Determination of the local structure of the La0.5-xNa0.5+xFe2As2 superconductor

We have studied the local structure of La0.5-xNa0.5+xFe2As2 superconductor by As K-edge x-ray absorption fine structure measurements as a function of temperature in a wide range of hole doping (x = 0.0 to 0.35). A substantial change in the local structure around the arsenic atom has been observed with doping. The axial As-Asa bond length is found to be >similar to 3.12 angstrom, indicating that the system is in its collapsed tetragonal phase. The temperature-dependent mean square relative displacements reveal that the As-Fe bond is relatively weak in the undoped sample; however, it gets stronger with hole doping. On the other hand, the axial As-Asa bond tends to get weaker in the optimally doped regime (x = 0.25-0.35), suggesting the possible role of increased As atom displacements in the axial direction in the superconducting phase of these materials. Consistently, the arsenic height from the Fe-Fe plane shows a gradual increase in the optimally doped regime. Apart from providing a quantitative information on the local structure of La0.5-xNa0.5+xFe2As2, the results suggest a vital role of electron-lattice coupling and axial As atom displacements in the superconductivity of these iron-based materials.