Effects of Charge Disproportionation on the Phonon Density of States in Fe Perovskites

Iron perovskites CaFeO 3 and La 0.33 Sr 0.67 FeO 3 show charge disproportionation, resulting in charge-ordered states with “Fe 3+ ”:“Fe 5+ ” =1:1 and =2:1, respectively. We have performed nuclear resonant scattering measurements of phonon density of states in CaFeO 3 and La 0.33 Sr 0.67 FeO 3 . With decreasing temperature, we found changes in the phonon density of states in the region of 30–60 meV in CaFeO 3 , while only a small change was observed in La 0.33 Sr 0.67 FeO 3 . This finding gives support to the previous finding that the charge disproportionation is driven by electron–phonon interaction in CaFeO 3 , and electronically in La 0.33 Sr 0.67 FeO 3 .