mater.scichina.com  link.springer.com Published online 23 December 2016 | doi: 10.1007/s40843-016-5150-xSci China Mater  2017, 60(1): 83–89

Crystal structure and superconductivity at about 30 Kin ACa2Fe4As4F2 (A = Rb, Cs)Zhicheng Wang1, Chaoyang He1, Zhangtu Tang1, Siqi Wu1 and Guanghan Cao1,2*

ABSTRACT  We have synthesized two iron fluo-arsenidesACa2Fe4As4F2 with A = Rb and Cs, analogous to the newlydiscovered superconductor KCa2Fe4As4F2. The quinary inor-ganic compounds crystallize in a body-centered tetragonallattice with space group I4/mmm, which contain doubleFe2As2 layers that are separated by insulating Ca2F2 layers.The electrical and magnetic measurements on the polycrys-talline samples demonstrate that the new materials undergosuperconducting transitions at Tc = 30.5 and 28.2 K, respec-tively, without extrinsic doping. The correlations between Tc

and structural parameters are discussed.

Keywords:   iron-based superconductors, crystal structure, su-perconductivity, intergrowth

Since the discovery of high-temperature superconductivityin iron pnictides [1,2], considerable efforts have beendevoted to explore new iron-based superconductors (IBS).To date dozens of IBS, in diverse structure types, have beendiscovered, all of which contain Fe2X2 (X = As or Se) layersthat are essential for the emergence of superconductivity.Some of the structures are relatively simple because feweratoms connect the superconductive Fe2X2 layers [1–7].Other complex structures consist of thicker connectingblocks between the Fe2X2 layers [8–13]. These material dis-coveries greatly enrich the related research on IBS [14–16].

In 2013, we designed several new structures containingFe2X2 layers, in view of the crystal chemistry of IBS [17].This year one of the candidate structure “KLaFe4As4” wasrealized in the form of AkAeFe4As4 (Ak = K, Rb, Cs; Ae =Ca, Sr) [18] and AkEuFe4As4 (Ak = Rb, Cs) [19–21]. Veryrecently, another proposed structure A3Fe4X4Z2 (see Ref.[17] for details of the definition of A, X and Z) was imple-mented in KCa2Fe4As4F2 (hereafter called 12442) [22]. Inthese new IBS above, the Fe2X2 layers are asymmetric, not

being seen in previous IBS. Furthermore, these materialsare self-doped (hence superconductivity emerges) in theirstoichiometric form. For the 12442-type IBS, in particu-lar, there are double Fe2As2 layers (with alkali metal atomssandwiched) separated by Ca2F2 layers, which has not beenseen in previous IBS either.

In this work, we synthesized ACa2Fe4As4F2 (A = Rb,Cs), two sister compounds of KCa2Fe4As4F2, which canbe viewed as an intergrowth of AFe2As2 (A = Rb, Cs) andCaFeAsF. The crystallographic parameters were deter-mined. The physical property measurements demonstratethat RbCa2Fe4As4F2 and CsCa2Fe4As4F2 exhibit bulk super-conductivity at 30.5 and 28.2 K, respectively.

Polycrystalline samples of RbCa2Fe4As4F2 andCsCa2Fe4As4F2 were prepared by conventional solid-statereactions in sealed Ta tubes. Details of the sample prepa-ration are presented in the following EXPERIMENTALSECTION.

Powder X-ray diffraction (XRD) patterns of the as-pre-pared RbCa2Fe4As4F2 and CsCa2Fe4As4F2 are displayed inFigs 1a and b, respectively. The former shows no obviousimpurities, and the latter indicates nearly pure 12442-typephase with weak reflections (3% of the strongest reflection)from unreacted CaF2. These XRD data were then employedfor Rietveld analyses using software Rietan-FP [23]. Therefinement was based on the 12442-type structural modelshown as Fig. 1c in which the occupation factor of eachatom was fixed to 1.0. The weighted reliability factor Rwp

are 3.47% and 4.61%, while the goodness-of-fit parameterS are 1.49 and 1.74, for RbCa2Fe4As4F2 and CsCa2Fe4As4F2

respectively. The resulting crystallographic data are tabu-lated in Table 1 in comparison with those of KCa2Fe4As4F2

[22].As  an  intergrowth  between   RbFe2As2   and  CaFeAsF,

1 Department of Physics and Stat Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, China2 Collaborative Innovation Centre of Advanced Microstructures, Nanjing 210093, China* Corresponding author (email: ghcao@zju.edu.cn)

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Figure 1   XRD patterns and their Rietveld refinement profiles for RbCa2Fe4As4F2 (a) and CsCa2Fe4As4F2 (b) polycrystalline samples; (c) the 12442-typecrystal structure, projected along the [100] direction.

