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Structural and magnetic ordering in the VxNb1+yS2 system
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Journal of Physics and Chemistry of Solids 69 (2008) 49–54 www.elsevier.com/locate/jpcs
Structural and magnetic ordering in the VxNb1+yS2 system Makoto Nakanishia,, Yukiyasu Matsunoa, Tatsuo Fujiia, Jun Takadaa, Kazuyoshi Yoshimurab a
Department of Material Chemistry, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan b Division of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa Oiwake-cho, Kyoto 606-8502, Japan Received 5 December 2006; received in revised form 20 July 2007; accepted 22 July 2007
Abstract The influence of composition on the structural ordering and magnetism in the VxNb1+yS2 system has been investigated by X-ray diffraction and magnetic measurements. Stoichiometric V1/3NbS2 did not exhibit the structural ordering of vanadium between the NbS2 layers. In the ordered structure, the vanadium composition deviated from the ideal value of x ¼ 13 to both higher and lower values, while the niobium composition was in the range of 0.05pyp0.18. Excess niobium, y40, is thought to play an essential role in the structural ordering in this system. For samples with excess niobium and ordered structures, a magnetic transition was observed at 20–50 K, depending on the composition. The spontaneous magnetization of 3–5 103 mB/V atom is thought to be intrinsic to this system. The magnetization curves consisted of a constant and a proportional parts of the magnetic field, which correspond to the spontaneous magnetization and high-field susceptibility, respectively. The magnetization curves and the temperature dependencies of the high-field susceptibility were quite similar to those of the canted antiferromagnetic NiS2. A correlation between the structural and magnetic ordering is suggested. r 2007 Elsevier Ltd. All rights reserved. Keywords: A. Chalcogenides; A. Magnetic materials; C. X-ray diffraction; D. Crystal structure; D. Magnetic properties
1. Introduction Layered transition metal chalcogenides, for example, Ba(Co,Ni)S2y [1] and Tl(Co,Ni)2S2 [2], exhibit both metallic conductivity and magnetic ordering and have been investigated from the point of view of comparison with high-Tc superconductors and other strongly correlated electron systems. NbS2 is a typical two-dimensional conductor that exhibits superconductivity at 6.2 K [3]. The structure of this compound consists of NbS2 layers, each niobium ion being surrounded by six sulfur atoms in a trigonal prismatic arrangement. The NbS2 layers are bonded by van der Waals forces, and various ions or compounds can intercalate between the layers. When a 3d-transition metal intercalates into the van der Waals gap, Corresponding author. Tel.: +81 86 251 8107; fax: +81 86 251 8087.
E-mail address: [email protected] (M. Nakanishi). 0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2007.07.124
metallic conductivity and a variety of magnetism are realized [4–8]. It was revealed by means of susceptibility [4] and electrical conductivity measurements [5] that V1/3NbS2 exhibits metallic conductivity and ferromagnetic ordering at 55 K. The structure of V1/3NbS2 is composed of alternative stacking of vanadium layers and NbS2 blocks. Vanadium occupies one-third of the octahedral sites between the NbS2 layers in an ordered arrangement, as shown in Fig. 1(a). The magnetism of this system is unusual. The temperature dependence of magnetic susceptibility increases abruptly at 50 K with decreasing temperature, but decreases gradually below 50 K. The magnetism in the system is attributed to vanadium. The effective moment in the paramagnetic state is estimated to be approximately 2.9 mB/V atom; hence, vanadium is expected to be trivalent with S ¼ 1. However, the magnitude of the spontaneous magnetization appears to be almost two orders smaller than that expected from
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Fig. 1. Schematic illustration of the crystal structure of V1/3NbS2 composed of NbS2 trigonal prismatic layers and vanadium ions. The figure also shows the ordered (a) and random (b) arrangements of vanadium ions between these layers.
