Dependences on RE of superconducting properties of transition metal co-doped (Ca, RE)FeAs2 with RE = La–Gd

•We synthesized Co or Ni co-doped (Ca, RE)FeAs2 with RE = La–Gd.•Co or Ni co-doping improved superconducting properties of all (Ca, RE)FeAs2 samples.•Tc of (Ca, RE)FeAs2 decreased with decreasing ionic radii of RE3+.•Eu doped samples showed exceptionally low Tc and long interlayer distance.•Long interlayer distance of Eu doped samples suggested co-existence of Eu2+ and Eu3+.

Dependence of superconducting properties of (Ca, RE)(Fe, TM)As2 [(Ca, RE)112, TM: Co, Ni)] on RE elements (RE = La–Gd) was systematically investigated. Improvement of superconducting properties by Co or Ni co-doping was observed for all (Ca, RE)112, which is similar to Co-co-doped (Ca, La)112 or (Ca, Pr)112. Tc of Co-co-doped samples decreased from 38 K for RE = La to 29 K for RE = Gd with decreasing ionic radii of RE3+. However, Co-co-doped (Ca, Eu)112 showed exceptionally low Tc = 21 K probably due to the co-existence of Eu3+ and Eu2+ suggested by longer interlayer distance dFe–Fe of (Ca, Eu)112 than other (Ca, RE)112.