Superconductivity in a 122-type Fe-based compound (La,Na,K)Fe2As2.

We synthesized a Fe-based superconductor (FeSC), (La,Na,K)Fe2As2, and characterized its superconducting properties. It was found that (La,Na,K)Fe2As2 has a 122-type (ThCr2Si2-type) structure with a space group I4/mmm (No. 139), identical to (Ba,K)Fe2As2 and (La,Na)Fe2As2 but distinct from so-called 1144-type FeSCs such as CaKFe4As4 and (La,Na)CsFe4As4. The results demonstrate that the formation of the 1144-type phase necessitates the large ionic radius mismatch among the so-called A-site constituent elements of the AFe2As2 formula. The lattice constants are a = 3.850(1) Å and c = 13.21(1) Å. The La, Na, and K ions occupy the same atomic site of Wyckoff position 1a. Electrical resistivity and magnetic susceptibility show the superconducting transition at 22.5 K. The transition temperature (Tc) of (La,Na,K)Fe2As2 is comparable with that of 122-type (La,Na)Fe2As2 and 1144-type (La,Na)AFe4As4 (A = Rb, Cs), while being more than 10 K lower than those of typical 122- and 1144-type FeSCs. The results suggest that the random distribution of La3+ and Na+ ions is the main reason for lower Tc in the AE = (La,Na) 122-type and 1144-type FeSCs.

Introduction

The discovery of superconductivity in LaFeAs(O,F) in 2008 has triggered the search for new Fe-based superconductors[1]. A large number of Fe-based superconductors (FeSCs) with various crystal structures have been reported, such as LnFeAs(O,F) (Ln corresponds to rare earth elements) — a 1111-type compound[1-3], (AE1−A)Fe2As2 (AE = Ca, Sr, Ba, Eu, A = Na, K, Rb) — a 122-type compound[4,5], AEAFe4As4 (AE = Ca, Sr, Ba, Eu, A = Na, K, Rb) — a 1144-type compound[6-9], AFeAs (A = Li, Na) — a 111-type compound[10], (Ca,Ln)FeAs2 — a 112-type compound[11,12], and perovskite Fe nictide[13]. The superconducting transition temperature (Tc) reaches as high as ~55 K in bulk materials, and several reports have indicated that Tc is close to 100 K in thin film samples[14,15].

Usually, the 122-type and the 1144-type compounds contain divalent (mostly alkali earth) elements. On the other hand, we have recently demonstrated that the combination of monovalent Na and trivalent La successfully substitute the divalent ions and form the 122- and 1144-type superconductors, such as La0.5−Na0.5+Fe2As2 ((La,Na)Fe2As2) and (La,Na)AFe4As4 (A = Rb, Cs). For 122-type (La,Na)Fe2As2, La0.4Na0.6Fe2As2 (x = 0.1) is a non-superconductor and exhibits a structural phase transition at 130 K[16]. Superconductivity appears between as a consequence of hole carrier doping, with the highest Tc of 27.0 K for x = 0.3[17]. On the other hand, 1144-type (La,Na)AFe4As4 exhibit superconductivity by themselves, with Tc around 25 K for (La,Na)RbFe4As4 and 24 K for (La,Na)CsFe4As4, respectively[18].

The previous results indicate that the combinations of (La,Na) yields the 122-type structure while those of (La,Na,Rb) and (La,Na,Cs) yield the 1144-type structure. From the results, we have pointed out two parameters which possibly determine the preferred crystal structure. The first parameter is the ionic size difference between the AE ions and the A ions, namely, Δr = rAE − rA, where rAE and rA are ionic radii of AE (=Ca, Sr, Ba, Eu, (La,Na)) and A (=Na, K, Rb, Cs) ions. Here large Δr favors the 1144-type structure. The second parameter is the lattice mismatch of the original 122-type compound crystal structure, namely, Δa = |aAE122 − aA122|, where aAE122 and aA122 are the lattice constants of AEFe2As2 and AFe2As2. Here small Δa favors the 1144-type structure. According to the criteria, (La,Na) prefers the 122-type while (La,Na,Rb(Cs)) prefers the 1144-type, respectively.

The combination of (La,Na,K) is interesting in the above regard, since its parameters are close to the border of the above criteria. More specifically, the 122-type structure is expected judging from Δr, while the 1144-type is favored based on Δa. In order to extend the material verify of FeSCs, and more importantly, to determine which parameter indeed governs the crystal structure of real materials, we tried to synthesize (La,Na,K)Fe2As2. In this paper, we show that (La,Na,K)Fe2As2 forms the 122-type crystal structure. The observed Tc is 22.5 K, which is close to that of 122-type (La,Na)Fe2As2 and 1144-type (La,Na)AFe4As4 (A = Rb, Cs) FeSCs. We discuss the phase stability of (La,Na,K)Fe2As2 and possible reasons for their lower Tc based on the experimental observations.

