Modulation Effect of Interlayer Spacing on the Superconductivity of

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Modulation Effect of Interlayer Spacing on the Superconductivity of Electron-Doped FeSe-Based Intercalates Fumitaka Hayashi,†,∥ Hechang Lei,†,⊥ Jiangang Guo,†,@ and Hideo Hosono*,†,‡,§ †

Frontier Research Center, Tokyo Institute of Technology, 4259-S2-13 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan Materials Research Center for Element Strategy, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan § Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ‡

S Supporting Information *

ABSTRACT: FeSe-based intercalates are regarded as promising candidates for high-critical temperature (Tc) superconductors. Here we present new Na- and Sr-intercalated FeSe superconductors with embedded linear diamines (H2N)CnH2n(NH2) (abbreviated as DA; n = 0, 2, 3, or 6) prepared using a low-temperature ammonothermal method to investigate the effect of interlayer spacing on the superconductivity of electron-doped FeSes. The embedded DA formed a monolayer or bilayer in the interlayer of FeSe. The interlayer spacing between nearest FeSe layers could be tuned from 0.87 to 1.14 nm without significant change in the Na/Sr content or the ratio of Fe to Se. Importantly, bilayer phases Na/ethylenediamine- and Sr/hydrazine-FeSe show improved structural stability compared to that of Na/NH3-FeSe. The series of Na- and Sr-intercalated FeSe samples exhibited nearly the same high Tc values of 41−46 and 34−38 K, respectively, irrespective of rather different interlayer spacing d. The peculiar insensitivity for both series can be ascribed to the negligible dispersions of bands along the c axis; i.e., Fermi surfaces are nearly two-dimensional when d is larger than a certain threshold value (dsat) of ∼0.9 nm. The Fermi surface shape is already optimal for Tc, and a larger d will not enhance Tc further. On the other hand, the difference in Tc between two series may be explained by the higher carrier doping level in Na/ DA-FeSes compared to that in Sr/DA-FeSes, resulting in the increased density of states at the Fermi level and superconducting pairing strength.

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INTRODUCTION Exploration of new superconductors (SC) with high transition temperatures Tc, especially above 77 K, is a challenging subject in materials science. Since the discovery of superconductivity in ZrCuSiAs-type LaFeAs(O,F) with a Tc of ∼26 K,1 various kinds of FeAs- and FeSe-based superconductors (iron-based SCs), including RAEFeAsO(F,H) (RAE can be alkaline earth and rare earth metals),2−5 AFe2As2 (A is an alkaline earth),6−8 and FeSe,9 have been extensively studied as candidates for high-Tc SCs. These SCs have quasi-two-dimensional FeAs or FeSe layers. Among Fe-based SCs, FeSe forms by alternate stacking of the anti-PbO-type FeSe layers, which is the simplest structure. The members of this family of materials have received much attention for two main reasons. First, the Tc of bulk FeSe is low (∼8 K),9 and it could be enhanced up to 30− 45 K upon high-pressure10 or alkali/alkaline earth metal intercalation treatments.6,11,12 Second, an FeSe monolayer grown on a SrTiO3 substrate was reported to show a high Tc, close to the boiling point of liquid nitrogen (∼77 K), with tuning the charge carrier concentrations, which was identified by scanning tunnelling spectroscopy and angle-resolved photoemission spectroscopy experiments.13−15 Therefore, such FeSe-based SCs are ideal systems for further increasing Tc significantly and for improving our understanding of the origin of high Tc. © XXXX American Chemical Society

