Synthesis, crystal structure and physical properties of FeV₄S₈ and KFe₂V₈S₁₆

Abstract

Two compounds with the formulae FeV₄S₈ and KFe₂V₈S₁₆ were successfully synthesized via a melting salt method. The FeV₄S₈ crystallizes in the defective NiAs-like structure type of a monoclinic I2/m space group, while the KFe₂V₈S₁₆ belongs to the pseudo-hollandite chalcogenide family and crystallizes in a monoclinic C2/m space group. The structure of the FeV₄S₈ is composed of [V₄S₈]³⁻ layers which are connected by [FeS₆]⁹⁻ octahedra to form a 3D extended framework. The structure of KFe₂V₈S₁₆ is composed of a [Fe₂V₈S₁₆]⁻ one-dimensional (1D) tunnel-type framework, where the Fe atoms partially occupy the V1 site. In the KFe₂V₈S₁₆ compound, the K⁺ ions reside in the tunnels. Magnetic measurements show that the two compounds are both paramagnetic at high temperature. Weak ferromagnetic contributions are observed at low temperature for both of the compounds. The resistivity of FeV₄S₈ is measured to be 5.1 × 10⁻² Ω cm at room temperature. With the decrease in temperature, the compound shows a clear metal-to-insulator phase transition at 163 K.

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Introduction

Transition metal chalcogenides have aroused more and more interest owing to their various structures and abundant physical properties including thermoelectricity,^1–3 magnetoresistance,^4–6 superconductivity,⁷ as well as their potential applications in photovoltaics,⁸ battery electrodes,⁹ and catalysts.¹⁰ Many of the transition metal chalcogenides have a NiAs-type or metal deficient NiAs-type structure.^11–13 For instance, the VS compound belongs to the NiAs structure type with the space group P6₃/mmc (Fig. 1a).¹⁴ The V sites in the VS structure can be partially replaced by vacancies (□) to produce the metal deficient NiAs-type structures (V_1−xS). There are a series of vanadium sulfides including V₇S₈ (x = 1/8, Fig. 1b),¹⁵ V₃S₄ (x = 1/4, Fig. 1c),¹⁶ V₅S₈ (x = 3/8, Fig. 1e),¹⁷ and VS₂ (x = 1/2, Fig. 1g).¹⁸

Fig. 1 Crystal structures of NiAs and defective NiAs. (a) VS. (b) V₇S₈, only half of the unit cell is shown. The V sites with 75% occupation are highlighted by blue. (c) V₃S₄. (d) A_xV₆S₈. (e) V₅S₈. (f) A_xV₅S₈. (g) VS₂. (h) A_xVS₂.

These vanadium sulfides are very interesting due to their various physical properties.^19–21 It is believed that the 3d-electrons, which vary widely, ranging from itineration to localization, are responsible for the metallic conductivity and magnetism of these compounds.^12,22 Besides, the localization of the 3d electrons can be changed by varying the composition.²² Therefore, varying the compositions and 3d-electron configurations are of scientific significance not only for exploring new materials with unique physical properties, but also for understanding the electron correlations in these narrow band metals. In addition, the physical properties of this type of materials can be further tuned by intercalating other metal ions into the defective sites in the metal deficient NiAs-type structure.^23,24 There exist the A_xV₅Q₈ (A = alkali metal, alkaline-earth metal, and Tl; Q = S, Se, Te) which belongs to the pseudo-hollandite chalcogenide family.^25,26 The structure is composed of a V₅Q₈ one-dimensional (1D) tunnel-type framework and A ions located in the tunnels (Fig. 1f). The intercalated A ions act as electron donors which also can change the 3d-electron configurations, hence they could lead to fantastic transport and magnetic properties.^27,28 Furthermore, the substitution of the V sites in the pseudo-hollandite chalcogenides by other transition metal ions is an easy way to change the 3d-electron configurations. Besides, it is believed that the introduction of other components can lead to the formation of structural/functional units, which is beneficial for multi-functional device applications.^29–32

