High T c electron doped Ca 10 ðPt 3 As 8 ÞðFe 2 As 2 Þ 5 and Ca 10 ðPt 4 As 8 ÞðFe 2 As 2 Þ 5 superconductors with skutterudite intermediary layers Ni Ni a , Jared M. Allred a , Benny C. Chan b , and Robert Joseph Cava a,1 a Department of Chemistry, Princeton University, Princeton, NJ 08544; and b Department of Chemistry, College of New Jersey, Ewing, NJ 08628 Edited by Zachary Fisk, University of California, Irvine, CA, and approved September 26, 2011 (received for review June 29, 2011) It has been argued that the very high transition temperatures of the highest T c cuprate superconductors are facilitated by enhanced CuO 2 plane coupling through heavy metal oxide intermediary layers. Whether enhanced coupling through intermediary layers can also influence T c in the new high T c iron arsenide superconduc- tors has never been tested due the lack of appropriate systems for study. Here we report the crystal structures and properties of two iron arsenide superconductors, Ca 10 ðPt 3 As 8 ÞðFe 2 As 2 Þ 5 (the “10-3-8 phase”) and Ca 10 ðPt 4 As 8 ÞðFe 2 As 2 Þ 5 (the “10-4-8 phase”). Based on -Ca-ðPt n As 8 Þ-Ca-Fe 2 As 2 - layer stacking, these are very similar com- pounds for which the most important differences lie in the struc- tural and electronic characteristics of the intermediary platinum arsenide layers. Electron doping through partial substitution of Pt for Fe in the FeAs layers leads to T c of 11 K in the 10-3-8 phase and 26 K in the 10-4-8 phase. The often-cited empirical rule in the arsenide superconductor literature relating T c to As-Fe-As bond angles does not explain the observed differences in T c of the two phases; rather, comparison suggests the presence of stronger FeAs interlayer coupling in the 10-4-8 phase arising from the two- channel interlayer interactions and the metallic nature of its inter- mediary Pt 4 As 8 layer. The interlayer coupling is thus revealed as important in enhancing T c in the iron pnictide superconductors. I n early 2008, superconductivity near 26 K was reported (1) in LaFeAsO 0.9 F 0.1 , marking the discovery of a second family of high T c superconductors in addition to the cuprates. Since then, intensive effort has been devoted to the search for new Fe-based superconductors, not only in the hope that technologically applic- able superconductors may exist in this class, but also because comparison between the Fe pnictides and the cuprates may help to specify the essential ingredients that give rise to high T c super- conductivity. Several new families of high T c pnictide supercon- ductors have been discovered (2–4). All of these materials are based on layers of edge-sharing FeAs 4 or FeSe 4 tetrahedra (5), virtually always stacked with other intermediary layers in their crystal structures. The intermediary layers range from simple alkali or alkaline earth ions (2, 3) to more complex but still che- mically trivial perovskite layers such as Sr 4 V 2 O 6 (4). Recently, Nohara et al. reported the existence of new superconductors with T c up to 38 K in the Ca-Fe-Pt-As chemical system (6). Although layered -Ca-(Fe, Pt, As)-Ca-(Fe 2 As 2 )- stacking was suggested for the structures of the superconductors, neither the crystal structures nor the detailed physical properties were character- ized. This report is intriguing because Fe 122 arsenides, such as BaFe 2 As 2 (2), form in the ThCr 2 Si 2 structure type while Pt 122 arsenides, such as SrPt 2 As 2 (7), form in the CaBe 2 Ge 2 struc- ture type, reflecting fundamental chemical differences between FeAs and PtAs layers. Here we isolate, identify and report the crystal structures and physical properties of two superconductors in the Ca-Pt-Fe-As system. These materials raise the chemical complexity of the superconducting pnictides to a new level. The first, Ca 10 ðPt 3 As 8 ÞðFe 2 As 2 Þ 5 (10-3-8), has triclinic symmetry, which is extremely rare among superconductors. The second, Ca 10 ðPt 4 As 8 ÞðFe 2 As 2 Þ 5 (10-4-8), is a higher symmetry tetragonal phase. These two superconductors have novel structure types containing platinum arsenide intermediary layers, with formulas Pt 3 As 8 and Pt 4 As 8 , respectively, which are based on the skutter- udite structure, a common crystal structure type in binary pnic- tides. As-As dimers are present in these layers and are critical to understanding the electronic state of the superconductors. For the triclinic 10-3-8 phase, we find that T c can be tuned from 0 to 11 K through 6(1)% to 13(1)% of Pt substitution on the Fe site in the ðFe 2 As 2 Þ 5 layers, which dopes the layers with elec- trons. Although the ground state of the undoped 10-3-8 phase remains unclear, the Pt doping study provides evidence for simi- larities between this new superconducting family and the fre- quently studied 1111 (such as LaFeAsðO 1-x F x Þ) (1) and 122 (such as BaðFe 1-x Co x Þ 2 As 2 ) (8) families of arsenide superconductors. For the tetragonal 10-4-8 phase, superconductivity occurs at a much higher temperature, 26 K. These two chemically and structurally similar compounds, with significantly different T c s, provide a particularly interesting plat- form for studying superconductivity in the pnictides. Comparison between them indicates that simple rigid band filling ideas and the frequently invoked empirical rule relating T c to the As-Fe-As bond angle in superconducting iron pnictides are not enough to account for the different T c s here. Rather the data and analysis suggest that the difference is due to the stronger FeAs interlayer coupling in the 10-4-8 phase, which arises from the two-channel interlayer interactions present in the 10-4-8 structure and the metallic nature of its intermediary Pt 4 As 8 layer. This stronger interlayer coupling is thus believed to be an important factor in enhancing T c in the Fe pnictides. What’s more, bearing in mind that doping the FeAs layers in Fe pnictide superconductors leads to lower T c than doping the intermediary layers, it may be that T c can be pushed even higher in these systems through doping on the Ca or Pt n As 4 (n ¼ 3, 4) layers exclusively. Results Crystal Structure. The crystal structures of the Ca 10 ðPt 3 As 8 Þ ðFe 2 As 2 Þ 5 (10-3-8) and Ca 10 ðPt 4 As 8 ÞðFe 2 As 2 Þ 5 (10-4-8) super- conducting phases, which have -Ca-ðPt n As 8 Þ-Ca-ðFe 2 As 2 Þ- stack- ing, as well as selected single crystal and powder X-ray diffraction patterns, are shown in Fig. 1. The detailed crystallographic data determined from the single crystal structure refinements are summarized in Tables 1 and 2; appropriate twin laws were incor- porated into the structure model. Some streaking of diffraction spots along the c axis, indicative of stacking faults, was always observed in the 10-4-8 phase crystals. The systematic change in reflection intensities caused by stacking faults may introduce Author contributions: N.N. and R.J.C. designed research; N.N., J.M.A., and B.C.C. performed research; N.N., J.M.A., B.C.C., and R.J.C. analyzed data; and N.N., J.M.A., and R.J.C. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission. 1 To whom correspondence should be addressed. E-mail: [email protected]. See Author Summary on page 18201. www.pnas.org/cgi/doi/10.1073/pnas.1110563108 PNAS ∣ November 8, 2011 ∣ vol. 108 ∣ no. 45 ∣ E1019–E1026 PHYSICS PNAS PLUS Downloaded by guest on September 7, 2020
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High Tc electron doped Ca10ðPt3As8ÞðFe2As2Þ5 andCa10ðPt4As8ÞðFe2As2Þ5 superconductors withskutterudite intermediary layersNi Nia, Jared M. Allreda, Benny C. Chanb, and Robert Joseph Cavaa,1
aDepartment of Chemistry, Princeton University, Princeton, NJ 08544; and bDepartment of Chemistry, College of New Jersey, Ewing, NJ 08628
Edited by Zachary Fisk, University of California, Irvine, CA, and approved September 26, 2011 (received for review June 29, 2011)
It has been argued that the very high transition temperatures ofthe highest Tc cuprate superconductors are facilitated by enhancedCuO2 plane coupling through heavy metal oxide intermediarylayers. Whether enhanced coupling through intermediary layerscan also influence Tc in the new high Tc iron arsenide superconduc-tors has never been tested due the lack of appropriate systems forstudy. Here we report the crystal structures and properties of twoiron arsenide superconductors, Ca10ðPt3As8ÞðFe2As2Þ5 (the “10-3-8phase”) and Ca10ðPt4As8ÞðFe2As2Þ5 (the “10-4-8 phase”). Based on-Ca-ðPtnAs8Þ-Ca-Fe2As2- layer stacking, these are very similar com-pounds for which the most important differences lie in the struc-tural and electronic characteristics of the intermediary platinumarsenide layers. Electron doping through partial substitution ofPt for Fe in the FeAs layers leads to Tc of 11 K in the 10-3-8 phaseand 26 K in the 10-4-8 phase. The often-cited empirical rule in thearsenide superconductor literature relating Tc to As-Fe-As bondangles does not explain the observed differences in Tc of the twophases; rather, comparison suggests the presence of stronger FeAsinterlayer coupling in the 10-4-8 phase arising from the two-channel interlayer interactions and the metallic nature of its inter-mediary Pt4As8 layer. The interlayer coupling is thus revealed asimportant in enhancing Tc in the iron pnictide superconductors.