In FeAs-based SCs, the Tc is well-correlated with the FeAs interlayer distance.16,17 For example, F-doped SmFeAsO (dFeAs = 8.7 Å) exhibits the highest Tc of 55 K, while LiFeAs (dFeAs = 6.4 Å) shows a smaller Tc of 18 K.18 These phenomena were more obvious in the FeSe-based SCs.6,9−12 Interestingly, a similar situation is observed in cuprate SCs as well where Tc is increased with an increment in the effective spacer layers of CuO planes.19 For instance, the Tcs of (La1−xSrx)2CuO4 and Bi2Sr2CuO6 that possess a single spacer layer are ∼38 K, while those of YBa2Cu3O7 and Bi2Sr2CaCu2O8 with double-spacer layers are 85−92 K; that of Bi2Sr2Ca2Cu3O10 with triple layers is ∼123 K. One underlying approach is therefore to control the interlayer distance of the FeSe plane. Herein, we report two series of new superconducting FeSe intercalates, Na/DA-FeSe and Sr/DA-FeSe [DA = (H2N)CnH2n(NH2), where n = 0, 2, or 3], using a low-temperature ammonothermal method, wherein Na/Sr ions and the DA molecules were intercalated together within the interlayers. Seven new Na/DA-FeSe and Sr/DA-FeSe superconducting phases were identified with the same high Tc values of 41−46 and 34−38 K, respectively. The chemical compositions and structural stabilities were evaluated using various characterReceived: December 21, 2014

A

DOI: 10.1021/ic503033k Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry ization techniques. We found that the interlayer distances of the two series of intercalates varied from 0.87 to 1.14 nm, with little change in the contents of Na/Sr ions and the DAs, by controlling the preparation conditions and kinds of DA. We discuss the correlation between the Tc and the properties of various electron-doped FeSe-based intercalates.

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EXPERIMENTAL SECTION

Materials and Methods. Na and Sr metals and linear diamines such as anhydrous hydrazine (N2H4), ethylenediamine (EDA, C2H8N2), 1,3-propanediamine (PDA, C3H10N2), and 1,6-heptanediamine (HDA, C6H16N2) were used as electron sources and molecular spacers, respectively. The Na and Sr metals and the linear diamines were received from Kojyundo and Aldrich, respectively, without further purification. The starting FeSe sample was synthesized using a modified high-temperature solid-state method according to the literature12 that implemented iron granules (Alfa, 99.98%) and selenium grains (Kojundo, 99.99%). The FeSe intercalates were prepared in liquid NH3 at room temperature and at ∼1 MPa according to a modified method in the literature.12 Typically, 200 mg of FeSe powders, ∼10−15 mg of Na or Sr metal, and 60−100 mg of linear diamine, except for N2H4, were loaded into a Taiatsu Glass TVS-N2 high-pressure vessel (10, 30, or 50 mL) with a stop valve for vacuum treatment, which was sealed in the argon-filled glovebox to prevent air and water contamination. Before the introduction of NH3, the vessel was connected to a vacuum/NH3 gas line equipped with a turbo molecular pump and mass-flow controller. Typically, 4−6 g of NH3 was condensed by cooling the vessel to 223 K. For N2H4-intercalated samples, approximately 60 mg of N2H4 was added after the introduction of liquid NH3. As a cautionary note, N2H4 should not react directly with metallic Na, Sr, and FeSe to avoid the undesired explosive decomposition reaction of N2H4. Subsequently, the reaction vessel was closed and kept at 273−298 K for 2−3 h. After the intercalation reaction, the introduced NH3 was removed by opening the stop valve at 313 K, and then the samples were recovered in the glovebox. As needed, the evacuation treatment was performed at 303− 373 K using an oil-sealed rotary vacuum pump to yield the single phase of DA-intercalated FeSes. It is worth noting that almost all the intercalated ammonia is removed by vacuum treatment as reported in the literature.12 Characterization. The powder X-ray diffraction (PXRD) patterns of the products were measured by a Bruker diffractometer D8 ADVANCE with a Cu Kα anode (λ = 1.5408 Å). The XRD patterns were collected using the airtight specimen holder with a domelike Xray transparent cap, for environmentally sensitive materials (Bruker, A100B33). The DC magnetization was measured by a vibrating sample magnetometer (SVSM, Quantum Design) at 10 Oe. The chemical composition of the samples was determined by an electron probe microscope analyzer (EPMA, JEOL, JXA-8530F) and a carbon/ hydrogen/nitrogen (CHN) analyzer (J-SCIENCE, MICRO CORDER, JM10). The molar ratio of carbon to nitrogen in the FeSe intercalates was close to that for the DA used as a molecular spacer, indicating there was little NH3 in the samples.

