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

Highlights: • 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. • T_c of (Ca, RE)FeAs₂ decreased with decreasing ionic radii of RE³⁺. • Eu doped samples showed exceptionally low T_c and long interlayer distance. • Long interlayer distance of Eu doped samples suggested co-existence of Eu²⁺ and Eu³⁺. - Abstract: 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. T_c 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 T_c = 21 K probably due to the co-existence of Eu³⁺ and Eu²⁺ suggested by longer interlayer distance d_Fe–Fe of (Ca, Eu)112 than other (Ca, RE)112.