In early 2008, superconductivity near 26 K was reported (1) inLaFeAsO0.9F0.1, marking the discovery of a second family of
high Tc superconductors in addition to the cuprates. Since then,intensive effort has been devoted to the search for new Fe-basedsuperconductors, not only in the hope that technologically applic-able superconductors may exist in this class, but also becausecomparison between the Fe pnictides and the cuprates may helpto specify the essential ingredients that give rise to high Tc super-conductivity. Several new families of high Tc pnictide supercon-ductors have been discovered (2–4). All of these materials arebased on layers of edge-sharing FeAs4 or FeSe4 tetrahedra (5),virtually always stacked with other intermediary layers in theircrystal structures. The intermediary layers range from simplealkali or alkaline earth ions (2, 3) to more complex but still che-mically trivial perovskite layers such as Sr4V2O6 (4). Recently,Nohara et al. reported the existence of new superconductors withTc up to 38 K in the Ca-Fe-Pt-As chemical system (6). Althoughlayered -Ca-(Fe, Pt, As)-Ca-(Fe2As2)- stacking was suggestedfor the structures of the superconductors, neither the crystalstructures nor the detailed physical properties were character-ized. This report is intriguing because Fe 122 arsenides, suchas BaFe2As2 (2), form in the ThCr2Si2 structure type while Pt122 arsenides, such as SrPt2As2 (7), form in the CaBe2Ge2 struc-ture type, reflecting fundamental chemical differences betweenFeAs and PtAs layers. Here we isolate, identify and report thecrystal structures and physical properties of two superconductorsin the Ca-Pt-Fe-As system. These materials raise the chemicalcomplexity of the superconducting pnictides to a new level.The first, Ca10ðPt3As8ÞðFe2As2Þ5 (10-3-8), has triclinic symmetry,which is extremely rare among superconductors. The second,Ca10ðPt4As8ÞðFe2As2Þ5 (10-4-8), is a higher symmetry tetragonal
phase. These two superconductors have novel structure typescontaining platinum arsenide intermediary layers, with formulasPt3As8 and Pt4As8, respectively, which are based on the skutter-udite structure, a common crystal structure type in binary pnic-tides. As-As dimers are present in these layers and are critical tounderstanding the electronic state of the superconductors.
For the triclinic 10-3-8 phase, we find that Tc can be tunedfrom 0 to 11 K through 6(1)% to 13(1)% of Pt substitution on theFe site in the ðFe2As2Þ5 layers, which dopes the layers with elec-trons. Although the ground state of the undoped 10-3-8 phaseremains unclear, the Pt doping study provides evidence for simi-larities between this new superconducting family and the fre-quently studied 1111 (such as LaFeAsðO1-xFxÞ) (1) and 122 (suchas BaðFe1-xCoxÞ2As2) (8) families of arsenide superconductors.For the tetragonal 10-4-8 phase, superconductivity occurs at amuch higher temperature, 26 K.
These two chemically and structurally similar compounds, withsignificantly different Tcs, provide a particularly interesting plat-form for studying superconductivity in the pnictides. Comparisonbetween them indicates that simple rigid band filling ideas andthe frequently invoked empirical rule relating Tc to the As-Fe-Asbond angle in superconducting iron pnictides are not enough toaccount for the different Tcs here. Rather the data and analysissuggest that the difference is due to the stronger FeAs interlayercoupling in the 10-4-8 phase, which arises from the two-channelinterlayer interactions present in the 10-4-8 structure and themetallic nature of its intermediary Pt4As8 layer. This strongerinterlayer coupling is thus believed to be an important factorin enhancing Tc in the Fe pnictides. What’s more, bearing in mindthat doping the FeAs layers in Fe pnictide superconductors leadsto lower Tc than doping the intermediary layers, it may be that Tccan be pushed even higher in these systems through doping on theCa or PtnAs4 (n ¼ 3, 4) layers exclusively.
ResultsCrystal Structure. The crystal structures of the Ca10ðPt3As8ÞðFe2As2Þ5 (10-3-8) and Ca10ðPt4As8ÞðFe2As2Þ5 (10-4-8) super-conducting phases, which have -Ca-ðPtnAs8Þ-Ca-ðFe2As2Þ- stack-ing, as well as selected single crystal and powder X-ray diffractionpatterns, are shown in Fig. 1. The detailed crystallographic datadetermined from the single crystal structure refinements aresummarized in Tables 1 and 2; appropriate twin laws were incor-porated into the structure model. Some streaking of diffractionspots along the c axis, indicative of stacking faults, was alwaysobserved in the 10-4-8 phase crystals. The systematic change inreflection intensities caused by stacking faults may introduce
Author contributions: N.N. and R.J.C. designed research; N.N., J.M.A., and B.C.C.performed research; N.N., J.M.A., B.C.C., and R.J.C. analyzed data; and N.N., J.M.A., andR.J.C. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.1To whom correspondence should be addressed. E-mail: [email protected].
some uncertainty in the refined structural parameters, but thegood agreement factor obtained between model and data,R1 ¼ 5.78%, implies that these uncertainties are small.