Table 1 Crystallographic data of ACa2Fe4As4F2 (A = K, Rb, Cs). The space group is I4/mmm (No. 139). The atomic coordinates are as follows: K/Rb/Cs2a (0, 0, 0); Ca 4e (0.5, 0.5, z); Fe 8g (0.5, 0, z); As1 4e (0.5, 0.5, z); As2 4e (0, 0, z); F 4d (0.5, 0, 0.25). α1 and α2 denote the angles of As1–Fe–As1 andAs2–Fe–As2, respectively.

Lattice parameters Coordinates (z) Bond distances (Å)As height from

Fe plane (Å) Bond angles (°)A

a (Å) c (Å) V (Å3) Ca Fe As1 As2 Fe–As1 Fe–As2 As1 As2 α1 α2

K 3.8684(2) 31.007(1) 463.99(3) 0.2085(2) 0.1108(1) 0.0655(1) 0.1571(1) 2.390(3) 2.409(3) 1.405(3) 1.436(3) 108.0(2) 106.8(2)

Rb 3.8716(1) 31.667(1) 474.66(1) 0.2089(2) 0.1140(1) 0.0697(1) 0.1592(1) 2.391(2) 2.407(2) 1.403(1) 1.431(1) 108.1(2) 107.0(2)

Cs 3.8807(1) 32.363(1) 487.38(1) 0.2100(2) 0.1172(1) 0.0739(1) 0.1614(1) 2.393(2) 2.410(2) 1.401(1) 1.430(1) 108.3(2) 107.2(2)

RbCa2Fe4As4F2 indeed shows a lattice constant a very closeto the mean value of those of RbFe2As2 (3.863 Å) [24] andCaFeAsF (3.878 Å) [25]. Meanwhile the c axis is almostequal to the expected value (2cCaFeAsF + cRbFe2As2 = 31.633 Å).Similarly, the lattice constants of CsCa2Fe4As4F2 are relatedto those of CaFeAsF and CsFe2As2 [26]. It should be notedthat the apparent Fe valence in the hybrid 12442 phase be-come 2.25+, an average of 2.5+ (in RbFe2As2) and 2+ (inCaFeAsF). Along with charge homogenization, the con-stituent building blocks in the 12442-type structure suffersubtle modifications. For example, the CaFeAsF block be-comes slightly slimmer, as is also seen in KCa2Fe4As4F2 [22].

Fig. 2a shows the temperature dependence of electri-cal resistivity (ρ(T)) for ACa2Fe4As4F2 (A = Rb, Cs) sam-ples. Overall, the ρ(T) curves show a metallic behavior.No anomaly associated with the spin-density wave orderingthat often appears in the parent compounds of IBS [14–16]can be seen, consistent with the self-hole doping at 25%.Nevertheless, there is a hump-like anomaly at around 150

K, which is commonly seen in hole-doped IBS [3,27]. No-tably, the resistivity decreases almost linearly below ~75 Kuntil a superconducting transition takes place. The on-set superconducting transition temperature (Tc

onset ) is de-fined as the interception of linear extrapolations from su-perconducting and normal-state sides. For A = Rb and Cs,Tc

onset are 30.5 and 28.5 K, respectively, which are somewhatlower than that of KCa2Fe4As4F2 (33.0 K) [22].

Upon applying magnetic field, the superconductingtransitions shift to lower temperatures with a pronouncedtail, as shown in Figs 2b and c. Consequently, the zero-re-sistance temperature Tc

zero decreases much faster than Tconset ,

suggesting high anisotropy in the 12442-type materials.Taken 90% and 1% of ρn (the extrapolated normal-statevalue at Tc) as the criteria, the upper critical field (Hc2)and the irreversible field (Hirr) are extracted, both of whichare plotted in the insets of Figs 2b and c. The large gapsbetween Hc2(T) and Hirr(T) confirm the enhanced two-di-mensionality. Additionally, the slopes of Hc2(T) for RbCa2-

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Figure 2   Temperature dependence of electrical resistivity for the RbCa2Fe4As4F2 and CsCa2Fe4As4F2 polycrystalline samples. The insets of (b and c) arederived upper critical field (Hc2) and irreversible field (Hirr) as functions of temperature.

Fe4As4F2 and CsCa2Fe4As4F2 achieve as high as 13.9 and14.2 T K–1, respectively, suggesting small superconductingcoherence lengths at zero temperature. Further measure-ments using single crystalline samples are needed to revealthe anisotropy directly.