Hence, in this work, we presented two new sulfides FeV₄S₈ and KFe₂V₈S₁₆, which were synthesized by melting salt method. The FeV₄S₈ compound, which belongs to the defective NiAs-type structure, crystalizes in a monoclinic I2/m space group. The KFe₂V₈S₁₆ compound belongs to the pseudo-hollandite chalcogenides family, whose structure is composed of a [Fe₂V₈S₁₆]⁻ one-dimensional (1D) tunnel-type framework and K⁺ ions residing in the tunnels. However, the variation of compositions and intercalation of different transition or alkali metal ions into the valium sulfides not only induce the structure change from V₅S₈ but also significantly change the 3d configurations, leading to the variation of their physical properties. Both the FeV₄S₈ and KFe₂V₈S₁₆ compounds show Curie–Weiss behavior at high temperature and weak ferromagnetic ordering at low temperature. Temperature dependent resistivity measurements indicate that the FeV₄S₈ compound is a metal at room temperature. A metal-to-insulator transition occurred at 163 K. The physical properties of the two compounds are quite different from the parent compound V₅S₈. Therefore, the syntheses of the two new compounds are important for basic research in this area.

Experimental

Synthesis of FeV₄S₈ and KFe₂V₈S₁₆ single crystals

Single crystals of KFe₂V₈S₁₆ were prepared via melting salt method. Starting materials of K₂S powder (99.7%, Alfa), Fe powder (99.98%, Alfa), V powder (99.9%, Alfa), S powder (99.5%, SCRC), and KI powder (99%, SCRC) were used without further treatment. To synthesize the KFe₂V₈S₁₆ single crystals, a starting material of K₂S, Fe, V, S, and KI were grounded uniformly in the molar ratio of 1 : 4 : 16 : 32 : 100. Then the mixture was loaded into a silica tube, followed by flame-sealing under vacuum (10⁻³ mbar). The tube was slowly heated to 1073 K, and was held at this temperature for 3 days. Afterwards, the tube was slowly cooled to 873 K at the ratio of 3 K h⁻¹. Finally, the furnace was turned off to cool the tube to room temperature. The melt was washed and sonicated by water for several times, and the obtained black crystals were dried by acetone. To synthesize the FeV₄S₈ single crystals, similar procedure was performed, while the molar ratio of starting material of Fe, V, S, and KI was 4 : 16 : 32 : 100.

Single crystal X-ray diffraction

Suitable crystals were chosen to perform the data collections. Single crystal X-ray diffraction was performed on a Bruker D8QUEST diffractometer equipped with Mo K_α radiation. The diffraction data were collected at room temperature by the ω- and φ-scan methods. The crystal structures were solved and refined using APEX2 program.³³ Absorption corrections were performed using the multi-scan method (SADABS).³⁴ The detailed crystal data and structure refinement parameters are summarized in Table 1. Selected bond lengths are summarized in Table 2.

Table 1 Crystal data and structure refinement parameters for FeV₄S₈ and KFe₂V₈S₁₆ crystal

Formula	FeV4S8	KFe2V8S16
Fw (g mol^−1)	516.09	1071.28
T (K)	293(2)	295(2)
λ (Å)	0.71073	0.71073
Crystal system	Monoclinic	Monoclinic
Space group	I2/m	C2/m
a (Å)	7.9000(11)	17.465(5)
b (Å)	6.6425(11)	3.293(1)
c (Å)	8.0327(11)	8.473(3)
α (deg.)	90	90
β (deg.)	91.202(5)	103.82(1)
γ (deg.)	90	90
V (Å^3)	421.43(11)	473.2(3)
Z	2	1
ρ (g cm^−3)	4.067	3.759
μ (mm^−1)	67.221	7.215
F(000)	492	511
R(int)	0.0507	0.0245
Refinement method	Full-matrix least-squares on F^2
R(I > 2σ(I))	0.0308	0.0161
wR^2 (all data)	0.0642	0.0423
GOF	1.074	1.102

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Table 2 Selected bond distances (Å) of FeV₄S₈ and KFe₂V₈S₁₆

FeV4S8		KFe2V8S16
Fe(1)–S(2)	2.3678	V(1)|Fe1–S(1)	2.2795
Fe(1)–S(3)	2.4055	V(1)|Fe1–S(1)	2.3331
Fe(2)–S(1)	2.2824	V(1)|Fe1–S(4)	2.5079
Fe(2)–S(2)	2.2995	V(1)|Fe1–S(3)	2.5119
Fe(2)–S(3)	2.481	V(2)–S(3)	2.3819
V(1)–S(1)	2.3119	V(2)–S(2)	2.4005
V(1)–S(2)	2.3771	V(3)–S(3)	2.2897
V(1)–S(3)	2.4447	V(3)–S(2)	2.3136
V(2)–S(1)	2.3116	V(3)–S(4)	2.4812
V(2)–S(2)	2.3699/2.4065
V(2)–S(3)	2.4267

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Characterization

The obtained crystals were investigated with a JEOL (JSM6510) scanning electron microscope equipped by energy dispersive X-ray spectroscopy (EDXS, Oxford Instruments). Powder X-ray diffraction of the FeV₄S₈ samples were collected on a Bruker D8QUEST diffractometer equipped with mirror-monochromated Mo K_α radiation.