Figure 1. PXRD patterns of evacuated (a) Na/N2H4-, (b) Na/ C2H8N2-, (c) Na/C3H10N2-, (d) Na/C6H16N2-, and (e) Na/NH3intercalated FeSe and those of (f) as-prepared Na/C2H8N2intercalated FeSe and (g) parent FeSe. Samples e and g are from ref 12. Asterisks denote diffraction of basal FeSe planes. The preparation conditions are summarized in Table 1.

a single DA-intercalated phase, heat treatment around ∼373 K in an evacuated environment is needed for certain samples (denoted “evacuated phase”, Figure 1a−c, and entries 1−3 in Table 1); however, the single 1,6-heptanediamine-intercalated phase was not obtained irrespective of treatment conditions (Figure 1d). It is important to point out the difference in phase stability between Na/DA-FeSe and Na/NH3-FeSe. It is well-known that NH3-rich and NH3-poor phase (Na,Li)/NH3-FeSe are readily decomposed or transformed to another phase through evacuation or storage at room temperature because NH3 molecules easily desorb from the interlayers.12,20 In contrast, the structures of present Na/DA-FeSe intercalates are maintained upon the evacuation treatments around 373 K because of the strong interaction of DA with FeSe layers. An electron probe micro analyzer (EPMA) and a carbon/ hydrogen/nitrogen (CHN) determinator were used to evaluate the chemical composition of Na/DA-FeSes. The nominal and experimentally determined chemical compositions are summarized in entries 1−4 of Table 1. For comparison, the reported composition of Na/NH3-FeSe12 is shown in entry 5 of Table 1. The x values in the chemical formula Nax(DA)yFe2−zSe2 are 0.82−0.93, irrespective of the carbon numbers and the intercalation degrees of the DA (entries 1−4). All results are quite consistent with previous results of Ax(NH3)yFe2Se2 (A = Li, Na, Ba, Sr, Ca, Yb, and Eu) prepared by a low-temperature ammonothermal method.11b,12 The y values in the formula Nax(DA)yFe2−zSe2 were 0.31−0.45 (entries 1−3 in Table 1). In contrast, the y value for the as-prepared EDA-intercalated phase was 0.73, which is nearly twice that for the evacuated EDAintercalated phase. The z values in the formula Nax(DA)yFe2−zSe2 were in the range of 0.05−0.16, indicating very few Fe vacancies. Here we discuss the variation in interlayer distance d with the increasing carbon numbers of DA. The d values of the evacuated phases for Na/hydrazine (N2 H4 )-, Na/EDA (C2H8N2)-, and Na/1,3-propanediamine (PDA, C3H10N2)FeSe were 0.895, 0.951, and 1.000 nm, respectively (entries 1−

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RESULTS AND DISCUSSION Structural Evolution of FeSe Intercalates by Addition of Linear DAs (H2N)CnH2n(NH2) (n = 0, 2, 3, or 6) as a Molecular Spacer. Traces a−d and f of Figure 1 show PXRD patterns of the Na/DA-intercalated FeSe samples (denoted Na/DA-FeSe). For comparison, the patterns of the Na/NH3intercalated FeSe (NH3-poor phase)12 and parent FeSe sample are shown in traces e and g of Figure 1, respectively, together with the peak assignment (based on a body-centered and primitive tetragonal cell).12 Peaks marked with an asterisk in Figure 1 could be attributed to the basal spacing of the FeSe layer. Table 1 summarizes the synthesis conditions and the interlayer distance d between the nearest FeSe layers. To obtain B

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Table 1. Preparation Conditions and Chemical Properties of Na/Diamine- and Sr/Diamine-Intercalated FeSe Samples entry

nominal composition

evacuation temp (K)

experimental compositiona

db (nm)

