Effect of fluorination on the structure and superconducting properties of the hg-1201 phase

A fluorination of the reduced $\mathrm{HgBa}{}_{2}{\mathrm{CuO}}_{4+\ensuremath{\delta}}(\mathrm{Hg}\ensuremath{-}1201)$ with ${T}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}61\mathrm{K}$ 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 $\mathrm{HgBa}{}_{2}{\mathrm{CuO}}_{4}\mathrm{F}{}_{\ensuremath{\delta}}$ showed twice the amount of extra fluorine in comparison with those for the oxygenated $\mathrm{Hg}\ensuremath{-}1201$ phases with close ${T}_{c}$. This supports the ionic model of doping in $\mathrm{Hg}\ensuremath{-}1201$: two holes per oxygen and one hole per fluorine. The apical distances in $\mathrm{Hg}\ensuremath{-}1201$ 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 $\mathrm{Cu}\ensuremath{-}\mathrm{O}$ distances, while the in-plane distances and ${T}_{c}$ do not vary.