Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 868−873
pubs.acs.org/JPCL
Superconductivity in a New 1144-Type Family of (La,Na)AFe4As4 (A = Rb or Cs) Kenji Kawashima,*,†,‡ Shigeyuki Ishida,‡ Hiroshi Fujihisa,‡ Yoshito Gotoh,‡ Kunihiro Kihou,‡ Yoshiyuki Yoshida,‡ Hiroshi Eisaki,‡ Hiraku Ogino,‡ and Akira Iyo‡ †
IMRA Material R&D Co., Ltd., 2-1 Asahi-machi, Kariya, Aichi 448-0032, Japan National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan
‡
ABSTRACT: We discovered novel Fe-based superconductors (FeSCs) (La,Na)AFe4As4, where A = Rb or Cs, and characterized their superconducting properties. (La,Na)AFe4As4 is a so-called 1144-type compound with a tetragonal unit cell classified into space group P4/ mmm (no. 123). The lattice constants are a = 3.861(1) Å and c = 13.26(1) Å for (La,Na)RbFe4As4 and a = 3.880(1) Å and c = 13.60(1) Å for (La,Na)CsFe4As4. The Rietveld refinement results on the powder X-ray diffraction suggest that the La/Na ratio is rather fixed as La:Na = 0.44(5):0.56(5). The electrical resistivity and magnetic susceptibility show superconducting transition at 25.5 K for (La,Na)RbFe4As4 and 24.0 K for (La,Na)CsFe4As4. The superconducting transition temperature (Tc) of (La,Na)AFe4As4 is comparable with that of 122-type (La,Na)Fe2As2 and lower than that of typical 122-type or 1144-type FeSCs by more than 10 K. The possible reasons for lower Tc are discussed in terms of the structural modification, carrier concentration, and chemical disorder.
S
those of Rb+ (rRb = 1.61 Å) and Cs+ (rCs = 1.74 Å).15 This fact led us to expect that the combination of (La,Na)Fe2As2 and AFe2As2 (A = Rb, Cs) structural units could result in forming the 1144-type compound with the chemical formula (La,Na)AFe4As4. In this Letter, we report on the successful synthesis of novel 1144-type FeSCs, (La,Na)AFe4As4 (A = Rb or Cs). The observed Tc is 25.5 K for A = Rb and 24.0 K for A = Cs. It is found that their Tc values are lower than that of typical 122type and 1144-type FeSCs and comparable with that of (La,Na)Fe2As2. We discuss the possible reasons for their lower Tc in terms of the structural modification, carrier concentrations, and chemical disorder. Polycrystalline samples of (La,Na)AFe4As4 were synthesized using the stainless steel (SS) pipe and cap method.12 First, the precursors, LaAs, AAs (A = Na, Rb, Cs), FeAs, and Fe2As were prepared via the reaction of La, A, or Fe with As. Then, the precursor powders were mixed in the molar ratio corresponding to La0.4Na0.6AFe4As4 together with 5% excess AAs (A = Na, Rb, Cs) 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 performed in a nitrogen-filled glovebox. The SS pipe was heated to 860−890 °C for 2 h in a box furnace and quenched to room temperature. The synthesized sample was characterized by a powder X-ray diffraction technique using a diffractometer with Cu Kα radiation (Rigaku, Ultima IV) equipped with a high-speed detector system (Rigaku, D/teX
