Fe-Based Superconductors of (Ln0.5–xNa0.5+x)Fe2As2 (Ln = Ce, Pr)

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Fe-Based Superconductors of (Ln0.5−xNa0.5+x)Fe2As2 (Ln = Ce, Pr) Akira Iyo,*,† Kenji Kawashima,†,‡ Shigeyuki Ishida,† Hiroshi Fujihisa,† Yoshito Gotoh,† Yoshiyuki Yoshida,† and Hiroshi Eisaki† †

National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan IMRA Material R&D Co., Ltd., 2-1 Asahi-machi, Kariya, Aichi 448-0032, Japan

Inorg. Chem. Downloaded from pubs.acs.org by STEPHEN F AUSTIN STATE UNIV on 07/27/18. For personal use only.

‡

ABSTRACT: Recently, we succeeded in synthesizing (La0.5−xNa0.5+x)Fe2As2 ((La,Na)122) with a solid solution range of 0 ≤ x ≤ 0.35. Superconductivity was induced for 0.15 ≤ x ≤ 0.35, with the highest transition temperature Tc = 27.0 K for x = 0.3. Here, we report the synthesis and physical properties of analogous compounds (Ln0.5−xNa0.5+x)Fe2As2 ((Ln,Na)122) (Ln = Ce, Pr). Samples were synthesized by precisely tuning the reaction temperature according to Ln and x. The solid solution ranges, 0.1 ≤ x ≤ 0.3 (Ln = Ce) and 0.15 ≤ x ≤ 0.25 (Ln = Pr), become narrower with increasing atomic number of Ln (which decreases the ionic radius of Ln3+). Bulk superconductivity emerged for 0.2 ≤ x ≤ 0.3 and 0.15 ≤ x ≤ 0.25 with the highest Tc of 25.6 K (x = 0.3) and 24.7 K (x = 0.25) for Ln = Ce and Pr, respectively. Crystal structures refined via the Rietveld analysis method showed that the (Ln,Na)122 compounds (Ln = La, Ce, Pr) with the highest Tc have almost the same As−Fe−As bond angles (∼107°) and As heights from Fe planes (∼1.43 Å). In addition to the solid solution ranges, the phases in the samples changed depending on the ionic radius of Ln3+. The (Ln,Na)122 phase competes with the non-superconducting CaFe4As3(143)-type phase of (Ln,Na)Fe4As3 for Ln = Ce and Pr, whereas only the (Ln,Na)122 phase was stable for Ln = La. The 143-type phase alone was observed for Ln = Nd, and neither 122- nor 143-type phases were observed for Ln = Sm and Gd.

1. INTRODUCTION The first discovered Fe-based superconductor was the 1111type compound LaFeAs(O,F)1 with a transition temperature (Tc) of 26 K. Soon afterward, Tc was enhanced to over 50 K by substitution of heavier lanthanide (Ln) elements of Ce to Sm for La.2−4 Moreover, high-pressure synthesis was used to extend this method to much heavier Ln elements (Gd to Er) to prepare LnFeAsO1−y and LnFeAs(O,H).5−10 The 112-type compounds (Ca,Ln)FeAs2 and their superconductivity were first discovered for Ln = La and Pr.11,12 Then, the range of Ln was immediately extended to Ce, Nd, Sm, Eu, and Gd.13 The recently discovered Fe-based superconductors of ALn2Fe4As4O2 (A = K, Rb, and Cs) also have rich variation in Ln, such as Nd, Sm, Gd, Tb, Dy, and Ho.14 Other than Fe-based superconductors, such Ln substitutions can be found in Cu-based ones. For example, after the discovery of high-Tc superconductor YBa2Cu3O7−y,15 Y was replaced with almost all Ln elements. Thus, once superconductors including Ln are discovered, replacing one Ln with another is a promising way to explore new materials. Materials with such rich Ln variations display systematic changes in their physical properties, because the ionic radius of Ln3+ decreases with its atomic number (NLn) (known as lanthanide contraction). Varying Ln in the materials can improve Tc and help to clarify the mechanism of superconductivity.16,17 The chemical and mechanical properties also change with Ln, which is also useful by allowing the selection of superconducting materials for different applications. © XXXX American Chemical Society