Results

Figure 1 shows the powder X-ray diffraction (PXRD) pattern of the synthesized sample. The main phase can be indexed as a tetragonal unit cell with a space group I4/mmm (No. 139), which corresponds to the 122-type (ThCr2Si2-type) structure. Extra reflections, assigned to a minor impurity phase, are identified as LaAs and LaOFeAs (non-superconducting phase)[1,3,19]. Most importantly, there are no peaks corresponding to h + k + l = odd number, which are the signature of the 1144-type compounds. La, Na, and K occupy the same atomic position (Wyckoff position 1a) in its crystal structure. The lattice constants of (La,Na,K)Fe2As2 are a = 3.850(1)Å and c = 13.21(1)Å. The a and c lattice parameters of (La,Na,K)Fe2As2 are between those of La0.4Na0.6Fe2As2 (a = 3.8669(1) Å and c = 12.108(1) Å) and KFe2As2 (a = 3.8414(1) Å and c = 13.837(1) Å)[16,17,20]. More specifically, a-axis lengths of these compounds are: KFe2As2 < (La,Na,K)Fe2As2 < (La,Na,K)Fe2As2, while c-axis lengths are (La,Na)Fe2As2 < (La,Na,K)Fe2As2 < KFe2As2, respectively.

Figure 1

Powder X-ray patterns of (La,Na,K)Fe2As2.

We synthesized samples with various La/Na ratio and determined their lattice parameters and the chemical compositions (Table S1 and Fig. S1 in Supplementary Information). It turned out that the actual chemical composition does not depend on the initial compositions and yields close to La0.2Na0.3K0.5Fe2As. Correspondingly, the lattice constants do not depend on the initial composition (Table S1 and Fig. S2). Indeed, the change of the lattice constant is, if any, smaller by one order or more compared with the case of (La,Na)Fe2As2[17]. The results suggest that the 122-type (La,Na,K)Fe2As2 is stable only under fixed chemical composition. We note that Tc shows no La/Na (nominal) composition dependence, in contrast to the continuous change in Tc for 122-type (La,Na)Fe2As2[17].

Figure 2 shows the temperature (T) dependence of the magnetic susceptibility of (La,Na,K)Fe2As2 under an applied magnetic field of H = 10 Oe. The magnetic susceptibility exhibits a marked drop at 22.5 K in both ZFC and FC processes. Since the possible impurity phase LaAs and LaOFeAs do not show superconductivity[1,3,19], the superconductivity comes from the main (La,Na,K)Fe2As2 phase. The shielding volume fraction is 102%, a reasonable value as a bulk superconductor[21].

Figure 2

Temperature dependence of the magnetic susceptibility of (La,Na,K)Fe2As2.

Figure 3 shows the electrical resistivity of (La,Na,K)Fe2As2 as a function of T. The resistivity data shows the metallic behavior with convex curvature down to low-T. This behavior is also observed in other 122- and 1144-type superconductors[4-9,17,18]. The residual resistivity ratio, RRR = ρ300/ρ0, is 5.21, indicating the absence of strong scattering arising from impurities and/or grain boundaries, suitable for investigating their physical properties. As seen in the inset of Fig. 3, the resistivity sharply decreases at 23 K and reaches zero resistivity at 22 K.

Figure 3

Temperature dependence of the electrical resistivity of (La,Na,K)Fe2As2. Inset shows a magnified view near Tc.

Figure 4 shows the electrical resistivity data for (La,Na,K)Fe2As2 under various magnetic fields (H) as functions of T. The onset Tc and the zero resistivity temperature decrease systematically with increasing H. The transition width does not change with H up to the highest H. The superconducting transitions are not completely suppressed under H ≦ 90 kOe, indicating that the upper critical field of Hc2 is very large. A plot of Hc2(T) versus Tc(H) is shown in Fig. 5. Here Tc(H) is defined as the midpoint of the superconducting transition for each H, and the horizontal bar indicates the T-range between 10% and 90% of the resistivity transition. Hc2(T) shows the linear T-dependence within the measured T- and H- range. The slope, dHc2/dT, is −5.015 T/K. Using the Werthamer-Helfand-Hohenberg (WHH) formula, Hc2(0) = −0.69 (dHc2/dT)|Tc for a type-II superconductor[22], the upper critical magnetic field at 0 K is estimated to be 80 T. The corresponding coherence length (ξ0) is calculated to be 20.3 Å, estimated from the relationship between Hc2 ~ Φ0/2πζ02, where Φ0 is quantum flux.

Figure 4

Temperature dependence of the electrical resistivity of (La,Na,K)Fe2As2 under various magnetic fields up to 90 kOe.

Figure 5

H-T phase diagram of (La,Na,K)Fe2As2. Dotted lines show the linear fitting result. Inset shows a magnified view near Tc.

