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We synthesized a Fe-based superconductor (FeSC), (La,Na,K)Fe 2 As 2 , and characterized its superconducting properties. It was found that (La,Na,K)Fe 2 As 2  has a 122-type (ThCr 2 Si 2 -type) structure with a space group  I 4/ mmm  (No. 139), identical to (Ba,K)Fe 2 As 2  and (La,Na)Fe 2 As 2  but distinct from so-called 1144-type FeSCs such as CaKFe 4 As 4  and (La,Na)CsFe 4 As 4 . The results demonstrate that the formation of the 1144-type phase necessitates the large ionic radius mismatch among the so-called  A -site constituent elements of the  A Fe 2 As 2  formula. The lattice constants are  a  = 3.850(1) Å and  c  = 13.21(1) Å. The La, Na, and K ions occupy the same atomic site of Wyckoff position 1 a . Electrical resistivity and magnetic susceptibility show the superconducting transition at 22.5 K. The transition temperature ( T  c ) of (La,Na,K)Fe 2 As 2  is comparable with that of 122-type (La,Na)Fe 2 As 2  and 1144-type (La,Na) A Fe 4 As 4  ( A  = Rb, Cs), while being more than 10 K lower than those of typical 122- and 1144-type FeSCs. The results suggest that the random distribution of La 3+  and Na +  ions is the main reason for lower  T  c  in the  AE  = (La,Na) 122-type and 1144-type FeSCs.

Superconductivity in a 122-type Fe-based compound (La,Na,K)Fe2As2