Effects of Iron Doping on the Physical Properties of Quaternary

Article pubs.acs.org/IC

Effects of Iron Doping on the Physical Properties of Quaternary Ferromagnetic Sulfide: Ba2Fe0.6V1.4S6 Kejun Bu,† Jianqiao He,† Xiaofang Lai,*,§ Changsheng Song,† Dong Wang,† JiJian Xu,† Sishun Wang,† and Fuqiang Huang*,†,‡ †

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China ‡ State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China § School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou 510006, P. R. China S Supporting Information *

ABSTRACT: The mixed-metal sulfide compound with the formula Ba2Fe0.6V1.4S6 was successfully synthesized via solid-state reaction. Ba2Fe0.6V1.4S6 has a quasi-one-dimensional structure and crystallizes in the hexagonal space group P63/mmc. The structure is composed of face-sharing anion octahedron [MS6]8− (M = V or Fe) units to construct infinite chains along the c axis, in which the Fe atoms randomly occupy the V sites. The Ba2+ ions reside between adjacent chains. Magnetic susceptibility measurements reveal a transition between paramagnetism and ferromagnetism around 25 K. The small polaron hopping (SPH) conduction behavior has been observed in the higher temperature region (75−300 K), while in the lower temperature region (25−74 K), the resistivity features a variable range hopping mechanism (VRH). The analysis of density of states indicates that Fe-3dz2 and S-3p states mainly dominate the valence band maximum, while Fe-3dz2 states contribute significantly to the magnetic susceptibility.

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INTRODUCTION Transition metal chalcogenides have aroused much attention due to their novel physical properties such as superconductivity,1−3 magnetoresistance,4,5 and thermoelectricity,6−8 and their potential use in battery electrodes,9−11 catalysis,12,13 and photovoltaics.14,15 The NiAs-type (space group P63/mmc) or metal deficient NiAs-type structure is one of the most common structures in the transition metal chalcogenides.16,17 The vanadium sulfides belong to the NiAs-type structure, and vacancies can partially substitute the V sites to produce the metal deficient NiAs-type structure (V1−xS).17 Therefore, there are a series of vanadium sulfides that have been explored, including VS(x = 0),17 V3S4(x = 1/4),18 V5S8(x = 3/8),18 and VS2(x = 1/2)19 (shown in Figure 1). Vanadium sulfide derived compounds have aroused much more interest because of their novel physical properties.20,21 Due to variations widely ranging from itineration to localization, 3d electrons contribute much to the magnetic property © 2017 American Chemical Society

and metallic conductivity of vanadium sulfide derived compounds.22 Furthermore, varying the structure of compounds is an effective way to change the configuration of the 3d electrons, which may result in new materials with unique physical properties.22 Hence, in order to obtain materials with special physical properties, intercalating alkali metal ions or doping other 3d metal ions in the deficient NiAs-type structure are always adopted. The 3d electron configurations can be changed via intercalating alkali metal ions which act as electron donors,23−25 as shown in Figure 1. Besides, replacing of the V sites by other 3d metal ions is also effective in varying the 3d electron configurations.26 Hence, we use solid-state methods to synthesize novel physical properties of Ba2Fe0.6V1.4S6 compounds which have a ferromagnetic phase transition at 25 K. The Ba2Fe0.6V1.4S6 Received: April 19, 2017 Published: July 5, 2017 8302