Tc (K)

SVFc (at 2 K/%)

1 2 3 4 5d 6 7 8 9

Na(N2H4)3.0Fe2Se2 Na(C2H8N2)1.5Fe2Se2 Na(C3H10N2)1.5Fe2Se2 Na(C2H8N2)1.5Fe2Se2 Na(NH3)xFe2Se2 Sr0.5(N2H4)3.0Fe2Se2 Sr0.5(PDA)1.5Fe2Se2 Sr0.5(N2H4)xFe2Se2 Sr0.5(EDA)xFe2Se2

373 373 363 as prepared as prepared 353 373 303 as prepared

Na0.82(N2H4)0.45Fe1.95Se2.00 Na0.84(C2H8N2)0.31Fe1.88Se2.00 Na0.85(C3H10N2)0.42Fe1.84Se2.00 Na0.93(C2H8N2)0.73Fe1.89Se2.00 Na0.80(NH3)0.60Fe1.86Se2.00 Sr0.22(N2H4)0.54Fe1.82Se2.00 Sr0.23(C3H10N2)0.17Fe1.79Se2.00 Sr0.23(N2H4)1.18Fe1.85Se2.00 Sr0.28(C2H8N2)0.53Fe1.86Se2.00

0.895 0.951 1.000 1.116 0.871 0.891 0.874 1.144 1.105

42 46.5 45.5 45 45 35 35.5 33.5 37

∼13 ∼10 ∼11 ∼10 ∼80 ∼10 ∼10 ∼10 ∼11

a The chemical composition is the average of 10−16 recorded data, and the error range was within 1%. bInterlayer spacing between the nearest Fe layers. cShielding volume fraction determined from M(T) curves in ZFC mode. dData from ref 12.

3, respectively, in Table 1), indicating the increase in d value with carbon number. These d values are greater than that (0.871 nm) for the NH3-poor (NH3-monolayer) phase of Na/ NH3-FeSe.12 The steric structures and sizes of N2H4 and EDA are summarized in Figure S1 of the Supporting Information, in which the N−H, C−H, N−N, and C−N bond lengths in N2H4, EDA, and PDA are ∼0.10, ∼0.10, ∼0.14, and ∼0.15 nm, respectively. On the basis of the van der Waals dimension, the molecular sizes of N2H4 and EDA in the lengthwise direction (perpendicular to the N···N direction) are almost the same (∼0.2 nm), although the lateral sizes of two amines are different [0.29 nm for N2H4 vs 0.59 nm for EDA (see Figure S1)]. There are two points to be noted with respect to the variation of d values. First, on the basis of the small difference in d (0.02−0.13 nm) between the NH3 monolayer phase of Na/ NH3-FeSe and evacuated phases of Na/DA-FeSe, we can safely conclude that the intercalated diamines for the evacuated phase of Na/DA-FeSes form a monolayer in the FeSe interlayer. Note that little is known about the orientation of intercalated DAs. Second, the d value of the as-prepared EDA-intercalated phase was 1.116 nm, which is larger by 0.22 nm than that of the evacuated phases of Na/EDA-FeSe. This difference in d is very close to the size in lengthwise direction thickness (∼0.2 nm) of EDA, as explained above and shown in Figure S1 of the Supporting Information. It is highly likely that the intercalated EDA forms a bilayer in the interlayers, as in the case of NH3rich FeSe-based intercalates.12,20 It is worth noting that the d value for the EDA-bilayer phase is the largest value among the Na/(DA,NH 3 )-intercalated FeSe family reported thus far.11,12,20 Sr/DA-intercalated FeSe samples (Sr/DA-FeSes) were prepared and characterized, as well. Figure 2 shows the PXRD patterns of Sr/DA-FeSes. For comparison, Sr/NH3FeSe was prepared, as well (Figure 2c). The preparation conditions are summarized in entries 6−9 of Table 1. The degassing treatment yielded single evacuated phases of Sr/DAFeSes (entries 6−8). The broad peak around 10° for the evacuated Sr/N2H4-FeSe was due to the several orientations of N2H4 in the interlayers (Figure 2a). In contrast, the single phase of EDA-intercalated FeSe was obtained without evacuation treatment. The d values for the evacuated Sr/ N2H4-FeSe at room temperature and the as-prepared Sr/EDAFeSe were 1.105 and 1.144 nm, respectively (entries 8 and 9, respectively). Again, these values were greater by ∼0.25 nm than that for the evacuated Sr/N2H4-FeSe at 353 K. When the steric size of N2H4 and EDA shown in Figure S1 of the Supporting Information is taken into account, the introduced

