Room Temperature Electrochemical Redox Reactions of the Defect Perovskite SrFeO_2.5+x

Abstract

The electrochemical and chemical oxidation of the defect perovskite SrFeO 2.5 to the cubic perovskite SrFeO 3 at ambient temperature in alkaline electrolyte are shown to be in principle reversible processes with the appearance of intermediate compounds; the oxidation can be described by the formation of line phases SrFeO 2.75 and SrFeO 2.87 and one solid solution range SrFeO 3-x with 0 e x e 0.03. The structure of the brownmillerite type parent phase is characterized by the presence of stacking faults in the octahedral and tetrahedral sequence of layers whose concentration increases significantly upon formation of the intermediate phases. Models based on observed microscopic features are discussed in order to explain the unusual phenomenon of room temperature oxygen transport.

Investigation of the microstructural features of SrCo0.8Fe0.2O3−δ perovskite

“…It is known that brownmillerites exhibit characteristic spectra: two magnetically ordered sextets (Fig. 3a) related to antiferromagnetic ordering of Fe 3+ cations in octahedral and tetrahedral positions [13]. However, the spectrum of asprepared material SrCo 0.8 Fe 0.2 O 2.66 is only composed of diamagnetic doublets; magnetically ordered sextets that would have noticeable integral intensity~20% are absent.…”

“…However, electron diffraction patterns and high resolution images of SrCo 0.8 Fe 0.2 O 2.66 samples show the presence of only one brownmillerite phase. This is evidently related to the fact that under the reductive conditions of the electron microscope column, local heating of the sample by electron beam and due to anomalously high oxygen Convenient methods to study the phase composition of perovskite related oxides with mixed conductivity depending on oxygen stoichiometry are chronopotentiometry and potential step scanning techniques [13,14]. To carry out electrochemical experiments, we prepared the material of the working electrode SrCo 0.8 Fe 0.2 O 2.48 by heating asprepared SrCo 0.8 Fe 0.2 O 2.66 perovskite to 900°C in vacuum, followed by quenching into liquid nitrogen according to the procedure described in [14].…”

“…The transferred charge n is related to the amount of intercalated/deintercalated oxygen through a simple expression: δ0n/2 [13,14]. Thus, if we know the current passing through the cell and the time of the process, we can easily determine the change in oxygen nonstoichiometry.…”

Effect of Sonication Output Power on the Crystal Structure and Magnetism of SrFe12O19 Nanoparticles

“…The XRD patterns at 37.04° of 2θ presented a peak that increased its intensity in proportion to the increase of SOP; this phase was identified as a cubic type Perovskite structure SrFeO2.5 (PDF-2 00-033-0677). This phase formed from the reaction between SrCO3 and α-Fe2O3, which occurred at heat in a range from 1053 to 1173 K [5,26]. Moreover, peaks at 23.57° and 34.6 8° of 2θ were identified as a polymorphic phase of hematite, Fe2O3 (PDF-2 00-021-0920) [27].…”

Influence of Phase Transformations on Crystal Growth of Stoichiometric Brownmillerite Oxides: Sr2ScGaO5 and Ca2Fe2O5

“…However, the attribution of the space group in some cases remains ambiguous between Pnma, Imma, and I2mb. The associated structural difference is important, however, since Pnma and I2mb result into ordered (BO 4 ) ∞ -tetrahedral chains, while Imma yields an average and Some oxides with brownmillerite type structure-such as SrFeO3−x or SrCoO3−x-have been shown to exhibit oxygen mobility down to room temperature [13][14][15]. This makes brownmillerite-type oxides an attractive class of compounds for many technologically important applications in the field of solid state electrolytes, and more specifically for membranes or pure electrolytes in solid oxide fuel cells (SOFC) or for air separation, but equally important as sensor and electro catalysts.…”