DOI: 10.1021/acs.inorgchem.7b00960 Inorg. Chem. 2017, 56, 8302−8310

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Inorganic Chemistry

Figure 1. Crystal structures of NiAs-type and metal deficient NiAs-type in vanadium sulfides. temperature by the ω- and φ-scan methods. The crystal structures were solved and refined using APEX3 program.28 Absorption corrections were performed using the multiscan method (ASDABS). The detailed crystal data and structure refinement parameters are summarized in Table 1. Fractional atomic coordinates parameters, atomic displacement parameters, and geometric parameters are summarized in Tables S1−S3 in Supporting Information. Characterization. Inductively coupled plasma mass spectrometry (ICP-MS) was used to determine the Fe/V element ratio in the compound. The obtained crystals were investigated with a JEOL (JSM6510) scanning electron microscope equipped with energy dispersive X-ray spectroscopy (EDXS, Oxford Instruments). Powder X-ray diffraction data of the Ba2Fe0.60V1.40S6 samples were collected on a Bruker D8QUEST diffractometer equipped with mirror-monochromated Cu Kα radiation (λ = 0.15406 nm). The patterns were recorded in a slow-scanning mode with 2θ from 5° to 100° at a scan rate of 6°/ min. Simulated patterns were generated by using the FULLPROF program29 and CIF file of the refined single-crystal structure. The solid-state ultraviolet−visible (UV−vis) light diffuse-reflectance spectra of the fine powders of Ba2Fe0.6V1.4S6 were measured on a UV-4100 spectrophotometer operating from 1200 to 350 nm at room temperature. Physical Properties Measurements. Temperature variation of the resistance, R(T), was measured using the standard four-probe technique with the Resistivity model and collected on a Physical Properties Measurement System (PPMS, Quantum Design). For the electric properties measurements, powders were ground and pressed into disks, followed by calcination at 573 K for 5 h. Silver paste was applied which acts as the contact electrode. Magnetic properties were measured on the PPMS. Temperature-dependent direct-current (dc) magnetic susceptibility (M−T) curve of the powder sample was measured from 2 to 300 K under the magnetic field of 1 T in the zerofield-cooling (ZFC) and field-cooling (FC) processes. The field dependence of magnetization was measured at 2, 10, 30, 50, 100, 200, and 300 K under the applied magnetic field from −1 to 1 T. X-ray Photoelectron Spectroscopy. The XPS spectra of the compound were obtained with an Axis Ultra spectrometer to

compound is isostructural with BaVS3, the structure of which is composed of a [VS6]8− one-dimensional structure with Ba2+ ions in the tunnels. V and Fe occupy the same sites. The intercalation of the Ba2+ ions and doping of the Fe ions into the vanadium sulfides not only induce structure change but also significantly vary the 3d electron configuration, which cause the variation of magnetic properties. The Ba2Fe0.6V1.4S6 compound shows weak ferromagnetism at low temperature (2−25 K) and obeys Curie−Weiss behavior at high temperature. The SPH conduction behavior has been observed in the higher temperature region (75−300 K), while the resistivity follows a VRH model in the lower temperature region (25−74 K). Density functional theory (DFT) calculations reveal that Fe3dz2, S-3p states contribute to the VBM, while Fe-3dz2 states contribute to the magnetic susceptibility. The physical properties of the Ba2Fe0.6V1.4S6 compound are quite different from those of the parent BaVS3, implying that it is worthy for us to explore these compounds in this area.27

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EXPERIMENTAL SECTION

Synthesis. The compound Ba2Fe0.6V1.4S6 was prepared by use of solid-state reaction. A combination of the pure elements, V powder (99.999%, Alfa Aesar Puratronic), Fe powder (99.999%, Alfa Aesar Puratronic), S powder (99.999%, Alfa Aesar Puratronic), and BaS powder (99.999%, Alfa Aesar Puratronic) were mixed in a fused silica tube in an V:Fe:S:BaS molar ratio of 0.70:0.30:2:1. The tube was evacuated to 0.1 Pa, sealed, and heated gradually (60 K h−1) to 1173 K, where it was kept for 2 days. The tube was cooled to 773 K at a rate of 3 K h−1 and then quenched to room temperature. The crystals are stable in air and alcohol. Single-Crystal X-ray Crystallography. Suitable crystals were chosen to perform the data collections. Single-crystal X-ray diffraction was performed on a Bruker D8QUEST diffractometer equipped with Mo Kα radiation. The diffraction data were collected at room 8303


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(VASP).30−32 The Perdew−Burke−Ernzehof (PBE) version of the generalized gradient approximation (GGA)33 was used to describe the exchange correlation functional, and the projector augmented wave (PAW) method was used in the present work. Here the cutoff energy of the plane wave was chosen at 400 eV. For the structure optimizations, 6 × 6 × 6 k-points were used for the conventional cell. The convergence criteria are that the changes in total energies between two successive electronic steps are less than 10−5 eV, and all the Hellmann−Feynman force acting on each atom is less than 0.01 eV/Å.

