Local Structure of ReFeAsO (Re=La, Pr, Nd, Sm) Oxypnictides Studied by Fe K-edge EXAFS

Local structure of ReOFeAs (Re=La, Pr, Nd, Sm) system has been studied as a function of chemical pressure varied due to different rare-earth size. Fe K-edge extended X-ray absorption fine structure (EXAFS) measurements in the fluorescence mode has permitted to compare systematically the inter-atomic distances and their mean square relative displacements (MSRD). We find that the Fe-As bond length and the corresponding MSRD hardly show any change, suggesting the strongly covalent nature of this bond, while the Fe-Fe and Fe-Re bond lengths decrease with decreasing rare-earth size. The results provide important information on the atomic correlations that could have direct implication on the superconductivity and magnetism of ReOFeAs system, with the chemical pressure being a key ingredient.