Different routes to charge disproportionation in perovskite-type Fe oxides

Iron perovskites (formula presented) and (formula presented) show charge disproportionation, resulting in charge-ordered states with (formula presented) and (formula presented) respectively. We have made photoemission and unrestricted Hartree-Fock band-structure calculations of (formula presented) and compared it with (formula presented) With decreasing temperature, a gradual decrease of the spectral weight near the Fermi level occurred in (formula presented) as in (formula presented) although lattice distortion occurs only in (formula presented) Hartree-Fock calculations have indicated that both the breathing and tilting distortions are necessary to induce the charge disproportionation in (formula presented) while no lattice distortion is necessary for the charge disproportionation in (formula presented).