Physical property measurements

For resistivity measurement the compact sample was obtained by sintering polycrystalline pellet at 500 °C for 20 h in vacuumed silica tube. The temperature dependence of the resistivity was measured using the standard four-probe technique by the ETO model on the Physical Property Measurement System (PPMS, Quantum Design, DynaCOOL). The temperature dependence of magnetization was measured both under zero-field-cooled (ZFC) and field-cooled (FC) models in magnetic fields of 1000 Oe, 5000 Oe and 10 000 Oe from 2 K to 300 K on PPMS. The field dependence of magnetization was measured at 2 K, 10 K, and 300 K under the applied magnetic field from −2 T to 2 T.

Results and discussion

Synthesis and crystal structure description

Melting salt method is a good way to grow well defined single crystals at relatively low temperature. The SEM of the as-synthesized FeV₄S₈ and KFe₂V₈S₁₆ single crystals are shown in Fig. 2a and c. In order to check the homogeneity of their compositions, elemental analyses of the crystals were performed (Fig. 2b and d). The Fe/V/S ratio is 1/3.8/7.7, while the K/Fe/V/S ratio is 1/1.8/7.5/15.1.

Fig. 2 SEM images of the FeV₄S₈ (a) and KFe₂V₈S₁₆ (c) crystal. EDX spectra of the FeV₄S₈ (b) and KFe₂V₈S₁₆ (d) crystals.

The FeV₄S₈ compound, which belongs to the metal-deficient NiAs structure type, crystallizes in a monoclinic space group I2/m (Fig. 3a). FeV₄S₈ contains two independent Fe sites (2d, 4e), two independent V sites (4i and 4g), and three independent S sites (4i, 4i, and 8j). Both Fe and V locate in the octahedral coordination environments. The structure of FeV₄S₈ compound is constructed by [V₄S₈]³⁻ layers (Fig. S1a in ESI†) which are connected by [FeS₆]⁹⁻ octahedra. The occupations of the two Fe sites are 89% (Fe1) and 11% (Fe2), respectively. Therefore, 37.5% octahedral sites remain unoccupied. The average distance of Fe–S is 2.3673(3) Å, while the Fe–S distance in the reported structures is 2.453 Å in FeS, and 2.277 Å in Ba₂FeS₃. The average V–S distance is 2.3799(3) Å, comparable to that in V₅S₈ (2.390 Å).

Fig. 3 (a) Crystal structure of FeV₄S₈ view along the b axis. (b) Crystal structure of KFe₂V₈S₁₆ view along the b axis.

The KFe₂V₈S₁₆ has the similar structure with the pseudo-hollandite chalcogenide K_xV₅S₈ (Fig. 3b). The structure of KFe₂V₈S₁₆ is composed of a [Fe₂V₈S₁₆]⁻ one-dimensional (1D) tunnel-type framework and K⁺ ions residing in the tunnels (Fig. 3b). The structure contains three independent V sites (V1: 4i; V2: 2d; V3: 4i), in which the V1 site is half replaced by Fe atoms. The [Fe₂V₈S₁₆]⁻ framework has the [V₈S₁₆]⁷⁻ layers (Fig. S1b in ESI†) which have similar structure with the [V₄S₈]³⁻ layers in FeV₄S₈ compound. However, the [V₈S₁₆]⁷⁻ layers are connected by [Fe/VS₃]³⁻ double chains but not [FeS₆]⁹⁻ octahedra. The average V–S distance is 2.360 Å, which is comparable to that in the reported structure (2.40 Å in K_xV₅S₈). In addition, the average V1/Fe1–S bond length is 2.41 Å, slightly larger than the average distance of V–S (2.36 Å).

