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) 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 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 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.
Highlights
The tetragonal mineral schafarzikite (FeSb2O4; P42/mbc a = 8.62 Å c = 5.91 Å)[1] is of significant interest because of its unusual one-dimensional (1-D) structural characteristics and its ability to display a high degree of compositional flexibility
The formation of a defect cluster comprising one 5-coordinate Sb5+ ion, two interstitial O2− ions, and two 4-coordinate Sb3+ ions is suggested and is consistent with all experimental observations
With respect to the formation of Sb5+ ions, it is relevant to note that an attempt to partially substitute Sb3+ by Pb2+ in MnSb2O411 resulted in oxidation of Sb3+ rather than the desired Mn2+, and the resulting Sb5+ ions entered the octahedral sites by replacing some of the Mn2+ ions. This possibility was rejected for the oxidations studied here for two reasons: (1) substitution of Sb5+ ions in the chains of octahedra would result in significant impurity phases; (2) Powder X-ray diffraction (PXRD) and NPD are both sensitive to such mixing and both confirm that no detectable substitution of Sb within the chains occurs
Summary
The tetragonal mineral schafarzikite (FeSb2O4; P42/mbc a = 8.62 Å c = 5.91 Å)[1] is of significant interest because of its unusual one-dimensional (1-D) structural characteristics and its ability to display a high degree of compositional flexibility. In FeSb2O4, the chains of octahedra contain Fe2+ (A-site), and Sb3+ (B-site) provides the link. The B-site has trigonal pyramidal coordination, SbO3, which can be regarded as pseudotetrahedral, SbO3E, by including the stereochemically active lone pair of electrons (E). These electron pairs appear crucial for stabilizing the second key structural feature: channels that exist along [001], parallel to the chains of octahedra. Sb3+ ions form the channel walls and their lone pairs, E, are directed into the channels
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