Synthesis, Crystal Structure and Superconductivity in RbLn2Fe4As4O2 (Ln = Sm, Tb, Dy, and Ho)

We have synthesized four iron-based oxyarsenide superconductors RbLn(2)Fe(4)As(4)O(2) (Ln = Sm, Tb, Dy and Ho) resulting from the intergrowth of RbFe2As2 and LnFeAsO. It is found that the lattice match between RbFe2As2 and LnFeAsO is crucial for the phase formation. The structural intergrowth leads to double asymmetric Fe2As2 layers that are separated by insulating Ln(2)O(2) slabs. Consequently, the materials are intrinsically doped at a level of 0.25 holes/Fe-atom, and bulk superconductivity emerges at T-c = 35.8, 34.7, 34.3, and 33.8 K, respectively, for Ln = Sm, Tb, Dy, and Ho. Investigation of the correlation between crystal structure and T-c suggests that interlayer couplings may play an additional role for optimization of superconductivity.