Table 1. Crystal Data and Structure Refinement for Ba2Fe0.60V1.40S6 chemical formula Mr cryst syst, space group a, b, c (Å)

Ba2Fe0.60V1.40S6 571.85 hexagonal, P63/mmc 6.7246(8), 6.7246(8), 5.6159(8) 219.93(6) 1 Mo Kα black 4.318 12.58 0.025 × 0.020 × 0.005

V (Å3) Z radiation type cryst color ρc (g cm−3) μ (mm−1) cryst size (mm3) data collection diffractometer Bruker D8 QUEST absorption correction multiscan Tmin, Tmax 0.84, 0.94 no. of measured, indep, and obsd [I > 2σ(I)] 3371, 93, 81 reflns 0.037 Rint (sin θ/λ)max (Å−1) 0.594 refinement R[F2 > 2σ(F2)], wR(F2), GOF 0.021, 0.050, 1.22 no. reflns 93 no. params 11 Δρmax, Δρmin (e Å−3) 0.720, −0.40

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RESULTS AND DISCUSSION Synthesis and Crystal Structure Description. Using a solid-state reaction, we synthesized a new compound, Ba2Fe0.6V1.4S6. A scanning electron microscopy (SEM) image of well-defined Ba2Fe0.6V1.4S6 crystal is presented in Figure 2a. The presence of Ba, Fe, V, and S is confirmed by semiquantitative energy dispersive X-ray analysis (EDX) as shown in Figure 2b,c. The Ba/Fe/V/S ratio is 1/0.27/0.68/ 3.19 for Ba2Fe0.6V1.4S6 in Table 2, which is in accord with the Table 2. EDX Results of Ba2Fe00.60V1.40S6

investigate the oxidation states of the Fe, V, and S. The spectra were collected for the C 1s, Fe 2p, V 2p, and S 2s regions, and the binding energies were corrected against the C 1s reference of 284.8 eV. Electronic Structure Calculation. DFT calculations were performed using the Vienna Ab Initio Simulation Package

element

wt %

atomic %

SK VK Fe K Ba M total

35.36 11.99 5.24 47.40 100.00

62.06 13.25 5.28 19.42 100.00

ICP-MS results Fe/V = 0.31(2)/0.72(3). The phase purity of the Ba2Fe0.6V1.4S6 crystals is examined by powder X-ray diffractometer (XRD). The measured pattern of the compound

Figure 2. (a) SEM images of the Ba2Fe0.6V1.4S6 crystal. (b) EDX spectra of the Ba2Fe0.6V1.4S6. (c) Element mapping of Ba, V, Fe, and S in Ba2Fe0.6V1.4S6 compound. (d) Powder XRD pattern of Ba2Fe0.6V1.4S6. The simulated pattern is obtained by the FULLPROF program. 8304


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Figure 3. (a) [MS6]8− chains observed in the one-dimensional structure of the Ba2Fe0.6V1.4S6 compound along the c axis. (b) The structure of Ba2Fe0.6V1.4S6 viewed along the c axis. (c) Coordination polyhedra of the cations in Ba2Fe0.6V1.4S6.

Figure 4. (a) Zero-field-cooling (ZFC) and field-cooling (FC) magnetization curves and the inverse magnetic susceptibility with applied field of 1 T for the Ba2Fe0.6V1.4S6 compound. (b) Temperature derivative of the magnetic susceptibility, T2dχ/dT. Anomalies are seen at 25 K. (c) Magnetic hysteresis of the Ba2Fe0.6V1.4S6 compound at 2, 10, 30, 50, 100, 200, and 300 K. (d) The zoomed magnetic hysteresis of the Ba2Fe0.6V1.4S6 compound at 2, 10, 30, 50, 100, 200, and 300 K.

