No abstract

This paper discusses the fluorination characteristics of phases related to FeSbO, by reporting the results of a detailed study of MgFeSbO and CoFeSbO. 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 Fe ions are present within the chains of edge-linked octahedra (FeO in FeSbO) that form the structural channels. Oxidation of Fe to Fe 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 Fe 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.

The structures and magnetic properties of Fe1−xMgxSb2O4 and their products from low-temperature oxidation, Fe1−xMgxSb2O4+y, are described.

MxCo1−xSb2O4 (M = Mn, Fe) shows rotation of magnetic moments within the structure, tuneable with increasing x.

The structure of the mineral schafarzikite, FeSbO, has one-dimensional channels with walls comprising Sb cations; the channels are separated by edge-linked FeO octahedra that form infinite chains parallel to the channels. Although this structure provides interest with respect to the magnetic and electrical properties associated with the chains and the possibility of chemistry that could occur within the channels, materials in this structural class have received very little attention. Here we show, for the first time, that heating selected phases in oxygen-rich atmospheres can result in relatively large oxygen uptakes (up to ∼2% by mass) at low temperatures (ca. 350 °C) while retaining the parent structure. Using a variety of structural and spectroscopic techniques, it is shown that oxygen is inserted into the channels to provide a structure with the potential to show high one-dimensional oxide ion conductivity. This is the first report of oxygen-excess phases derived from this structure. The oxygen insertion is accompanied not only by oxidation of Fe to Fe within the octahedral chains but also Sb to Sb in the channel walls. The formation of a defect cluster comprising one 5-coordinate Sb ion (which is very rare in an oxide environment), two interstitial O ions, and two 4-coordinate Sb ions is suggested and is consistent with all experimental observations. To the best of our knowledge, this is the first example of an oxidation process where the local energetics of the product dictate that simultaneous oxidation of two different cations must occur. This reaction, together with a wide range of cation substitutions that are possible on the transition metal sites, presents opportunities to explore the schafarzikite structure more extensively for a range of catalytic and electrocatalytic applications.