Abstract
In this study we demonstrate the feasibility of following up a chemical reaction by single crystal x-ray (synchrotron) diffraction under operando conditions, carried out in a specially designed electrochemical cell mounted on the BM01A at the European Synchrotron Radiation Facility (ESRF). We investigated in detail the electrochemical oxidation of SrFeO2.5 to SrFeO3 on a spherical single crystal of 70 µm diameter by in situ diffraction at an ambient temperature. Complete data sets were obtained by scanning the whole reciprocal space using a 2M Pilatus detector, resulting in 3600 frames with a resolution of 0.1° per data set, each obtained in 18 min. The crystal was mounted in a specially designed electrochemical cell with 1N KOH used as the electrolyte. During the electrochemical oxidation, the reaction proceeds following the phase sequence SrFeO2.5/SrFeO2.75/SrFeO2.875/SrFeO3, structurally accompanied by establishing a complex series of long-range oxygen vacancy ordering, which gets instantly organized at ambient temperature. The topotactic reaction pathway is discussed in terms of the evolution of the twin domain structure. The formation of SrFeO2.875 is accompanied by the formation of diffuse streaks along the [1 0 0]-direction of the perovskite cell, reaching high d-spacings. The diffuse streaks are discussed and are thought to originate from a modified twin structure induced by the SrFeO2.75 to SrFeO2.875 transition, and the associated changes in the domain structure, developed during the oxygen intercalation. We equally analysed and discussed in detail the twin structure of all the title compounds. We confirm the ground state of SrFeO2.5 is able to adopt the Imma space group symmetry, showing stacking faults of the tetrahedral layers along the stacking axis of the brownmillerite unit cell, indicated by the 1D diffuse rods. We showed that in situ single crystal diffraction has huge potential in the study of non-stoichiometric compounds under operando conditions, in order to obtain structural information i.e. about diffuse scattering, and microstructural information related to domain effects such as twinning—information far beyond that which powder diffraction methods allow us to obtain.
Highlights
Due to the high performance of modern x-ray and neutron diffractometers, the chemical reactivity of solids is studied today routinely under in situ conditions on polycrystalline samples in specific reaction chambers [1,2,3,4]
We report here on the electrochemical oxygen intercalation reaction into SrFeO2.5 single crystals, followed up in situ by x-ray diffraction in a dedicated, miniaturized electrochemical cell, which was mounted on the BM01A goniometer
SrFeO3−x belongs to the few systems where the oxygen stoichiometry can be adjusted at elevated temperatures under controlled oxygen partial pressure [27], but which can be electrochemically oxidized or reduced between 0 ⩽ x ⩽ 0.5 already at ambient temperature in a reversible topotactic reaction [26]
Summary
Due to the high performance of modern x-ray and neutron diffractometers, the chemical reactivity of solids is studied today routinely under in situ conditions on polycrystalline samples in specific reaction chambers [1,2,3,4]. This technique became the standard characterization for battery systems and for catalysis and many other applications. The main reasons for this are related to the reaction kinetics, which usually leads to inhomogeneous samples, especially for larger grain sizes becoming difficult to quantify; other reasons concern the related structural complexity, as any type of disorder, twinning, stacking faults or domain effects usually demand sophisticated and time-consuming data analysis
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