Topotactic redox cycling in SrFeO2.5+δ explored by 3D electron diffraction in different gas atmospheres

Abstract

For oxygen conducting materials applied in solid oxide fuel cells and chemical-looping processes, the understanding of the oxygen diffusion mechanism and the materials' crystal structure at different stages of the redox reactions is a key parameter to control their performance. In this paper we report the first ever in situ 3D electron diffraction (ED) experiment in a gas environment and with it uncover the structure evolution of SrFeO_2.5 as notably different from that reported from in situ X-ray and in situ neutron powder diffraction studies in gas environments. Using in situ 3D ED on submicron sized single crystals, we observe the transformation under O₂ flow of brownmillerite SrFeO_2.5 with an intra- and interlayer ordering of the left and right twisted (FeO₄)_∞ tetrahedral chains (space group Pcmb) into consecutively SrFeO_2.75 with space group Cmmm (at 350 °C, 33% O₂) and SrFeO_3−δ with space group Pmm (at 400 °C, 100% O₂). Upon reduction in H₂ flow, the crystals return to the brownmillerite structure with intralayer order, but without regaining the interlayer order of the pristine crystals. Therefore, redox cycling of SrFeO_2.5 crystals in O₂ and H₂ introduces stacking faults into the structure, resulting in an I2/m(0βγ)0s symmetry with variable β.