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 0.8Fe 1.6Se 2 chalcogenide (T c=31.8K) by temperature dependent polarized extended x-ray absorption fine structure (EXAFS) at the Fe and SeK-edges. We find that the system is characterized by a large local disorder. The FeSe and FeFe 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 FeSe bondlength (k∼5.8eV Å 2) is similar to the one found in the binary FeSe superconductor, however, the FeFe bondlength appears to be flexible (k∼2.1eV Å 2) in comparison to the binary FeSe (k∼3.5eV Å 2), an indication of partly relaxed FeFe networks in K 0.8Fe 1.6Se 2. The results suggest a glassy nature for the title system, with the superconductivity being similar to that in the granular materials. © 2012 IOP Publishing Ltd.