Charge disproportionation in La1-xSrxFeO3 probed by diffraction and spectroscopic experiments

The crystal and local structures of La1-xSrxFeO3-delta (0 <= x <= 1) samples have been studied by x-ray diffraction and x-ray absorption spectroscopy techniques. The Fe-O bond length decreases with increasing x. Accordingly, the x-ray absorption near edge spectroscopy (XANES) spectra reveal a chemical shift of the iron K edge to higher energies. Both results agree with an Fe valence increase as La is substituted with Sr. Extended x-ray absorption fine structure spectroscopy and XANES show that the chemical state of Fe atoms in intermediate compositions can be described either by a bimodal distribution of formal Fe3+ and Fe4+ ions or by an Fe3.x+ intermediate valence. The large value of the Debye-Waller factors obtained for intermediate compositions indicates that hole doping produces local disorder around the Fe ions. These factors show unusually large values below the metal-insulator (MI) transition for x=2/3 or 3/4. We show that a significant charge disproportionation of the type 2Fe(4+) -> Fe3+ + Fe5+ cannot account for the local structure observed below the MI transition temperature of these samples. We suggest that an electronic localization arises from an order-disorder transition between dynamic and static distortions, resulting in the opening of a gap at the Fermi level.