Fluorine insertion reactions of the brownmillerite materials Sr2Fe2O5, Sr2CoFeO5, and Sr2Co2O5

The brownmillerite structure provides an ordered vacancy lattice into which fluorine anions should easily be accommodated, with full occupation of the vacant sites resulting in a simple perovskite structure. Low-temperature fluorine insertion reactions with the brownmillerite Sr2Co2O5 and related materials Sr2CoFeO5 and Sr2Fe2O5 have been investigated with a view to using fluorine insertion reactions to fill the oxygen vacancies in these materials, thus forming cubic perovskite-type phases with mixed 3+/4+ B cation oxidation states. The synthesis and magnetic properties of the new perovskite materials SrCoO2.5F0.5 and SrCo0.5Fe0.5O2.5F0.5 are presented here, supplemented by a consideration of Madelung energy calculations in order to explain the dissimilar fluorination behaviour of Sr2Fe2O5.

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► The brownmillerites Sr2Fe2O5, Sr2CoFeO5 and Sr2Co2O5 are amenable to fluorine insertion.
► Fluorination of Sr2Co2O5 and Sr2CoFeO5 forms cubic perovskite phases.
► Nominal compositions: SrCoO2.5F0.5 (a = 3.8574(3) Å) and SrCo0.5Fe0.5O2.5F0.5 (a = 3.872(2) Å).
► Sr2Fe2O5 forms a 1:1 mixture of SrFeO2F (a = 3.9481(3) Å) and SrFeO3−δ (a = 3.8775(3) Å).