Recently, we reported superconductivity in the 122-type compound18 of (La0.5−xNa0.5+x)Fe2As2 ((La,Na)122) with the highest Tc of 27.0 K for x = 0.3.19 Then, a natural question is whether La can be replaced with other Ln, and, if yes, how the physical properties change with Ln. To answer these questions, in this study, we tried to replace La in (La,Na)122 with heavier Ln elements (Ce, Pr, Nd, Sm, and Gd). Two new superconductors were successfully synthesized: (Ln,Na)122 (Ln = Ce and Pr). Moreover, we found that the CaFe4As3-type (143-type) phase of (Ln,Na)Fe4As3 ((Ln,Na)143) could be formed for Ln = Ce, Pr, and Nd. In this paper, we report on the synthesis and physical properties such as the crystal structures, superconductivity, and phase diagram of (Ln,Na)122.

2. EXPERIMENTAL PROCEDURE Polycrystalline samples of (Ln,Na)122 (Ln = Ce, Pr) were synthesized with the same process used for (La,Na)122 and (Sr,Na)122.19,20 In brief, starting materials of LnAs, NaAs, Fe2As, and FeAs were mixed in a mortar at a composition of (Ln0.5−xNa0.5+x)Fe2As2 in a glovebox (with 5 at. % excess As to compensate for its evaporation from the sample during heating). Note that the x values given in this paper are nominal. The mixed powder was pressed into a pellet and wrapped in a Ta foil. The pellet was then placed into a stainless steel (SST) pipe and both ends of the pipe were sealed with tube-fitting caps. The SST pipe was then placed in a furnace preheated to a set temperature (Ts) Received: May 7, 2018

A

DOI: 10.1021/acs.inorgchem.8b01247 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Powder XRD patterns of (a) (Ce0.5−xNa0.5+x)Fe2As2 (0 ≤ x ≤ 0.4) and (b) (Pr0.5−xNa0.5+x)Fe2As2 (0 ≤ x ≤ 0.3). Peaks assigned to (Ln,Na)122 (Ln = Ce, Pr) are indicated by circles (diffraction indices hkl are attached for x = 0.2 only). and maintained there for 2.5−3 h, followed by quenching the SST pipe in water. Ts was tuned precisely according to Ln (Ce or Pr) as well as x, because 122- and 143-type phases compete with each other at high temperatures, whereas only the 122-type phase is stable for Ln = La. (Ln,Na)143 forms rapidly at above 920 and 860 °C for Ln = Ce and Pr, respectively. On the other hand, (Ln,Na)122 does not form efficiently below 840 °C. Considering the above situations, Ts (°C) of 900 (x = 0), 920−200x (0.1 ≤ x ≤ 0.3), and 850−840 (x = 0) were used for Ln = Ce. The limited Ts range of 840−850 °C (0 ≤ x ≤ 0.3) had to be used for Ln = Pr. Note that we did not prepare samples with x < 0, because such compositions are apparently outside the solid solution ranges, as will be shown later. We also tried to synthesize (Ln,Na)122 (Ln = Nd, Sm, Gd). Samples with nominal compositions of (Ln0.3Na0.7)Fe2As2 (x = 0.2, Ln = Nd, Sm, Gd) were heated at Ts = 860−840 °C. Powder XRD patterns were measured at room temperature using a diffractometer with CuKa radiation (Rigaku, Ultima IV). Lattice parameters of the 122- and 143-type phases in the samples were calculated by least-squares fitting using the d-values of the diffraction peaks. Compositions of the samples were analyzed by an energy dispersive X-ray spectrometer (SwiftED3000) equipped in an electron microscope (TM3000; Hitachi High-Technologies). The crystal structure of the sample was refined via Rietveld analysis using BIOVIA’s Materials Studio Reflex software (version 2017 R2).21

Magnetization (M) measurements were performed under a magnetic field (H) of 10 Oe using a SQUID magnetometer (Quantum Design, MPMS-XL7). The electrical resistivity was measured using a fourprobe method.

