Structure and superconductivity of ${\mathrm{HgBa}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{8+\mathrm{\delta }}$

We have investigated the structure, defect concentration, and superconducting properties of ${\mathrm{HgBa}}_2$ ${\mathrm{Ca}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{8+\mathrm{\delta }}$ by neutron powder diffraction, thermogravimetric analysis, and ac susceptibility. Samples with three oxygen contents and ${\mathit{T}}_{\mathit{c}}$’s ranging from 135 to 94 K, obtained through different annealing conditions, were studied. Two defect sites were observed in all samples. The concentration of defect oxygen atoms located at a site in the center of the Hg-${\mathrm{O}}_{\mathrm{\delta }}$ plane, is found to vary from 0.18(2) to 0.10(2) oxygen atoms per formula unit as ${\mathit{T}}_{\mathit{c}}$ decreases from 135 to 94 K. An additional defect, which we interpret as oxygen atoms associated with the substitution of Cu on the Hg site, is found to have a fixed concentration of ∼0.09(2) oxygen atoms per formula unit.