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Crystal structures and ferromagnetism of FexWN2 (x ∼ 0.74, 0.90) with defective iron triangular lattice
Journal of Alloys and Compounds 593 (2014) 154–157
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Crystal structures and ferromagnetism of FexWN2 (x 0.74, 0.90) with defective iron triangular lattice Akira Miura a,⇑, Takahiro Takei a, Nobuhiro Kumada a, Eisuke Magome b, Chikako Moriyoshi b, Yoshihiro Kuroiwa b a b
Center for Crystal Science and Technology, University of Yamanashi, Kofu, 400-8511, Japan Department of Physical Science, Hiroshima University, Higashihiroshima, 739-8526, Japan
a r t i c l e
i n f o
Article history: Received 23 July 2013 Received in revised form 8 December 2013 Accepted 6 January 2014 Available online 18 January 2014 Keywords: Nitride materials Spin glass X-ray diffraction Magnetic measurements
a b s t r a c t The crystal structures of FexWN2 (x 0.74, 0.90) and their magnetic properties were investigated. The structural studies reveal the formation of ion defects with different concentrations in the two-dimensional iron triangular lattice by analyzing the synchrotron radiation X-ray diffraction data. The magnetic measurements demonstrate that the Fe0.74WN2 phase shows ferromagnetism at 300 K, while the less defective Fe0.90WN2 phase exhibits no ferromagnetism. A decrease of temperature to 5 K significantly suppresses the room-temperature ferromagnetism of the Fe0.74WN2 phase. We attribute the emergence of ferromagnetism in FexWN2 to the increases of defect concentration and thermal displacement in the iron triangular lattice. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction A triangular lattice containing magnetic atoms has a simple two-dimensional form in which there is a geometrical frustration of spins that can form ordered nearest-neighbor spin coupling [1], longer-range coupling, or disordered spins [2]. For instance, NiGa2S4 shows spin-disordered states that have been stabilized by geometrical frustration even at low temperature [2]. On the other hand, Sr5Ru5 xO15 (x 0.90) shows ferromagnetism with an extremely large coercivity (12 T) at 1.7 K [3]. Although the defective ruthenium triangular lattice has been proposed as a clue to its ferromagnetism, a further examination is necessary to clarify their correlation. Iron nitrides, such as Fe3N1+y, Fe4N1 z, and Fe16N2, are principal ferromagnetic materials, and many reports have indicated their nonstoichiometry, impurities, and magnetic properties [4–6]. However, the effects of nonstoichiometry and impurities on the magnetic properties of ternary nitrides with a two-dimensional iron triangular lattice have not been intensively studied. Stoichiometric FeWN2 is a layered nitride that has iron triangular lattices sandwiched between trigonal W–N slabs as determined by Rietveld analysis using laboratory X-ray diffraction data (Fig. 1) [7–9]. It was proposed that FexWN2 (x 0.7) is similar in structure, but it contains defective iron triangular lattices; however, Rietveld analysis was not performed [10]. Temperature dependencies of the ⇑ Corresponding author. Tel.: +81 55 220 8614; fax: +81 55 254 3035. E-mail address: [email protected] (A. Miura). 0925-8388/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2014.01.030
susceptibilities of FeWN2 [7] and FexWN2 (x 0.7) [10] were reported, though no detailed measurement conditions of zero-field cooling or field cooling were described. The relationship between the magnetic field strength and their magnetization is yet to be examined. Therefore, a systematic analysis is necessary in order to understand the details of their magnetic properties. In this report, we examine the crystal structure and magnetic properties of FexWN2 with two different iron concentrations using synchrotron X-ray diffraction and a vibrating sample magnetometer; this should provide clues as to how the defects in the triangular lattice affect their magnetic properties. 2. Method The following chemicals were used without further treatment: FeCl24H2O (Kanto Kagaku >99%), Na2WO42H2O (Kanto Kagaku >99%), ammonia gas (Sumitomo Seika >99.9%), 3 M H2SO4 (Kanto Kagaku 99.9999%), and distilled water. Fe0.74WN2 and Fe0.90WN2 powders were synthesized according previously described methods [7–10]. The FeCl2 and Na2WO4 aqueous solutions were mixed and FeWO4 was obtained as a precipitate. This oxide was washed several times with water and dried. Fe0.90WN2 was obtained by heating 0.15 g of FeWO4 powder in an ammonia stream at 700 °C, and it was subsequently quenched (with 3% of Fe3N1+y impurity as described later). The Fe0.74WN2 was synthesized by soaking the Fe0.90WN2 powder in 1 M sulfuric acid at 50 °C for 60 h. Synchrotron X-ray powder diffraction measurements were performed at room temperature on a Debye– Scherrer camera at the BL02B2 experimental station of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI; Proposal No. 2013A1299). The wavelength of the radiation beam was 0.41336(5) Å, and the step size was 0.01°. The Rietveld refinement was performed using the RIETAN-FP software package [11], and the crystal structures were drawn in VESTA [12]. The Fe/W ratio was measured by energy-dispersive X-ray spectroscopy (EDX) using
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A. Miura et al. / Journal of Alloys and Compounds 593 (2014) 154–157 Table 1 Summary of Rietveld refinements of FexWN2 (x 0.74, 0.90).