3. RESULTS AND DISCUSSION 3.1. X-ray Diffraction Analysis. We found that, like the case of Ln = La, for Ln = Ce and Pr, there exist solid solution ranges in which almost single-phase (Ln,Na)122 samples were synthesized. On the other hand, we never observed diffraction peaks corresponding to (Ln,Na)122 for Ln = Nd, Sm, or Gd, despite testing several combinations of x and Ts. Figure 1a,b shows powder XRD patterns of (Ln0.5−xNa0.5+x)Fe2As2 samples with 0 ≤ x ≤ 0.4 and 0 ≤ x ≤ 0.3 for Ln = Ce and Pr, respectively. Diffraction peaks assignable to (Ln,Na)122 are indicated by circles and diffraction indices (hkl). Note that superstructure peaks such as h + k + l = odd numbers were not observed. Therefore, there is no ordering of Ln and Na, unlike the 1144-type compounds of AeAFe4As4 (Ae = Ca, Sr; A = K, Rb, Cs) and (La,Na)AFe4As4 (A = Rb, Cs).22,23 Impurity phases of LnFeAsO were formed most likely due to oxygen contamination of the starting materials. B

DOI: 10.1021/acs.inorgchem.8b01247 Inorg. Chem. XXXX, XXX, XXX−XXX

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type compounds were detected for Ln = Sm and Gd, as shown in Figure 2b for Ln = Sm as a representative. The x-dependence of the a- and c-axis lattice parameters for (Ln,Na)122 is represented in Figure 3a,b for Ln = Ce and Pr,

There were a relatively large amount of impurity phases in (Ln,Na)122 samples with x = 0, as shown in Figure 1. This means that the solid solution ranges do not include x = 0 for Ln = Ce and Pr, although a parent compound (x = 0) with a formal Fe valence of +2 had been synthesized for Ln = La.19 We estimated the solid solution ranges from Figure 1 to be 0.1 ≤ x ≤ 0.3 and 0.15 ≤ x ≤ 0.25 for Ln = Ce, and Pr, respectively. Impurity phases similar to the case of (La,Na)122, such as 111-type compounds of NaFeAs (Na111) and LnAs, appeared in the (Ln,Na)122 (Ln = Ce, Pr) samples when the compositions were outside of the solid solution ranges. In addition, diffraction peaks that were never observed for (La,Na)122 appeared in (Pr,Na)122 samples for x = 0.1 and 0, as indicated by up triangles in Figure 1b. We assigned these peaks to the compound (Pr,Na)143 (a = 11.90(7) Å, b = 3.74(4) Å, c = 11.66(3) Å) from a similar analysis of the sample Ln = Nd, as shown below. XRD patterns of samples with nominal compositions of (Ln0.3Na0.7)Fe2As2 (Ln = Nd and Sm) are shown in Figure 2a,b, respectively. A complex diffraction pattern appeared for

Figure 3. Dependence of a- and c-axis lattice parameters (left and right y-axis, respectively) on the nominal composition (x) for the (Ln0.5−xNa0.5+x)Fe2As2 (Ln = Ce and Pr) phases with 0 ≤ x ≤ 0.4. Shaded areas represent the solid solution ranges estimated by XRD analysis.

respectively. The lattice parameters change significantly in the solid solution ranges (shaded areas) but very little elsewhere, like in the case of (La,Na)122.19 It means that actual composition x changed only inside the solid solution ranges. 3.2. Crystal Structure Refinement. We carried out crystal structure refinement for (Ce,Na)122 (x = 0.3) and (Pr,Na)122 (x = 0.25) samples, which have the highest Tc for each (Ln,Na)122 system. First, by assuming their sum to be 1, the occupancy ratios of Ce3+/Na+ and Pr3+/Na+ were optimized to be 0.23(3)/0.77(3) and 0.24(3)/0.76(3), respectively, which were close to the nominal composition ratios. Therefore, the actual x values of the samples in the solid solution ranges were almost the same as the nominal values. As a final refinement, a virtual chemical species comprising 23% Ce3+ (24% Pr3+) mixed with 77% Na+ (76% Na+) was placed at a CeNa (PrNa) site. Figure 4 shows the powder XRD pattern and Rietveld refinement profile for (Ce,Na)122 as a representative. The crystal structure of (Ln,Na)122 is illustrated in the inset. The refined structural parameters of (Ce,Na)122 and (Pr,Na)122 are summarized in Table 1, together with those of (La,Na)122 for comparison.19 The volume of a unit cell (V) decreased with increasing NLn, which is reasonable because the ionic radius of Ln3+ decreases as NLn increases. The As−Fe−As bond angle in the FeAs tetrahedra (αAs−Fe−As) and the As height from the Fe planes (hAs), two important factors governing Tc in Fe-based superconductors,25−27 were almost the same for La, Ce, and Pr. 3.3. Superconductivity. Figure 5a,b shows the temperature dependence of susceptibility for (Ln,Na)122 samples in the solid solution ranges for Ln = Ce and Pr, respectively. (Ce,Na)122 samples with x = 0.1 and 0.15 exhibited only a trace of superconductivity (not clearly visible in Figure 5a).