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Figure 5. (a) Temperature dependence of electrical resistivity for the Ba2Fe0.6V1.4S6 compound. (b) Plots of ln(R/T) vs T−1. (c) Plots of −ln(R) vs T−1/4.

as the dominant oxidation state with corresponding doublets at 711.06 and 724.31 eV, respectively.36 Therefore, the title compound can be denoted as [Ba2+][(V0.44+/V0.35+/Fe0.33+)4+][S2−]3. Magnetic Properties. Vanadium sulfide derived compounds are of significance because of their interesting magnetic properties, such as the considerably localized ferromagnetism, and the itinerant antiferromagnetism.37 It is reported that V3S4 and VS show weak temperature-independent paramagnetism,21,38 while V5S8 shows an antiferromagnetic ordering below the Neel temperature (TN).20 In addition, quasi-onedimensional compound BaVS3 has semiconductor character with high conductivity and has a transition between paramagnetism and antiferromagnetism at 70 K.27 The observed antiferromagnetism in BaVS3 has been explained as the gradual appearance of a regular arrangement of electrons in magnetic V ions,25 or gradual electrons which transfer from localized magnetic states to nonmagnetic band states.39 Owing to a change in 3d electron configurations, vanadium sulfide derived compounds can be tuned to have intriguing magnetic properties. Intercalation of alkali or doping of another 3d metal is one of the reasonable ways to modify the 3d electron configurations. By doping Fe ions, Ba2Fe0.6V1.4S6 shows quite different magnetism from the isostructural compound BaVS3, with antiferromagnetic ordering disappearing and weak ferromagnetic ordering observed at low temperature. Several single crystals of Ba2Fe0.6V1.4S6 are manually picked to test the magnetic properties. No significant differences between zero-field-cooling (ZFC) and field-cooling (FC) curves are observed at the magnetic field of 1 T in Figure 4a.

matches well with the simulated one obtained from singlecrystal data, with only small amounts of Ba2S3 and BaS impurities (Figure 2d and Figure S7 in Supporting Information). Note that the pattern of Ba2Fe0.6V1.4S6 slightly shifts to lower diffraction angles compared to that of BaVS3, due to the increase of the lattice constants. The title compound, Ba2Fe0.6V1.4S6, is isostructural with BaVS3 and crystallizes in a hexagonal space group P63/mmc (Figure 3a). Ba2Fe0.6V1.4S6 contains one independent V/Fe site (2a), one independent S site (6h), and one independent Ba site (2c). The structure consists of face-sharing anion octahedron [MS6]8− (M = V or Fe) units to construct infinite chains along the crystallographic c axis. The adjacent chains are separated by Ba2+ ions to form quasi-one-dimensional structures (Figure 3b). The V and Fe atoms possess 6-fold coordination with S atoms and randomly occupy the M sites. The occupations of the M sites are 70% (V) and 30% (Fe), respectively. The average distance of the M−S bond length is 2.389(3) Å, while the M−S (M = V or Fe) distance in the reported structure is 2.277 Å in Ba2FeS3, 2.385 Å in BaVS3, and 2.453 Å in FeS (Figure 3c). The average V−V distance is 2.8080(4) Å, compared to that for BaVS3 (2.84 Å), suggesting a weak interaction for V−V bonding. The XPS V 2p spectra and Fe 2p spectra are shown in Figure S1 in Supporting Information. The observed V 2p peaks are usually very broad and, in most case, can be deconvoluted into two peaks. The peaks with binding energies of 517.19 and 525.40 eV correspond to the 2p3/2 and 2p1/2, respectively, of the V5+ oxidation states.34 The peaks at 516.90 and 523.80 eV may be attributed to V4+ ions.35 Besides, Fe3+ ions are detected 8306


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Figure 6. (a) Electronic band structure of Ba2Fe0.6V1.4S6. (b) Total DOS of Ba2Fe0.6V1.4S6. (c) Partial DOS of Ba2Fe0.6V1.4S6. (d) Partial DOS of Fe3d orbitals. The inset illustrates splitting of the Fe-3d orbitals in a nonperfect octahedral crystal field.

Electrical Transport Properties. Temperature-dependent resistance of the Ba2Fe0.6V1.4S6 compound is shown in Figure 5a. The resistance increases with the decrease of the temperature, indicating the semiconducting behavior of the compound. The room temperature resistivity of compound is 1.040 Ω cm. The resistivity of the Ba2Fe0.6V1.4S6 compound increases sharply at the whole range of temperature, while the temperature-dependent resistance of BaVS3 sharply increases from 70 to 2 K.27 Therefore, the doping Fe atoms significantly change the electrical transport properties, compared to BaVS3. The resistivity of the Ba2Fe0.6V1.4S6 measured under different external magnetic fields (Figure 5a) suggests no magnetoresistance effect over the whole temperature range. It is known that the semiconducting transport can be described via the small polaron hopping (SPH) model and variable range hopping (VRH) model.40,41 According to polaron hopping conduction, the electrical resistivity satisfies the Arrhenius-like