Fig. 1. Crystal structure of FexWN2 with Fe triangular lattice. W and N are located at center and corner of trigonals, respectively. FeWO4 and Fe2W3O12 powders as the standards on a scanning electron microscope system (SEM; JEOL; JSM-6500). The magnetic moment was measured using a physical property measurement system (Quantum Design; PPMS).
3. Results and discussion Fig. 2 shows the refinement profiles of the FexWN2 before and after the acid treatment at 50 °C for 60 h. Both X-ray diffraction peaks correspond to a hexagonal cell with space group P63/mmc, which is consistent with previous reports on nonstoichiometric FexWN2 and stoichiometric FeWN2 [9,10]. The preliminary analysis was performed without fixing all the atomic displacement factors (Biso), but these converged to an abnormally low value of 0.01 Å2 for tungsten. Therefore, further refinements were performed with the value for tungsten fixed at 0.1 Å2, which gave comparable lattice parameters and occupancies and brought about slightly higher refinement parameters. The final refinement data are listed in Tables 1–3. The refinements converged to the following values: Rwp = 6.31%, Rp = 4.73%, and S = 3.04 for Fe0.90WN2 and Rwp = 6.41%, Rp = 4.93%, and S = 2.53 for Fe0.74WN2. These values of R and S are somewhat higher than expected. Although the refinement results indicate the average structures of FexWN2, the fitting difficulty might be caused by multiple phases with slightly different lattice parameters or inhomogeneity, which are described later. While the diffraction pattern of Fe0.74WN2 does not show any impurities, the pattern of Fe0.90WN2 shows approximately 3% of Fe3N1+y impurity. No superlattice peaks were detected for either of the FexWN2 compounds; this indicates a statistic distribution of Fe vacancies in the triangle lattice. The increase in Fe vacancy, x, expands the a-axis but shrinks the c-axis. This trend is similar to that of one previously reported [10],
Fig. 2. Rietveld profiles of synchrotron X-ray diffraction patterns of Fe0.74WN2 and Fe0.90WN2. Green bars represent allowed reflections for Fe0.74WN2 and Fe0.90WN2. Bars below those of Fe0.90WN2 show reflections for Fe3N1+y impurity. Bottom lines give difference between observed and calculated profiles. Insets give expansions between 9° and 12° showing Fe3N1+y impurity peaks.