Fig. 3a shows the temperature dependence of directcurrent (dc) magnetic susceptibility measured under a lowfield of 10 Oe. The diamagnetic transitions occur at 30.5and 28.2 K, consistent with the resistivity measurement.After a demagnetization correction, the volume fractionsof magnetic shielding, measured in zero-field-cool-ing (ZFC) mode, reach 90% and 65%, respectively, forRbCa2Fe4As4F2 and CsCa2Fe4As4F2. The Meissner vol-ume fractions, measured in field-cooling (FC) mode, stillachieve 23% and 25%, which unambiguously manifestbulk superconductivity. Figs 3b and c show the isothermalmagnetization curves at 2 K for the two superconductors.The lower critical magnetic fields Hc1 (about 400 Oe) aremuch smaller than the upper critical magnetic fields (seeabove), and therefore, ACa2Fe4As4F2 (A = Rb, Cs) belong toextremely type-II superconductors. The superconductingmagnetic hysteresis loops indicate strong magnetic fluxpinning effect. Given the samples’ dimensions with e cm ×

e cm × h cm, the critical current density Jc can be derivedfrom the hysteresis loops using Bean critical state model[28], which gives Jc(H) = 30 × [M+(H)–M–(H)]/e (A cm−2).Consequently, Jc values of the RbCa2Fe4As4F2 sample areestimated to be 2.2 × 105 A cm−2 at H = 0 and 8.0 × 104 Acm−2 at H = 10 kOe, while Jc values of the CsCa2Fe4As4F2

sample are approximately 5.2 × 105 A cm−2 at H = 0 and 2.8× 105 A cm−2 at H = 10 kOe.

As stated above, the hole doping level remains 25% forACa2Fe4As4F2 (A = K, Rb, Cs) assuming ideal stoichiomet-ric ratios. Therefore, it is meaningful to investigate thecrystal-structure origin for the Tc variations. Fig. 4 plots Tc

as functions of different structural parameters. One seesthat Tc inversely correlates with the lattice parameters aand c. This is in sharp contrast to the variations of Tc

max inthe optimally hole-doped (Ae, Ak)Fe2As2 systems (Tc

max in-creases with the lattice parameters) [15,16]. Then, weconsider the relevance of the spacing between Fe2As2 lay-ers. While Tc decreases with dintra (the spacing of Fe planeswithin the double Fe2As2 layers), in contrast, Tc increasesalmost linearly with the spacing of Fe planes separated bythe Ca2F2 layers, dinter (Fig. 1c). Given that the effectiveinterlayer coupling increases with decreasing the spacing,

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the result suggests that either the intra-bilayer couplingenhances Tc, or, the inter-bilayer coupling suppresses Tc. Ifthis is the case, Tc values of IBS are determined not only bysingle Fe2As2 layer itself, as widely believed, but also by thecoupling between different Fe2As2 layers, which calls forfurther theoretical investigations.

We note that the most commonly used parameters forthe crystal-structure relationship with Tc in IBS are theAs–Fe–As bond angle, α [29,30] and the As height fromthe Fe plane, hAs [31]. In the 12442-type structure, thereare two different values for the above parameters due tothe asymmetric Fe2As2 layers. The possible correlationswith Tc are shown in Figs 4e and f. As can be seen, bothparameters deviate from the “ideal” values (α = 109.5° andhAs = 1.38 Å) marked by thick gray lines. Contrarily, Tc

tends to increase when the parameters deviate more fromthe ideal values. The As height dependence resembles theTc variation in FeSe under high pressures [13].

In summary, we have synthesized and characterized twonew quinary iron-arsenide fluorides RbCa2Fe4As4F2 andCsCa2Fe4As4F2 with separate double Fe2As2 layers. The two

compounds superconduct at 30.5 and 28.2 K, respectively,due to self-doping effect. The Tc trend violates the empir-ical rule of structural parameters. Instead, the interlayercoupling seems to be relevant to optimizingTc. The successin obtaining new superconductors by incorporating largealkali-metal ions [18–22] is reminiscence of the recentlydiscovered Cr-based superconductors A2Cr3As3 [32–34],which suggests that the alkali-metal elements may havebeen overlooked in the exploration of superconductingmaterials. In the 12442-type IBS, alkali-metal cations seemessential, and therefore, one may expect more 12442-typesuperconductors will be found by incorporation of al-kali-metal cations in the near future.

