  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.

Superconductivity;Other Condensed Matter