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 RbFe 2 As 2 and LnFeAsO. It is found that the lattice match between RbFe 2 As 2 and LnFeAsO is crucial for the phase formation. The structural intergrowth leads to double asymmetric Fe 2 As 2 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.