ince the discovery of superconductivity in LaFeAs(O,F) in 2008, a large number of Fe-based superconductors (FeSCs) with various crystal structures have been reported, such as LnFeAs(O,F) (Ln = rare earth elements) (1111-type compounds), (Ae1−xAx)Fe2As2 (Ae = Ca, Sr, Ba, Eu; A = Na, K, Rb) (122-type compounds), AFeAs (A = Li, Na) (111-type compounds), (Ca,Ln)FeAs2 (112-type compounds), and perovskite Fe nictide.1−9 The superconducting transition temperature (Tc) increased up to ∼55 K in bulk materials, and several reports indicate that Tc reaches close to 100 K in thin-film samples.10,11 The AeAFe4As4 (Ae = Ca, Sr, and A = K, Rb, Cs) superconductors are some of the most recent FeSCs, discovered by the present authors in 2016.12 AeAFe4As4, a so-called 1144-type compound, has a hybrid structure composed of two alternating 122-type structures, namely, AeFe2As2 (Ae = Ca, Sr) and AFe2As2 (A = K, Rb, Cs). Here, the large contrast between the ionic radii of the Ae2+ and A+ ions does not allow them to occupy the same atomic positions and form solid solutions as realized in conventional 122-type FeSCs such as Ba1−xKxFe2As2.4 As a consequence, the Ae/A ratio is fixed at 1:1. We recently discovered a new 122-type superconductor, La0.5−yNa0.5+yFe2As2 ((La,Na)Fe2As2). The compound does not include alkali earth metal elements (Ae) in its composition. La0.4Na0.6Fe2As2 (y = 0.1) is a nonsuperconductor and exhibits a structural phase transition at 130 K.13 Superconductivity is observed between 0.15 ≤ y ≤ 0.35 with its highest Tc = 27.0 K for y = 0.3,14 which is relatively lower than the Tc of approximately 40 K seen in typical 122-type FeSCs. Notably, the ionic radii of La3+ (rLa = 1.16 Å) and Na+ (rNa = 1.18 Å) are close to that of Ca2+ (rLa = 1.12 Å) and much smaller than © XXXX American Chemical Society
Received: January 17, 2018 Accepted: February 2, 2018
868
DOI: 10.1021/acs.jpclett.8b00162 J. Phys. Chem. Lett. 2018, 9, 868−873
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The Journal of Physical Chemistry Letters Ultra). The intensity data were collected with Cu Kα radiation over a 2θ range from 5° to 140° at 0.01° step width. The lattice constants and the atomic positions were refined via Rietveld analysis using BIOVIA’s Materials Studio Reflex software (version 2017 R2).16 Magnetic susceptibility measurements were performed by a SQUID magnetometer (Quantum Design, MPMS-XL) at a temperature between 5 and 50 K under an applied magnetic field of H = 10 Oe. This measurement was performed 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 in the temperature range from 5 to 300 K at applied magnetic fields up to 90 kOe using PPMS (Quantum Design). Figure 1 shows the powder X-ray diffraction patterns of (La,Na)AFe4As4 (A = Rb, Cs). The main phase can be indexed
Figure 2. Reitveld refinement results of (La,Na)RbFe4As4 (Iobs., observed; Ical., calculated). The inset shows the crystal structure of (La,Na)RbFe4As4 drawn using the refined structural parameters of Rietveld analysis (the program VESTA was used20).
Table 1. Atomic Coordinates of (La,Na)RbFe4As4 at Room Temperaturea atom
WP
x
y
s
occupancy
La/Na Rb Fe As1 As2
1a 1d 4i 2g 2h
0 0.5 0 0 0.5
0 0.5 0.5 0 0.5
0 0.5 0.2317(1) 0.3361(1) 0.1222(1)
1 1 1 1 1
a
Space group, P4/mmm (no. 123); lattice constant, a = 3.8606(1) Å, c = 13.2624(1) Å; V = 197.67(1) Å3; Z = 1; Rwp = 13.53%; Re = 10.18%; S = 1.33. Preferred orientation parameter (R0) = 1.662(7), direction = ⟨0.309, 0.070, 0.948⟩. The occupancy was fixed at 1 for all atomic sites. The global isotropic temperature factor U was refined to be 0.0063(2) Å2 .
