Large local disorder in superconducting K0.8Fe1.6Se2 studied by extended x-ray absorption fine structure

We have measured the local structure of superconducting K 0.8 Fe 1.6 Se 2 chalcogenide ( T c  = 31.8 K) by temperature dependent polarized extended x-ray absorption fine structure (EXAFS) at the Fe and Se K-edges. We find that the system is characterized by a large local disorder. The Fe–Se and Fe–Fe distances are found to be shorter than the distances measured by diffraction, while the corresponding mean square relative displacements reveal large Fe-site disorder and relatively large c -axis disorder. The local force constant for the Fe–Se bondlength ( k  ~ 5.8 eV Å −2 ) is similar to the one found in the binary FeSe superconductor, however, the Fe–Fe bondlength appears to be flexible ( k  ~ 2.1 eV Å −2 ) in comparison to the binary FeSe ( k  ~ 3.5 eV Å −2 ), an indication of partly relaxed Fe–Fe networks in K 0.8 Fe 1.6 Se 2 . The results suggest a glassy nature for the title system, with the superconductivity being similar to that in the granular materials.