Valence-state mixing and separation inSmBaFe2O5+w

A mixed-valence state, formally denoted as ${\mathrm{Fe}}^{2.5+},$ is observed in the 300 K Mössbauer spectra of the most reduced samples of ${\mathrm{SmBaFe}}_2{\mathrm{O}}_{5+w}.$ Upon cooling below the Verwey-type transition temperature ${(T}_{\mathrm{V}}\approx 200\mathrm{K}),$ the component assigned to ${\mathrm{Fe}}^{2.5+}$ separates into a high-spin ${\mathrm{Fe}}^{3+}$ state and an ${\mathrm{Fe}}^{2+}$ state with an unusually low internal field. The separation of the mixed-valence state at $T_{\mathrm{V}}$ 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.