Effect of aliovalent substitution on the local structure of CaKFeAssuperconductor.

We have investigated the local structure of the iron-based CaKFeAssuperconductor featuring distinct aliovalent substitutions at the Ca and K sites, that is CaKFeAs, CaKSrFeAs, CaKBaFeAsand CaNaKBaFeAs. Temperature-dependent Fe K-edge extended x-ray absorption fine structure (EXAFS) measurements are used to determine the near-neighbors bondlengths and their stiffness. The EXAFS analysis reveals that the Fe-As bondlength undergoes negligible changes by substitution, however, the Fe-Fe bondlength and the As height are affected by the Sr substitution.