Topotactic Fluorine Insertion into the Channels of FeSb₂O₄-Related Materials

This paper discusses the fluorination characteristics of phases related to FeSb₂O₄, by reporting the results of a detailed study of Mg_0.50Fe_0.50Sb₂O₄ and Co_0.50Fe_0.50Sb₂O₄. Reaction with fluorine gas at low temperatures (typically 230 °C) results in topotactic insertion of fluorine into the channels, which are an inherent feature of the structure. Neutron powder diffraction and solid state NMR studies show that the interstitial fluoride ions are bonded to antimony within the channel walls to form Sb-F-Sb bridges. To date, these reactions have been observed only when Fe²⁺ ions are present within the chains of edge-linked octahedra (FeO₆ in FeSb₂O₄) that form the structural channels. Oxidation of Fe²⁺ to Fe³⁺ is primarily responsible for balancing the increased negative charge associated with the presence of the fluoride ions within the channels. For the two phases studied, the creation of Fe³⁺ ions within the chains of octahedra modify the magnetic exchange interactions to change the ground-state magnetic symmetry to C-type magnetic order in contrast to the A-type order observed for the unfluorinated oxide parents.