assumption that the sum of their ratios is 1. The occupancy ratios are close to the nominal composition of La:Na = 0.4:0.6. As a final refinement, a virtual chemical species composed of 44% La3+ mixed with 56% Na+ was placed at the 1a site of the Wyckoff position with an occupancy ratio of 1. The crystal structure of (La,Na)RbFe4As4 is illustrated in the inset of Figure 2 using the obtained structure parameters. Note that the heights of the As atoms relative to the Fe planes in the Rb side and La/Na side are clearly different, namely, h1 = 1.385(3) Å (Rb side) and h2 = 1.451(3) Å (La/Na side). As a result, the As−Fe−As bond angles (α) of the two sides become different 108.7(1)° for α1 = As1−Fe−As1 (Rb side) and 106.1(1)° for α2 = As2−Fe−As2 (La/Na side). The final refined structural parameters of (La,Na)RbFe4As4 are summarized in Table 2, together with those of CaRbFe4As4.12 It is seen that the a-axis length of (La,Na)RbFe4As4 is shorter than CaRbFe4As4, whereas the c-axis length is longer. Correspondingly, the bond angle α2 of (La,Na)RbFe4As4 is smaller compared to that of CaRbFe4As4. We will discuss the difference later. Figure 3 shows the electrical resistivity of (La,Na)AFe4As4 (A = Rb, Cs) as a function of temperature (T). The resistivity data shows the metallic behavior with convex curvature down to low-T. This behavior is also observed in other 1144-type superconductors.12,21−23 The residual resistivity ratio, RRR = ρ300/ρ0, is estimated to be approximately 5.8 for (La,Na)-
Figure 1. Powder X-ray patterns of (La,Na)AFe4As4 (A = Rb, Cs). Arrows indicate the characteristic diffraction peaks of the 1144-type structure (h + k + l = odd).
as a tetragonal unit cell with a space group P4/mmm (no. 123), which corresponds to the 1144-type structure.12 There are extra reflections, which are assigned to the minor impurity phase and are identified as La0.4Na0.6Fe2As2, AFe2As2 (A = Rb, Cs), and LaOFeAs (nonsuperconducting phase).1,13,17−19 The amount of impurity phase in this sample is estimated as ≃10%, judging from the intensity ratio between the target phase and the impurity phase. The lattice constants are a = 3.861(1) Å and c = 13.26(1) Å for (La,Na)RbFe4As4 and a = 3.880(1) Å and c = 13.60(1) Å for (La,Na)CsFe4As4. The Rietveld refinement result for (La,Na)RbFe4As4 is shown in Figure 2. The diffraction pattern is well-fitted with a weighted-profile reliability factor (Rwp) of 13.53% and an expected reliability factor (Re) of 10.18%. These values are sufficiently small, thus ensuring that the assumed crystal structural model is appropriate. The resulting parameters are listed in Table 1. The occupancy ratios of La3+ and Na+ are determined as 0.44(5) and 0.56(5), respectively, under the 869
DOI: 10.1021/acs.jpclett.8b00162 J. Phys. Chem. Lett. 2018, 9, 868−873
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The Journal of Physical Chemistry Letters Table 2. Structural Parameters of (La,Na)RbFe4As4 and CaRbFe4As4a crystal system space group a (Å) c (Å) dFe−As1 (Å) dFe−As2 (Å) hAs1 (Å) hAs2 (Å) α1 = As1−Fe−As1 (deg) α2 = As2−Fe−As2 (deg)
(La,Na)RbFe4As4
CaRbFe4As412
tetragonal P4/mmm 3.8606(1) 13.2624(1) 2.376(2) 2.415(2) 1.385(3) 1.451(3) 108.7(1) 106.1(1)
tetragonal P4/mmm 3.8778(1) 13.1088(1) 2.408(2) 2.380(2) 1.429(3) 1.381(3) 107.2(1) 109.1(1)
a
As1 and As2 denote the As atoms at Rb and (La,Na) or Ca side, respectively.
Figure 4. Temperature dependence of the magnetic susceptibility of (La,Na)AFe4As4 (A = Rb, Cs). The right horizontal axis shows the shielding volume fraction (SVF) without considering the demagnetization factor.
confirmed that the results themselves are reproducible, although we do not understand the reason. Figure 5 shows the electrical resistivity data for (La,Na)AFe4As4 (A = Rb, Cs) under various magnetic fields (H) as
Figure 3. Temperature dependence of the electrical resistivity for (La,Na)AFe4As4 (A = Rb, Cs). The inset shows the enlarged view near Tc.