S ¼ 1 and that of Mn1/3NbS2 [4]. It is expected that the magnetic structure in this system is as complicated as that in Cr1/3NbS2, which exhibits a helical spin ordering [9]. On the other hand, according to another report, there is no magnetic transition in V1/3NbS2 down to 2 K [6]. Most of the vanadium sulfides such as V3S4 [10], V5S8 [11], and NaVS2 [12], order antiferromagnetically; however, this might be one of those few systems in which vanadium orders ferromagnetically, as in BaVS3–d [13]. The MnxNbS2 system is reported to have a wide composition range (0.19pxp0.52) and exhibits a temperature dependence of susceptibility similar to that of ferromagnetic V1/ 3NbS2 in the composition range of 0.4pxp0U45 [14]. It is possible that the metal compositions in V1/3NbS2 are nonstoichiometric and that the magnetic ordering in V1/3NbS2 has some correlation with this. Hence, it is interesting to study the effects of vanadium and niobium compositions on magnetism. We investigated the VxNb1+yS2 system to understand the ordered state of V1/3NbS2 and the influence of compositions on the magnetism in this system. 2. Experimental procedure Appropriate amounts of vanadium powder (99.9%), niobium powder (99.9%), and crystalline sulfur (99.999%) were mixed and sealed in an evacuated silica tube and heated at 1000 1C for 3 days. Then, the mixture was heated at 800 1C for 3 days to promote the structural ordering, and
quenched in water. Samples obtained from mixture were characterized by powder X-ray diffraction measurements using Cu Ka radiation. The heat treatment was repeated until the X-ray diffraction profiles remained unchanged. The temperature dependencies of the magnetic susceptibility from 5 to 300 K were measured by using a SQUID magnetometer (Quantum Design MPMS2) under a field of 100 Oe after zero-field cooling. The field dependencies of magnetization up to 10 kOe were also measured at various temperatures ranging from 5 to 55 K. 3. Results and discussion 3.1. Structural ordering Initially, VxNbS2 was synthesized in the range of 0.2pxp0.4, changing the number of vanadium atoms between the NbS2 blocks. Though niobium oxide NbO2 was detected in some samples as an impurity by the X-ray diffraction measurements, almost all the samples were single phase. However, the X-ray diffraction profiles were different from those reported previously [4,6]. Vanadium occupies one-third of the octahedral sites in order, whereas in our synthesized samples, the octahedral sites were randomly occupied, as shown in Fig. 1(b). The lattice constants of the samples changed with the vanadium content x. V1/3NbS2–d samples (0.2pdp0.3) were synthesized to clarify the effects of deviation from the stoichiometry of
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Fig. 3. Variation of lattice constants a and c for V1/3NbS2–d samples. Fig. 2. X-ray diffraction profiles of V1/3NbS2–d (VxNb1+yS2) samples—a: d ¼ 0 (x ¼ 0.33, y ¼ 0); b: d ¼ 0.1 (x ¼ 0.35, y ¼ 0.05); c: d ¼ 0.2 (x ¼ 0.37, y ¼ 0.11); and d: d ¼ 0.3 (x ¼ 0.39, y ¼ 0.18). The marked peaks correspond to the superlattice reflections.
sulfur on the constituent phases and crystal structure. Fig. 2 shows the X-ray diffraction profiles for these samples. No impurity was detected in any of the samples, and ordered peaks were observed in the composition range of 0.10pdp0.20. Small amounts of sulfur remained in the silica tubes, and the lattice constants of the samples were unchanged in the case of do0, as shown in Fig. 3. In the case of d40, the c-axis monotonically increased with d. On the other hand, the a-axis increased abruptly at d ¼ 0.05 and then decreased at d ¼ 0.2. So far, there are no reports which state that excess S2 are intercalated between the NbS2 blocks, as (M,S)xNbS2. Usually, all the sulfur ions arrange themselves into closely packed layers, and the metal ions M occupy the sites between these layers, as shown in the case of Vx(Nb,V)S2 [15]. In the case of niobium sulfides Nb1+xS2 [15], niobium occupies the octahedral sites between the NbS2 block layers as NbxNbS2, instead of forming vacancies in the sulfur sites as NbS2–d. We first described the chemical formula of this system as V1/3NbS2–d. The existence of sulfur vacancies was not investigated directly, e.g., by the Reitveld analysis, but it appears that the excess niobium occupies the octahedral sites between the NbS2 layers as (Nb,V)xNbS2 when d40. Our results apparently indicated that vanadium ions could hardly occupy the trigonal prismatic sites between the sulfur layers and that the excess sulfur was excluded from the samples when do0.
