Different routes to charge disproportionation in perovskite-type Fe oxides

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Iron perovskites ${\mathrm{CaFeO}}_{3}$ and ${\mathrm{La}}_{0.33}{\mathrm{Sr}}_{0.67}{\mathrm{FeO}}_{3}$ show charge disproportionation, resulting in charge-ordered states with ${\mathrm{Fe}}^{3+}:{\mathrm{Fe}}^{5+}=1:1$ and $=2:1,$ respectively. We have made photoemission and unrestricted Hartree-Fock band-structure calculations of ${\mathrm{CaFeO}}_{3}$ and compared it with ${\mathrm{La}}_{0.33}{\mathrm{Sr}}_{0.67}{\mathrm{FeO}}_{3}.$ With decreasing temperature, a gradual decrease of the spectral weight near the Fermi level occurred in ${\mathrm{CaFeO}}_{3}$ as in ${\mathrm{La}}_{0.33}{\mathrm{Sr}}_{0.67}{\mathrm{FeO}}_{3}$ although lattice distortion occurs only in ${\mathrm{CaFeO}}_{3}.$ Hartree-Fock calculations have indicated that both the breathing and tilting distortions are necessary to induce the charge disproportionation in ${\mathrm{CaFeO}}_{3},$ while no lattice distortion is necessary for the charge disproportionation in ${\mathrm{La}}_{0.33}{\mathrm{Sr}}_{0.67}{\mathrm{FeO}}_{3}.$