Samples
Fe0.74WN2
Fe0.90WN2
Number of phases Phase Phase ratio (%) Crystal system Space group Lattice parameter (Å)
1 Fe0.74WN2 100.0 Hexagonal P63/mmc a = 2.87461(4) c = 10.8367(3) 1.87 0.96 6.31 4.73 3.04
2 Fe0.90WN2 97.0 Hexagonal P63/mmc a = 2.87186(4) c = 10.9527(3) 1.60 0.81 6.41 4.94 2.53
RB (%) RF (%) RWP (%) RP (%) S
Fe3N1+y 3.0 Hexagonal P63/mmc a = 4.797 c = 4.424 1.90 0.87
Table 2 Atomic positions and occupancies of Fe0.74WN2. Atom
Site
x
y
z
Occupancy
Biso (Å2)
Fe W N
2a 2b 4f
0 0 1/3
0 0 2/3
0 1/4 0.3743(6)
0.74(2) 1.00(2) 1
0.60(3) 0.1 0.6(2)
Table 3 Atomic positions and occupancies of Fe0.90WN2. Atom
Site
x
y
z
Occupancy
Biso (Å2)
Fe W N
2a 2b 4f
0 0 1/3
0 0 2/3
0 1/4 0.3713(7)
0.90(2) 1.01(2) 1
0.49 (3) 0.1 0.3(2)
but a detailed comparison exhibits minor but noticeable differences. The lattice parameters of Fe0.74WN2 (a = 2.87461(4) Å, c = 10.8368(3) Å) are close to those reported for FexWN2 (x 0.7) after acid treatment for 24 h: a = 2.875(1) Å, c = 10.829(9) Å [10]. The values for Fe0.90WN2 (a = 2.87187(4), c = 10.9529(2) Å) are similar to those previously reported for FeWN2, but some deviations were observed even though they have been synthesized using the same synthesis procedure of heating a FeWO4 precursor under an ammonia flow: a = 2.8724(1), c = 10.973(4) Å [7], a = 2.87630(5), c = 10.9320(4) Å [9], and a = 2.8702(5), c = 10.989(4) Å [10]. The deviations among these samples possibly originate from the existence of several phases with slightly different lattice parameters, which are difficult to control over their syntheses. Therefore, all the analyses in this work were performed using the Fe0.74WN2 and Fe0.90WN2 powders synthesized from the same batch that was treated at a high temperature under an ammonia flow. The lattice parameters of Fe0.90WN2 were also close to those of the FexWN2 (x 0.75: a = 2.8722(4), c = 10.950(4) Å) synthesized by short-term acid treatment (1 h) of FeWN2 that had the values of 2.8702(5) for the a axis and 10.989(4) Å for the c axis [10]. This is presumably attributed to the inhomogeneous feature of the short-term acid-treated sample derived from FeWN2 that had slightly different parameters [10]. Though the occupancies of the tungsten were uniform within standard deviation, those of the iron were determined to be 0.74(2) and 0.90(2) for the two nitrides. Thus, the triangular lattices have iron vacancies at different concentrations. The iron contents determined from Rietveld analysis agreed with those from EDX analysis: Fe/W = 0.76(7) for Fe0.74WN2 and Fe/W = 0.97(7) for Fe0.90WN2. Nonetheless, the occupancy of 0.90(2) in Fe0.90WN2 is inconsistent with the previously reported stoichiometric content for FeWN2 [7–10]. This discrepancy can be explained by the 3% Fe3N1+y impurity in the Fe0.90WN2 sample, which could only be detected by synchrotron X-ray experiments. We would like
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A. Miura et al. / Journal of Alloys and Compounds 593 (2014) 154–157
Fig. 3. Temperature dependences of magnetic susceptibilities of Fe0.90WN2 and Fe0.74WN2. M/H expresses per mol of Fe, assuming Fe0.90WN2 with 3% of Fe3N and single-phase Fe0.74WN2. ZFC and FC represent zero field cooling and field cooling, respectively.
to note that the total ratio of Fe/W in Fe0.90WN2 with 3% Fe3N1+y within standard deviation agrees with that of the previously reported stoichiometric FeWN2 [7–10]. Fig. 3 shows the temperature dependence of the susceptibilities of FexWN2 (x 0.74, 0.90). The susceptibilities of Fe0.74WN2 and Fe0.90WN2 show broad peaks around 40 and 20 K, respectively. Deviations between zero field cooling (ZFC) and field cooling (FC) curves for each nitride appear at around 70 and 100 K, respectively. These trends suggest the possibility of spin-glass behavior of the FexWN2 phases [13], but they are unlikely attributed to the impurity phase of ferromagnetic Fe3N1+y. Fig. 4 shows the magnetic field vs. magnetic moment of FexWN2. Fe0.74WN2 exhibits a hysteresis loop at 300 K, which indicates ferromagnetism. The saturation magnetization is 0.02 lB/ Fe, which is estimated by the y-axis intercept of the extrapolated linear regression from the high-field region. Its magnetic coercivity at 300 K is 0.8 T. When the temperature is decreased to 200 K, its ferromagnetism is slightly enhanced in terms of its saturation magnetization and coercivity, but any further decrease reduces