The top panel of Fig. 1 shows that the powder X-ray pattern forthe 10-3-8 phase can be described well by the determined crystalstructure. For both compounds, the strongest reflections reveala simple tetragonal basal plane subcell with a0 ∼ 3.91 Å as shownin the (hk0) single crystal X-ray diffraction pattern. However,weaker superlattice reflections, which correspond to a squarestructural supercell in the real space basal plane, oriented in the(210) direction, are also observed and lead to a ¼ b ¼ a0∕
ffiffiffi
5p
.This unusual superlattice condition is mathematically equivalentto that seen in K0.8Fe2-δSe2 (9) but arises from a completelydifferent chemical mechanism—the commensurability conditionof the (Pt3As8) or (Pt4As8) skutterudite layers with the Fe2As2layers, rather than partial vacancy ordering. Inspection of the sin-gle crystal diffraction patterns in the c direction indicates that10-4-8 phase has a primitive tetragonal cell, while the 10-3-8phase shows significant shifting of the stacking between neigh-boring layers. This layer shift results in a triclinic unit cell, despitethe fact that the basal plane cell is essentially square—i.e., witha ¼ b and γ ≈ 90°.
The 10-3-8 and 10-4-8 phases are new structure types that canbe classified as derivatives of the SrZnBi2 and SrZnSb2 struc-tures, respectively (10), as shown in the top left panel of Fig. 2.As variants of the more common ThCr2Si2 structure, whichaccounts for many of the high Tc Fe pnictide superconductors,the SrZnSb2 and SrZnBi2 structure types have every other M2X2
tetrahedral layer in AM2X2 replaced with a square X2 layer ofthe same size, as shown in Fig. 2. The only difference betweenthe SrZnSb2 and SrZnBi2 structures is that one is body centeredwhile the other is primitive. This difference in centering isallowed chemically because the X2 layer is not sensitive to theadjacent A cation arrangement, resulting in the fact that both
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Fig. 1. (Upper) The X-ray powder diffraction pattern of the 10-3-8 phasetaken at 300 K. This pattern shows that single phase 10-3-8 can be obtainedbut the powder X-ray diffraction patternwas not employed in the crystal struc-ture determination. Upper black dots, observed pattern; upper red curve, cal-culated pattern; tic marks, calculated peak positions using the crystal structurein Table 1; green (lower) curve, difference between observed and calculatedpattern. (Inset) The crystal structures of the 10-3-8 and 10-4-8 phases. In the10-3-8 phase, the occupancy of the Pt sites at the corners is 50%. (Lower Panels)The single crystal X-ray diffraction patterns for the ðh0lÞ zone of the 10-3-8phase and the ðhk0Þ as well as ðh0lÞ zones of the 10-4-8 phase. a0, b
0, a
and b are described in the text.
Table 1. Crystal structure of 10-3-8 phase at 100 K
Space group P − 1 (# 2) Total reflection 2,229Z 1 Absorption coefficient 39.203∕ mmUnit cell parameters a ¼ 8.759ð4Þ Å , b ¼ 8.759ð4Þ Å , c ¼ 10.641ð5Þ Å , V ¼ 788.1ð6Þ Å3 α ¼ 94.744ð5Þ°,
R1 ¼ 0.0450 and wR2 ¼ 0.1102 for [F0 > 4σF0] and R1 ¼ 0.0902 for all reflections.*Fe1, Fe2, Fe3, Fe4 and Fe5 sites represent Fe1-xPtx with refined x ¼ 0.040 (2); the site occupancy of Pt3 is 0.5.
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staggered and nonstaggered stackings can form around this layer.The difference in stacking has a profound effect in the supercon-ducting phases, in which the X2 layer is replaced by a more com-plex Pt-As layer where some potential Pt sites are forced to bevacant depending on the A site stacking.
The X2 layers are transformed to Pt3As8 and Pt4As8 layers inCa10ðPt3As8ÞðFe2As2Þ5 and Ca10ðPt4As8ÞðFe2As2Þ5, respectively.This transformation is illustrated schematically in the top leftpanel in Fig. 2. Starting from the square lattice of As atoms,the substitution of 1∕5 of the As with Pt and the insertion ofinterstitial Pt lead to strong displacements of the As from their
ideal positions. This occurs so that intralayer As-As dimers areformed and PtnAs8 (n ¼ 3, 4) skutterudite-like layers emerge.The periodicity of these layers is based on the size of thethe Pt sublattice, which matches the FeAs lattice size witha ¼ ffiffiffi
5p
× aFe2As2 . Thus, the Fe2As2 and PtnAs8 layers becomecommensurate in the (210) Fe2As2 direction (Fig. 2, upper right).The resulting Pt3As8 intermediary layer for the 10-3-8 super-conductor is illustrated in the right bottom panel of Fig. 2. Thelayer consists of a square lattice of corner-sharing PtAs4 squareswith a rotation of approximately 25° about an axis perpendicularto the plane, governed by the formation of intraplanar As-Asdimers. This arrangement of atoms is unique in the superconduct-ing iron pnictides but is fairly common in platinum group pnic-tides. A simple example of a compound where such rotationsof corner shared MX4 squares is dictated by the formation ofpnictide-pnictide bonds is skutterudite IrAs3 (11), shown in thelower panel of Fig. 2. In this compound the arrangement of Asatoms has the same projected in-plane structure, but all the Iratoms are octahedrally coordinated rather than having the lowercoordination observed for Pt in the 10-3-8 and 10-4-8 phases.Platinum-based compounds with very similar structures havealso been observed, such as BaPt4As6 (12), shown in Fig. 2, where1∕2 of the Pt are octahedrally coordinated and the rest are squareplanes. In both cases the rotations have an out-of-plane compo-nent, which causes the spacing between Pt atoms to contract. TheAs-As bond distance in the dimers is quite similar in these twocompounds, 2.47 Å–2.54 Å in IrAs3 and 2.41 Å–2.42 Å inBaPt4As6. These distances are comparable to the ones in thenew Ca-Fe-Pt-As superconductors where they are 2.48 Å–2.49 Åin the 10-3-8 phase and 2.50 Å in the 10-4-8 phase.
The limited range of the As-As bond lengths present in thenew superconductors indicates that the size of the Pt sublatticeis constrained by both the As-As dimer size and the Fe2As2 sub-lattice size. The net result of these strong constraints is that only1∕2 of the total number of square sites in the plane are largeenough to contain a Pt atom in a simple square planar coordina-tion with As. The other 1∕2 Pt cannot sit exactly in the middleof the remaining squares but instead must be displaced to a posi-tion about 0.5 Å above or below the plane. This displacementcauses a conflict with the Ca ions that are adjacent to the Pt-Aslayers, resulting in the difference in symmetry and formula forthe primitive and body centered structures. This situation is illu-strated in Fig. 3, which compares the Pt3As8 and Pt4As8 layersin detail. The Pt atoms shown in blue are the ones called substi-tutional Pt atoms in Fig. 2. The Pt atoms shown in red (the onescalled interstitial Pt atoms in Fig. 2) are critical to the Pt stoichio-
Fig. 2. (Top Left) The crystal structures of SrZnSb2 and SrZnBi2 (10), whichcan be considered parent phases of the new Ca-Fe-Pt-As superconductors.(Top Right) Schematic illustration of the transformation from the X2 layerin the parent phases to the PtnAs8 skutterudite-like layers in the supercon-ductors (the center interstitial Pt is only present in the Pt4As8 layer). Displace-ments are exaggerated for clarity. (Bottom) Comparison of the crystalstructures of skutterudite-structure IrAs3 (11), the platinum arsenide layerin BaPt4As6 (12), and the Pt3As8 intermediary layer in the 10-3-8 supercon-ducting phase. As-As bonds are shown bolded.
