Temperature Dependent Local Structure of LaFeAsO_{1-x}F_x: Probing the Bond Correlations

The local structure of the parent and doped LaFeAsO1-xFx (pnictide) compounds were studied by x-ray absorption spectroscopy. In the doped system, the Fe-As and Fe-Fe correlations are well modeled by an Einstein model with no low temperature anomalies. 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. For the Fe-Fe correlation, doping enhances the effective Einstein temperature of Fe-Fe atom correlation. The results suggest that the onset of superconductivity in the F doped system may be related to enhanced magnetic correlations. Density functional calculations of the charge density reveal strong bonding between neighboring As ions but metal-like behavior in the Fe layers.