the ferromagnetism. This change of hysteresis curves were reproduced through repeated measurements between 5 and 300 K. On the other hand, Fe0.90WN2 shows ferromagnetism with a small coercivity of 0.01 T at 300 K, and saturation magnetization is increased with a decrease in temperature. These small coercivities and temperature dependence agree with those of the Fe3N1+y phase [6], which has been detected as an impure phase in Fe0.90WN2. Therefore, the Fe0.90WN2 phase is unlikely to show ferromagnetism at 5 and 300 K. These hysteresis curves suggest that room-temperature ferromagnetism occurred in Fe0.74WN2 with iron vacancies as high as 1/4. The relatively high magnetic coercivity (0.8 T) eliminates the likelihood of the presence of the following reported Fe-based impurities: a-Fe (0.047 T) [4], Fe16N2 (0.1 T) [4], Fe4N1 z (0.0005 T) [5], Fe3N1+y (0.054 T) [14], and c-Fe2O3 (0.006 T) [15]. The saturation magnetization and coercivity of a-Fe2O3 at 295 K are 0.25 emu/g and 0.3 T, respectively [16]. Therefore, there is no possibility for the a-Fe2O3 impurity to be the main ferromagnetic phase because the saturation magnetization of 0.25 emu/g can be found only under the impossible assumption that all Fe0.74WN2 is a-Fe2O3. Additionally, even if the Fe-based impurities are formed as trace impurities, they are mostly dissolved during the acid treatment of Fe0.74WN2 synthesis. Because the synchrotron diffraction pattern of Fe0.74WN2 suggests a single-phase structure without any impurities and superlattice peaks, the triangular iron lattice with roughly a quarter of the statistical vacancies is perhaps responsible for the room-temperature ferromagnetism. The low saturation magnetization of 0.02 lB/Fe in Fe0.74WN2 at 300 K suggests a weak ferromagnetic-like ordering. The interplanar and intraplanar distances between the iron atoms are 2.87462(4) and 5.4184(2) Å, respectively; thus, both interactions are possible. The increase of vacancy concentration from x 0.90 to x 0.74 decreases the Fe–N bond distance from 2.175(5) Å to 2.147(4) Å, which is suggestive of possible oxidation state of Fe. The bond valence sum of Fe site increases from 2.00 to 2.16, which corresponds to the increase of the average effective charge of iron atom from 2.22 to 2.91. The spin-glass characteristics within the triangular geometry and statistical vacancies are likely related to the ferromagnetism of relatively large coercivity observed in Sr5Ru5 xO15 [3]. However, the origin of this ferromagnetism remains open to debate, and further investigation is needed to determine its Fe valence and magnetic structures at different
Fig. 4. Magnetization hysteresis loops of Fe0.74WN2 and Fe0.90WN2 at 5, 100, 200 and 300 K. Magnetic susceptibilities of Fe0.74WN2 and Fe0.90WN2 express lB per Fe, assuming single-phase Fe0.74WN2 and Fe0.90WN2 with 3% of Fe3N1+y.
A. Miura et al. / Journal of Alloys and Compounds 593 (2014) 154–157
temperatures. The appearance of ferromagnetism within the iron triangular lattice is at least partially attributable to the disorders induced by both iron vacancies and thermal displacement, and this provides useful clues for designing new ferromagnetic materials. Acknowledgements A.M. acknowledge Profs. M. Nagao, K. Nakashima, N. Yoneyama (University of Yamanashi), K. Yamaki (Utsunomiya University), and Y. Tsujimoto (NIMS) for their valuable comments and fruitful discussion. References [1] J.E. Greedan, J. Mater. Chem. 11 (2001) 37–53. [2] S. Nakatsuji, Y. Nambu, H. Tonomura, O. Sakai, S. Jonas, C. Broholm, H. Tsunetsugu, Y. Qiu, Y. Maeno, Science 309 (2005) 1697–1700. [3] A. Yamamoto, D. Hashizume, H.A. Katori, T. Sasaki, E. Ohmichi, T. Nishizaki, N. Kobayashi, H. Takagi, Chem. Mater. 22 (2010) 5712–5717.
