Oxygen insertion reactions within the 1-D channels of phases related to FeSb2O4

Abstract

The structure of the mineral schafarzikite, FeSb2O4, has one-dimensional channels with walls comprising Sb3+ cations; the channels are separated by edge-linked FeO6 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) whilst 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 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 Fe2+ to Fe3+ within the octahedral chains but also Sb3+ to Sb5+ in the channel walls. The formation of a defect cluster comprising one 5-coordinate Sb5+ ion (which is very rare in an oxide environment), two interstitial O2- ions and two 4-coordinate Sb3+ 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 the 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.

The neutron diffraction data are contained in two folders corresponding to the instruments:
1. HRPT with one data file and the instrument parameter file.
2. D20 with one instrument parameter file and five sub-folders containing the datasets for the temperature ranges indicated. The individual file names can be used to deduce the temperature for that particular measurement. For example, the sub-folder 50-145_degC contains 20 datasets for which the first (run 895262) relates to 50\(^o\)C and the last (run 895281) to 145\(^o\)C. All data files have had background due to the quartz tube subtracted. The inclusion of run number in the file name means that they can immediately be used for sequential refinement using seqgsas.
The Mössbauer data contain two columns that correspond with the experimental x-axis (mm s-1) and y-axis (−absorption / %) points for the collected raw data. The heating temperature is included in the file name.