Large local disorder in the superconducting K_{0.8}Fe_{1.6}Se_{2} studied by extended x-ray absorption fine structure

TL;DR

This study reveals significant local disorder in superconducting K_{0.8}Fe_{1.6}Se_{2} using EXAFS, showing shorter local bond lengths and large displacements, indicating a glassy, granular-like superconducting state.

Contribution

It provides the first detailed local structural analysis of K_{0.8}Fe_{1.6}Se_{2} revealing disorder and bond flexibility, advancing understanding of its superconducting nature.

Findings

Large local disorder characterized by short Fe-Se and Fe-Fe distances.

High Fe-site and c-axis disorder indicated by mean square displacements.

Fe-Fe bonds are more flexible compared to binary FeSe, suggesting relaxed Fe networks.

Abstract

We have measured 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 Fe-Se bondlength (k ~ 5.8 eV/\AA^{2}) is similar to the one found in the binary FeSe superconductor, however, the Fe-Fe bondlength appears to get flexible (k ~ 2.1 eV/\AA^{2}) in comparison to the binary FeSe (k ~ 3.5 eV/\AA^{2}), an indication of partly relaxed Fe-Fe networks in K_{0.8}Fe_{1.6}Se_{2}. The results suggest glassy nature of the title system, with the superconductivity being similar to the one in the granular materials.