The valance state of the V1 atoms in the V₅S₈ is +3, while the average valance of the V2 and V3 is +3.25. From the point view of crystal structure, the average V1–S distance is 0.233 Å similar with the average distance of V2–S and V3–S. For the FeV₄S₈ compounds, the average Fe–S and V–S distances are 2.3673(3) Å and 2.3799(3) Å, respectively, indicating the similar radii of Fe and V ions. Therefore, we can assume the valance state of Fe ions is also +3 (3d⁵ configuration).

Magnetic properties

Vanadium sulfides that belong to the defective NiAs type compounds are of significance due to their abundant physical properties, such as the itinerant antiferromagnetism, and considerably localization antiferromagnetism. It is reported that the VS and V₃S₄ show weak temperature-independent paramagnetism, whereas V₅S₈ shows Curie–Weiss behavior above the Neel temperature (T_N). In addition, the V₃S₄ and V₅S₈ order antiferromagnetically below T_N = 8 and 32 K. It is believed that the tunable physical properties of this series of vanadium sulfides are due to the change of 3d electron configurations. Therefore, further tuning the 3d electron configurations in this system may produce intriguing physical properties. The reasonable way to change the 3d electron configurations is intercalation of other 3d metal ions or alkali metal ions. The physical properties of the two compounds are quite different from the parent compound V₅S₈. By intercalation of Fe ions, the antiferromagnetic ordering disappear, while the weak ferromagnetic ordering shows up at low temperature.

Single crystals of FeV₄S₈ are picked manually for the magnetic susceptibility measurements. Fig. 4a shows the zero-field cooled (ZFC) and field-cooled (FC) curves measured at the magnetic fields of 1000 Oe. Obviously, the FeV₄S₈ compound shows clear Curie–Weiss behavior in the whole temperature range. Curie–Weiss fitting of the magnetic susceptibility yields values of Curie constant C and Weiss constant θ are 0.16 emu K mol⁻¹ and −2.1 K, respectively. The effective magnetic moments (μ_eff) can be evaluated from the following equation: . The derived effective magnetic moments are 1.13 μ_B. The negative value of the Weiss constant indicates that there are weak ferromagnetic interactions at low temperature. Besides, by increasing external magnetic field, the temperature dependent magnetic susceptibility disobeys the Curie–Weiss law at low temperature (Fig. S2 in ESI†). In addition, separations between ZFC and FC curves are also observed at low temperature which can further confirm the existence of weak ferromagnetic contributions. The hysteresis loops of FeV₄S₈ compound are shown in Fig. 4b. At high temperature, the M–H curves show linear dependence, which is consist with the paramagnetic behavior. However, the M–H curves bend slightly at 2 K, implying the weak ferromagnetic contributions. It is known that the V₅S₈ have an antiferromagnetic phase transition at 32 K, which is due to the itineration of 3d electrons. The absence of antiferromagnetic ordering might due to the random distribution of Fe atoms which break down the itineration of electrons.

Fig. 4 (a) Temperature-dependence of the magnetization of the FeV₄S₈ compound. (b) Magnetic hysteresis of the FeV₄S₈ compound at 2 K, 10 K and 300 K. (c) Temperature-dependence of the magnetization of the KFe₂V₈S₁₆ compound. Inset: the inverse magnetic susceptibility vs. temperature plot. The blue line is the linear fit of the magnetic susceptibility data from 300 K to 50 K. (d) Magnetic hysteresis of the KFe₂V₈S₁₆ compound at 2 K, 10 K and 300 K.

The temperature dependent magnetic susceptibility of KFe₂V₈S₁₆ is shown in Fig. 4c. At high temperature, the KFe₂V₈S₁₆ compound also shows Curie–Weiss behavior. However, a separation of ZFC and FC shows up at low temperature, which indicates the presence of ferromagnetic interactions. The M–H curves of KFe₂V₈S₁₆ also bend slightly at 2 K, implying the weak ferromagnetic contributions. The high temperature susceptibility of KFe₂V₈S₁₆ is fitted to Curie–Weiss law (Fig. 4c inset). The obtained C and θ is 3.9 emu K mol⁻¹ and −295 K, respectively.

