Effect of 3d transition metal doping on the superconductivity in quaternary fluoroarsenide CaFeAsF

We examined the doping effect of 3d transition metal (TM) elements (Cr, Mn, Co, Ni and Cu) at the Fe site of a quaternary fluoroarsenide CaFeAsF, an analogue of 1111-type parent compound LaFeAsO. The anomaly at ∼120 K observed in resistivity (ρ) versus temperature (T) plot for the parent compound is suppressed by the doping of each TM element. Furthermore, Co and Ni doping (CaFe_1-xTM_xAsF, TM = Co, Ni) induces superconductivity with a transition temperature maximized at the nominal x = 0.10 for Co (22K) and at x = 0.05 for Ni (12K). These optimal doping levels may be understood by considering that Ni²⁺(3d⁸) adds double electrons to the FeAs layers compared with Co²⁺ (3d⁷). Increased x for Co or Ni breaks the superconductivity, while metallic nature dρ/dT > 0 is still kept. These observations indicate that Co and Ni serve as electron donors. In contrast, Cr, Mn and Cu doping does not induce superconductivity, yielding dρ/dT < 0 below the ρ-T anomaly temperature, indicating that these TM ions act as scattering centers. The two different types of behavior of TM replacing the Fe site are discussed in relation to the changes in the lattice constants with doping.