Charge disproportionation inLa1−xSrxFeO3probed by diffraction and spectroscopic experiments

The crystal and local structures of ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x\mathrm{Fe}{\mathrm{O}}_{3-\delta }$ $(0⩽x⩽1)$ 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 ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Fe}}^{4+}$ ions or by an ${\mathrm{Fe}}^{3.\mathrm{x}+}$ 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 $2{\mathrm{Fe}}^{4+}\to {\mathrm{Fe}}^{3+}+{\mathrm{Fe}}^{5+}$ 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.