 0.00 < x < 0.15, where the unit cell parameters were found to vary from a=3.8685 (1) Angstrom, c=9.4667(2) Angstrom for x = 0.00 to a=3.9010(1) Angstrom and c = 9.3048(6) Angstrom for a nominal composition x = 0.30. The as-prepared phases with 0.00 < x < 0.05 were found to be nonsuperconducting due to overdoping, while superconductivity appeared at x = 0.10 (Tc,onset=40K). Tc increases with the amount of sulfate substitution due to the decrease of the average copper valence. For the annealed samples, the variation is opposite to that found for the as-prepared samples, Tc,onset being higher for the sulfur-free phase. Rietveld refinements of neutron powder diffraction data has determined ∼ 18% of the HgO22- dumbbells to be substituted by SO42- groups. In this structure the SO42- anion adopts an average S-O bond length that is slightly higher than those in BaSO4