Large local disorder in the superconducting K_{0.8}Fe_{1.6}Se_{2}   studied by extended x-ray absorption fine structure
  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. 
Antonella Iadecola, Boby Joseph, Laura Simonelli, Alessandro Puri,   Yoshikazu Mizuguchi, Hiroyuki Takeya, Yoshihiko Takano,...
cond-mat.supr-con physics.chem-ph