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[4] S. Yamashita, Y. Masubuchi, Y. Nakazawa, T. Okayama, M. Tsuchiya, S. Kikkawa, J. Solid State Chem. 194 (2012) 76–79. [5] S.K. Chen, S. Jin, T.H. Tiefel, Y.F. Hsieh, E.M. Gyorgy, J.D.W. Johnson, J. Appl. Phys. 70 (1991) 6247–6249. [6] A. Leineweber, H. Jacobs, F. Hüning, H. Lueken, W. Kockelmann, J. Alloys Comp. 316 (2001) 21–38. [7] D.S. Bem, J.D. Houmes, H.C. zur Loye, In: A.R. Barron, G.S. Fischman, M.A. Fury, A.F. Hepp (Eds.), MRS Symposium Proceeding; Covalent Ceramics II: NonOxides; Materials Research Society, Boston 1993, pp. 153–162. [8] D.S. Bem, H.C. zur Loye, J. Solid State Chem. 104 (1993) 467–469. [9] D.S. Bem, C.M. LampeOnnerud, H.P. Olsen, H.C. zur Loye, Inorg. Chem. 35 (1996) 581–585. [10] A. Miura, X.-D. Wen, H. Abe, G. Yau, F.J. DiSalvo, J. Solid State Chem. 183 (2010) 327–331. [11] F. Izumi, K. Momma, Solid State Phenom. 130 (2007) 15–20. [12] K. Momma, F. Izumi, J. Appl. Crystallogr. 41 (2008) 653–658. [13] K. Binder, A.P. Young, Rev. Mod. Phys. 58 (1986) 801–976. [14] Y. Zhong Qing, Z. Jian Rong, D. You Wei, J. Magn. Magn. Mater. 159 (1996) L8– L10. [15] V. Chhabra, P. Ayyub, S. Chattopadhyay, A.N. Maitra, Mater. Lett. 26 (1996) 21– 26. [16] A.H. Hill, F. Jiao, P.G. Bruce, A. Harrison, W. Kockelmann, C. Ritter, Chem. Mater. 20 (2008) 4891–4899.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Crystal structures and ferromagnetism of FexWN2 (x 0.74, 0.90) with defective iron triangular lattice Akira Miura a,⇑, Takahiro Takei a, Nobuhiro Kumada a, Eisuke Magome b, Chikako Moriyoshi b, Yoshihiro Kuroiwa b a b
Center for Crystal Science and Technology, University of Yamanashi, Kofu, 400-8511, Japan Department of Physical Science, Hiroshima University, Higashihiroshima, 739-8526, Japan
a r t i c l e
i n f o
Article history: Received 23 July 2013 Received in revised form 8 December 2013 Accepted 6 January 2014 Available online 18 January 2014 Keywords: Nitride materials Spin glass X-ray diffraction Magnetic measurements
a b s t r a c t The crystal structures of FexWN2 (x 0.74, 0.90) and their magnetic properties were investigated. The structural studies reveal the formation of ion defects with different concentrations in the two-dimensional iron triangular lattice by analyzing the synchrotron radiation X-ray diffraction data. The magnetic measurements demonstrate that the Fe0.74WN2 phase shows ferromagnetism at 300 K, while the less defective Fe0.90WN2 phase exhibits no ferromagnetism. A decrease of temperature to 5 K significantly suppresses the room-temperature ferromagnetism of the Fe0.74WN2 phase. We attribute the emergence of ferromagnetism in FexWN2 to the increases of defect concentration and thermal displacement in the iron triangular lattice. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction A triangular lattice containing magnetic atoms has a simple two-dimensional form in which there is a geometrical frustration of spins that can form ordered nearest-neighbor spin coupling [1], longer-range coupling, or disordered spins [2]. For instance, NiGa2S4 shows spin-disordered states that have been stabilized by geometrical frustration even at low temperature [2]. On the other hand, Sr5Ru5 xO15 (x 0.90) shows ferromagnetism with an extremely large coercivity (12 T) at 1.7 K [3]. Although the defective ruthenium triangular lattice has been proposed as a clue to its ferromagnetism, a further examination is necessary to clarify their correlation. Iron nitrides, such as Fe3N1+y, Fe4N1 z, and Fe16N2, are principal ferromagnetic materials, and many reports have indicated their nonstoichiometry, impurities, and magnetic properties [4–6]. However, the effects of nonstoichiometry and impurities on the magnetic properties of ternary nitrides with a two-dimensional iron triangular lattice have not been intensively studied. Stoichiometric FeWN2 is a layered nitride that has iron triangular lattices sandwiched between trigonal W–N slabs as determined by Rietveld analysis using laboratory X-ray diffraction data (Fig. 1) [7–9]. It was proposed that FexWN2 (x 0.7) is similar in structure, but it contains defective iron triangular lattices; however, Rietveld analysis was not performed [10]. Temperature dependencies of the ⇑ Corresponding author. Tel.: +81 55 220 8614; fax: +81 55 254 3035. E-mail address: [email protected] (A. Miura). 0925-8388/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2014.01.030
