Order, Disorder, and Dynamics in Brownmillerite Sr₂Fe₂O₅

Abstract

The room-temperature structure of brownmillerite-type Sr2Fe2O5 remains controversial, despite numerous published crystallographic studies utilising x-ray, neutron and electron diffraction data collected on single-crystalline and powder samples. The main disagreements concern the ordering of twisted FeO4 tetrahedral chains within and between the layers stacked along the b axis, and the impact of this ordering on oxide-ionic conductivity. Here, we present new data along with a reinterpretation of previously published diffraction images, including the re-assignment of satellite reflections, which harmonise the results of past studies in a unified description of tetrahedral chain ordering in Sr2Fe2O5 at length scales relevant to x-ray and neutron diffraction. Implications for the prevailing model of oxide ion transport in this material are also discussed.

Quantum materials with strong spin–orbit coupling: challenges and opportunities for materials chemists

“…also be used to identify and study stacking faults, through the rods and streaks of diffuse scattering they generate in single crystal diffraction patterns (see e.g. 67,[125][126][127] for a range of examples). Stacking faults can also influence the behaviour of the electrons in a material.…”

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

“…Also other stackings were observed, fitting in a general 3+1D superspace model with I2/m(0βγ)0s with varying β for the different stacking order, and with γ=1/2 indicating the L-R intralayer ordering. 15 The Pcmb structure was afterwards confirmed as the main phase using neutron and synchrotron X-ray diffraction studies on single crystals of SrFeO 2.5 17,18,25 and established to be a thermodynamic ground state of SrFeO 2.5 by density functional theory (DFT) calculations. 26,27 The structure could be refined from powder Xray data, but with insignificant difference in R-values versus a disordered Imma structure.…”

“…[10][11][12][13][14] The SrFeO 2.5 phase has a brownmillerite structure that consists of alternating layers of FeO 4 tetrahedra and FeO 6 octahedra. [15][16][17][18] In brownmillerites, the tetrahedra form (FeO 4 ) ¥ chains that can be twisted in two orientations, labelled left (L) and right (R), depending on the chemical composition. 19,20 The SrFeO 2.5 brownmillerite is subject to complex twinning, 16 which made its exact structure a topic of disagreement, [21][22][23][24] with contradicting results until electron diffraction (ED) and high resolution transmission electron microscopy (TEM) studies performed on SrFeO 2.5 submicron single crystals 15 clearly showed the alternation between L and R chains within the tetrahedral layers (so-called intralayer ordering).…”

Evolution of the oxygen vacancy order during oxidation and reduction of SrFeOx followed by in situ 3D electron diffraction

“…Also other stackings were observed, fitting in a general 3+1D superspace model with I2/m(0βγ)0s with varying β for the different stacking order, and with γ=1/2 indicating the L-R intralayer ordering. 15 The Pcmb structure was afterwards confirmed as the main phase using neutron and synchrotron X-ray diffraction studies on single crystals of SrFeO2.5 17,18,25 and established to be a thermodynamic ground state of SrFeO2.5 by density functional theory (DFT) calculations. 26,27 The structure could be refined from powder Xray data, but with insignificant difference in R-values versus a disordered Imma structure.…”

“…[10][11][12][13][14] The SrFeO2.5 phase has a brownmillerite structure that consists of alternating layers of FeO4 tetrahedra and FeO6 octahedra. [15][16][17][18] In brownmillerites, the tetrahedra form (FeO4)¥ chains that can be twisted in two orientations, labelled left (L) and right (R), depending on the chemical composition. 19,20 The SrFeO2.5 brownmillerite is subject to complex twinning, 16 which made its exact structure a topic of disagreement, [21][22][23][24] with contradicting results until ED and high resolution TEM studies performed on SrFeO2.5 submicron single crystals 15 clearly showed the alternation between L and R chains within the tetrahedral layers (so-called intralayer ordering).…”