The compound partly shows Curie−Weiss behavior at 25−100 K, and Curie−Weiss fitted curves deviate far from the experimental data near 25 K (Curie constant C = 0.78 emu K mol−1 and Weiss constant θ = −7.80 K) (Figure S6 in Supporting Information), while the compound obeys the Curie−Weiss law between 100 and 300 K. According to Curie−Weiss fitting of the magnetic susceptibility between 100 and 300 K, the value of Curie constant C is 1.46 emu K mol−1, and the Weiss constant θ is −99.28 K, respectively. From the equation μeff = 8C μB, the effective magnetic moments are calculated as 3.42 μB. The total theoretical magnetic moments are calculated from μeff = [0.8 μeff(V4+)2 + 0.6 μeff(Fe3+)2]1/2μB, where Fe3+ ions may exist predominantly in the intermediatespin state (S = 3/2).44 The theoretical effective magnetic moments μeff are 3.87 μB which is close to the experimental results. The magnetic interactions present antiferromagnetic ordering at this temperature range due to the negative value of the Weiss constant. From the dχ/dT × T2 plot in Figure 4b, we can see pronounced jumps at 25 K, suggesting that weak ferromagnetic ordering occurs below 25 K. The hysteresis loops of Ba2Fe0.6V1.4S6 compounds are shown in Figure 4c,d. The M− H curves display a linear field dependence above 25 K which is consistent with the paramagnetic behavior, and bend slightly at 10 and 2 K, suggesting the weak ferromagnetic ordering. It is known that BaVS3 has an antiferromagnetic phase transition at 70 K, caused by itinerant 3d electrons of V.27 At low temperature, the random distribution of Fe atoms may be gradually localized and break down the itineration 3d electrons of V−V pairs, which cause the absence of antiferromagnetic ordering.

Ep

( ), where E

relation, namely ρT = ρ0 exp

KBT

p

is the total

activation energy of a polariton. The experimental resistivity data matches well with the SPH model in the temperature range 75−300 K, rather than over the whole temperature range investigated. Likewise, the compound follows the VRH model in the lower temperature region, and the resistivity can be T0 1/4 , T

( )

expressed as ρT = ρ0 exp

where T0 =

β . κg (μ)a3

g(μ) is

the density of states at the Fermi level; a is the localization radius of states near the Fermi level, and β is a numerical coefficient. As shown in Figure 5c, the aforementioned plots for the compound present straight lines between 25 and 74 K, which satisfy the VRH in the low temperature region. The 8307


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Inorganic Chemistry activation energy ε0(T) can be expressed as ε0 = ε0(T ) =

(kT )3/4 [g (u)a3]1/4

=

1 k T (T /T )−3/4 . 4 B 0 0

contribution of the localized Fe-3d electrons and part of broken itineration V-3d electrons at low temperature.

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This expression

CONCLUSIONS In summary, we have successfully synthesized, via a solid-state reaction method, a new compound Ba2Fe0.6V1.4S6, which crystallized in the hexagonal P63/mmc space group. The structure of Ba2Fe0.6V1.4S6 is composed of face-sharing anion octahedron [MS6]8− (M = V or Fe) units to construct infinite chains along the c axis. Also, the Ba2+ ions reside between adjacent chains. Magnetic susceptibility measurements reveal a transition between paramagnetism and ferromagnetism around 25 K. The SPH conduction mechanism has been observed in the higher temperature region (75−300 K), while the resistivity follows a VRH model in the lower temperature region (25−74 K). The analysis of DOS indicates that the valence band maximum consists mainly of Fe-3dz2 and S-3p states, while Fe3dz2 states contribute much to the magnetic susceptibility.

