Microsoft Word - 211.docx CHEMICAL ENGINEERING TRANSACTIONS VOL. 61, 2017 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Petar S Varbanov, Rongxin Su, Hon Loong Lam, Xia Liu, Jiří J Klemeš Copyright © 2017, AIDIC Servizi S.r.l. ISBN 978-88-95608-51-8; ISSN 2283-9216 A New Structure of the Environment-Friendly Material Fe16N2 Lei Fenga,b,*, Dongmei Zhanga,b, Fei Wangc, Li Dongb, Shuang Chenb, Jie Liub, Xuesong Huib a Metallurgy and Energy College, North China University of Science and Technology, Tangshan, China b Qian'an College, North China University of Science and Technology, Tangshan, China c Institute of Power Source & Ecomaterials Science, Hebei University of Technology, Tianjin, China feng_lei2000@163.com In this paper, a new compound c-Fe16N2 is obtained using a new crystal structure prediction software CALYPSO (Crystal Structure Analysis by Particle Swarm Optimization). The compound has a structure of C2/m (the twelfth space group), and a magnetic moment of 17 µB per unit cell. The enthalpy of c-Fe16N2 is - 8.620 eV/atom, which is little to that of α′′-Fe16N2 (-8.287 eV/atom), so the former is more stable from the view of energy. The average magnetic moment of c-Fe16N2 is 2.13 µB per iron atom which is small to that of α′′- Fe16N2 (2.8 µB). The c-Fe16N2 is a new rare earth-free permanent magnet. No rare earth means no environmental pollution in the the rare earth mining. The α′′-Fe16N2 has a highest saturation magnetic flux density, but it is difficult to mass production, because there are other several N-Fe iron compounds (such as Fe3N, Fe4N) in the obtained resultants. The newly found compound c-Fe16N2 may replace α′′-Fe16N2 because the former can be easily obtained compared to the latter, and there maybe a new way to mass production of α′′-Fe16N2 from c-Fe16N2. 1. Introduction The magnetic material is an important functional material. At present, the permanent magnetic materials are all made of rare earth, such as SmCo, Sm2Co17 and Nd2Fe14B. Rare earth metal is non-renewable, and the source reserves is less and less. The mining, refining, recycling (Balintova et al., 2014) of rare earth metal are damaging to the environment (Elena et al., 2015). It is known that the rare earth mining is harmful to environment, and the purification of rare earth can generate air and water pollutions, so it is necessary to develop a new type of strong magnetic material which does not contain rare earth. In recent years, many attentions have been paid to nitriding magnetic materials. Iron nitride FeN (Rissanen et al., 1998), Fe2 ~ 3N, Fe3N, Fe4N have high magnetization, and they are chemical stable. The discovery of these nitride magnetic materials makes researchers who study the magnetic materials have a great interest in N element. Jack (1951) first discovered α′′-Fe16N2 and its high magnetization was reported by Kim and Takahashi (1972). The α′′-Fe16N2 has the largest saturation magnetic field strength of 2.83T in all the magnetic materials. The α′′-Fe16N2 compound has a body centred tetragonal (bct) structure (I4/mmm space group) (Jack., 1995) which is illustrated in the Figure 1 (Benea et al., 2016), the lattice parameters are a = b = 5.72Å, c= 6.29Å, α = β = γ = 90° (Robert et al., 1994). The site positions of all the atoms in the α′′-Fe16N2 crystal structure are listed in the Table 1. In the structure, Fe atoms occupy three different inequivalent Wyckoff sites (4e, 8h, 4d), and the N enters the 2a crystal site. The structure can be seen composing of a 2 × 2 × 2 super cell of bcc Fe with two additional N atoms locating at interstitial octahedral positions. The cell of bcc Fe is deformed, the length of c axis is twice of the α-Fe, so one unit cell of a α′′-Fe16N2 is 205.8 Å 3. The Fe atoms occupy the six vertexes of an octahedral structure, the upper and the lower vertexes are the 4e sites, and the horizontal vertexes are the 4h sites. The nearest neighbour of N atom is Fe2, and the furthest neighbour is Fe3. The preparation methods of α′′-Fe16N2 are mainly the following several kinds. The first method is the nitriding annealing method, this is the earliest method for preparing α′′-Fe16N2 (Huang et al., 1994) and is also the main method for the preparation of bulk α" -Fe16N2 material. This method is also used in the preparation of α′′- DOI: 10.3303/CET1761248 Please cite this article as: Feng L., Zhang D., Wang F., Dong L., Chen S., Liu J., Hui X., 2017, A new structure of the environment-friendly material fe16n2, Chemical Engineering Transactions, 61, 1501-1506 DOI:10.3303/CET1761248 1501 Fe16N2 thin films. At about 750 °C, α-Fe is nitrided in nitrogen to form a nitrogen austenite phase, and then quenched at a temperature below 0 °C to obtain a martensitic α ' phase and anneal at about 120 °C for an hour to obtain α′′-Fe16N2 phase (Jack, 1994). The second method is implanting the N2 + ion into the iron film (Leroy et al., 1995). The third is the vapor phase epitaxy method. The iron nitride film is grown by