Discussion

The present study demonstrates that (La,Na,K)Fe2As2 crystalize into a 122-type crystal structure rather than a 1144-type structure. Figure 6 shows the variation of the 122-type and 1144-type compounds in terms of the ionic size (VIII) difference of Δr = rAE − rA (rAE and rA are ionic radii of AE and A ions) and the lattice constant difference of the end materials, Δa = |aAE122 − aA122|, (aAE122 and aA122 are the a-axis lattice constants of AEFe2As2 and AFe2As2). From previous case studies, we have shown that the 1144-type structure is formed when Δa < 0.07 Å and Δr < −0.35 Å. In the case of (La,Na,K)Fe2As2, Δa = |a(La,Na)122 − aK122| is 0.03 Å, which possibly favors the 1144-type structure. On the other hand, Δr = r(La,Na) − rK = −0.34 Å, which is at the boundary between the 122-type and the 1144-type phases. Giving that (La,Na,K)Fe2As2 forms the 122-type structure, one can conclude that the critical parameter which determines the real crystal structure is Δr, rather than Δa.

Figure 6

Plot of the difference between the ionic radius (VIII) of AE2+ and A+ (Δr = rAE − rA) and the difference (absolute value) between the a-axis lengths of AEFe2As2 (AE122) and AFe2As2 (A122) (Δa = |aAE122 − aA122|) for 122- and 1144-type Fe-based superconductors[4–9,17,18]. Yellow diamond shows (La,Na,K)Fe2As2.

In Fig. 7 we summarize Tcmax of 122- and 1144-type superconductors with various alkali metal elements (A = Na, K, Rb, Cs). Clearly Tc with AE = (La,Na) is significantly lower, by nearly 10 K, than other 122- and 1144-type FeSCs. Furthermore, Tc of (La,Na,K)Fe2As2 is 22.5 K, much lower than Tc = 27 K for (La,Na)Fe2As2. In our previous paper, we pointed out two possible origins for lower Tc in the materials containing AE = (La,Na)[18], namely, the difference in the As-FeAs bond angles, and the random distribution of trivalent La3+ and monovalent Na+ ions, which possibly causes the strong potential disorder and/or local lattice distortion[23]. The local lattice distortion is even larger for (La,Na,K)Fe2As2 because of the large contrast in the ionic radii of La3+ (rLa = 1.16 Å), Na+ (rNa = 1.18 Å) and K+ (rK = 1.51 Å)[24]. The present results suggest that the random distribution of the A/AE ions is responsible for the low Tc in the AE = (La,Na) 122-type and 1144-type FeSCs.

Figure 7

Tcmax of 122-type AE1−AFe2As2 (circles) and Tc of 1144-type AEAFe4As4 (squares) (AE = Ca, Sr, Eu, Ba, (La,Na)) as a function of alkali metal element (A = Na, K, Rb, Cs).

Conclusion

We synthesized (La,Na,K)Fe2As2, which has a 122-type structure with Tc = 22.5 K. This case study successfully demonstrate that namely, the formation of the 1144-type crystal structure requires sufficient amount of ionic radius difference between the constituent AE (=Ca, Sr, Eu, (La,Na)) and A (=Na, K, Rb, Cs) ions. Tc of (La,Na,K)Fe2As2 is much lower than that of other 122- and 1144-type FeSCs that do not include trivalent La3+ in their composition, and is comparable but slightly lower compared with (La,Na)Fe2As2 and (La,Na)AFe4As4 (A = Rb, Cs), presumably due to the mixture of three elements (La,Na,K) with different valence/ionic radius within a same site.

Methods

Preparation of (La,Na,K)Fe2As2 samples

Polycrystalline samples of (La,Na,K)Fe2As2 were synthesized using the stainless steel (SS) pipe and cap method[6,17,18] as we have employed in synthesizing the 122-type and the 1144-type compounds. First, the precursors, LaAs, AAs (A = Na, K), FeAs, and Fe2As, were prepared via the reaction of La, A, or Fe with As. Then the precursor powders were mixed in a molar ratio of La:Na:K:Fe:As = 0.4:0.6:1:4:4 (nominal composition: La0.2Na0.3K0.5Fe2As2, ((La,Na,K)Fe2As2) together with 5 at% excess AAs (A = Na, K) to compensate for the evaporation of A and As during the heating process. The mixed powder was pressed into a pellet and put into a SS pipe, which was then sealed using tube-fitting caps. The process was carried out in a nitrogen-filled glove box. The SS pipe was heated to 1143 K for 2 h in a box furnace and quenched to room temperature.

Material characterization

The synthesized sample was characterized by powder X-ray diffraction using a diffractometer with CuKα radiation (Rigaku, Ultima IV) equipped with a high-speed detector system (Rigaku, D/teX Ultra). Intensity data were collected with CuKα radiation over a 2θ range from 5° to 80° at a 0.01° step width. The compositions of the samples were analyzed by an energy dispersive X-ray spectrometry (SwiftED3000) equipped in an electron microscope (TM3000, Hitachi High-Technologies). Magnetic susceptibility measurements were performed using a SQUID magnetometer (Quantum Design, MPMS-XL) at temperatures from 5 to 50 K under an applied magnetic field of H = 10 Oe. This measurement was carried out on warming after zero-field cooling (ZFC process) and then on cooling in a field (FC process). The electrical resistivity was measured using a conventional DC four-probe method at temperatures from 5 to 300 K at applied magnetic fields up to 90 kOe, using PPMS (Quantum Design).