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

We have measured the local structure of superconducting K0.8Fe1.6Se2 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 similar to 5.8 eV angstrom(-2)) is similar to the one found in the binary FeSe superconductor, however, the Fe-Fe bondlength appears to be flexible (k similar to 2.1 eV angstrom(-2)) in comparison to the binary FeSe (k similar to 3.5 eV angstrom(-2)), an indication of partly relaxed Fe-Fe networks in K0.8Fe1.6Se2. The results suggest a glassy nature for the title system, with the superconductivity being similar to that in the granular materials.