Abstract
The crystal and local structures of samples have been studied by x-ray diffraction and x-ray absorption spectroscopy techniques. The Fe-O bond length decreases with increasing . Accordingly, the x-ray absorption near edge spectroscopy (XANES) spectra reveal a chemical shift of the iron 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 and ions or by an 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 or . We show that a significant charge disproportionation of the type 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.
2 More- Received 6 July 2007
DOI:https://doi.org/10.1103/PhysRevB.77.054107
©2008 American Physical Society