Lack Of Atomic Charge Localization In Transition Metal Mixed Valence Oxides

We review our recent studies on the occurrence of atomic charge localization in magnetite and half-doped manganites. X-ray resonant scattering experiments at the iron K-edge of magnetite and X-ray absorption spectroscopy at the Mn K-edge of Ca-doped manganites show the lack of charge localization on the transition metal atom, either temporal or spatial. The evolution of the intensity of (002) and (006) forbidden reflections in magnetite across the Verwey transition, as a function of the incoming X-ray beam energy, the azimuthal angle and the polarization of the scattering process, indicates that the atomic anomalous scattering factors for the octahedral iron ions are identical to each other, i.e. all of them show an identical electronic state. High resolution XANES spectra ofRE1-xCaxMnO3series cannot be reproduced by a linear combination of REMnO3andCaMnO3spectra. Accordingly, the electronic state of the Mn atom in these intermediate compounds cannot be described as a bimodal distribution ofMn3+andMn4+pure ionic states. The claimed experimental evidence of charge ordering inMn3+/Mn4+=1manganites proposed from X-ray resonant scattering experiments has also been critically re-examined. We show that although these experiments prove the existence of two kinds of Mn atoms, they cannot be identified asMn3+andMn4+ions. Moreover, the so-called charge-ordering (CO) and orbital-ordering (OO) resonant scattering reflections are naturally explained due to the presence of local distortions without including any ordering of the Mn d-orbitals.