Thus, we described the above-mentioned formula as VxNb1+yS2 (yX0) thereafter. The irregular change in the a-axis with the amount of excess niobium was similar to the case of MnxNbS2 [14]; however, we could not reveal the reasons for the same. The structural ordering in the VxNb1+yS2 system was investigated systematically by changing the compositions of vanadium and niobium. It was revealed by X-ray diffraction measurements that NbO2 was present in some samples, but almost monophasic samples of VxNb1+yS2 were obtained. These results are summarized in Fig. 4. From the results, we see that the ordered structures exist over a wide range of composition. In the ordered structures, the vanadium composition deviated from the ideal value of x ¼ 13 to both higher and lower values, whereas the niobium composition was in the range of 0.05pyp0.18. From the above considerations, the excess niobium as well as vanadium occupies the octahedral sites between the NbS2 layers in the ordered arrangement; moreover, the excess niobium is thought to play an essential role in the structural ordering in this system. 3.2. Magnetic ordering The susceptibilities of the samples with y ¼ 0 in VxNb1+yS2 obeyed the Curie–Weiss law between 5 and 300 K and did not exhibit magnetic transitions. Their Weiss temperatures were low, e.g., 3 K for x ¼ 0.33, and the effective moments were estimated to be 2.1–2.5 mB, which is less than 2.93–2.97 mB [4]. The values of the effective moments indicate that vanadium exists in divalent or trivalent states. Our results indicate that VxNbS2 samples
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Fig. 4. Phase diagram for structural ordering in the VxNb1+yS2 system. Open squares and filled triangles indicate ordered and random arrangements of vanadium ions, respectively, as shown in Fig. 1.
Table 1 Magnetic parameters in the VxNb1+yS2 system x
y
Y (K)
Tt (K)
meff (mB/V)
S
0.20 0.27 0.30 0.33 0.40 0.35 0.36 0.37
0 0 0 0 0 0.05 0.08 0.11
2 4 3 3 6 23 41 32
– – – – – 49 43 25
2.5 2.4 2.4 2.3 2.1 2.6 2.5 2.4
0.8 0.8 0.8 0.8 0.7 0.9 0.9 0.8
in the composition range 0.2pxp0.4 are in the paramagnetic state down to 5 K, which differs from the previous results [4]. The Weiss temperature Y, effective moment meff, and spin quantum number S estimated from the effective moments are summarized in Table 1. For samples with excess niobium, VxNb1+yS2 (y40), and ordered structures, the susceptibilities increased abruptly at 20–50 K depending on their compositions. Fig. 5 shows the temperature dependencies of the susceptibilities of these samples, in comparison with that of paramagnetic V1/3NbS2. The behavior is similar to that of the ferromagnetic samples reported previously, except for the decreasing tendency of the susceptibility with decreasing temperature. The Weiss temperatures of the samples were negative in the range from 23 to 41 K, and the effective moments in the paramagnetic state were 2.3–2.6 mB. Moreover, the spontaneous magnetization at
Fig. 5. Temperature dependencies of magnetization for VxNb1+yS2 samples under a field of 100 Oe—a: x ¼ 0.33, y ¼ 0; b: x ¼ 0.35, y ¼ 0.05; and c: x ¼ 0.37, y ¼ 0.11. Transition temperatures determined by the temperature dependences of spontaneous magnetization are indicated.
