Abstract
A mixed-valence state, formally denoted as Fe 2 . 5 + , is observed in the 300 K Mössbauer spectra of the most reduced samples of SmBaFe 2 O 5 + w . Upon cooling below the Verwey-type transition temperature (TV≈ 200 K ) , the component assigned to Fe 2 . 5 + separates into a high-spin Fe 3 + state and an Fe 2 + state with an unusually low internal field. The separation of the mixed-valence state at TV 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.
© 1999 American Physical Society