Table 2. Crystal structure of 10-4-8 phase at 100 K
Space group P4∕n (# 85) Total reflection 895Z 1 Absorption coefficient 41.902∕ mmUnit cell parameters a ¼ 8.733ð3Þ Å , a ¼ 8.733ð3Þ Å , c ¼ 10.481ð4Þ Å , V ¼ 799.3ð5Þ Å3 α ¼ 90°, β ¼ 90°, γ ¼ 90°
R1 ¼ 0.0578 and wR2 ¼ 0.1559 for [F0 > 4σF0] and R1 ¼ 0.0809 for all reflections.*Fe1 and Fe2 sites represent Fe1-xPtx with refined x ¼ 0.03ð1Þ; the site occupancy of the Pt1 site is 0.877(9). The refined chemical formulais Ca10ðPt4-δAs8ÞððFe0.97Pt0.03Þ2As2Þ5 (δ ¼ 0.246).
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metry of the layers. The differences can be seen by focusing onthe parts of the structures that are encircled by the dashed ovalsin the figure. In the primitive 10-4-8 case (top panel), for eachinterstitial Pt site, only one side of the plane is blocked by Ca, sothat Pt atoms can sit above the plane on one site and below theplane on the other. In the 10-3-8 case, on the other hand (bottompanel), one of the potential interstitial Pt sites is blocked on bothsides of the plane by Ca ions, and thus no Pt atoms can sit onthese sites; for the other potential interstitial sites, however,Ca does not interfere, and Pt can occupy a position on either sideof the plane. Pt cannot occupy both sides of the plane at once,because then the Pt-Pt separations would be too small; thus theabove-plane and below-plane sites are randomly occupied with a50% probability. We do not have any evidence that there is anylong range ordering to lift this disorder. This difference in theblocking of the interstitial Pt sites, due to the different arrange-ments of neighboring Ca, accounts for the difference in formulasof the two new superconductors because the 10-3-8 structure canonly accommodate filling of 1∕2 of the interstitial Pt sites whilethe 10-4-8 structure allows filling of all of them.
Physical Properties. In all preparations of the 10-3-8 and 10-4-8superconductors, we found some fraction of Pt substitution onthe Fe site in the FeAs layers. (The determination of the Ptdoping concentration is described in detail in Methods. To easilycompare the physical properties of the new superconductors withthe other Fe pnictide superconductors, which have much simplerformulas, the units of molar susceptibility, magnetization, andheat capacity presented are normalized to one ðFe1-xPtxÞ2 per for-mula unit.)
As a representative of the 10-3-8 phase, the picture of the mm-size single crystals and the physical properties of the x ¼ 0.09ð1Þsample are shown in Fig. 4. Fig. 4A shows the electrical resistivity,ρðTÞ, from 2 to 300 K. As temperature decreases, the resistivityslowly decreases from 0.7 mΩ-cm at 300 K to 0.61 mΩ-cm near190 K, indicated with the red arrow, and then increases monoto-nically to 1.1 mΩ-cm at 12 K, followed by a sharp decrease tozero at the superconducting transition. The magnitude of thenormal-state resistivity is similar to that seen in other Fe pnictidesuperconductors, an indication of the “poor metal” nature of thephase. Fig. 4B presents the normal-state magnetic properties. At10 K, the magnetization M is linearly proportional to H andextrapolates to 0 at H ¼ 0, indicating that no ferromagneticimpurities are present in the single crystal sample. Thus the tem-
perature-dependent susceptibility, χðTÞ, was measured at 5 Tandcalculated as MðTÞ∕H. Magnetic anomalies, such as are oftenassociated with structural or magnetic phase transitions in thepnictide superconductors, were not observed for the 10-3-8 com-pound, with either H∥ab or H⊥ab. The upturn in χ below 80 Kmay be attributed to paramagnetic impurities in the sample. From80 to 300 K, χðTÞ increases approximately linearly with tempera-ture. The ratio of χ∥ab over χ⊥ab at 300 K is approximately 2. Thisvalue is larger than the ratio of 1.6 observed in CaFe2As2, indi-cating higher anisotropy in the 10-3-8 superconductor. The Hallcoefficient, RH , shown in the inset of Fig. 4C, is negative at alltemperatures, indicating that electron carriers are dominant inthis compound. If a single band model is assumed, then thecarrier concentration nðTÞ can be estimated as −1∕eRH . This isplotted as a function of temperature in Fig. 4C. n decreases from5.5 × 1021 cm−3 at 300 K (3.2 times of that of LaFeAsO0.89F0.11)to 0.74 × 1021 cm−3 at 12 K. A slope change is observed in nðTÞat around 190 K, indicated by the red arrow, at the same tem-perature where the minimum normal-state resistivity is found(Fig. 4A). The Seebeck coefficient, shown in Fig. 4D, is negativethroughout the measured temperature range, with a room tem-perature value of −24.3 μV∕K and minimum value of −34.3 μV∕K near 150 K; this again indicates the dominant role of electronsin the transport.
Fig. 5 presents the physical properties of the superconductingstate for the triclinic 10-3-8 phase. The zero-field-cooled (ZFC)and field-cooled (FC) DC susceptibility measurements wereperformed at 2 mTwith H∥ab so that the demagnetization effectcould be minimized. The diamagnetic signal observed below 9 Kin both ZFC and FC measurements confirms the bulk supercon-ductivity in this compound and is consistent with the resistivitymeasurements. The shielding fraction estimated from the ZFCdata is around 120%, similar to what is observed in transitionmetal doped BaFe2As2 (8). The Meissner fraction inferred fromthe FC data is only about 6% due to the flux pinning, whichis the usual case in the Fe pnictide superconductors (8). Thetemperature-dependent CpðTÞ∕T data is presented in Fig. 4B. Afeature in Cp∕T associated with the superconducting transition isobserved near 9 K, confirming the bulk superconductivity. Thesubtle character of this feature could be due to a distributionof Tcs in the sample or could be intrinsic. An empirical ΔCp∕TjTc
∝ T2c relationship has been observed for unannealed Fe pnic-
Fig. 3. (Top) The detailed structure of the Pt4As8 layer in Ca10ðPt4As8ÞðFe2As2Þ5. (Bottom) The detailed structure of the Pt3As8 layer in Ca10ðPt3As8ÞðFe2As2Þ5. The dashed lines show the out-of-plane Pt-As bonding. The dashedovals show the regions of Pt-Ca positional conflict. The bolded black linesshow the As-As dimers. The green lines show the basal plane edges inone unit cell.
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Fig. 4. The characterization of the normal-state properties for the 10-3-8phase Ca10ðPt3As8ÞððFe1-xPtxÞ2As2Þ5 with x ¼ 0.09ð1Þ. (A) Temperature-de-pendent in-plane electrical resistivity ρ. (B) The temperature dependentmagnetic susceptibility, MðTÞ∕H, taken at 5 T with H∥ab and H⊥ab. (Inset)The field dependent magnetization data, MðHÞ, taken at 10 K with H∥ab.(Inset) 10-3-8 single crystals and 1 mm scale. (C) The estimated temperaturedependent carrier density n. (Inset) The temperature dependent Hall coeffi-cient RH. (D) The temperature dependent in-plane Seebeck coefficient, S.
