Topotactic redox cycling in <tex>SrFeO_{2.5+ð}$</tex> explored by 3D electron diffraction in different gas atmospheres

For oxygen conducting materials applied in solid oxide fuel cells and chemical-looping processes, the understanding of the oxygen diffusion mechanism and the materials&#39; 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 SrFeO2.5 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 O-2 flow of brownmillerite SrFeO2.5 with an intra- and interlayer ordering of the left and right twisted (FeO4)(infinity) tetrahedral chains (space group Pcmb) into consecutively SrFeO2.75 with space group Cmmm (at 350 degrees C, 33% O-2) and SrFeO3-delta with space group Pm3m (at 400 degrees C, 100% O-2). Upon reduction in H-2 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 SrFeO2.5 crystals in O-2 and H-2 introduces stacking faults into the structure, resulting in an I2/m(0 beta gamma)0s symmetry with variable beta.