Electrical transport properties

Phase purity of the FeV₄S₈ compact disk was checked by powder X-ray diffraction, as shown in Fig. 5a. All the peaks can be indexed, indicating high degree of purity crystallinity. The electrical transport property of the FeV₄S₈ disk is depicted in Fig. 5b. In the high temperature region, the resistivity decreases with the deceasing temperature, which indicates the metallic behavior of the FeV₄S₈ disk. The room temperature resistivity is 0.024 Ω cm. The resistivity increases sharply at 163 K, which demonstrates the metal-to-insulator phase transition. Therefore, the substitution of V1 by Fe atoms significantly changes the electrical transport properties from the V₅S₈ which shows metallic conductivity in the whole temperature range (300–2 K). Besides V₅S₈, many other vanadium sulfides, including VS, V₃S₄, and VS₂, also show metallic behaviors. However, the FeV₄S₈ has a metal-to-insulator phase transition around 163 K which is rare in this series of compounds. The magnetic resistances of the FeV₄S₈ are also measured under different external magnetic field (Fig. 5b and c). At low temperature, the FeV₄S₈ disk shows slightly negative magnetoresistance with ∼0.9% of resistance reduction. However, the positive to negative transition in the magnetoresistance that occurred in V₅S₈ at 4.2 K is not observed in our case, indicating the absence of spin flopping transition.³⁵

Fig. 5 (a) Powder X-ray diffraction of the FeV₄S₈ disk. (b) Temperature-dependent resistivity of the FeV₄S₈ disk under different external magnetic field. (c) Magnetic field dependence of the magnetoresistance of the FeV₄S₈ disk.

Conclusions

In summary, we successfully synthesized two compounds, namely the FeV₄S₈ and KFe₂V₈S₁₆, via melting salt method. The FeV₄S₈ crystallizes in the defective NiAs-like structure type of a monoclinic I2/m space group, while the KFe₂V₈S₁₆ belongs to the pseudo-hollandite chalcogenide family. The structure of FeV₄S₈ features 3D extended framework, which is composed of [V₄S₈]³⁻ layers and [FeS₆]⁹⁻ octahedra. The structure of KFe₂V₈S₁₆ is composed of a [Fe₂V₈S₁₆]⁻ one-dimensional (1D) tunnel-type framework. Different from the FeV₄S₈ framework, the [Fe₂V₈S₁₆]⁻ in KFe₂V₈S₁₆ is composed of [V₈S₁₆]⁷⁻ layers and [Fe/VS₃]³⁻ double chains. The two compounds are both paramagnetic at high temperature, and possess weak ferromagnetic contribution at low temperature. Electrical transport and magnetoresistance properties of the FeV₄S₈ were investigated with the room temperature resistivity of 5.1 × 10⁻² Ω cm. Besides, the FeV₄S₈ compound also shows clear metal-to-insulator phase transition at 163 K.

Acknowledgements

This work was financially supported by Innovation Program of the CAS (Grant KJCX2-EW-W11), “Strategic Priority Research Program (B)” of the Chinese Academy of Sciences (Grants XDB04040200), NSF of China (Grants 91122034, 51125006, 51202279, 61376056, 21201012, 51402341 and 51402335).