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tide superconductor families (13). For the 122 superconductorswith similar Tc to the 10-3-8 phase, such as BaðFe0.885Co0.115Þ2As2(Tc ∼ 9 K) and BaðFe0.965Co0.035Þ2As2 (Tc ∼ 7 K), the heatcapacity feature at Tc is also small (14). The inset shows the lowtemperature Cp∕T vs. T2 plot, from 2 to 5 K. The data can befitted with Cp ¼ γT þ βT3, where γ ¼ 4.5ð1Þ mJ∕mole-K2 andβ ¼ 1.06ð1Þ mJ∕mole-K4. ρðTÞ∕ρð300 K;0 TÞ, measured at 9,7, 5, 3, 2, 1, 0.5, 0.2, 0 T with H∥ab and H⊥ab is presented inFig. 5C. With applied field, Tc is suppressed to lower tempera-tures and the resistive transition broadens, indicating the pre-sence of strong thermal fluctuation of the vortices. This isdifferent from what is observed in the BaðFe1-xCoxÞ2As2 super-conductors, where no broadening was observed (8) but is remi-niscent of that in RFeAsðO1-xFxÞ and cuprates (15). To determineTc at each field, 90%, 50% and 10% of the normal-state resis-tance at 16 K are used as the criteria. At 0 T, T90%
c ¼ 11.31 K,T50%c ¼ 9.64 K, and T10%
c ¼ 9.22 K. For all three criteria, theH==ab
c2 curves show roughly linear behavior. The H⊥abc2 curve, how-
ever, changes from a concave shape for the 90% criterion to aconvex shape for the 10% criterion. As a compromise, we focuson the Hc2 values inferred from the 50% criterion, shown inFig. 5D. With 9 T applied field, the Tc was suppressed to 0.9Tc0with H∥ab and 0.5Tc0 with H⊥ab. The single band WHH theory,without taking into account the effects of spin paramagnetismand spin-orbit scattering is used to fit the Hc2 curves (16). Thisfit is shown as the solid curve in the panel. The resultingH==ab
c2 ð0Þ ¼ 55 T, and H⊥abc2 ð0Þ ¼ 13 T. The anisotropy parameter
Γ ¼ ðH==abc2 ∕H⊥ab
c2 Þ2 decreases from 100 near Tc0 to 25 at 0.9Tc0, asshown in the inset of Fig. 5D. In accordance with the Ginzburg–Landau theory, H⊥ab
c2 ¼ ϕ0∕2πξ2==ab and H==abc2 ¼ ϕ0∕2πξ==abξ⊥ab,
the coherence lengths are estimated to be ξ==abð0Þ ¼ 50 Å andξ⊥abð0Þ ¼ 12 Å.
We have been successful in tuning the ground state of the10-3-8 phase from normal to superconducting by controlling thePt concentration. The effect of Pt doping in the 10-3-8 phase issummarized in Fig. 6. Because the samples are easily exfoliated,and thus the direct comparison of the resistivities among themare not suitable (8, 17) (the resistivities at 300 K are all in the1 mΩ-cm range), the normalized resistivity, ρ∕ρð300 KÞ, is em-ployed in the figure. The enlarged ρ∕ρð300 KÞ near Tc is shownin Fig. 6A. Zero resistivity was not observed in x ¼ 0.06ð1Þ samplebut does appear at higher Pt concentrations; superconductivity
shows up in the x ¼ 0.07ð1Þ sample with T50%c ¼ 5.9 K and in-
creases to T50%c ¼ 9.6 K in the x ¼ 0.09ð1Þ sample, T50%
c ¼ 9.9 Kin x ¼ 0.13ð1Þ sample. The nature of the bulk superconductivityin these samples is confirmed by the AC susceptibility data,presented in Fig. 6B, which shows large diamagnetic throws withsimilar magnitudes. The Tcs inferred from both types of measure-ments are consistent with each other and summarized in Table 3.Fig. 6C shows the evolution of ρ∕ρð300 KÞ from 2 to 300 K withdoping [from x ¼ 0.06ð1Þ to 0.13(1), each subsequent dataset isshifted upward by 0.3 for clarity]. With decreasing temperature,the resistivity of the x ¼ 0.06ð1Þ sample slowly decreases to aminimum around 170 K and then monotonically increases; aslope break is observed around 60 K. This resistivity shape is re-miniscent of the one in underdoped BaðFe1-xCoxÞ2As2 (8), whichis associated with the structural and magnetic phase transits. Nomagnetic anomaly was observed from 2 to 300 K for thiscompound, however. For x ¼ 0.07ð1Þ and 0.09(1) compounds,upon cooling, the resistivity decreases slowly first and then in-creases before dropping to zero; no slope break is observed. Thex ¼ 0.13ð1Þ sample shows a quite different resistivity shape, whichdecreases continuously from 300 to 50 K, followed by a subtleincrease and then a drop to zero. This shape is reminiscent ofthe nearly optimally doped BaðFe1-xCoxÞ2As2 (8). The Seebeckcoefficient data from 40 to 300 K is presented in Fig. 6D. SðTÞare all negative in measured temperature, implying the dominantrole of the electron carriers. At 300 K, S decreases from−17 μV∕K for x ¼ 0.07ð1Þ to −37 μV∕K for x ¼ 0.13ð1Þ, indicat-ing that the Pt doping is electron doping.
The single crystal X-ray refinement shows that there is Ptdeficiency on the Pt4As8 layer and there is also Pt substitutionfor Fe in the FeAs layer, resulting in the formula for this sampleof Ca10ðPt4-δAs8ÞððFe0.97Pt0.03Þ2As2Þ5 (δ ¼ 0.246). ρðTÞ from 2 to300 K is shown in Fig. 7. The resistivity monotonically decreaseswith decreasing temperature from 0.6 mΩ-cm to 0.24 mΩ-cmfollowed by a sharp drop to zero at T90%
c ¼ 26.7 K and T10%c ¼
26.2 K. The bulk superconductivity is confirmed by the low fieldDC susceptibility, shown in Fig. 7B. Tc is inferred as 26 K. Theestimated shielding fraction is around 100%. The normalizedresistivity, ρðTÞ∕ρð300 K;0 TÞ, with field along the ab planeand the c axis is shown in Fig. 7C. At 0 T, T50%
c is 26.4 K. The
0 4 8 12
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0 8 16
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10
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1000 250
40
T (K)
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T / Tc
T (K)
ρ/ρ(300K, 0T
)
ρ (mΩ
-cm)
4πχ
FC
ZFC
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D
CA
B
T (K)
Cp/
T (
J/m
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Fe
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t x)2
K2 )
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8 0
H // ab
H ⊥ abH // ab
HC
2 (T)
WHH
Γ
T / Tc
Cp/T
(m
J/m
ol-(
Fe 1-
xPt x) 2 K
2 )
T2 (K2)
Fig. 5. The characterization of the superconducting properties for the10-3-8 phase Ca10ðPt3As8ÞððFe1-xPtxÞ2As2Þ5 with x ¼ 0.09ð1Þ. (A) Enlargementof the temperature dependent electrical resistivity near Tc and the field-cooled/zero-field-cooled DC susceptibility MðTÞ∕H with H∥ab. (B) CpðTÞ∕Tvs. T. (Inset) CpðTÞ∕T vs. T2. The solid line shows the fit using CpðTÞ∕T ¼γ þ β T2. (C) The normalized resistivity, ρ∕ρð300 K;0 TÞ taken at 9, 7, 5, 3, 2,1, 0.5, 0.2 and 0 T with H∥ab and H⊥ab. The 10%, 50% and 90% criteriaare shown. (D) The inferred anisotropic Hc2ðTÞ for H∥ab and H⊥ab usingthe 50% criterion. The solid line is the single band WHH fit (16). (Inset)The anisotropic parameter ΓðTÞ ¼ ðH==ab
c2 ∕H⊥abc2 Þ2.