5 K was 0.003–0.005 mB/V atom. The small spontaneous magnetization might be attributed to magnetic impurities. However, NbO2 included in some samples is paramagnetic [16] and ferromagnetic or ferrimagnetic phases in V–Nb–S or V–Nb–O systems have not been reported yet. The transition temperatures depended on the compositions, as shown in Fig. 5. Hence, the small spontaneous magnetization should be intrinsic to this system. The Weiss temperature, transition temperature Tt determined from the temperature dependencies of spontaneous magnetization, effective moment, and spin quantum number in the VxNb1+yS2 (y40) system are also summarized in Table 1. The transition temperature decreased with increase in the amount of excess niobium between the NbS2 blocks. The differences between the present and previous results [4,6] may be related to the structural ordering and/or compositional deviations due to the difference in the preparation techniques. It was revealed by magnetization measurements below the transition temperature that the magnetization curves consisted of a constant value M0 and a proportional part of the magnetic field wH, i.e., M ¼ M0+wH, at higher fields, as shown in Fig. 6(a). These values are thought to correspond to the spontaneous magnetization and the high-field susceptibility, respectively. The temperature dependencies of M/H and w estimated from the
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investigate the origin of these magnetic behaviors in more detail by microscopic methods such as NMR measurements. The MnxNbS2 system has been extensively investigated, especially with regard to the compositional dependencies of the magnetic and electric properties [14]. In the range xX0.4, the magnetic susceptibility of this system is quite similar to that of the ordered phase of VxNb1+yS2; thus, the same magnetic interactions and ordering states may also exist in MnxNbS2. 4. Conclusion The effect of composition on the structural ordering and magnetism in the VxNb1+yS2 system has been investigated by X-ray diffraction and magnetic measurements. Excess niobium is thought to play an essential role in the structural ordering in this system. For samples with excess niobium and ordered structures, magnetic transitions and spontaneous magnetization of 3–5 103 mB/V atom have been observed. The magnetization curves and temperature dependences of high-field susceptibility are quite similar to those of the canted antiferromagnetic NiS2. Acknowledgment We are grateful to Prof. M. Fukuhara, Okayama University of Science, for providing assistance in performing the magnetic measurements. References
Fig. 6. Magnetization curves up to 10 kOe (a) and temperature dependencies of M/H at 100 Oe and high field susceptibility w (b) for V0.35Nb1.05S2.
magnetization curves are shown in Fig. 6(b). The magnetization curves and temperature dependencies of the high-field susceptibility are similar to those of NiS2, which exhibits canted antiferromagnetic ordering [17]. The negative Weiss temperatures estimated by fitting the susceptibilities also indicate the existence of antiferromagnetic interactions in the structurally ordered phase. We will
[1] L.S. Martinson, J.W. Schweitzer, N.C. Baenziger, Metal-insulator transitions in BaCo1xNixS2y, Phys. Rev. Lett. 71 (1993) 125–128. [2] G. Huan, M. Greenblatt, K.V. Ramanujachary, Magnetic ordering in metallic TlCo2xNixS2 (0pxp2.0) with the ThCr2Si2 structure, Solid-State Commun. 71 (1989) 221–228. [3] M.H. van Maaren, G.M. Schaeffer, Superconductivity in group Va dichalcogenides, Phys. Lett. 20 (1966) 131. [4] S.S.P. Parkin, R.H. Friend, 3d transition-metal intercalates of the niobium and tantalum dichalcogenides I. Magnetic properties, Philos. Mag. B 41 (1980) 65–93. [5] S.S.P. Parkin, R.H. Friend, 3d transition-metal intercalates of the niobium and tantalum dichalcogenides II. Transport properties, Philos. Mag. B 41 (1980) 95–112. [6] F. Hulliger, E. Pobitschka, On the magnetic behavior of new 2H–NbS2-type derivatives, J. Solid State Chem. 1 (1970) 117–119. [7] J.M. van den Berg, P. Cossee, Structural aspects and magnetic behavior of NbS2 and TaS2 containing extra metal atoms of the first transition series, Inorg. Chim. Acta 2 (1968) 143–148. [8] K. Anzenhofer, J.M. van den Berg, P. Cossee, J.N. Helle, The crystal structure and magnetic susceptibilities of MnNb3S6, FeNb3S6, CoNb3S6 and NiNb3S6, J. Phys. Chem. Solids 31 (1970) 1057–1067. [9] T. Miyadai, K. Kikuchi, H. Kondo, S. Sakka, M. Arai, Y. Ishikawa, Magnetic properties of Cr1/3NbS2, J. Phys. Soc. Jpn. 52 (1983) 1394–1401. [10] Y. Kitaoka, H. Yasuoka, Y. Oka, K. Kosuge, S. Kachi, Observation of the antiferromagnetic order in metallic compounds V3S4 and V3Se4, J. Phys. Soc. Jpn. 46 (1979) 1381–1382.
