Superconductivity at 33-37 K in ALn2Fe4As4O2 (A = K and Cs; Ln = lanthanides)

We have synthesized ten iron oxyarsenides, KLn(2)Fe(4)As(4)O(2) (Ln = Gd, Tb, Dy, and Ho) and CsLn(2)Fe(4)As(4)O(2)(Ln = Nd, Sm, Gd, Tb, Dy, and Ho), with the aid of the lattice-match approach. The resultant compounds possess hole-doped conducting double FeAs layers [AFe(4)As(4)](2-) that are separated by the insulating [Ln(2)O(2)](2+)slabs. Measurements of electrical resistivity and dcmagnetic susceptibility demonstrate bulk superconductivity at T-c = 33-37 K. We find that T-c correlates with the axial ratio c/a for all 12442-type superconductors discovered. Also, T-c tends to increase with the lattice mismatch, implying that lattice instability plays a role in the enhancement of superconductivity.