0 5 10 15-0.02
0.00
0 5 10 150
1
2
0 100 200 300
-40
0
0 100 200 3000
2
T (K)
I // ab
x=0.06(1)
0.13(1)0.09(1)
0.07(1)
χ' (
emu/
g)
H=0.5mTH // ab
D
C
B
A
T (K)
0.13(1)
0.13(1)
0.09(1)
0.07(1)ρ/ρ (
300K
, 0T
)
x=0.06(1)
10-3-8 phase
T (K)
x=0.06(1)
0.09(1)
S (
µV/K
)
T (K)
x=0.06(1)
0.13(1)
0.09(1)
0.07(1)
ρ/ρ (
300K
, 0T
)
Fig. 6. The characterization of the doping-dependent properties for the10-3-8 phase Ca10ðPt3As8ÞððFe1-xPtxÞ2As2Þ5. (A) The enlarged normalizedresistivity, ρðTÞ∕ρð300 KÞ, near Tc for the 10-3-8 series. (B) Temperature-dependent AC susceptibility data, MðTÞ∕H, taken at 0.5 mTwith H∥ab in the10-3-8 phase. (C) The normalized resistivity, ρðTÞ∕ρð300 KÞ, from 2 to 300 K.Each subsequent dataset is shifted upward by 0.3 for clarity. (D) The Seebeckcoefficient SðTÞ.
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Hc2 data was inferred using the 50% criterion and is summarizedin Fig. 7D. The Tc was only suppressed to 0.9Tc0 with H∥ab and0.7Tc0 with H⊥ab at 9 T. Comparing with the x ¼ 0.13ð1Þ 10-3-8phase, less anisotropy was observed. The anisotropic parameter Γshown in the inset is 32 at 0.9Tc0 and 14 near Tc0 while this valueis 25 at 0.9Tc0 and 100 near Tc0 in x ¼ 0.13ð1Þ 10-3-8 phase.
DiscussionAlthough systematic composition-dependent physical propertiesmeasurements of the 10-4-8 phase are currently not available, thelower Tc triclinic 10-3-8 phase can be readily compared to theother Fe pnictides: (i) From 80 to 300 K, a linear temperaturedependence of MðTÞ∕H has also been observed in all the 1111and 122 Fe pnictide superconductors, such as LaFeAsðO1-xFxÞand BaðFe1-xCoxÞ2As2 (8, 18). In the latter case, the linear tem-perature dependence persists up to 700 K, the highest tempera-ture measured. It has been suggested that the linear temperaturedependence of MðTÞ∕H is related to antiferromagnetic spinfluctuations (19). (ii) Upon doping, the resistivity changes froma “semiconducting” appearance in the underdoped region to a“metallic” appearance near the optimal doping concentration.This systematic change is reminiscent of what is seen in otherFe pnictides, where the “semiconducting” appearance comesfrom the formation of an SDW gap (1, 8). However, no magneticanomalies implying a magnetic phase transition were observedfrom 2 to 300 K in the 10-3-8 phase. It is possible that the dopinglevel is still too high, even in the lowest Pt-content samples, andthat the SDW transition exists but has already been fully sup-pressed. Further investigation is needed to clarify the groundstate of the undoped or very underdoped phase. (iii) The aniso-tropy parameter Γ ¼ ðH==ab
c2 ∕H⊥abc2 Þ2 ranges from 100 near Tc0 to
25 at 0.9Tc0. This is much larger than the 1 to 4 in ðBa0.55K0.45ÞFe2As2 (20) and even larger than the ones in RFeAsO0.8F0.2 (15)and indicates a highly anisotropic 2D nature for the 10-3-8 super-conductor.
The Tc in the 10-4-8 phase is almost twice the highest Tc weobserved in the 10-3-8 phase. Although we have not yet success-fully overdoped the 10-3-8 phase, the almost linear normal-stateresistivity of the x ¼ 0.13ð1Þ 10-3-8 compound, indicates that thiscompound is very close to the optimal doping and thus representsthe nearly maximum Tc obtainable in the triclinic 10-3-8 phasein the Ca-Pt-Fe-As system. Although we never observed a Tchigher than 26 K in the 10-4-8 phase, a Tc of 38 K was reportedby Nohara et al., for a phase of unreported formula with a crystal-lographic cell corresponding to our tetragonal 10-4-8 phase (6),suggesting that Tc can be even higher in the 10-4-8 phase when anoptimal doping condition is obtained. Simple rigid band fillingideas is not enough to account for the Tc difference in thesetwo compounds. If we assign Ca10ðPt3As8ÞðFe2As2Þ5 as the par-ent compound, the optimally doped Tc ¼ 11 K 10-3-8 sample has0.26 doped electrons per Fe, while the Tc ¼ 26 K 10-4-8 samplehas 0.21 doped electrons per Fe, which is not enough to accountfor the difference in Tc.
It is thus of interest to compare these two chemically andstructurally similar compounds, which show quite different Tcs,in more detail. From the chemical point of view, we can straight-forwardly model the effective charges in these compounds. Thecommon assignments of Fe2þ and As3− ions in the FeAs layergives the chemical formula ½ðFe2As2Þ510−. In the PtnAs8 layer,due to the formation of strong As-As dimers in both compounds,the As in PtnAs8 layer contributes 1 electron to the As-As bond,forming As2
4−. For reduced compounds such as skutteruditesand those studied here, the stability of the d8 configuration forPt rules, and the only oxidation state observed for platinum is+2; this d8 configuration is consistent with the observed Pt-Ascoordination polyhedra in the superconductors. In their undopedform, this yields an intermediary layer of ½Pt3As810− for the 10-3-8 phase and ½Pt4As88− for the 10-4-8 phase, leading to a majorchemical and electronic difference between the two superconduc-tors. Ca10ðPt3As8ÞðFe2As2Þ5 is a valence satisfied compoundthrough the Zintl concept (21)—the ½Pt3As810− layer is perfectlycharge balanced by the ½Ca1020þ and ½Fe10As1010−, leading to asemiconducting nature for the Pt3As8 intermediary layer (i.e.,it will not contribute density of states at Ef ) and thus will resultin weak FeAs interlayer coupling through this intermediary layer(22). The 10-4-8 phase on the other hand has one more Pt atomin the Pt4As8 intermediary layer, exceeding its valence satisfac-tion requirements, indicating that this layer is likely to have statesat Ef and is therefore metallic in character—thus leading tostronger FeAs interlayer coupling. Enhanced coupling throughintermediary layers has been suggested as the origin of the veryhigh transition temperatures in the highest Tc cuprates (22), butin the iron arsenide superconductors this is the only system wherethis possibility could be tested.
Structural differences may also have an impact on the super-conductivity, but the usual considerations in the arsenide super-
Table 3. Starting material ratio in sample growth
Phase Starting ratio Ca∶Fe∶Pt∶As EDS Ca∶Fe∶Pt∶As x TRc (K) TM
The EDS measured ratio of Ca∶Fe∶Pt∶As and the resulting x in Ca10ðPt3As8ÞððFe1-xPtxÞ2Þ5 for the 10-3-8 phase. TRc is the Tc obtained
from the 50% criterion from the resistivity data. TMc is the Tc obtained from the AC susceptibility data.
*The chemical formula of this compound is Ca10ðPt4-δAs8ÞððFe0.97Pt0.03Þ2As2Þ5 (δ ¼ 0.246), determined from the single crystal structurerefinement.