molecular beam epitaxy in NH3 / H2 to prepare α "-Fe16N2. But up to now, the reported coercive force of α′′-Fe16N2 compound was still too small to practical use. The low coercive force is attributed to inappropriate grain size which is with obvious grain boundaries. Unfortunately, the grain size and the grain boundaries in the compound are hard to control by traditional methods. As we said above, the structure of α′′-Fe16N2 is a martensite structure, its structure is sensitive to temperature changes, and stable only under 214 °C. But according the literature, the lowest temperature to obtain α′′-Fe16N2 is 350 °C, so the traditional annealing method is not applicable for α′′-Fe16N2. So, a low-temperature approach is needed to prevent the martensitic phase breaking down. Recently, Jiang et al. (2015) invented a new method to prepare α′′-Fe16N2. They firstly used ball milling method to prepare α′′-Fe16N2 powder, the ratio of ball mass to sample mass was 10:1, the ammonium nitrate (NH4NO3) was used to as nitrogen source in the reaction during ball milling，the experiment was carried out in a nitrogen gas environment inside a glove box, the ball mill rotation speed was 600 rpm in planetary mode. After 60 h of milling, the α′′-Fe16N2 was obtained with a purity of 70 %, the saturation magnetization was up to 210 emu/g, and the coercivity was 854 Oe at room temperature. A strained-wire method of production of α′′- Fe16N2 was proposed in 2016 (Jiang et al., 2016). First, the bulk iron was melted with a prefixed ratio of urea, second, the FeN mixture was heated to 660°C for 4 h, then quenched into water at room temperature, after that, 0.2–0.3 mm2 and 10 mm long samples which were cut from the mixture were strained using a loading device to apply an external force which could elongate the length of the lattice. The prepared FeN magnet was with a coercivity of 1,220 Oe, and the saturation magnetization at the 2-T field was 220 emu/g. But the prepared sample was a mixture of Fe16N2, Fe8N, Fe4N, and Fe. In this article, by using a new crystal structure prediction software CALYPSO, a new compound c-Fe16N2 is obtained, and it is different to α′′-Fe16N2 which is difficult to mass production. The c-Fe16N2 has a lower enthalpy than α′′-Fe16N2, so the former is more stable than the latter in energy, in other words, the former can be easily obtained compared to the latter. The c-Fe16N2 is also a magnet with large magnetic moment, so it can replace the α′′-Fe16N2 and is an important rare earth-free permanent magnet. There is no rare earth in c- Fe16N2, so there is no occurrence of environmental pollution in the production of this compound. Table 1: The site positions of the Fe and N in α′′-Fe16N2 crystal structure Site Site symmetry x/a y/b z/c First [second] coordination Fe1 4d 0 1/2 1/4 8 Fe3, 2.55 Å; [4 Fe2, 2.86 Å] 2 N, 3.26 Å Fe2 4e 0 0 0.31 1 N ,1.95 Å, 4 Fe3, 2.33 Å, 1 Fe2, 2.39 Å [4 Fe3, 2.80 Å, 4 Fe1, 2.86 Å] Fe3 8h 0.25 0.25 0 1 N, 2.01 Å, 2 Fe2, 2.33 Å, 4 Fe1, 2.55 Å 1 [2 Fe2, 2.80 Å, 4 Fe3, 2.84 Å] N 2a 0 0 0 2 Fe2, 1.95 Å, 4 Fe3, 2.01 Å [8 Fe1, 3.24 Å] Figure 1: The structure of the α′′-Fe16N2 compound 1502 2. Method and calculation details CALYPSO is a short name of “Crystal Structure Analysis by Particle Swarm Optimization” (Wang et al., 2015), it has been developed by professor Ma and his team in Jilin university (Su et al., 2017). It is designed to predict crystal structures of materials ranging from 0-dimensional (0D) to 1D, 2D, and 3D (Wang et al., 2017). In the software, the Particle Swarm Optimization (PSO) algorithm is applied, and this algorithm is inspired by team organization pattern of a bird flock which can be regarded as a distributed algorithm in multidimensional searching and can be seen as an unbiased global optimization method. This algorithm can quickly search for all the potential energy surface, overcome the energy barrier, and not cause the loss of the smallest energy structure. The software can employ the structure relaxation software such as Vasp, Pwscf, Castep, Gaussian, etc. In the prediction of structures of a material, the CALYPSO can randomly generate a group structures according to the material formula and take care of the symmetry requirements, a few random structures can be generated in every generation, the number of generation can be set according to the number of atoms, for each structure, the geometry optimization will be make, the number of the geometry optimization is usually set as three or four, the reasonable structures will be reserved and make further optimization, the unreasonable will be discarded, and this procedure will be repeat many times. The software can also be used to search for functional materials according to a few parameters, such as semiconductors, superconductor, superhard materials, lithium-ion battery materials, and study the binary phase diagram, clusters (Liu et al., 2016), crystal structures under high pressure (Harran et al., 2016), and so on. The preparation of Fe16N2 involves α-Fe to α′-N-martensite as one adds N atoms gradually in the structure and tempers it. But the prepared sample is always a mixture. So, the more stable phase of Fe16N2 should be explored and the CALYPSO software was used to study other new structures of Fe16N2. For Fe16N2, the number of element specie, name of the element Fe, N, the atom numbers, the number of formula, an estimate volume of the formula and the distance of the atoms were all supplied to the CALYPSO software, and the maximum step of the revolution in the procedure was 30. The density of states (DOS) and bands structures were obtained using a generalized-gradient approximation (GGA) method proposed by Perdew et al. (1996). 