Oxygen Insertion Reactions within the One-Dimensional Channels of Phases Related to FeSb<sub>2</sub>O<sub>4</sub>

The structure of the mineral schafarzikite, FeSb<sub>2</sub>O<sub>4</sub>, has one-dimensional channels with walls comprising Sb<sup>3+</sup> cations; the channels are separated by edge-linked FeO<sub>6</sub> 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<sup>2+</sup> to Fe<sup>3+</sup> within the octahedral chains but also Sb<sup>3+</sup> to Sb<sup>5+</sup> in the channel walls. The formation of a defect cluster comprising one 5-coordinate Sb<sup>5+</sup> ion (which is very rare in an oxide environment), two interstitial O<sup>2–</sup> ions, and two 4-coordinate Sb<sup>3+</sup> 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.