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[11] A.B. De Vries, C. Haas, Magnetic susceptibility and nuclear magnetic resonance of vanadium sulfides, J. Phys. Chem. Solids 34 (1973) 651–659. [12] G.A. Wiegers, C.F. Van Bruggen, Magnetic and structural phase transitions in sodium intercalates NaxVS2 and NaxVSe2, Physica B&C 86–88 (1977) 1009–1011. [13] O. Massenet, R. Buder, J.J. Since, C. Schlenker, J. Mercier, J. Kelber, D.G. Stucky, BaVS3, a quasi one dimensional ferromagnet or antiferromagnet depending on stoichiometry, Mater. Res. Bull. 13 (1978) 187–195.
[14] K. Miwa, H. Ikuta, H. Hinode, T. Uchida, M. Wakihara, Synthesis and physical properties of MnxNbS2, J. Solid State Chem. 125 (1996) 178–181. [15] W.G. Fisher, M.J. Sienko, Stoichiometry, structure, and physical properties of niobium disulfide, Inorg. Chem. 19 (1980) 39–43. [16] K. Sakata, Electrical and magnetic properties of NbO2, J. Phys. Soc. Jpn. 26 (1969) 867. [17] K. Kikuchi, Magnetic study of NiS2 single crystals, J. Phys. Soc. Jpn. 47 (1979) 484–490.
Journal of Physics and Chemistry of Solids 69 (2008) 49–54 www.elsevier.com/locate/jpcs
Structural and magnetic ordering in the VxNb1+yS2 system Makoto Nakanishia,, Yukiyasu Matsunoa, Tatsuo Fujiia, Jun Takadaa, Kazuyoshi Yoshimurab a
Department of Material Chemistry, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan b Division of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa Oiwake-cho, Kyoto 606-8502, Japan Received 5 December 2006; received in revised form 20 July 2007; accepted 22 July 2007
Abstract The influence of composition on the structural ordering and magnetism in the VxNb1+yS2 system has been investigated by X-ray diffraction and magnetic measurements. Stoichiometric V1/3NbS2 did not exhibit the structural ordering of vanadium between the NbS2 layers. In the ordered structure, the vanadium composition deviated from the ideal value of x ¼ 13 to both higher and lower values, while the niobium composition was in the range of 0.05pyp0.18. Excess niobium, y40, is thought to play an essential role in the structural ordering in this system. For samples with excess niobium and ordered structures, a magnetic transition was observed at 20–50 K, depending on the composition. The spontaneous magnetization of 3–5 103 mB/V atom is thought to be intrinsic to this system. The magnetization curves consisted of a constant and a proportional parts of the magnetic field, which correspond to the spontaneous magnetization and high-field susceptibility, respectively. The magnetization curves and the temperature dependencies of the high-field susceptibility were quite similar to those of the canted antiferromagnetic NiS2. A correlation between the structural and magnetic ordering is suggested. r 2007 Elsevier Ltd. All rights reserved. Keywords: A. Chalcogenides; A. Magnetic materials; C. X-ray diffraction; D. Crystal structure; D. Magnetic properties
1. Introduction Layered transition metal chalcogenides, for example, Ba(Co,Ni)S2y [1] and Tl(Co,Ni)2S2 [2], exhibit both metallic conductivity and magnetic ordering and have been investigated from the point of view of comparison with high-Tc superconductors and other strongly correlated electron systems. NbS2 is a typical two-dimensional conductor that exhibits superconductivity at 6.2 K [3]. The structure of this compound consists of NbS2 layers, each niobium ion being surrounded by six sulfur atoms in a trigonal prismatic arrangement. The NbS2 layers are bonded by van der Waals forces, and various ions or compounds can intercalate between the layers. When a 3d-transition metal intercalates into the van der Waals gap, Corresponding author. Tel.: +81 86 251 8107; fax: +81 86 251 8087.