Figure 2. Powder XRD patterns of samples with the nominal compositions of (a) (Nd0.3Na0.7)Fe2As2 and (b) (Sm0.3Na0.7)Fe2As2 heated at Ts = 860 °C. Peaks assigned to a CaFe4As3-type compound, (Nd,Na)Fe4As3, are indicated by up triangles. Backgrounds are subtracted from the data.

Ln = Nd, whereas most peaks for Ln = Sm were identified as known compounds. We were able to index most of the unknown peaks other than those from Na111 and NdAs by assuming an orthorhombic system with lattice parameters of a = 11.915(2) Å, b = 3.7026(3) Å, and c = 11.70(2) Å, as indicated by the up triangles in Figure 2a. We analyzed compositions of grains in the sample and found a compound with the atomic composition ratio of (Nd + Na):Fe:As = 15(1):49(3):36(3). The measured lattice parameters and composition ratio are similar to those of CaFe4As3 (a = 11.895(1) Å, b = 3.7430(3) Å, c = 11.610(1) Å; Ca:Fe:As = 12.5:50:37.5).24 We concluded that the compound crystallized in the sample is most likely to be the 143-type compound of (Nd,Na)143. Note that the Nd:Na atomic ratio in (Nd,Na)143 was measured to be 35(3):65(5), which is close to the nominal value (Nd:Na = 3:7). It suggests that the composition ratio of Nd3+ and Na+ (valence state of Fe) in (Nd,Na)143 can be more or less changed, whereas it is fixed in CaFe4As3. Neither 122- nor 143C

DOI: 10.1021/acs.inorgchem.8b01247 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Powder XRD pattern and Rietveld refinement profile of (Ce0.2Na0.8)Fe2As2 (Obs.: Observed, Cal.: Calculated). Inset: Crystal structure of (Ln,Na)Fe2As2 illustrated using the VESTA software.28

Table 1. Refined Structural Parameters at Room Temperature for (Ln0.5−xNa0.5−x)Fe2As2 (Ln = La, Ce, Pr) Having the Highest Tc for Each (Ln,Na)122 Systema Ln x (nominal) x (refined) Tc (K) a (Å) c (Å) V (Å3) LnNa (2a) Fe (4d) As (4e) 0,0,z dFe−As (Å) hAs (Å) αAs−Fe−As (deg)

La

Ce

compositions and Tc 0.3 0.3 0.29(3) 0.27(3) 27.0(1) 25.6(1) lattice parameters 3.8409(1) 3.8408(1) 12.3245(2) 12.2394(2) 181.81(1) 180.55(1) atomic coordinates 0,0,0 0,0,0 0,1/2,1/4 0,1/2,1/4 0.3659(1) 0.3660(1) selected structural parameters 2.393(2) 2.388(2) 1.428(3) 1.420(3) 106.7(1) 107.0(1)

Pr 0.25 0.26(3) 24.7(1)

Figure 5. Temperature dependence of mgnetization (M) for (a) (Ce0.5−xNa0.5−x)Fe2As2 (x = 0.1−0.3) and (b) (Pr0.5−xNa0.5−x)Fe2As2 (x = 0.15−0.25) measured in a magnetic field (H) of 10 Oe. Measurements were performed with zero-field-cooled (ZFC) and field-cooled (FC) procedures.

