The perovskites LaMnO 3 and CaFeO 3 consisting of high-spin d 4 transition metal ions undergo different types of distortions, i.e., a Jahn−Teller distortion in LaMnO 3 and a charge disproportionation in CaFeO 3 . We investigated the electronic factor causing this difference on the basis of first principles spin-polarized electronic band structure calculations for their ideal cubic structures and also tight-binding electronic band structure calculations for their ideal cubic and distorted structures. Our study shows that a charge disproportionation is favored over a Jahn− Teller distortion in CaFeO 3 because the covalent character is strong in the Fe−O bond, while the opposite is true for LaMnO 3 because the covalent character is weak in the Mn−O bond. In spin-polarized electronic band structure calculations, the covalency of the M−O (M) Fe, Mn) bond is enhanced in the up-spin bands but is reduced in the down-spin bands. Our analysis shows that electron−electron repulsion causes the energy gap between the metal 3d and the oxygen 2p orbitals to become larger for the down-spin than for the up-spin−orbital interactions. Thus in the d-block e g bands of both LaMnO 3 and CaFeO 3 the metal 3d orbital contribution is larger in the down-spin than in the up-spin bands.
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