Temperature Dependent Local Structure of LaFeAsO_1-xF_x: Probing the Bond Correlations
The local structure of the parent and doped LaFeAsO1−xFx (pnictide) compounds were studied by x-ray absorption spectroscopy and density functional methods. In the doped system, the Fe–As and Fe–Fe correlations are both well modeled by an Einstein model. For the Fe–As bonds, the Einstein temperatures are identical for the doped (11%) and undoped samples but the doped sample is found to have a lower level of static disorder. Doping is found to increases the effective Einstein temperature of Fe–Fe atomic correlation. The results suggest that the onset of superconductivity in the F doped system may be related to enhanced magnetic interactions. Density functional calculations of the total charge density reveal strong bonding between neighboring As ions but metal-like behavior in the Fe layers. It is also seen directly that the replacement of oxygen by fluorine modifies the electron charge density mainly on the Fe sites.