3. Results and discussions According the computational results, a new monoclinic structure (C2/m space group) of Fe16N2 was found, the lattice parameters are a = 5.0716 Å, b = 7.3262 Å, c = 5.0404 Å, α = γ = 90°, β = 92.5146°, and the volume is 187.0985 Å3. The site positions of all the atoms in the crystal structure are listed in the Table 2. The structure is illustrated in the Figure 2. The N atoms are all in the centre of four plane, the N atoms in the up and lower plane have four nearest Fe atoms, the N atoms in the left and right plane have two first nearest Fe atoms and four second nearest Fe atoms. Two Fe1, Fe5, Fe7 atoms in the body centre of the structure form an octahedron. Table 2: The site positions of the atoms in Fe16N2 with C2/m crystal structure Site Site symmetry x/a y/b z/c Fe1 4h 0 0.7568 0.5 Fe3 4g 0 0.2454 0 Fe5 4i 0.7681 0 0.7674 Fe7 4i 0.2440 0 0.7615 N1 2d 0 0.5 0.5 The calculated total density of states (DOS) are presented in Figure 3. For the total DOS, the states are mainly between -10 ~ 17 eV, the shapes of the upper and lower halves are asymmetry, especially around the Fermi level, and there are two leading peaks below and above the Fermi level. The calculated total magnetic moment of Fe16N2 with C2/m crystal structure is 17µB per unit cell, 2.13 µB per Fe atom, this result is small to that of α′′-Fe16N2 obtained by Kim and Takahashi (1972), and their result is 2.8 µB. The magnetic moment of one N atom is -0.02 µB, which is little and anti-parallel with the other Fe atoms. The enthalpy of c-Fe16N2 is -8.620 eV/atom, which is little to that of α′′-Fe16N2 (-8.287 eV/atom), so the former is more stable from the view of energy. 1503 Figure 2: The structure of the Fe16N2 with C2/m crystal structure Figure 3: The total density of states of Fe16N2 with C2/m crystal structure The spin-dependent energy bands along high-symmetry directions in the Brillouin zone for Fe16N2 with C2/m crystal structure are shown in Figure 4. The band structures distribute in the range of -8 eV to 17 eV, and very dense in the range of -5 eV to the Fermi level, implies that the electron in this range is rather local, and the effective mass is relatively large. The bands below -5 eV are relatively wide, the shapes look like parabolas, so they are sp-like bands. The valence bands intersect with the conduction bands. In this article, the crystal structure and the electric structure were studied using GGA method. If one who had the experimental condition could try to prepare the alloy c-Fe16N2. Because X-ray diffraction method is usually used to confirm the crystal structure, so the calculated X-ray diffraction was done, and the data are plotted in the Figure 5. 1504 Figure 4: The majority spin band structures of Fe16N2 with C2/m crystal structure Figure 5: The x-ray diffraction result of Fe16N2 with C2/m crystal structure 4. Conclusions In this paper, using a new crystal structure prediction software CALYPSO, a new compound of c-Fe16N2 is obtained which is different to α′′-Fe16N2. It has a C2/m symmetry, and the c-Fe16N2 is a new magnet with a magnetic moment of 17 µB. The density of states and the band structures are all obtained using GGA method. The x-ray diffraction simulation result of c-Fe16N2 is also obtained. The newly found compound c-Fe16N2 may replace α′′-Fe16N2 as a new rare earth-free permanent magnet. There is no rare earth in the compound, implying no occurrence of environmental pollution in the the rare earth mining. The magnetic moment of c- Fe16N2 is smaller than that of α′′-Fe16N2, and this may limit some applications of this material. Acknowledgments The authors acknowledge the support by the Department of Science and Technology of Hebei Province ( No. 15211034), China and the Innovation Project of Qian'an College of University Student of North China University of Science and Technology, Tangshan, China. This research was also supported by the National Natural Science Foundation of China (No. 51404085), and the Key Technology R&D Program of Tianjin city (No. 15ZCZDSF00030). 1505 References Benea D., Isnard O., Pop V., 2016, Electronic structure and magnetic properties of the Fe16N2 doped with Ti, Journal of Magnetism and Magnetic Materials, 420, 75-80. 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