Notes and references

1. G. Chen, M. S. Dresselhaus, G. Dresselhaus, J. P. Fleurial and T. Caillat, Int. Mater. Rev., 2003, 48, 45–66.
2. G. D. Mahan, J. Appl. Phys., 1989, 65, 1578–1583.
3. G. J. Snyder and E. S. Toberer, Nat. Mater., 2008, 7, 105–114.
4. R. Xu, A. Husmann, T. F. Rosenbaum, M. L. Saboungi, J. E. Enderby and P. B. Littlewood, Nature, 1997, 390, 57–60.
5. Y. Q. Guo, J. Dai, J. Y. Zhao, C. Z. Wu, D. Q. Li, L. D. Zhang, W. Ning, M. L. Tian, X. C. Zeng and Y. Xie, Phys. Rev. Lett., 2014, 113, 157202.
6. T. Block and W. Tremel, J. Alloys Compd., 2006, 422, 12–15.
7. J. D. Weiss, C. Tarantini, J. Jiang, F. Kametani, A. A. Polyanskii, D. C. Larbalestier and E. E. Hellstrom, Nat. Mater., 2012, 11, 682–685.
8. S. E. Habas, H. A. S. Platt, M. F. A. M. van Hest and D. S. Ginley, Chem. Rev., 2010, 110, 6571–6594.
9. Y. H. Liao, K. S. Park, P. H. Xiao, G. Henkelman, W. S. Li and J. B. Goodenough, Chem. Mater., 2013, 25, 1699–1705.
10. V. Iliev, L. Prahov, L. Bilyarska, H. Fischer, G. Schulz-Ekloff, D. Wohrle and L. Petrov, J. Mol. Catal. A: Chem., 2000, 151, 161–169.
11. K. Motizuki, J. Magn. Magn. Mater., 1987, 70, 1–7.
12. S. P. Farrell and M. E. Fleet, Phys. Chem. Miner., 2001, 28, 17–27.
13. M. Y. C. Teo, S. A. Kulinich, O. A. Plaksin and A. L. Zhu, J. Phys. Chem. A, 2010, 114, 4173–4180.
14. M. Knecht, H. Ebert and W. Bensch, J. Alloys Compd., 1997, 246, 166–176.
15. S. Brunie, M. Chevreto and J. M. Kauffman, Mater. Res. Bull., 1972, 7, 253.
16. I. Kawada, M. Nakanoonoda, M. Ishii, M. Saeki and M. Nakahira, J. Solid State Chem., 1975, 15, 246–252.
17. I. Kawada, M. Nakanoonoda, M. Ishii and M. Saeki, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr., 1975, 31, S68.
18. K. Friese, O. Jarchow and K. Kato, Z. Kristallogr., 1997, 212, 648–655.
19. S. Funahashi, H. Nozaki and I. Kawada, J. Phys. Chem. Solids, 1981, 42, 1009–1013.
20. A. Fujimori, M. Saeki and H. Nozaki, Phys. Rev. B: Condens. Matter Mater. Phys., 1991, 44, 163–169.
21. Y. Kitaoka and H. Yasuoka, J. Phys. Soc. Jpn., 1980, 48, 1949–1956.
22. A. Muller, R. Sessoli, E. Krickemeyer, H. Bogge, J. Meyer, D. Gatteschi, L. Pardi, J. Westphal, K. Hovemeier, R. Rohlfing, J. Doring, F. Hellweg, C. Beugholt and M. Schmidtmann, Inorg. Chem., 1997, 36, 5239–5250.
23. Y. Sahoo and A. K. Rastogi, Phys. B, 1995, 215, 233–242.
24. M. A. Ruman and A. K. Rastogi, J. Phys. Chem. Solids, 2003, 64, 77–85.
25. S. Petricek, H. Boller and K. O. Klepp, Solid State Ionics, 1995, 81, 183–188.
26. K. D. Bronsema, R. Jansen and G. A. Wiegers, Mater. Res. Bull., 1984, 19, 555–562.
27. W. Bensch and E. Worner, Solid State Ionics, 1992, 58, 275–283.
28. W. Bensch, E. Worner, M. Muhler and U. Ruschewitz, Eur. J. Solid State Inorg. Chem., 1993, 30, 645–658.
29. X. Zhang, J. He, W. Chen, K. Zhang, C. Zheng, J. Sun, F. Liao, J. Lin and F. Huang, Chem.–Eur. J., 2014, 20, 5977–5982.
30. X. Zhang, W. Chen, D. J. Mei, C. Zheng, F. H. Liao, Y. T. Li, J. H. Lin and F. Q. Huang, J. Alloys Compd., 2014, 610, 671–675.
31. X. Zhang, Q. Wang, Z. Ma, J. He, Z. Wang, C. Zheng, J. Lin and F. Huang, Inorg. Chem., 2015, 54, 5301–5308.
32. S. Meng, X. Zhang, G. H. Zhang, Y. M. Wang, H. Zhang and F. Q. Huang, Inorg. Chem., 2015, 54, 5768–5773.
33. B. Optics, Chem. Eng. News, 2014, 16.
34. A. B. Bruker, Zh. Obshch. Khim., 1957, 27, 2593–2598.
35. H. Nozaki and Y. Ishizawa, Phys. Lett. A, 1977, 63, 131–132.

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Footnote

† Electronic supplementary information (ESI) available: The [V₄S₈]³⁻ layers in FeV₄S₈ and [V₈S₁₆]⁷⁻ layers in KFe₂V₈S₁₆ crystal structures. M–T curves of FeV₄S₈ single crystals measured at different magnetic field. CCDC 1407280 and 1432650. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra22619h

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