Superconductivity in a 122-type Fe-based compound (La,Na,K)Fe(2)As(2)

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 I4/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 AFe(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 1a. 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)AFe(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.