susceptibilities of FeWN2 [7] and FexWN2 (x 0.7) [10] were reported, though no detailed measurement conditions of zero-field cooling or field cooling were described. The relationship between the magnetic field strength and their magnetization is yet to be examined. Therefore, a systematic analysis is necessary in order to understand the details of their magnetic properties. In this report, we examine the crystal structure and magnetic properties of FexWN2 with two different iron concentrations using synchrotron X-ray diffraction and a vibrating sample magnetometer; this should provide clues as to how the defects in the triangular lattice affect their magnetic properties. 2. Method The following chemicals were used without further treatment: FeCl24H2O (Kanto Kagaku >99%), Na2WO42H2O (Kanto Kagaku >99%), ammonia gas (Sumitomo Seika >99.9%), 3 M H2SO4 (Kanto Kagaku 99.9999%), and distilled water. Fe0.74WN2 and Fe0.90WN2 powders were synthesized according previously described methods [7–10]. The FeCl2 and Na2WO4 aqueous solutions were mixed and FeWO4 was obtained as a precipitate. This oxide was washed several times with water and dried. Fe0.90WN2 was obtained by heating 0.15 g of FeWO4 powder in an ammonia stream at 700 °C, and it was subsequently quenched (with 3% of Fe3N1+y impurity as described later). The Fe0.74WN2 was synthesized by soaking the Fe0.90WN2 powder in 1 M sulfuric acid at 50 °C for 60 h. Synchrotron X-ray powder diffraction measurements were performed at room temperature on a Debye– Scherrer camera at the BL02B2 experimental station of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI; Proposal No. 2013A1299). The wavelength of the radiation beam was 0.41336(5) Å, and the step size was 0.01°. The Rietveld refinement was performed using the RIETAN-FP software package [11], and the crystal structures were drawn in VESTA [12]. The Fe/W ratio was measured by energy-dispersive X-ray spectroscopy (EDX) using
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A. Miura et al. / Journal of Alloys and Compounds 593 (2014) 154–157 Table 1 Summary of Rietveld refinements of FexWN2 (x 0.74, 0.90).
Fig. 1. Crystal structure of FexWN2 with Fe triangular lattice. W and N are located at center and corner of trigonals, respectively. FeWO4 and Fe2W3O12 powders as the standards on a scanning electron microscope system (SEM; JEOL; JSM-6500). The magnetic moment was measured using a physical property measurement system (Quantum Design; PPMS).
3. Results and discussion Fig. 2 shows the refinement profiles of the FexWN2 before and after the acid treatment at 50 °C for 60 h. Both X-ray diffraction peaks correspond to a hexagonal cell with space group P63/mmc, which is consistent with previous reports on nonstoichiometric FexWN2 and stoichiometric FeWN2 [9,10]. The preliminary analysis was performed without fixing all the atomic displacement factors (Biso), but these converged to an abnormally low value of 0.01 Å2 for tungsten. Therefore, further refinements were performed with the value for tungsten fixed at 0.1 Å2, which gave comparable lattice parameters and occupancies and brought about slightly higher refinement parameters. The final refinement data are listed in Tables 1–3. The refinements converged to the following values: Rwp = 6.31%, Rp = 4.73%, and S = 3.04 for Fe0.90WN2 and Rwp = 6.41%, Rp = 4.93%, and S = 2.53 for Fe0.74WN2. These values of R and S are somewhat higher than expected. Although the refinement results indicate the average structures of FexWN2, the fitting difficulty might be caused by multiple phases with slightly different lattice parameters or inhomogeneity, which are described later. While the diffraction pattern of Fe0.74WN2 does not show any impurities, the pattern of Fe0.90WN2 shows approximately 3% of Fe3N1+y impurity. No superlattice peaks were detected for either of the FexWN2 compounds; this indicates a statistic distribution of Fe vacancies in the triangle lattice. The increase in Fe vacancy, x, expands the a-axis but shrinks the c-axis. This trend is similar to that of one previously reported [10],
Fig. 2. Rietveld profiles of synchrotron X-ray diffraction patterns of Fe0.74WN2 and Fe0.90WN2. Green bars represent allowed reflections for Fe0.74WN2 and Fe0.90WN2. Bars below those of Fe0.90WN2 show reflections for Fe3N1+y impurity. Bottom lines give difference between observed and calculated profiles. Insets give expansions between 9° and 12° showing Fe3N1+y impurity peaks.