Figure 2. PXRD patterns of evacuated (a and d) Sr/N2H4-, (b) Sr/ C3H10N2-, and (c) Sr/NH3-intercalated FeSe and that of (e) asprepared Sr/C2H8N2-FeSe. The preparation conditions for these intercalates are summarized in entries 6−9 of Table 1, in which the evacuation temperatures for samples a and d are 353 and 298 K, respectively. Asterisks denote diffraction of basal FeSe planes. Broad peaks for sample a were due to the several orientations of hydrazine.

N2H4 and EDA could form a bilayer state in the interlayers again. The chemical formulas SrxDAyFe2−zSe2 of prepared intercalates are listed in entries 6−9 of Table 1. The x values corresponding to the Sr content were 0.22−0.28 irrespective of the type of DA used as a molecular spacer. These values are ∼30% of those for Na/DA-intercalated samples (0.82−0.93) mainly because of the difference in the charge state [Na(I) vs Sr(II)]. The y values varied significantly from 0.17 to 1.18 for the Sr/DA-FeSes, which was attributed to the difference in the preparation conditions, especially the evacuation temperature. The z values in SrxDAyFe2−zSe2 were 0.14−0.21 (entries 6−9 of Table 1), indicating very few Fe vacancies. From these characterization results, it is observed that the interlayer distances (d) for both FeSe intercalates were tuned from 0.87 to 1.11 nm without significant changes in Na, Sr, and Fe content. This result presents a good platform for studying simply the effect of interlayer spacing on the superconductivity of electron-doped FeSe-based SCs without a change in the electron doping levels. C

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Figure 3. Magnetization curves of two series of (a) Na/DA- and (b) Sr/DA-intercalated FeSe samples at H = 10 Oe. The properties of samples are listed in Table 1.

Magnetic Properties of Na/DA- and Sr/DA-FeSe Intercalates. Figure 3a shows the magnetic susceptibility 4πχ(T) curves of Na/DA-FeSes at H = 10 Oe with zero-field cooling (ZFC) and field cooling (FC) modes in the lower temperature range. Sharp superconducting transitions were observed, with Tc values between 42 and 47 K. Figure 3b shows the magnetic susceptibility 4πχ(T) curves of the Sr/DA-FeSes, and again, sharp superconducting transitions were observed. The Tc values for the Sr/DA-FeSe samples ranged from 34 to 37 K, which are significantly lower than those in the Na/DAFeSe samples. On the other hand, the superconducting volume fractions (SVFs) of Na/DA- and Sr/DA-intercalated FeSes were 10−13%, according to ZFC measurements, and there were large positive values at the normal state. These positive values accompanying low SVFs suggest that there could be some magnetic impurities originating from Fe species extracted from the samples during the intercalation process or negligible impurity phase induced by the thermal treatment. This behavior has also been observed in (Li,Na)/EDA-intercalated FeSe.11e,21 The correlations between the Tc determined from the 4πχ(T) curves and interlayer distance d for Na/DA- and Sr/ DA-FeSes are shown in Figure 4. The most striking feature in