0 100 200 3000.00
0.65
15 20 250.0
0.4
0.80 0.85 0.90 0.95 1.000
10
0.95 1.000
40
0 10 20 30
-1
0
ρ (m
Ω-c
m)
CA
T (K)
I // ab I = 0.3 mA
10-4-8 phase
28 25 20 15
H=9 T7 T5 T3 T1 T0 Tρ /
ρ(30
0 K
, 0 T
)
T (K)
H // ab H ⊥ ab
50%
H ⊥ ab
H // ab
Hc2
(T)
T / Tc
T / Tc
Γ
FC
ZFC
DB
T (K)
4πχ H // ab
H=2 mT
Fig. 7. The characterization of the superconducting properties for the10-4-8 phase Ca10ðPt4-δAs8ÞððFe0.97Pt0.03Þ2As2Þ5 ðδ ¼ 0.246Þ. (A) The resistivitytaken at H ¼ 0 T from 2 to 300 K. (B) The field-cooled/zero-field-cooledDC susceptibility MðTÞ∕H with H∥ab. (C) The normalized resistivity,ρ∕ρð300 K;0 TÞ taken at 9, 7, 5, 3, 1, 0.5, and 0 T with H∥ab and H⊥ab.The 50% criterion is shown. (D) The inferred anisotropic Hc2ðTÞ for H∥aband H⊥ab using the 50% criterion. The dashed line is the guide for the eyes.(Inset) The anisotropic parameter ΓðTÞ ¼ ðH==ab
c2 ∕H⊥abc2 Þ2.
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conductors do not explain the observations in the present mate-rials. With a few exceptions, an empirical rule between Tc andαAs-Fe-As, the As-Fe-As bond angle, or Pn height, the distance be-tween the adjacent Fe and As layer, has been observed in the Fearsenides (23–25). This empirical rule implies that Tc is enhancedfor As-Fe-As bond angles near that of an ideal tetrahedron,109.47°, or for particular values of Pn ∼ 1.38 Å . For our com-pounds, αAs-Fe-As in the 10-3-8 phase has an average value of108.99° while αAs-Fe-As in the 10-4-8 phase has an average valueof 107.63°. Because the 10-4-8 phase has a higher Tc than the10-3-8 phase, our compounds do not follow the empirical ruleof angles. Similarly, the average Pn is 1.40 Å in the 10-3-8 phaseand 1.43 Å in the 10-4-8 phase, which also does not follow theempirical rule of heights. Thus the structural differences usuallycredited with governing the Tc in the arsenide superconductorscannot be operating in our phases, supporting our argument thatit is the metallicity and resulting enhanced intralayer coupling inthe 10-4-8 phase that determine its higher Tc.
Significant differences in the Pt-As interactions betweenPt3As8∕Pt4As8 intermediary layers and the neighboring FeAslayers are also present, further strengthening our argument aboutthe importance of the interlayer coupling in determining the Tcs.In the 10-3-8 phase there is only one interlayer Pt-As interactionchannel per unit cell with the Pt-As distance of 3.20 Å (bottomleft of Fig. 3, indicated by the dashed line), and it appears tobe random whether the Pt-As bond points “up” or “down.” Thisnot only leads to weaker interlayer coupling from the structuralperspective but also helps to electronically isolate the Pt3As8 in-termediary layers from the FeAs layers, making that layer moreelectronically blocking and reinforcing its semiconducting nature.In the 10-4-8 compound, on the other hand, due to two interlayerPt-As interaction channels rather than one (top left of Fig. 3, in-dicated by the dashed line), a shorter Pt-As distance of 3.08 Å ,and their fully structurally ordered character, stronger interlayercoupling is realized and the metallic nature of the Pt4As8 layer isreinforced. What’s more, the overall tetragonal symmetry of thephase promotes good orbital overlap between layers. The aniso-tropic Hc2 measurements in the superconducting state, whichshow the 10-3-8 phase to be much more anisotropic than the10-4-8 phase, support the chemical picture. Thus though onecan infer that both compounds should show strong 2D characterin their electronic structures, the 10-4-8 compound will exhibitmore and better hybridized electronic states associated with thestronger interlayer interactions, which we argue leads to its higherTc. Our study reveals the importance of strong FeAs interlayercoupling in enhancing Tc in the Fe pnictides and suggests thatfurther searches for superconducting iron arsenide phases withmetallic intermediary layers rather than the commonly found in-sulating intermediary layers may be a fruitful path for obtaininghigher Tcs in the pnictide superconductor family.
MethodsTo prepare the crystals, CaAs, FeAs, Fe, Pt, and As were mixed in an argon-filled glovebox, pressed into pellets, and put into alumina crucibles. The cru-cibles were then sealed in quartz tubes under 1∕3 atmosphere of Ar. For heat-ing temperatures between 700 °C and 950 °C, a polycrystalline mixture of oneor both superconducting phases together with a considerable amount ofPtAs2 was obtained. For heating temperatures above 1,100 °C, single crystal-line 10-3-8 or 10-4-8 phases intergrown with PtAs2 were obtained. For the10-3-8 phase, 3 × 2 × 0.5 mm3 size single crystals with no intergrown impu-rities could be obtained when excess CaAs was added to the mixture, with
the Pt doping concentration manipulated by tuning the Fe to Pt ratio; start-ing material ratios are summarized in Table 3. The crystal growth tubes wereheated to 1,100–1,180 °C, held for one week, furnace-cooled or cooled by5 °C∕h to 975 °C, and then water quenched. The plate-like 10-3-8 single crys-tals were then separated by washing out the ionic CaAs in distilled water. Forthe 10-4-8 phase, the tubes were heated to 1,100–1,180 °C, held for oneweek, furnace-cooled to 900 °C, held for 1 d, and then water quenched; pure10-4-8 phase single crystals with 0.5 × 0.5 × 0.03 mm3 size can be obtained.Growth details for the 10-4-8 phase are also summarized in Table 3.
Crystal structure determination for both new phases was performed onsingle crystals at 100 K using a Bruker Apex II single crystal X-ray diffract-ometer with graphite-monochromated Mo Kα radiation (λ ¼ 0.71073 Å ).The sample used for single crystal structure refinement of the 10-3-8 phasewas chosen from the x ¼ 0.07ð1Þ batch. The sample used for single crystalstructure refinement of the 10-4-8 phase was cut from the same piece usedfor the resistivity and anisotropic Hc2 measurement. Unit cell refinement anddata integration were performed with Bruker APEX2 software package. Unitcell determination was aided by the CELL NOW program (26). The crystalstructures were refined using SHELXL-97 (26) implemented through WinGX(27). X-ray powder diffraction patterns were collected on a Bruker D8 Focusdiffractometer employing Cu Kα (λ ∼ 1.5406 Å) radiation and a graphitediffracted beam monochromator. Rietveld refinement was carried out usingthe FULLPROF program suite (28). The Bragg peaks were refined using theThompson–Cox–Hastings pseudo-Voigt function convoluted with an axialdivergence asymmetric peak shape; [001] preferred orientation was includedin the powder refinements.