3.8388(1) 12.1927(3) 179.68(1)

The Tc values of (Ln,Na)122 (Ln = La, Ce, Pr) are plotted for x in Figure 6. Tc tends to rise as x increases for each

0,0,0 0,1/2,1/4 0.3667(1) 2.389(2) 1.422(3) 106.9(1)

Space group: I4/mmm. The occupancy was fixed to 1 at all atomic sites. For Ln = Ce, Rwp = 11.42%, Re = 10.79%, S = 1.06. Preferred orientation parameters: R0 = 1.387(5), direction = ⟨0.420, 0.167, 0.892⟩. The isotropic displacement parameter Uiso is 0.0038(1), 0.0017(1), and 0.0020(1) Å2 for CeNa, Fe, and As sites, respectively. For Ln = Pr, Rwp = 10.91%, Re = 10.36%, S = 1.05. Preferred orientation parameters: R0 = 0.713(2), direction = ⟨0.324, 0.120, 0.939⟩. Uiso is 0.0046(1), 0.0024(1), and 0.0028(1) Å2 for PrNa, Fe, and As sites, respectively. a

Figure 6. x-dependence of Tc for (Ln0.5−xNa0.5+x)Fe2As2 (Ln = La, Ce, Pr) in the solid solution ranges. Tc was defined as the temperature where the ZFC susceptibility is 10% that at 5 K.

Because there are no Ln1111-based superconductors composed of Ln, Na, Fe, As, and O synthesized at normal pressure, we could exclude the existence of a small amount of Ln1111type compound as an origin of the superconducting trace. Instead, the superconducting trace is most probably attributable to a small amount of superconducting component owing to the inhomogeneity in x that exists more or less in polycrystalline samples. (Ce,Na)122 samples with x = 0.2, 0.25, and 0.3 and all the (Pr,Na)122 samples in the solid solution ranges (x = 0.15, 0.2, and 0.25) showed clear superconducting transitions with large shielding volume fractions of approximately 60−100%. This bulk superconductivity is due to (Ln,Na)122 in consideration of the yield of (Ln,Na)122 in the sample, as can be seen from the XRD patterns (Figure 1).

(Ln,Na)122 system, and the highest values were 27.0 K (x = 0.3), 25.6 K (x = 0.3), and 24.7 K (x = 0.25) for Ln = La, Ce, and Pr, respectively. Thus, the (Ln,Na)122 system had relatively lower Tc compared to other 122-systems of (Ae,A)122 (Tc = 34−38 K).20,29−35 As pointed out in the (La,Na)122 case,19 there are the following two possible reasons for the lower Tc. First, the combination of Ln3+ and Na+ could induce greater potential disorder to the superconducting Fe2As2 layers than those for Ae2+ and A+. Second, the αAs−Fe−As (hAs) values in the (Ln,Na)122 system are much D

DOI: 10.1021/acs.inorgchem.8b01247 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

3.4. Phase Diagrams. The solid solution ranges and superconducting ranges for (Ln,Na)122 (Ln = La, Ce, Pr) are summarized in Figure 8, plotted against the ionic radius of

smaller (larger) than the optimal values (109.5° and 1.38 Å, respectively), compared to other 122-systems. In the (Ln,Na)122 system, the highest Tc slightly decreases with increasing NLn. It is unlikely that this trend is attributed to changes in αAs−Fe−As and hAs, which are almost independent of Ln. A possible reason for the Tc change is the increase in the magnitude of potential disorder as NLn increases. The chemical pressure (a decrease in V) can also help to suppress Tc, since it generally decreases when applying a physical pressure to 122type superconductors.36 Figure 7a,b shows the temperature-dependent normalized resistivity for (Ln,Na)122 (Ln = Ce and Pr) samples in the

Figure 8. Solid solution ranges (shaded areas) and superconductivity ranges of (Ln0.5−xNa0.5+x)Fe2As2 plotted against the ionic radius of Ln3+ (VIII). Diamonds and circles represent superconducting (SC) and non-superconducting (non-SC) compositions, respectively. For Ln = Nd, only (Nd,Na)Fe4As3 was formed, not (Nd,Na)Fe2As2.