intercalates showed that Tc increases with increasing d and then is saturated above a dsat of ∼0.9 nm, although the plot numbers in the publication are scarce (two samples).21 Because the d values for as-prepared and evacuated phases of Na/DA- and Sr/ DA-FeSe samples are similar to or larger than dsat (Figure 4), the study presented here clearly indicates that there is a threshold value dsat above which the Tc will be insensitive to the d values of FeSe SCs. Very recently, theoretical calculations by Guterding et al. also report that the Fermi surfaces (FSs) of Li0.5(NH3)Fe2Se2 with a c of 0.81 nm and Li0.5(NH3)2Fe2Se2 with a c of 1.03 nm are completely two-dimensional and there are no qualitative differences in superconducting pairing strengths for a c axis length between 0.81 and 1.03 nm.22 Thus, it suggests that once the large interlayer distance is beyond dsat (∼0.9 nm), the interlayer interaction becomes very small and the dispersions of bands along the c axis will be negligible; i.e., the FSs are nearly two-dimensional. This kind of FS shape has already been optimal for Tc, and a larger interlayer distance d will not further enhance Tc.22 Another noticeable point in Figure 4 is the difference in Tc of the two series of FeSe-based intercalates. The Tc values of the Sr/DA-FeSes were ∼10 K smaller than those of the Na/DAFeSes. This significant difference in Tc could be due to variation in the degree of electron doping for both series, judging from similar interlayer spacing d. Such phenomena have indeed been observed in K/NH3 co-intercalated FeSe samples,11f although there is an opposite trend of dependencies of Tc on carrier doping level between K/NH3- and (Na/Sr)/DA-FeSe systems. In the former, K0.3(NH3)yFe2Se2, K-intercalated compounds with the lower charge, shows a Tc (∼44 K) higher than that of the more carrier-doped compounds K0.6(NH3)yFe2Se2 (Tc ∼ 30 K).11f The difference in the trend of Tc dependencies between the two systems may be due to the different d values (0.74− 0.78 nm) of K/NH3-FeSes11f versus those (greater than dsat) of the present Na/DA- and Sr/DA-FeSes. Because the former members have smaller d values (0.74−0.78 nm < dsat) falling into the range in which Tc still increases with d,21 the higher K doping level could enhance the interlayer interaction of FeSe layers and thereby result in a lowering of Tc. On the other hand, in the latter cases, the d values of Na/ DA- and Sr/DA-FeSes are larger than dsat (∼0.9 nm). Recent theoretical density functional theory calculation clearly indicates that in this region (d > dsat) Tc is mainly controlled by the electron doping level when the Fermi surface is mostly two-dimensional as a result of a large interlayer distance.22 The source of the Tc enhancement with electron doping relies on an increased density of states at the Fermi level and enhanced

Figure 4. Correlation between Tc and interlayer distance d between FeSe layers for the Na/DA- and Sr/DA-FeSe intercalates.

the figure is that the Tc values are insensitive to the d values for both FeSe intercalates embedded with linear DAs with different carbon numbers. This feature is distinctly different from the case in cuprate SCs in which the Tc values increase with increasing interlayer distances.19 This result is also different from that in FeSe-based SCs with small d values in which the Tc values also increase with the value of d.11d On the other hand, the recent study by Noji and co-workers on the FeSe-based D

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larger than a certain threshold value dsat (∼0.9 nm), Tc will be insensitive to d. This peculiar insensitivity could be due to the negligible dispersions of bands along the c axis, i.e., nearly twodimensional Fermi surface when d > dsat as demonstrated in the recent theoretical study.22 Because the Fermi surface shape would be optimal for Tc, a larger d will not enhance Tc further in the molecular spacer intercalated FeSe system. The higher Tc (41−46 K) of Na/DA-FeSes may be due to the increased density of states at the Fermi level and enhanced superconducting pairing strength, resulting from the heavier electron doping. With respect to phase stability, the Na/EDA- and Sr/N2H4FeSe samples showed outstanding structural stability compared to those of the Na/NH3-FeSe and Li/NH3-FeSe intercalates. In particular, the structures of Sr/N2H4-FeSe were kept even after vacuum degassing treatment (