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

We have synthesized four iron-based oxyarsenide superconductors RbLn2Fe4­As4O2 (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 Ln2O2 slabs. Consequently, the materials are intrinsically doped at a level of 0.25 holes/Fe-atom, and bulk superconductivity emerges at Tc = 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 Tc suggests that interlayer couplings may play an additional role for optimization of superconductivity.