Effect of 3d Transition Metal Doping on the Superconductivity in Quaternary Fluoroarsenide CaFeAsF

We examined doping effect of 3d transition metal elements (TM: 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 (rho) vs. temperature (T) plot for the parent compound is suppressed by the doping of each TM element. Furthermore, Co- and Ni-doping (CaFe1-xTMxAsF, TM = Co, Ni) induces superconductivity with a transition temperature maximized at the nominal x = 0.10 for Co (22 K) and at x = 0.05 for Ni (12 K). 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 drho/dT>0 is still kept. These observations indicate that Co and Ni work as electron donors. In contrast, neither of Cr, Mn nor Cu-doping induce superconductivity, yielding drho/dT<0 in the below the rho-T anomaly temperature, indicating that these transition metal ions act as scattering centers. The two different behavior of TM replacing the Fe site is discussed in relation to the changes in the lattice constants with the doping.

They suffer a crystallographic transition from the tetragonal to orthogonal, accompanying with antiferromagnetic transition yielding anomaly in electrical resistivity at 140-200 K [1], [3], [7]- [11], [16]- [17]. It is now a consensus that the superconductivity in all the compounds is induced as a result of electron or hole doping to the (FeAs) δ− layer, which simultaneously suppress both transitions to occur. Thus, exploring efforts in the material studies have been focused on the synthesis of ZrCuSiAs-and related-type compounds containing the square iron lattice as well as on the carrier doping technique [19].
Four types doping methods have been reported to date i.e., (1) indirect electron-doping by an introduction of fluorine into the O sites and the oxygen-vacancy formation in insulating layers (RO) δ+ in RFeAsO [20], [21], (2)  AFeAsF (A = Ca, Sr), in which the (FeAs) δ− layer is sandwiched by (AF) δ+ layer in place of (RO) δ+ layer in RFeAsO compounds (figure 1a) and their superconductivity induced by partial substitution of Fe with cobalt [27], [28]. Following these report, the existence of AFeAsF (A = Ca, Sr and Eu) system was reported by a few groups [29], [30] and superconductivity induced by lanthanide metal doping with T c up to 56 K have been posted on arXiv/condmat preprint server [31], [32]. For the case of partial substitution of  [26]. Our discovery indicates that the Co-doping technique is a universal way to convert the FeAs-based layered compounds to superconductors. Further, the effectiveness of Co-doping suggests that the square Fe lattice in the FeAs layer is much more robust to impurities, than CuO 4 planes in High-T c cuprate. [33] In this study, we examined substitution of a series of 3d-transtion metals to the iron site on the emergence of superconductivity in CaFeAsF. Temperature dependence of electrical resistivity and lattice parameter changes upon substitution were measured.
As a consequence, it was revealed that Ni-substitution induces superconductivity similar to the Co while Cr-, Mn-or Cu-substitution does not yield superconductivity. The optimal concentration for Ni was almost a half of Co, suggesting that both ions with excess 3d electron serve as electron donor. wt.%). These products were then mixed in stoichiometric ratios, pressed, and heated in evacuated silica tubes at 1000 ˚C for 10 h to obtain sintered pellets. All the procedures until the sealing to silica glass tubes were carried out in an Ar-filled glove box (O 2 , H 2 O < 1 ppm).

Experimental
The crystal structure and lattice constants of the materials were examined by powder X-ray diffraction (XRD; Bruker D8 Advance TXS) using Cu Kα radiation from a rotating anode with an aid of Rietveld refinement using Code TOPAS3 [34].
Temperature dependence of DC electrical resistivity (ρ) at 2-300 K was measured by a four-probe technique using platinum electrodes deposited on samples Figure 1b shows powder XRD pattern of non-doped CaFeAsF. Except the several weak peaks arising from impurity phase (Fe 2 As, the volume fraction being 2 % at most), each of the major peaks was assigned to the CaFeAsF phase and room temperature lattice constants was evaluated as a = 0.3879 nm and c = 0.8593 nm. Figure 1c shows temperature dependence of electrical resistivity of CaFeAsF. With a decrease in temperature, ρ-T curves exhibit sudden decreases at ~120 K (T anorm ). This anomalous behavior is quite analogous to those in RFeAsO and BFe 2 As 2 , implying that the crystallographic transition takes place at T anorm . CaFeAsF also likely suffers a magnetic ordering in the similar temperature region.   In contrast, the increment of a-axis length due to Co-and Ni-substitution is smaller than that for the above cases (+0.04% for Co and +0.03% for Ni-doping for nominal x = 0.05), indicating Co-and Ni-substitution don't induce large distortion in the iron lattice.

Results and Discussion
As shown in lower column of figure

Summary
We examined the partial replacement of Fe site in CaFeAsF with 3d-transition metals (TM: Cr, Mn, Co, Ni and Cu) and obtained the following conclusion; (1) Only Co-or Ni-doping was effective for emergence of superconductivity. The optimal doping level for Ni was close to a half of that for Co-doping.
(2) Cr-, Mn-or Cu-doping led to enhanced resistivity at low temperatures, without inducing superconductivity, indicating that these ions act as scattering centers.  Triangles are due to impurity phases (CaF 2 , FeAs and CaMn 2 As 2 ). 10