Different routes to charge disproportionation in perovskite-type Fe oxides

Iron perovskites CaFeO3 and La0.33Sr0.67FeO3 show charge disproportionation, resulting in charge-ordered states with Fe3+:Fe5+=1:1 and =2:1, respectively. We have made photoemission and unrestricted Hartree-Fock band-structure calculations of CaFeO3 and compared it with La0.33Sr0.67FeO3. With decreasing temperature, a gradual decrease of the spectral weight near the Fermi level occurred in CaFeO3 as in La0.33Sr0.67FeO3 although lattice distortion occurs only in CaFeO3. Hartree-Fock calculations have indicated that both the breathing and tilting distortions are necessary to induce the charge disproportionation in CaFeO3, while no lattice distortion is necessary for the charge disproportionation in La0.33Sr0.67FeO3.