DOI:
10.1039/C5RA22619H
(Paper)
RSC Adv., 2016,
6, 8277-8281
Synthesis, crystal structure and physical properties of FeV4S8 and KFe2V8S16†
Received
28th October 2015
, Accepted 11th January 2016
First published on 14th January 2016
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.
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 (V1−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
![image file: c5ra22619h-f1.tif](/image/article/2016/RA/c5ra22619h/c5ra22619h-f1.gif) |
| 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) AxV6S8. (e) V5S8. (f) AxV5S8. (g) VS2. (h) AxVS2. | |
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 AxV5Q8 (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 Kα 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 (Å3) |
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 F2 |
R(I > 2σ(I)) |
0.0308 |
0.0161 |
wR2 (all data) |
0.0642 |
0.0423 |
GOF |
1.074 |
1.102 |
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 |
|
|
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 Kα 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.
![image file: c5ra22619h-f2.tif](/image/article/2016/RA/c5ra22619h/c5ra22619h-f2.gif) |
| 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).
![image file: c5ra22619h-f3.tif](/image/article/2016/RA/c5ra22619h/c5ra22619h-f3.gif) |
| 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 KxV5S8 (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 KxV5S8). 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 (TN). In addition, the V3S4 and V5S8 order antiferromagnetically below TN = 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 (μeff) can be evaluated from the following equation:
. The derived effective magnetic moments are 1.13 μB. 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.
![image file: c5ra22619h-f4.tif](/image/article/2016/RA/c5ra22619h/c5ra22619h-f4.gif) |
| 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
![image file: c5ra22619h-f5.tif](/image/article/2016/RA/c5ra22619h/c5ra22619h-f5.gif) |
| 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).
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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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