Different routes to charge disproportionation in perovskite-type Fe oxides

Iron perovskites CaFeO₃ and La_0.33Sr_0.67FeO₃ 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 La_0.33Sr_0.67FeO₃. With decreasing temperature, a gradual decrease of the spectral weight near the Fermi level occurred in CaFeO₃ as in La_0.33Sr_0.67FeO₃ 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 La_0.33Sr_0.67FeO₃.