⁵⁷Fe Mössbauer study of stoichiometric iron-based superconductor CaKFe₄As₄: a comparison to KFe₂As₂ and CaFe₂As₂

57_Fe Mössbauer spectra at different temperatures between ∼5 and ∼ 300 K were measured on an oriented mosaic of single crystals of CaKFe₄As₄. The data indicate that CaKFe₄As₄ is a well formed compound with narrow spectral lines, no traces of other, Fe–containing, secondary phases in the spectra and no static magnetic order. There is no discernible feature at the superconducting transition temperature in any of the hyperfine parameters. The temperature dependence of the quadrupole splitting approximately follows the empirical ‘T^3/2 law’. The hyperfine parameters of CaKFe₄As₄ are compared with those for KFe₂As₂ measured in this work, and the literature data for CaFe₂As₂, and were found to be in between those for these two, ordered, 122 compounds, in agreement with the gross view of CaKFe₄As₄ as a structural analog of KFe₂As₂ and CaKFe₄As₄ that has alternating Ca- and K-layers in the structure.