Valence-state mixing and separation in SmBaFe2O5+w

A mixed-valence state, formally denoted as Fe2.5+, is observed in the 300 K Mossbauer spectra of the most reduced samples of SmBaFe2O5 + w. Upon cooli ng below:the Verwey-type transition temperature (T-V approximate to 200 K), the component assigned to Fe2.5+ separates into a high-spin Fe3+ state and an Fe2+ state with an unusually low internal field. The separation of the mixed-valence state;at T-V is also confirmed by magnetic susceptibility mea surements and differential scanning calorimetry. A model is proposed which accounts for the variation of the amount of the mixed-valence state with th e oxygen content parameter w. [S0163-1829(99)00446-4].