| The 3d perovskite oxides have a relatively simple structure and contain in some cases 3d transition metals in unusually high valence states. The deviations from the perfect perovskite structure can be of various types, among which Jahn-Teller distortion is the most well known. In contrast, some crystals belonging to this family of compounds exhibit so called charge disproportionation[Woodward:2000] where two different metal sites in the low temperature phase are clearly identified using Mössbauer spectroscopy [Nasu:1992]. We present a simple tool based on the rotation of pairs of orbitals to express the wave function in optimal atomic-like orbitals [Sadoc:2007]. This unitary transformation allows one to interpret the wave function in terms of localized orbitals often used by chemists and physicists. The analysis of the obtained wave functions shows that the charge transfer configurations dominate the ground state of those 3d perovskite oxides that display charge disproportionation. The predominant configuration of iron in CaFeO3 is Fe3+d5 with holes in the oxygen octahedron instead of Fe4+d4 extracted from the ionic model. The same is observed for the formal Ni3+ ions (d7) in RNiO3 perovskites where charge transfer makes the Ni2+(3d8) the leading electronic configuration. Moreover, the calculated charges in the charge disproportionated phase show that the transition metal centers keep the same number of electrons in the d-shell at both crystallographic inequivalent sites. The experimental observations interpreted as charge disproportionation between the Fe-sites can also be explained with different ligand to metal charge transfer. |
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