EXPERIMENTAL SECTIONPolycrystalline samples of ACa2Fe4As4F2 (A = Rb, Cs) weresynthesized by solid-state reactions with the source mate-rials Rb ingot (99.75%), Cs ingot (99.5%), Ca shot (99.5%),Fe powders (99.998%), As pieces (99.999%) and CaF2 pow-ders (99%). The intermediate products of CaAs, FeAs, andFe2As were pre-synthesized  with their  corresponding ele-

Figure 3    (a) Temperature dependence of magnetic susceptibility measured at 10 Oe in FC and ZFC modes forACa2Fe4As4F2 (A = Rb, Cs); (b, c) isother-mal magnetization loops at 2 K for RbCa2Fe4As4F2 and CsCa2Fe4As4F2.

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Figure 4   Correlations between Tc and structural parameters in ACa2Fe4As4F2 (A = K, Rb, Cs). The parameters include lattice constants a (a) and c (b),intra-Fe2As2-bilayer spacing (c), inter-Fe2As2-bilayer spacing (d), As height from the Fe plane, hAs (e) and As–Fe–As bond angle, α (f). The thick graylines mark the expected/ideal values of hAs and α that generate maximum Tc.

ments at 1023 K for 12 h in evacuated quartz tube, respec-tively. Then Rb1.03Fe2As2 and Cs1.03Fe2As2 (3% excess in or-der to compensate the loss of alkali metal) were preparedby reacting Rb and Cs with FeAs at 923 and 873 K for 10 has another two intermediate products. Moreover, CaF2 washeated to 873 K for 24 h in air to remove adsorbed water.After that, RbFe2As2 (or CsFe2As2), CaAs, CaF2 and Fe2Aswere mixed homogeneously in the stoichiometric ratio andpressed into pellets. The pellets were loaded in an aluminatube with a cover. The sample-loaded alumina tube wassealed in a Ta tube which was jacketed with an evacuatedquartz ampoule. Subsequently, the sample was heated to1203 K in 5 h, holding for 36 h. The final products werestable in air. All the operations above were carried out inan argon-filled glovebox with the water and oxygen con-

tent below 1 ppm.Powder XRD was carried out at room temperature on a

PANalytical X-ray diffractometer (empyrean series 2) witha monochromatic CuKα1 radiation. The electrical trans-port measurements were carried out by a standard four-terminal method on a cryogenic limited vibrating samplemagnetometer (VSM) equipped with a Keithley 2400 digi-tal sourcemeter and a Keithley 2182 nanovoltmeter. The dcmagnetizations were measured on a quantum design mag-netic property measurement system (MPMS-XL5), and thesamples were cut into a rectangular shape so as to obtainthe demagnetization factor easily.

Received 26 October 2016; accepted 1 December 2016;published online 23 December 2016

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Acknowledgments     This work was supported by the National NaturalScience Foundation of China (90922002 and 11190023) and the NationalKey Research and Development Program of China (2016YFA0300202)

Author contributions      Cao G designed the experiment, discussed theresults and wrote the paper with Wang Z. Wang Z synthesized the samplestogether with He C. The structural characterizations and physical propertymeasurements were performed with assistance from Tang Z and Wu S. Allauthors contributed to the general discussion

Conflict of interest      The authors declare that they have no conflict ofinterest.

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Zhicheng Wang is currently a graduate student at the Department of Physics, Zhejiang University. His PhD research fo-cuses on the explorations and physical property measurements of novel superconductors

Guanghan Cao is currently a professor at the Department of Physics, Zhejiang University. He received his PhD degreefrom the University of Science and Technology of China in 1995, and then became a postdoctoral fellow in Zhejiang Uni-versity where he has stayed until present. He visited the National Institute for Materials Sciences, Japan, from 1999 to 2001,and he also worked as a visiting professor at the Institute of Solid State Physics of the University of Tokyo in 2011. Hisresearch interests include exploratory synthesis, structural characterizations and physical-property measurements of novelsuperconductors and other related materials.

ACa2Fe4As4F2 (A = Rb, Cs)的晶体结构以及大约30 K的超导电性王志成1, 何朝阳1, 汤章图1, 武思祺1, 曹光旱1,2*

摘要   本文合成了与新发现的KCa2Fe4As4F2超导体结构相同的两种铁砷氟化物ACa2Fe4As4F2 (A = Rb, Cs). 该五元无机物具有四方体心晶格,空间群为 I4/mmm, 包含被Ca2F2绝缘层分开的双Fe2As2层. 对多晶样品的电阻和直流磁化率的测量表明, ACa2Fe4As4F2 (A = Rb, Cs)样品无需外在掺杂即呈现出30.5和28.2 K的超导电性. 文中还讨论了其Tc与晶体结构参数间的关联.

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