Precision neutron-diffraction study of the highssuperconductors

A neutron-diffraction study of ${\mathrm{HgBa}}_{2}$ ${\mathrm{CuO}}_{4+\mathrm{\ensuremath{\delta}}}$ structure and its dependence on extra oxygen content for \ensuremath{\delta} between 0.06 and 0.19 has been carried out with four pure samples using high-resolution TOF and \ensuremath{\lambda}=const diffractometers at room temperature and T=8 K. Sample characterization was done by measuring ac susceptibility, x-ray and EPR spectra, and by iodometric titration. A detailed analysis of atomic structure and the occupancy factors of mercury and oxygen was performed and showed the stoichiometric cation content. Extra oxygen was found only in the (1/2,1/2,0) position of the basal plane. ${\mathrm{T}}_{\mathrm{c}}$ varied parabolically with \ensuremath{\delta} and exhibited a maximum of 98 K at \ensuremath{\delta}\ensuremath{\approx}0.12. The data obtained can be considered as evidence of the conventional anion doping mechanism with 2\ensuremath{\delta} holes per ${\mathrm{CuO}}_{2}$ layer.