justifies the initial assumption that the width of the band responsible for conduction is very narrow at low temperatures. The activation energy equals ε0(T) in order of magnitude, and it monotonically decreases as T3/4 with decreasing temperature.45 Therefore, activation energy values are 93.03 meV at 74 K and 41.2 meV at 25 K, respectively. Electronic Structure Calculation. DFT calculations of the Ba2Fe0.6V1.4S6 are performed to understand the electronic band structure of these materials. The band structure and total density of states (DOS) of Ba2Fe0.6V1.4S6 are shown in Figure 6a,b. The results show that the Ba2Fe0.6V1.4S6 compound has a semiconductor character with a direct bandgap of 0.06 eV, which fits well with all-optical absorption without a significant band feature in the UV−vis spectrum (Figure S2 in Supporting Information). We see that the electronic bands near the valence band maximum (VBM) in the ground-state structure of Ba2Fe0.6V1.4S6 have a nearly zero dispersion along the G(000)−K(1/31/30) direction but strong dispersion along K(1/31/30)−L(1/201/2) and G(000)−A(001/2) directions in Figure 6a, indicating small effective masses and high electron eτ mobility along the c axis (μ = mcn* ). Therefore, the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00960. Bond distances/angles, atomic coordinates, XPS, UV− vis, and electronic structure of Ba2Fe0.6V1.4S6 (PDF)

cn

Ba2Fe0.6V1.4S6 compound has a feature of the one-dimensional electronic structures.42 The experimental magnetic properties are explained by analysis of the total and partial density of states in Figure 6b,c. Total and partial DOS of Ba2+ ions (Figure S3 in Supporting Information) make little contribution to the VBM and conductive band minimum (CBM). Fe-3d, S-3p states contribute to the VBM, while the CBM is mainly composed of V-3d and S-3p states. For a perfect octahedra of the V/Fe environment, the c/a ratio equals 0.816. However, in the Ba2Fe0.6V1.4S6 compound c/a equals 0.835. This octahedral distortion makes the fundamental d wave functions split into a fundamental eg doublet and a higher energy singlet dz2, corresponding to two orbitals extending principally in the x− y plane and one orbital extending essentially along the c axis, respectively.39 As shown in Figure S5 in Supporting Information and Figure 6c, we can see V-3d states form a wide band around CBM and Fe-3dz2 absolutely dominate VBM around EF under the Jahn−Teller effect. It is proposed that the antiferromagnetic ordering of BaVS3 might be due to delocalization of the 3d state and formation of vanadium pairs (V↑−V↓).43 The electrons in the Fe-3dz2 band are essentially localized and break down the itineration electrons of V−V chains (Figure 6c), contributing very much to the magnetic susceptibility. Moreover, we get similar conclusions allowing for spin polarization (Figure S4 in Supporting Information). We can see not only Fe-3d states showing a rather large spin split of VBM, but also V-3d states which exhibit a large spin split in CBM. At high temperature, Fe−Fe shows antiferromagnetic ordering in the Ba 2 Fe 0.6 V 1.4 S 6 compound, and ferromagnetism is on coercively, while at low temperature the Fe-3d electrons are gradually localized and break down the itineration electrons of V−V pairs. Likewise, part of the itineration V-3d electrons become localized and contribute to ferromagnetic properties due to Fe randomly substituting V sites along the chains in low temperature. Therefore, the compound shows a weak ferromagnetism with relative low coercivity and saturation moment due to the

Accession Codes

CCDC 1555940 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Fuqiang Huang: 0000-0001-7727-0488 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research project was financially supported by “Strategic Priority Research Program (B)” of the Chinese Academy of Sciences (Grant XDB04040200), CAS Center for Excellence in Superconducting Electronics, National Key Research and Development Program (Grant 2016YFB0901600), NSF of China (Grants 11404358 and 51402341), and Science and Technology Commission of Shanghai (Grants 13JC1405700 and 14520722000).

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REFERENCES

(1) Lai, X.; Zhang, H.; Wang, Y.; Wang, X.; Zhang, X.; Lin, J.; Huang, F. Observation of Superconductivity in Tetragonal FeS. J. Am. Chem. Soc. 2015, 137, 10148−10151. (2) Takahashi, H.; Sugimoto, A.; Nambu, Y.; Yamauchi, T.; Hirata, Y.; Kawakami, T.; Avdeev, M.; Matsubayashi, K.; Du, F.; Kawashima, C.; Soeda, H.; Nakano, S.; Uwatoko, Y.; Ueda, Y.; Sato, T. J.; Ohgushi, K. Pressure-induced superconductivity in the iron-based ladder material BaFe2S3. Nat. Mater. 2015, 14, 1008−1012.

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