Effect of aliovalent substitution on the local structure of CaKFe 4 As 4 superconductor.

We have investigated the local structure of the iron-based CaKFe 4 As 4 superconductor featuring distinct aliovalent substitutions at the Ca and K sites, that is CaKFe 4 As 4 , CaK 0.9 Sr 0.1 Fe 4 As 4 , CaK 0.9 Ba 0.1 Fe 4 As 4 and Ca 0.9 Na 0.1 K 0.9 Ba 0.1 Fe 4 As 4 . 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 CaK 0.9 Sr 0.1 Fe 4 As 4 and CaK 0.9 Ba 0.1 Fe 4 As 4 are 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 CaKFe 4 As 4 . The mean As heights show a peculiar temperature dependence characteristic of CaKFe 4 As 4 system. 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 CaK 0.9 Sr 0.1 Fe 4 As 4 . The local structure results are discussed in relation to the differing transport properties of aliovalent substituted 1144 superconductor.