Superconductivity in a New 1144-Type Family of (La,Na)AFe4As4 (A = Rb or Cs).

We discovered novel Fe-based superconductors (FeSCs) (La,Na)AFe4As4, where A = Rb or Cs, and characterized their superconducting properties. (La,Na)AFe4As4 is a so-called 1144-type compound with a tetragonal unit cell classified into space group P4/mmm (no. 123). The lattice constants are a = 3.861(1) Å and c = 13.26(1) Å for (La,Na)RbFe4As4 and a = 3.880(1) Å and c = 13.60(1) Å for (La,Na)CsFe4As4. The Rietveld refinement results on the powder X-ray diffraction suggest that the La/Na ratio is rather fixed as La:Na = 0.44(5):0.56(5). The electrical resistivity and magnetic susceptibility show superconducting transition at 25.5 K for (La,Na)RbFe4As4 and 24.0 K for (La,Na)CsFe4As4. The superconducting transition temperature (Tc) of (La,Na)AFe4As4 is comparable with that of 122-type (La,Na)Fe2As2 and lower than that of typical 122-type or 1144-type FeSCs by more than 10 K. The possible reasons for lower Tc are discussed in terms of the structural modification, carrier concentration, and chemical disorder.