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. The superconducting transition temperatures of CaKSrFeAsand CaKBaFeAsare very similar even if the mean As heights are significantly different suggesting that the anion height may not be a unique parameter to describe the superconductivity in CaKFeAs. The mean As heights show a peculiar temperature dependence characteristic of CaKFeAssystem. Furthermore, the temperature-dependent mean square relative displacements reveal similar Fe-Fe bond stiffness in all samples, instead the Fe-As bond is substantially stiffer in case of CaKSrFeAs. The local structure results are discussed in relation to the differing transport properties of aliovalent substituted 1144 superconductor.