RbFe4As4 and 5.6 for (La,Na)CsFe4As4, indicating the high quality of the synthesized samples, which is sufficient for investigating their physical properties. As seen in the inset of Figure 3, the resistivity sharply decreases at 26.2 K for (La,Na)RbFe4As4 and 24.8 K for (La,Na)CsFe4As4 and reaches zero resistivity at 25.2 K for (La,Na)RbFe4As4 and 23.0 K for (La,Na)CsFe4As4. The superconducting transition should originate at the 1144 phase and not from the impurities, as Tc of the possible impurity phase is much lower (2.6 K for RbFe2As2 and 2.8 K for CsFe2As2) or absent (non-SC for LaOFeAs or La0.4Na0.6Fe2As2).1,13,17−19 Figure 4 shows the T-dependence of the magnetic susceptibility of (La,Na)AFe4As4 (A = Rb, Cs) under an applied magnetic field of H = 10 Oe. The magnetic susceptibility exhibits a marked drop at 25.5 K for (La,Na)RbFe4As4 and 24.0 K for (La,Na)CsFe4As4 in both ZFC and FC processes, indicating the occurrence of superconductivity. The Tc values are in good agreement with those confirmed from the electrical resistivity characteristics (Figure 3). The shielding volume fraction is 190% for (La,Na)RbFe4As4 and 130% for (La,Na)CsFe4As4, respectively, which are reasonable values as bulk superconductors by taking account of the demagnetizing factors of the present samples.24 Tc of (La,Na)RbFe4As4 and that of (La,Na)CsFe4As4 are almost the same, while the former is slightly (1−2 K) higher. We have
Figure 5. Temperature dependence of the electrical resistivity of (La,Na)AFe4As4 (A = Rb, Cs) under various magnetic fields up to 90 kOe.
functions of T. The transition width remains almost unchanged up to the highest H. The onset Tc and the zero resistivity temperature decrease systematically with increasing H, and the superconducting transitions of (La,Na)AFe4As4 are not completely suppressed under H < 90 kOe, indicating that the upper critical field of Hc2 is very large. Plots of Hc2(T) versus Tc(H) (H−T phase diagram) for (La,Na)AFe 4As4 and CaRbFe4As4 are shown in Figure 6. 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. The H−T phase diagram shows the linear T-dependence within the measured T- and H- ranges. The slopes, dHc2/dT, are −54.64 kOe/K for (La,Na)RbFe4As4 and −48.41 kOe/K for (La,Na)CsFe 4 As 4 . Using the 870
DOI: 10.1021/acs.jpclett.8b00162 J. Phys. Chem. Lett. 2018, 9, 868−873
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The Journal of Physical Chemistry Letters
Figure 6. H−T phase diagrams of (La,Na)AFe4As4 (A = Rb, Cs) and CaRbFe4As4. The filled circle and square indicate the obtained Hc2(0) value of (La,Na)AFe4As4 using the WHH formula. The dotted lines show the linear fitting result. The inset shows the enlarged view near Tc of (La,Na)AFe4As4.
Figure 7. 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.12,19−21 The yellow diamonds represent (La,Na)AFe4As4 (A = Rb, Cs).
