Temperature-dependent local structure of LaFeAsO_1-xF _x: Probing the atomic correlations

The local structure of the parent and doped LaFeAsO_1-xF _x(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.