Comment on “Structural and Mössbauer study of the brownmillerite oxides LaSrMn2−xFexO5 (0 ⩽ x ⩽ 0.5)”

Mohamed et al. recently reported in this Journal on the synthesis and characterisation of a series of oxides described as “LaSrMn2−xFexO5”. On the basis of Rietveld refinements against powder X-ray diffraction data, the authors stated that all synthesised members of the series crystallised in the brownmillerite structure with Pnma symmetry. However, the diffraction patterns presented in that paper clearly correspond to oxidised perovskite phases with small orthorhombic distortions, rather than brownmillerites. This is consistent with the authors' failure to report having performed any reaction steps under reducing conditions, as is normally required to obtain the mixed +2/+3 oxidation state of Mn and Fe in the expected brownmillerites. The materials synthesised under the reported conditions were most likely to have been oxidised pseudo-cubic perovskites LaSrMn2−xFexO6−δ (δ ≪ 1), which is consistent with their reported X-ray diffraction data. The interpretation of Mössbauer data in the original paper is therefore flawed.