Samples
Fe0.74WN2
Fe0.90WN2
Number of phases Phase Phase ratio (%) Crystal system Space group Lattice parameter (Å)
1 Fe0.74WN2 100.0 Hexagonal P63/mmc a = 2.87461(4) c = 10.8367(3) 1.87 0.96 6.31 4.73 3.04
2 Fe0.90WN2 97.0 Hexagonal P63/mmc a = 2.87186(4) c = 10.9527(3) 1.60 0.81 6.41 4.94 2.53
RB (%) RF (%) RWP (%) RP (%) S
Fe3N1+y 3.0 Hexagonal P63/mmc a = 4.797 c = 4.424 1.90 0.87
Table 2 Atomic positions and occupancies of Fe0.74WN2. Atom
Site
x
y
z
Occupancy
Biso (Å2)
Fe W N
2a 2b 4f
0 0 1/3
0 0 2/3
0 1/4 0.3743(6)
0.74(2) 1.00(2) 1
0.60(3) 0.1 0.6(2)
Table 3 Atomic positions and occupancies of Fe0.90WN2. Atom
Site
x
y
z
Occupancy
Biso (Å2)
Fe W N
2a 2b 4f
0 0 1/3
0 0 2/3
0 1/4 0.3713(7)
0.90(2) 1.01(2) 1
0.49 (3) 0.1 0.3(2)
but a detailed comparison exhibits minor but noticeable differences. The lattice parameters of Fe0.74WN2 (a = 2.87461(4) Å, c = 10.8368(3) Å) are close to those reported for FexWN2 (x 0.7) after acid treatment for 24 h: a = 2.875(1) Å, c = 10.829(9) Å [10]. The values for Fe0.90WN2 (a = 2.87187(4), c = 10.9529(2) Å) are similar to those previously reported for FeWN2, but some deviations were observed even though they have been synthesized using the same synthesis procedure of heating a FeWO4 precursor under an ammonia flow: a = 2.8724(1), c = 10.973(4) Å [7], a = 2.87630(5), c = 10.9320(4) Å [9], and a = 2.8702(5), c = 10.989(4) Å [10]. The deviations among these samples possibly originate from the existence of several phases with slightly different lattice parameters, which are difficult to control over their syntheses. Therefore, all the analyses in this work were performed using the Fe0.74WN2 and Fe0.90WN2 powders synthesized from the same batch that was treated at a high temperature under an ammonia flow. The lattice parameters of Fe0.90WN2 were also close to those of the FexWN2 (x 0.75: a = 2.8722(4), c = 10.950(4) Å) synthesized by short-term acid treatment (1 h) of FeWN2 that had the values of 2.8702(5) for the a axis and 10.989(4) Å for the c axis [10]. This is presumably attributed to the inhomogeneous feature of the short-term acid-treated sample derived from FeWN2 that had slightly different parameters [10]. Though the occupancies of the tungsten were uniform within standard deviation, those of the iron were determined to be 0.74(2) and 0.90(2) for the two nitrides. Thus, the triangular lattices have iron vacancies at different concentrations. The iron contents determined from Rietveld analysis agreed with those from EDX analysis: Fe/W = 0.76(7) for Fe0.74WN2 and Fe/W = 0.97(7) for Fe0.90WN2. Nonetheless, the occupancy of 0.90(2) in Fe0.90WN2 is inconsistent with the previously reported stoichiometric content for FeWN2 [7–10]. This discrepancy can be explained by the 3% Fe3N1+y impurity in the Fe0.90WN2 sample, which could only be detected by synchrotron X-ray experiments. We would like
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Fig. 3. Temperature dependences of magnetic susceptibilities of Fe0.90WN2 and Fe0.74WN2. M/H expresses per mol of Fe, assuming Fe0.90WN2 with 3% of Fe3N and single-phase Fe0.74WN2. ZFC and FC represent zero field cooling and field cooling, respectively.
