Effect of Fe content on atomic and electronic structure of complex oxides Sr(Ti,Fe)O3−δ

Abstract

We present a study of the electronic and atomic structure of two series of SrTi1-xFexO3-δ (STFO) powders with different Fe content produced by two different methods, spray pyrolysis or modified Pechini synthesis, by means of soft X-ray absorption spectroscopy. Partial substitution of Ti by Fe atoms in SrTiO3 were found to cause asymmetric distortion of TiO6 octahedrons which increases with increasing Fe content that may violate the cubic symmetry of STFO. The presence mainly of Fe 3+ states in octahedral environment at small concentration of Fe atoms along with essentially smaller content of Fe 4+ states in octahedral environment where the latter contribution increases with increasing Fe content was traced. It is found that the modified Pechini method allows to synthesize more stable structures but a tendency of the SrOx formation in the structure prepared by this technique was marked. The spray pyrolysis method gives the structure free of SrO precipitates but the presence of Fe 3+ states in tetrahedral environment with Fe content higher than 50% and even a certain amount of Fe 2+ ions in an octahedral environment at concentrations higher than 75% in the STFO prepared by this method was established. The O1s (K)-absorption spectra point to increase in oxygen vacancy concentration with increasing Fe content. The lowest degree of structure distortions with enough high oxygen vacancy concentration was traced in STFO (x= 0.25 to x = 0.35) produced by modified Pechini synthesis, which makes it mostly appropriate for technical applications, e.g., as gas sensors, oxygen separation membranes or as fuel cell material.

Atomic Modeling and Electronic Structure of Mixed Ionic–Electronic Conductor SrTi1–xFexO3–x/2+δ Considered as a Mixture of SrTiO3 and Sr2Fe2O5

“…Our calculated moments, as shown in Table 3, are more similar to those of Sr 2 Fe 2 O 5 for both 5-and 6-fold coordinated iron. 31 The small difference in the oxidation state between Fe ions in 5-fold and 6-fold coordinations can be explained by the nature of the bonding between Fe and O. Consistent with the covalent character of the Fe−O bond, the electrons appear to be shared by neighboring Fe and O atoms.…”

“…Accordingly, the Fe atoms exhibit effective charges slightly larger than +3 but smaller than +4 and an average charge of the oxygen atom lower in magnitude than −2. 26,29,40 Given this, we suggest that Fe 4+ states reported in experimental studies, 25,29,31,32 at least for the case of dilute iron, may more accurately be described as empty states of hybridized Fe−O bonding orbitals, which form the highest occupied energy levels and empty states at the VBM. (The effect of these states on the electronic structure and band gap will be discussed later.…”

“…20,26 Therefore, with a reasonable amount of Fe, STF exhibits both significant ionic and electronic conductivities. 28,30 Several studies have focused on establishing the mixed conducting properties of STF as a function of Fe concentration, 25,26 and experiments have revealed the material characteristics of STF such as electronic structure and magnetism, 25,26,28,31,32 surface oxygen exchange kinetics, 27,30,33,34 local atomic structure, 25,29,31−33 and oxygen nonstoichiometry. 35,36 There are also a large number of thermodynamic defect chemical models 27,28,36 and computational investigations 25,32,37−41 of the ionic and electronic properties of STF.…”

“…There have been extensive discussions of the oxidation state of Fe in STF. 25,28,29,31,32 Typically, the oxidation state of Fe is indicated as Fe 3+ when 5-fold coordinated and as Fe 4+ when 6fold coordinated by oxygen ions. 29,32,41 In experiments, Fe 3+ is observed in STF under a range of pressures and temperatures, and Fe 4+ is observed when the sample is oxidized.…”

Effect of sintering temperature on electrochemical performance of porous SrTi1-xFexO3-δ (x = 0.35, 0.5, 0.7) oxygen electrodes for solid oxide cells

“…An area specific resistance (ASR) of 0.020 Ω cm 2 at 650°C [42] has been reported for LSCF, whereas for SrTi 0.1 Fe 0.9 O 3 (STF90), an ASR of 0.067 Ω cm 2 at 650°C has been reported [43]. Some reports show that STFx material with x = 0.35 has the lowest degree of structural distortions, which makes it promising for technical applications [44] despite its relatively low electronic conductivity (~2 S cm −1 ) and higher ASR values (~0.1 Ω cm 2 at 800°C [40]). In general, there is only a limited number of publications related to STFx performance, and the materials require more studies.…”