E-mail address: [email protected] (M. Nakanishi). 0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2007.07.124
metallic conductivity and a variety of magnetism are realized [4–8]. It was revealed by means of susceptibility [4] and electrical conductivity measurements [5] that V1/3NbS2 exhibits metallic conductivity and ferromagnetic ordering at 55 K. The structure of V1/3NbS2 is composed of alternative stacking of vanadium layers and NbS2 blocks. Vanadium occupies one-third of the octahedral sites between the NbS2 layers in an ordered arrangement, as shown in Fig. 1(a). The magnetism of this system is unusual. The temperature dependence of magnetic susceptibility increases abruptly at 50 K with decreasing temperature, but decreases gradually below 50 K. The magnetism in the system is attributed to vanadium. The effective moment in the paramagnetic state is estimated to be approximately 2.9 mB/V atom; hence, vanadium is expected to be trivalent with S ¼ 1. However, the magnitude of the spontaneous magnetization appears to be almost two orders smaller than that expected from
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Fig. 1. Schematic illustration of the crystal structure of V1/3NbS2 composed of NbS2 trigonal prismatic layers and vanadium ions. The figure also shows the ordered (a) and random (b) arrangements of vanadium ions between these layers.
S ¼ 1 and that of Mn1/3NbS2 [4]. It is expected that the magnetic structure in this system is as complicated as that in Cr1/3NbS2, which exhibits a helical spin ordering [9]. On the other hand, according to another report, there is no magnetic transition in V1/3NbS2 down to 2 K [6]. Most of the vanadium sulfides such as V3S4 [10], V5S8 [11], and NaVS2 [12], order antiferromagnetically; however, this might be one of those few systems in which vanadium orders ferromagnetically, as in BaVS3–d [13]. The MnxNbS2 system is reported to have a wide composition range (0.19pxp0.52) and exhibits a temperature dependence of susceptibility similar to that of ferromagnetic V1/ 3NbS2 in the composition range of 0.4pxp0U45 [14]. It is possible that the metal compositions in V1/3NbS2 are nonstoichiometric and that the magnetic ordering in V1/3NbS2 has some correlation with this. Hence, it is interesting to study the effects of vanadium and niobium compositions on magnetism. We investigated the VxNb1+yS2 system to understand the ordered state of V1/3NbS2 and the influence of compositions on the magnetism in this system. 2. Experimental procedure Appropriate amounts of vanadium powder (99.9%), niobium powder (99.9%), and crystalline sulfur (99.999%) were mixed and sealed in an evacuated silica tube and heated at 1000 1C for 3 days. Then, the mixture was heated at 800 1C for 3 days to promote the structural ordering, and
quenched in water. Samples obtained from mixture were characterized by powder X-ray diffraction measurements using Cu Ka radiation. The heat treatment was repeated until the X-ray diffraction profiles remained unchanged. The temperature dependencies of the magnetic susceptibility from 5 to 300 K were measured by using a SQUID magnetometer (Quantum Design MPMS2) under a field of 100 Oe after zero-field cooling. The field dependencies of magnetization up to 10 kOe were also measured at various temperatures ranging from 5 to 55 K. 3. Results and discussion 3.1. Structural ordering Initially, VxNbS2 was synthesized in the range of 0.2pxp0.4, changing the number of vanadium atoms between the NbS2 blocks. Though niobium oxide NbO2 was detected in some samples as an impurity by the X-ray diffraction measurements, almost all the samples were single phase. However, the X-ray diffraction profiles were different from those reported previously [4,6]. Vanadium occupies one-third of the octahedral sites in order, whereas in our synthesized samples, the octahedral sites were randomly occupied, as shown in Fig. 1(b). The lattice constants of the samples changed with the vanadium content x. V1/3NbS2–d samples (0.2pdp0.3) were synthesized to clarify the effects of deviation from the stoichiometry of
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Fig. 3. Variation of lattice constants a and c for V1/3NbS2–d samples. Fig. 2. X-ray diffraction profiles of V1/3NbS2–d (VxNb1+yS2) samples—a: d ¼ 0 (x ¼ 0.33, y ¼ 0); b: d ¼ 0.1 (x ¼ 0.35, y ¼ 0.05); c: d ¼ 0.2 (x ¼ 0.37, y ¼ 0.11); and d: d ¼ 0.3 (x ¼ 0.39, y ¼ 0.18). The marked peaks correspond to the superlattice reflections.