Werthamer−Helfand−Hohenberg (WHH) formula, Hc2(0) = −0.69 (dHc2/dT)|TcTc, for a type-II superconductor,25 the upper critical magnetic field at 0 K is estimated as 980 kOe for (La,Na)RbFe4As4 and as 820 kOe for (La,Na)CsFe4As4. Thus, (La,Na)AFe4As4 has a large Hc2(0),26 as is the case for other FeSCs. The corresponding coherence lengths (ξ0) are calculated as 18 Å for (La,Na)RbFe4As4 and 20 Å for (La,Na)CsFe4As4 estimated from the relationship between Hc2 ∼ Φ0/2πξ02, where Φ0 is the quantum flux. The present study demonstrates that the (La,Na)Fe2As2 structural unit and the AFe2As2 (A = Rb or Cs) unit can stack with each other and resultantly form 1144-type (La,Na)AFe4As4. According to the empirical rules established by our previous studies,12,21−23 the 1144-type FeSCs are formed if the following conditions are satisfied: (1) the a-axis lattice constant of AeFe2As2 (aAe122) and that of AFe2As2 (aA122) are close to each other, namely, Δa = | aAe122 − aA122| < ∼ 0.07 Å, and (2) the ionic radius of the A+ ion (rA) is sufficiently larger than that of the Ae2+ ions (rAe), namely, Δr = rAe − rA < −0.35 Å. The situation is illustrated in Figure 7, in which the red-colored region fulfills the requirements. In the case of (La,Na)AFe4As4, the a-axis lattice constant of La0.4Na0.6Fe2As2 is 3.875 Å, which is close to a = 3.888(2) Å for RbFe2As2 and a = 3.8894(2) Å for CsFe2As2.13,17,18 Furthermore, the ionic radii (VIII) of La3+ (rLa = 1.16 Å) and Na+ (rNa = 1.18 Å) are significantly smaller than that of Rb+ (rRb = 1.61 Å) and Cs+ (rCs = 1.74 Å).15 Based on the empirical rule, it is quite natural that the 1144 crystal structures can be formed by the combination of the (La,Na)Fe2As2. On the other hand, the present success in synthesizing the 1144-type (La,Na)AFe4As4 reinforces the validity of the empirical rule. Table 3 lists the lattice parameters, Tc, and the valence of Fe ions of the known 1144-FeSCs. It is immediately noticed that Tc of (La,Na)AFe4As4 is significantly lower, by nearly 10 K, compared with other 1144-type FeSCs. One can think of several possibilities that account for the difference. First, the Fe valence of (La,Na)AFe4As4 is 2.28, which is slightly higher than the higher-Tc counterparts, which have 2.25. Another possibility is that the different 122 subunit, namely, AeFe2As2 (Ae = Ca,
Table 3. Lattice Constants, Tc, and Hole Concentration of 1144-Type FeSCs a (Å) (La,Na) RbFe4As4 (La,Na) CsFe4As4 CaKFe4As4 CaRbFe4As4 CaCsFe4As4 SrRbFe4As4 SrCsFe4As4 EuRbFe4As4 EuCsFe4As4
3.861(1)
c (Å) 13.26(1)
Tc (K) 25.5
Fe valence
ref
2.28
a
this work
a
this work
3.880(1)
13.60(1)
24
2.28
3.866(1) 3.8757(9) 3.891(1) 3.897(1) 3.910(1) 3.889(1) 3.901(1)
12.817(5) 13.104(3) 13.414(2) 13.417(5) 13.729(3) 13.30(1) 13.61(1)
35 35 31.6 35.1 36.8 36 35
2.25 2.25 2.25 2.25 2.25 2.25 2.25
12 12 12 12 12 19, 20 19, 21
a
Hole concentration of (La,Na)RbFe4As4 is estimated by assuming La:Na = 0.44:0.56
Eu, Sr) versus (La,Na)Fe2As2, yields different Tc. To see which of these reasons is more likely, we plot Tc of two hole-doped 122-type FeSCs, Ba1−xKxFe2As2 and La0.5−yNa0.5+yFe2As2, together with the 1144-type FeSCs as functions of the hole concentrations in Figure 8. (In Figure 8, we employ two notations as horizontal axes. The upper one is a conventionally used one corresponding to x in Ba1−xKxFe2As2. The lower one corresponds to the valence of Fe ions in a form of Fe2+y, which is used in the case of La0.5−yNa0.5+yFe2As2. The relationship 2y = x is fulfilled.) For the two 122-type FeSCs, Tc systematically changes with changing x or y,14,27 exhibiting so-called dome-like doping dependence. The highest Tc is 38 K at y ∼ 0.2 (x = 0.4) for Ba1−xKxFe2As2 and 27 K at y ∼ 0.3 for La0.5−yNa0.5+yFe2As2. One can notice that Tc of the two 1144-type superconductors lie very close to the Tc − x(y) curves of their 122 counterparts, namely, the series of AeAFe4As4 lie on Ba1−xKxFe2As2 and those of (La,Na)AFe4As4 lie on La0.5−yNa0.5+yFe2As2. The coincidence strongly suggests that Tc of the 1144 FeSCs reflect the Tc of AeFe2As2 (Ae = Ba, Sr, Ca, or La0.5−yNa0.5+y) subunits. 871