Ln3+. The solid solution range narrows with decreasing ionic radius of Ln3+. It does not reach Ln = Nd for which only the (Ln,Na)143 phase is stable, so the effective ionic radius of the (Nd,Na) site must be too small to allow the (Nd,Na)122 phase to crystallize. Note that the 143-type compound also has never been detected for Ln = La. These results mean that the crystallization of 143-type compounds depends sensitively on the effective ionic radius of the (Ln,Na) site in (Ln,Na)122. It is interesting that the thermodynamically stable phases as well as the solid solution ranges changed significantly with Ln, even though the difference in ionic radius from La3+ to Sm3+ is small (0.08 Å). (Ionic radii of Ln3+ (VIII) are 1.16, 1.14, 1.13, 1.11, and 1.08 Å for La, Ce, Pr, Nd, and Sm, respectively.38) The boundary between superconducting (SC) and nonsuperconducting (non-SC) samples is around x = 0.15 (i.e., (Ln0.35Na0.65)122), which corresponds to a formal Fe valence of +2.15. Almost the same formal Fe valence was observed at the boundary (x ∼ 0.3) for (Ae,A)122, e.g., (Ba0.7K0.3)122. Note that twice the number of holes were doped with regard to the same x for (Ln3+0.5−xNa+0.5+x)122 compared to (Ae2+1−xA+x)122. It will be interesting to further extend the Ln variation in the (Ln,Na)122 system, in order to investigate the relationship between Tc and the structure in more detail. High-pressure synthesis, which is a powerful method to append Ln with smaller ionic radius in LnFeAsO-based superconductors,6,9,10 will be also useful for the (Ln,Na)122 system.

Figure 7. Temperature dependence of normalized resistivity for (a) (Ce0.5−xNa0.5−x)Fe2As2 (x = 0.1, 0.2, and 0.3) and (b) (Pr0.5−xNa0.5−x)Fe2As2 (x = 0.15, 0.2, and 0.25). The resistivity anomaly is indicated by an arrow. The absolute values of resistivity (mΩ·cm) at 300 K were within the ranges of 1−3 and 3−6 for (Ce,Na)Fe2As2 and (Pr,Na)Fe2As2, respectively.

solid solution ranges, respectively. The resistivity anomaly, which often appears in non-superconducting Fe-based compounds, was observed at 108 K for x = 0.1, as indicated by the arrow in Figure 7a. The resistivity anomaly was also observed in the (La0.4Na0.6)Fe2As2 single crystal at 125 K,37 and shown to be caused by a structural phase transition from a high-temperature tetragonal to a low-temperature orthorhombic phase, accompanied by antiferromagnetic ordering of the Fe moments.37 Therefore, the anomaly observed in (Ce,Na)122 (x = 0.1) most probably has the same origin as (La0.4Na0.6)Fe2As2. The anomalous temperature of (Ce,Na)122 (x = 0.1) is slightly lower than that of the (La,Na)122 polycrystal (113 K, x = 0.1).19 The decrease in resistivity below 20 K is due to a small superconducting component detected by the susceptibility measurement. (Ce,Na)122 (x = 0.2 and 0.3) and (Pr,Na)122 (x = 0.15, 0.2, and 0.25) showed superconducting transitions with zero resistivity at 18−25 K.

4. SUMMARY In this study, we succeeded in appending Ln = Ce and Pr to the (Ln,Na)122 superconductor system. The solid solution ranges were found to become narrower with decreasing ionic radius of Ln3+. Bulk superconductivity emerged for 0.2 ≤ x ≤ 0.3 and 0.15 ≤ x ≤ 0.25 for Ln = Ce and Pr, respectively. The highest Tc (27.0, 25.6, and 24.7 K for Ln = La, Ce, and Pr, respectively) slightly decrease with Ln. The structure parameters of αAs−Fe−As and hAs are almost independent of Ln for the (Ln,Na)122 (Ln = La, Ce, Pr) with the highest Tc. E

DOI: 10.1021/acs.inorgchem.8b01247 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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The formed phases strongly depend on Ln: whereas only the (Ln,Na)122 phase is stable for Ln = La, (Ln,Na)122 and (Ln,Na)Fe4As3 phases compete with each other for Ln = Ce and Pr, while no (Ln,Na)122 phase was formed for Ln = Nd, Sm, and Gd. It would be interesting to use a further extension of Ln to investigate the relationship between Tc and the sample structure.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Akira Iyo: 0000-0002-9610-647X Kenji Kawashima: 0000-0003-4786-3498 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A part of this work was supported by JSPS KAKENHI Grant Numbers JP16H06439 and JP26247057.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b01247 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.inorgchem.8b01247 Inorg. Chem. XXXX, XXX, XXX−XXX