to note that the total ratio of Fe/W in Fe0.90WN2 with 3% Fe3N1+y within standard deviation agrees with that of the previously reported stoichiometric FeWN2 [7–10]. Fig. 3 shows the temperature dependence of the susceptibilities of FexWN2 (x 0.74, 0.90). The susceptibilities of Fe0.74WN2 and Fe0.90WN2 show broad peaks around 40 and 20 K, respectively. Deviations between zero field cooling (ZFC) and field cooling (FC) curves for each nitride appear at around 70 and 100 K, respectively. These trends suggest the possibility of spin-glass behavior of the FexWN2 phases [13], but they are unlikely attributed to the impurity phase of ferromagnetic Fe3N1+y. Fig. 4 shows the magnetic field vs. magnetic moment of FexWN2. Fe0.74WN2 exhibits a hysteresis loop at 300 K, which indicates ferromagnetism. The saturation magnetization is 0.02 lB/ Fe, which is estimated by the y-axis intercept of the extrapolated linear regression from the high-field region. Its magnetic coercivity at 300 K is 0.8 T. When the temperature is decreased to 200 K, its ferromagnetism is slightly enhanced in terms of its saturation magnetization and coercivity, but any further decrease reduces
the ferromagnetism. This change of hysteresis curves were reproduced through repeated measurements between 5 and 300 K. On the other hand, Fe0.90WN2 shows ferromagnetism with a small coercivity of 0.01 T at 300 K, and saturation magnetization is increased with a decrease in temperature. These small coercivities and temperature dependence agree with those of the Fe3N1+y phase [6], which has been detected as an impure phase in Fe0.90WN2. Therefore, the Fe0.90WN2 phase is unlikely to show ferromagnetism at 5 and 300 K. These hysteresis curves suggest that room-temperature ferromagnetism occurred in Fe0.74WN2 with iron vacancies as high as 1/4. The relatively high magnetic coercivity (0.8 T) eliminates the likelihood of the presence of the following reported Fe-based impurities: a-Fe (0.047 T) [4], Fe16N2 (0.1 T) [4], Fe4N1 z (0.0005 T) [5], Fe3N1+y (0.054 T) [14], and c-Fe2O3 (0.006 T) [15]. The saturation magnetization and coercivity of a-Fe2O3 at 295 K are 0.25 emu/g and 0.3 T, respectively [16]. Therefore, there is no possibility for the a-Fe2O3 impurity to be the main ferromagnetic phase because the saturation magnetization of 0.25 emu/g can be found only under the impossible assumption that all Fe0.74WN2 is a-Fe2O3. Additionally, even if the Fe-based impurities are formed as trace impurities, they are mostly dissolved during the acid treatment of Fe0.74WN2 synthesis. Because the synchrotron diffraction pattern of Fe0.74WN2 suggests a single-phase structure without any impurities and superlattice peaks, the triangular iron lattice with roughly a quarter of the statistical vacancies is perhaps responsible for the room-temperature ferromagnetism. The low saturation magnetization of 0.02 lB/Fe in Fe0.74WN2 at 300 K suggests a weak ferromagnetic-like ordering. The interplanar and intraplanar distances between the iron atoms are 2.87462(4) and 5.4184(2) Å, respectively; thus, both interactions are possible. The increase of vacancy concentration from x 0.90 to x 0.74 decreases the Fe–N bond distance from 2.175(5) Å to 2.147(4) Å, which is suggestive of possible oxidation state of Fe. The bond valence sum of Fe site increases from 2.00 to 2.16, which corresponds to the increase of the average effective charge of iron atom from 2.22 to 2.91. The spin-glass characteristics within the triangular geometry and statistical vacancies are likely related to the ferromagnetism of relatively large coercivity observed in Sr5Ru5 xO15 [3]. However, the origin of this ferromagnetism remains open to debate, and further investigation is needed to determine its Fe valence and magnetic structures at different
Fig. 4. Magnetization hysteresis loops of Fe0.74WN2 and Fe0.90WN2 at 5, 100, 200 and 300 K. Magnetic susceptibilities of Fe0.74WN2 and Fe0.90WN2 express lB per Fe, assuming single-phase Fe0.74WN2 and Fe0.90WN2 with 3% of Fe3N1+y.
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temperatures. The appearance of ferromagnetism within the iron triangular lattice is at least partially attributable to the disorders induced by both iron vacancies and thermal displacement, and this provides useful clues for designing new ferromagnetic materials. Acknowledgements A.M. acknowledge Profs. M. Nagao, K. Nakashima, N. Yoneyama (University of Yamanashi), K. Yamaki (Utsunomiya University), and Y. Tsujimoto (NIMS) for their valuable comments and fruitful discussion. References [1] J.E. Greedan, J. Mater. Chem. 11 (2001) 37–53. [2] S. Nakatsuji, Y. Nambu, H. Tonomura, O. Sakai, S. Jonas, C. Broholm, H. Tsunetsugu, Y. Qiu, Y. Maeno, Science 309 (2005) 1697–1700. [3] A. Yamamoto, D. Hashizume, H.A. Katori, T. Sasaki, E. Ohmichi, T. Nishizaki, N. Kobayashi, H. Takagi, Chem. Mater. 22 (2010) 5712–5717.
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