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Title: Short- and long-range modulated oxygen order in brownmillerite-type Sr2FeCoO5 and Ca2FeCoO5
Authors: Auckett, JE
Withers, RL
Studer, AJ
Avdeev, M
Edwards, AJ
McIntyre, GJ
Ling, CD
Keywords: Crystals
Electron diffraction
Ionic conductivity
Neutron diffraction
Issue Date: 23-Jul-2015
Publisher: Asia-Oceania Neutron Scattering Association
Citation: Auckett, J. E., Withers, R. L., Studer, A. J., Avdeev, M., Edwards, A. J., McIntyre, G. J., & Ling, C. D. (2015). Short- and long-range modulated oxygen order in brownmillerite-type Sr2FeCoO5 and Ca2FeCoO5. Paper presented at 2nd Asia Oceania Conference on Neutron Scattering (AOCNS 2015), Novotel, Manly Pacific, Sydney, Australia, 19-23 July 2015.
Abstract: Sr2FeCoO5 and Ca2FeCoO5 belong to the perovskite-derived group of oxides known as the brownmillerites (A2B2O5), which are characterised by layers of BO6 octahedra alternating with layers of chain-linked BO4 tetrahedra separated by oxygen vacancy channels. Brownmillerites have potential applications as diverse as combustion catalysts, cements, and solid oxide fuel cell cathodes. Oxide-ionic conductivity in brownmillerites is associated with the presence of oxygen vacancies adjacent to the tetrahedral chains, and is therefore influenced by the relative arrangements of oppositely twisted chains throughout the structure. Recently, a variety of modulated chain-ordering schemes have been identified in several brownmillerites that were previously thought to adopt only simple ordered or completely disordered chain arrangements. In this work, a brownmillerite chain-ordering modulation first identified in Sr2FeCoO5 by electron diffraction has been confirmed by single-crystal neutron diffraction on a large crystal grown by the floating-zone (FZ) method. Although incommensurate modulation vectors are identified on the short electron diffraction length scale, the neutron diffraction data show clearly that a commensurate arrangement dominates the crystal on average. This behaviour closely follows that of the series end-member Sr2Fe2O5, an important ionic-conductive brownmillerite, implying that Sr2FeCoO5 may display similarly favourable ionic-conductive properties. We also investigated Ca2FeCoO5, a relatively new brownmillerite characterised in 2010. The presence of well-resolved satellite reflections in single-crystal and powder neutron diffraction data support the choice of a commensurate chain-ordered arrangement to describe the structure. However, attempts to refine this structure against either data set yielded poor results. A detailed examination of neutron and x-ray precession images obtained for FZ-grown single crystals reveal the presence of certain reflections that are forbidden by the expected Pbma symmetry. Consideration of the apparent pseudo-symmetry yields evidence for a micro-intergrowth of regions with slightly different chain-ordering schemes within the dominating Pbma matrix, similar to an arrangement reported previously for Ca2MnGaO5.
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