sulfur on the constituent phases and crystal structure. Fig. 2 shows the X-ray diffraction profiles for these samples. No impurity was detected in any of the samples, and ordered peaks were observed in the composition range of 0.10pdp0.20. Small amounts of sulfur remained in the silica tubes, and the lattice constants of the samples were unchanged in the case of do0, as shown in Fig. 3. In the case of d40, the c-axis monotonically increased with d. On the other hand, the a-axis increased abruptly at d ¼ 0.05 and then decreased at d ¼ 0.2. So far, there are no reports which state that excess S2 are intercalated between the NbS2 blocks, as (M,S)xNbS2. Usually, all the sulfur ions arrange themselves into closely packed layers, and the metal ions M occupy the sites between these layers, as shown in the case of Vx(Nb,V)S2 [15]. In the case of niobium sulfides Nb1+xS2 [15], niobium occupies the octahedral sites between the NbS2 block layers as NbxNbS2, instead of forming vacancies in the sulfur sites as NbS2–d. We first described the chemical formula of this system as V1/3NbS2–d. The existence of sulfur vacancies was not investigated directly, e.g., by the Reitveld analysis, but it appears that the excess niobium occupies the octahedral sites between the NbS2 layers as (Nb,V)xNbS2 when d40. Our results apparently indicated that vanadium ions could hardly occupy the trigonal prismatic sites between the sulfur layers and that the excess sulfur was excluded from the samples when do0.
Thus, we described the above-mentioned formula as VxNb1+yS2 (yX0) thereafter. The irregular change in the a-axis with the amount of excess niobium was similar to the case of MnxNbS2 [14]; however, we could not reveal the reasons for the same. The structural ordering in the VxNb1+yS2 system was investigated systematically by changing the compositions of vanadium and niobium. It was revealed by X-ray diffraction measurements that NbO2 was present in some samples, but almost monophasic samples of VxNb1+yS2 were obtained. These results are summarized in Fig. 4. From the results, we see that the ordered structures exist over a wide range of composition. In the ordered structures, the vanadium composition deviated from the ideal value of x ¼ 13 to both higher and lower values, whereas the niobium composition was in the range of 0.05pyp0.18. From the above considerations, the excess niobium as well as vanadium occupies the octahedral sites between the NbS2 layers in the ordered arrangement; moreover, the excess niobium is thought to play an essential role in the structural ordering in this system. 3.2. Magnetic ordering The susceptibilities of the samples with y ¼ 0 in VxNb1+yS2 obeyed the Curie–Weiss law between 5 and 300 K and did not exhibit magnetic transitions. Their Weiss temperatures were low, e.g., 3 K for x ¼ 0.33, and the effective moments were estimated to be 2.1–2.5 mB, which is less than 2.93–2.97 mB [4]. The values of the effective moments indicate that vanadium exists in divalent or trivalent states. Our results indicate that VxNbS2 samples
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Fig. 4. Phase diagram for structural ordering in the VxNb1+yS2 system. Open squares and filled triangles indicate ordered and random arrangements of vanadium ions, respectively, as shown in Fig. 1.