DOI: 10.1021/acs.jpclett.8b00162 J. Phys. Chem. Lett. 2018, 9, 868−873
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The Journal of Physical Chemistry Letters
106.7°), CaRbFe4As4 (α1 = 107.2°, α2 = 109.1°), and (La,Na)RbFe4As4 (α1 = 108.7°, α2 = 106.1°). For the 1144type FeSCs, there are two inequivalent α’s, namely, α1 corresponding to the As1−Fe−As1 bonds close to the A atoms and α2 representing the As2−Fe−As2 bonds close to the Ae [or (La,Na)] atoms. Figure 9 shows that α of Ba0.6K0.4Fe2As2 and α2 of CaRbFe4As4 are close to the regular tetrahedral angle, whereas α of (La,Na)Fe2As2 and α2 of (La,Na)AFe4As4 are comparatively smaller. If we employ α2 as an indicator of the characteristic bond angle, a relationship between α and Tc is fulfilled. Currently, we do not have any reason why α2 but not α1 follows such general tendency. A presumably detailed atom-selective electronic structure calculation would be helpful in accounting for the phenomenological observation.30,31 One would also expect that Tc of (La,Na)AFe4As4 increases if one can optimize α2 through additional chemical substitution and/or application of external pressure. We successfully synthesized novel superconductors (La,Na)AFe4As4 (A = Rb, Cs), which have 1144-type structures. They were formed as line phase compounds with a fixed La/Na ratio of La:Na = 0.44(5):0.56(5). (La,Na)RbFe4As4 exhibits superconductivity at 25.5 K, whereas (La,Na)CsFe4As4 exhibits superconductivity at 24.0 K. Tc of (La,Na)AFe4As4 is lower than that of other 1144-type superconductors and comparable with that of (La,Na)Fe2As2. The results strongly suggest that the superconducting properties of the 1144-type FeSCs are subject to those of their constituent 122 subunits.
Figure 8. Hole concentration dependence of Tc for 122- and 1144type FeSCs.
Presently, the reasons for lower Tc in La0.5−yNa0.5+yFe2As2 and (La,Na)AFe4As4 are not clear. The most plausible explanation is that the random distributions of the monovalent Na+ ions and trivalent La3+ ions cause the local lattice distortion of the FeAs4 tetrahedron and/or strong potential disorder, which results in decreasing the Tc of the unconventional superconductors.28 From the structural point of view, according to the BCS theory, and really in several superconductors, materials with larger cell parameters tend to possess higher Tc due to larger density of states and/or to larger electron− phonon coupling. The fact that Tc of (La,Na)AFe4As4, which has smaller in-plane lattice parameters, is relatively lower is consistent with this line of thinking. More specific to the FeSCs, it is empirically shown that Tc of FeSCs strongly depends on the As−Fe−As bond angle (α); it tends to increase as α approaches 109.5°, which corresponds to a regular tetrahedron.29 Figure 9 shows the As−Fe−As bond angle (α) versus Tc of Ba0.6K0.4Fe2As2 (α = 109.5°), La0.2Na0.8Fe2As2 (α =
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kenji Kawashima: 0000-0003-4786-3498 Kunihiro Kihou: 0000-0003-3065-4147 Akira Iyo: 0000-0002-9610-647X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported by JSPS KAKENHI Grant Number JP16H06439. We are grateful to Profs Kohji Kishio, Shin-ichi Uchida, and Hideo Aoki at AIST for their fruitful discussions.
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REFERENCES
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Figure 9. Relationship between bond angle As−Fe−As (α) dependence of Tc of 122-type Ba0.6K0.4Fe2As2 and La0.2Na0.8Fe2As2 and 1144type CaRbFe4As4 and (La,Na)RbFe4As4. The inset shows the schematic diagram of the 1144-type structure. 872
DOI: 10.1021/acs.jpclett.8b00162 J. Phys. Chem. Lett. 2018, 9, 868−873
Letter
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DOI: 10.1021/acs.jpclett.8b00162 J. Phys. Chem. Lett. 2018, 9, 868−873
