Effect of Fluorination on the Structure and Superconducting Properties of the Hg-1201 Phase

A fluorination of the reduced HgBa₂CuO_4+δ(Hg-1201) with T_c = 61K resulted first in an increase in T_c up to 97 K and then in a decrease and even a suppression of superconductivity due to overdoping. Neutron powder refinements performed on fluorinated HgBa₂CuO₄F_δ showed twice the amount of extra fluorine in comparison with those for the oxygenated Hg-1201 phases with close T_c. This supports the ionic model of doping in Hg-1201: two holes per oxygen and one hole per fluorine. The apical distances in Hg-1201 depend on the number of dopant atoms rather than on their charge. The exchange of extra oxygen for a double amount of fluorine shortens the apical Cu-O distances, while the in-plane distances and T_c do not vary.