Table 1 Magnetic parameters in the VxNb1+yS2 system x
y
Y (K)
Tt (K)
meff (mB/V)
S
0.20 0.27 0.30 0.33 0.40 0.35 0.36 0.37
0 0 0 0 0 0.05 0.08 0.11
2 4 3 3 6 23 41 32
– – – – – 49 43 25
2.5 2.4 2.4 2.3 2.1 2.6 2.5 2.4
0.8 0.8 0.8 0.8 0.7 0.9 0.9 0.8
in the composition range 0.2pxp0.4 are in the paramagnetic state down to 5 K, which differs from the previous results [4]. The Weiss temperature Y, effective moment meff, and spin quantum number S estimated from the effective moments are summarized in Table 1. For samples with excess niobium, VxNb1+yS2 (y40), and ordered structures, the susceptibilities increased abruptly at 20–50 K depending on their compositions. Fig. 5 shows the temperature dependencies of the susceptibilities of these samples, in comparison with that of paramagnetic V1/3NbS2. The behavior is similar to that of the ferromagnetic samples reported previously, except for the decreasing tendency of the susceptibility with decreasing temperature. The Weiss temperatures of the samples were negative in the range from 23 to 41 K, and the effective moments in the paramagnetic state were 2.3–2.6 mB. Moreover, the spontaneous magnetization at
Fig. 5. Temperature dependencies of magnetization for VxNb1+yS2 samples under a field of 100 Oe—a: x ¼ 0.33, y ¼ 0; b: x ¼ 0.35, y ¼ 0.05; and c: x ¼ 0.37, y ¼ 0.11. Transition temperatures determined by the temperature dependences of spontaneous magnetization are indicated.
5 K was 0.003–0.005 mB/V atom. The small spontaneous magnetization might be attributed to magnetic impurities. However, NbO2 included in some samples is paramagnetic [16] and ferromagnetic or ferrimagnetic phases in V–Nb–S or V–Nb–O systems have not been reported yet. The transition temperatures depended on the compositions, as shown in Fig. 5. Hence, the small spontaneous magnetization should be intrinsic to this system. The Weiss temperature, transition temperature Tt determined from the temperature dependencies of spontaneous magnetization, effective moment, and spin quantum number in the VxNb1+yS2 (y40) system are also summarized in Table 1. The transition temperature decreased with increase in the amount of excess niobium between the NbS2 blocks. The differences between the present and previous results [4,6] may be related to the structural ordering and/or compositional deviations due to the difference in the preparation techniques. It was revealed by magnetization measurements below the transition temperature that the magnetization curves consisted of a constant value M0 and a proportional part of the magnetic field wH, i.e., M ¼ M0+wH, at higher fields, as shown in Fig. 6(a). These values are thought to correspond to the spontaneous magnetization and the high-field susceptibility, respectively. The temperature dependencies of M/H and w estimated from the
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investigate the origin of these magnetic behaviors in more detail by microscopic methods such as NMR measurements. The MnxNbS2 system has been extensively investigated, especially with regard to the compositional dependencies of the magnetic and electric properties [14]. In the range xX0.4, the magnetic susceptibility of this system is quite similar to that of the ordered phase of VxNb1+yS2; thus, the same magnetic interactions and ordering states may also exist in MnxNbS2. 4. Conclusion The effect of composition on the structural ordering and magnetism in the VxNb1+yS2 system has been investigated by X-ray diffraction and magnetic measurements. Excess niobium is thought to play an essential role in the structural ordering in this system. For samples with excess niobium and ordered structures, magnetic transitions and spontaneous magnetization of 3–5 103 mB/V atom have been observed. The magnetization curves and temperature dependences of high-field susceptibility are quite similar to those of the canted antiferromagnetic NiS2. Acknowledgment We are grateful to Prof. M. Fukuhara, Okayama University of Science, for providing assistance in performing the magnetic measurements. References
Fig. 6. Magnetization curves up to 10 kOe (a) and temperature dependencies of M/H at 100 Oe and high field susceptibility w (b) for V0.35Nb1.05S2.
magnetization curves are shown in Fig. 6(b). The magnetization curves and temperature dependencies of the high-field susceptibility are similar to those of NiS2, which exhibits canted antiferromagnetic ordering [17]. The negative Weiss temperatures estimated by fitting the susceptibilities also indicate the existence of antiferromagnetic interactions in the structurally ordered phase. We will
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