Valence-state mixing and separation in SmBaFe₂O_5+w

A mixed-valence state, formally denoted as (Formula presented) is observed in the 300 K Mössbauer spectra of the most reduced samples of (Formula presented) Upon cooling below the Verwey-type transition temperature (Formula presented) the component assigned to (Formula presented) separates into a high-spin (Formula presented) state and an (Formula presented) state with an unusually low internal field. The separation of the mixed-valence state at (Formula presented) is also confirmed by magnetic susceptibility measurements and differential scanning calorimetry. A model is proposed which accounts for the variation of the amount of the mixed-valence state with the oxygen content parameter w.