  While for a long time iron based superconductors have been investigated
looking at the average crystalline structure there is now growing interest on
the divergence of local from the average structure. Here we use advanced data
analysis of EXAFS, a fast and local probe, which has key advantages compared to
crystallographic measurements, in unveiling the dynamics of the local
structure. The results show that the Fe-As pair oscillates in the double-well
potential both in undoped and in cobalt doped LaFeAsO crystals. The parameters
and the characteristics of the double-well potential are determined by curve
fits of the polarized EXAFS data. Temperature dependent Fe-As pair distribution
and tunneling frequency are derived from selected model of two potential wells.
We observe the softening of the Fe-As oscillations at the tetragonal to
orthorhombic structural phase transition at 150 K in LaFeAsO. A weak broad
softening is observed while decreasing the temperature from 70 to 40 K in
superconducting LaFe0.89Co0.11AsO which could be correlated with the onset of
short range charge density wave order.
