Temperature dependent local structure of NdFeAsO1-xFx system using arsenic K-edge extended x-ray absorption fine structure

Local structure of NdFeAsO$_{1-x}$F$_{x}$ ($x$=0.0, 0.05, 0.15 and 0.18) high temperature iron pnictide superconductor system is studied using arsenic $K$-edge extended x-ray absorption fine structure measurements as a function of temperature. Fe-As bondlength shows only a weak temperature and F-substitution dependence, consistent with the strong covalent nature of this bond. The temperature dependence of the mean-square relative-displacements of the Fe-As bondlength are well described by the correlated-Einstein model for all the samples, but with different Einstein-temperatures for the superconducting and non-superconducting samples. The results indicate distinct local Fe-As lattice dynamics in the superconducting and non-superconducting iron-pnictide systems.