For the 10-3-8 phase, the Pt doping concentrations were determined usingenergy dispersive X-ray spectroscopy (EDS) in an FEI Quanta 200 FEG Envir-onmental-SEM through measuring the percentages of elements present.By making several EDS measurements on each sample characterized, theaverage percentage of each element was obtained. Because for the 10-3-8phase the single crystal X-ray measurements show there is neither Pt defi-ciency nor Fe substitution in the Pt3As8 layer, resulting in the formulaCa10ðPt3As8ÞððFe1-xPtxÞ2As2Þ5, the average Pt percentage measured by theEDS measurement leads to an x1, and the average Fe percentage leads toan x2 that were used to determined x as 1∕2ðx1 þ x2Þ and the error in com-position as 1∕2jx1 − x2j. The EDS results are summarized in Table 3. Using thesingle crystal growth method described in the paper, we have not successfullyobtained 10-3-8 crystals with x less than 0.06. For the 10-4-8 phase, becausethe crystals for single crystal X-ray, resistivity, anisotropic Hc2, and EDSmeasurements were cut from the same exact piece, the formula of the sam-ple was determined from the single crystal X-ray crystal structure refinement.
DC magnetization, MðHÞ and MðTÞ, were measured in a Quantum Design(QD) Magnetic Properties Measurement System (MPMS) superconductingquantum interface device (SQUID) magnetometer. For the 10-4-8 phase,due to the smallness of the single crystals, the sample used in the χðTÞ mea-surement was a composite of several single crystals that were carefullyaligned so that the applied field was parallel to the ab planes.
Heat capacity (relaxation method), AC susceptibility, resistivity, and Hallcoefficient measurements were performed in a QD Physical Properties Mea-surement System (PPMS). The standard four-probe technique was employedfor the resistivity measurements (I ¼ 1 or 0.3 mA). A four-wire geometry wasused in the Hall coefficient measurements. To remove the magnetoresistivecomponents, the polarity of the magnetic field (H⊥ab) was switched. In bothresistivity and Hall effect measurements, the four thin platinum/gold wiresemployed were attached to the sample with Epotek H20E silver epoxy.Seebeck coefficient measurements were performed with a modified MMRTechnologies SB100 Seebeck measurement system.
ACKNOWLEDGMENTS. The authors would like to thank E. Climent-Pascual,S. Jia, S. Dutton, M. Bremholm, M. Fuccillo, M. Ali, J. Krizan, K. Baroudi,and H. W. Ji for helpful discussions. This work was supported by the Air ForceOffice of Scientific Research Multidisciplinary Research Program for Univer-sity Research Initiative on superconductivity. Preliminary XRD data was col-lected with funds from the National Science Foundation Major ResearchInstrumentation program (0922931).
1. Kamihara Y, Watanabe T, Hirano M, Hosono H (2008) Iron-Based Layered Supercon-
ductor LaðO1-xFxÞFeAs (x ¼ 0.05–0.12) with Tc ¼ 26 K. J Am Chem Soc 130:3296–3297.
2. Rotter M, Tegel M, Johrendt D (2008) Superconductivity at 38 K in the iron arsenide
ðBa1-xKxÞFe2As2 . Phys Rev Lett 101:107006.
3. Wang XC, et al. (2008) The superconductivity at 18 K in LiFeAs system. Solid State
Commun 148:538–540.
4. Zhu X, et al. (2009) Transition of stoichiometric Sr2VO3FeAs to a superconducting state
at 37.2 K. Phys Rev B Condens Matter Mater Phys 79:220512(R).
5. Hsu FC, et al. (2008) Superconductivity in the PbO-type structure α-FeSe. Proc Natl AcadSci 105:14262–14264.
6. NoharaM, Kakiya S, Kudo K (2011) Superconductivity in Ca-Fe-Pt-As at 38 K. Proceedingsof the International Workshop on Novel Superconductors and Super Materials p a-41.
7. Kudo K, Nishikubo Y, Nohara M (2010) Coexistence of superconductivity and chargedensity wave in SrPt2As2. J Phys Soc Jpn 79:123710.
8. Ni N, et al. (2008) Effects of Co substitution on thermodynamic and transport proper-ties and anisotropic Hc2 in BaðFe1-xCoxÞ2As2 single crystals. Phys Rev B Condens MatterMater Phys 78:214515.
Ni et al. PNAS ∣ November 8, 2011 ∣ vol. 108 ∣ no. 45 ∣ E1025
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ICS
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020
9. Zavalij P, et al. (2011) On the structure of vacancy ordered superconducting alkalimetal iron selenide. Phys Rev B Condens Matter Mater Phys 83:132509.
10. Zheng Z, Hoffmann R (1986) Donor-acceptor layer formation and lattice site prefer-ence in the solid: the CaBe2Ge2 structure. J Am Chem Soc 108:3078–3088.
11. Kjekshus A, Rakke T (1974) Compounds with the skutterudite type crystal structure. III.Structural data for arsenides and antimonides. Acta Chem Scand 28A:99–103.
12. Wenski G, Mewis A (1987) Ternary pyrite-type derivatives: Preparation and crystalstructure of SrPt4P6 , SrPt4As6, BaPt4As6 and BaPt2P3. Z Naturforsch B 42:507–513.
14. Bud’ko SL, Ni N, Canfield PC (2009) Jump in specific heat at the superconductingtransition temperature in BaðFe1-xCoxÞ2As2 and BaðFe1-xNixÞ2As2 single crystals. PhysRev B Condens Matter Mater Phys 79:220516.
15. Hunte F, et al. (2008) Two-band superconductivity in LaFeAsO0.89F0.11 at very highmagnetic fields. Nature 453:903–905.
16. Werthamer N, Helfand RE, Hohenberg PC (1966) Temperature and purity dependenceof the superconducting critical field, Hc2. III. Electron spin and spin-orbit effects. PhysRev 147:295–302.
17. Tanatar M, et al. (2009) Anisotropy of the iron pnictide superconductorBaðFe1-xCoxÞ2As2 (x ¼ 0.074, Tc ¼ 23 K). Phys Rev B Condens Matter Mater Phys79:094507.
18. Wang XF, et al. (2009) Anisotropy in the electrical resistivity and susceptibility of super-conducting BaFe2As2 single crystals. Phys Rev Lett 102:117005.
19. Korshunov MM (2009) Nonanalytic spin susceptibility of a fermi liquid: The case ofFe-based pnictides. Phys Rev Lett 102:236403.
20. Altarawneh MM (2008) Determination of anisotropic Hc2 up to 60 T inBa0.55K0.45Fe2As2 single crystals. Phys Rev B Condens Matter Mater Phys 78:220505(R).
21. Müller U (1993) Inorganic Structural Chemistry (Wiley, Chichester, UK), pp 123–124.22. Sterne PA, Wang CS (1988) Higher Tc through metallic inter-layer coupling in
Bi2Sr2CaCu2O8. J Phys C Solid State Phys 21:949–955.23. Lee CH, et al. (2008) Effect of structural parameters on superconductivity in fluorine-
free LnFeAsO1-y (Ln ¼ La, Nd). J Phys Soc Jpn 77:083704.24. Johnston DC (2010) The puzzle of high temperature superconductivity in layered iron
pnictides and chalcogenides. Adv Phys 59:803–1061.25. Mizuguchi Y, et al. (2010) Anion height dependence of Tc for the Fe-based super-
conductor. Supercond Sci Tech 23:054013.26. Sheldrick GM (2008) A short history of SHELX. Acta Crystallogr A 64:112–122.27. Farrugia LJ (1999) WinGX suite for single crystal small molecule crystallography. J Appl
Crystallogr 32:837.28. Rodríguez-Carvajal J (1993) Recent advances in magnetic structure determination by
neutron powder diffraction. Physica B 192:55–69.
E1026 ∣ www.pnas.org/cgi/doi/10.1073/pnas.1110563108 Ni et al.
