A Layered Double Perovskite Ca₂FeMnO₆ with Unusually High Valence Fe⁴⁺ Obtained by Low-Temperature Topotactic Oxidation

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A new double perovskite Ca₂FeMnO₆ with a layered arrangement of Mn⁴⁺ and the unusually high valence Fe⁴⁺ was obtained by oxidizing the brownmillerite Ca₂FeMnO₅ with ozone at 200 °C. The low-temperature topotactic reaction kept the layered cation arrangement of the brownmillerite but oxidized Mn³⁺ to Mn⁴⁺ and Fe³⁺ to Fe⁴⁺. The crystal structure with a P2₁/c space group was analyzed with the synchrotron X-ray diffraction data. The changes in the valence states of Fe and Mn were confirmed by bond valence sum calculations, X-ray absorption spectroscopy, and Mössbauer spectra measurements.