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Crystal structure of Er2Fe17 nitride
Journal of Alloys and Compounds 266 (1998) 39–42
L
Crystal structure of Er 2 Fe 17 nitride a b b a, V.I. Voronin , A.V. Zinin , N.V. Kudrevatykh , A.N. Pirogov * a
Ural Branch of Russian Academy of Sciences, Institute of Metal Physics, Russian Academy of Sciences, S. Kovalevskaya St. 18, 620219 Ekaterinburg, Russia b Institute of Physics and Applied Mathematics at Ural State University, 620083 Ekaterinburg, Russia Received 30 July 1997; received in revised form 3 September 1997
Abstract The crystal structure of the Er 1.85 Fe 17.3 N 2.58 compound, which belongs to the family of R 2 Fe 17 N 3 -type nitrides has been studied by neutron diffraction. It was found that the structure of this compound should be described as a partially disordered Th 2 Ni 17 -type. Its peculiar feature is a random substitution of Fe-atoms (‘‘dumbbells’’) for Er atoms. It leads to a deviation from the precise 2:17 stoichiometry to a more Fe-rich ratio (up to 2:19). Nitrogen atoms in this nitride occupy mainly only one type of interstitial 6h-sites. 1998 Elsevier Science S.A. Keywords: Rare earth compounds nitrides; Disordered structure; Neutron diffraction
1. Introduction In the early 90s rare earth compounds of the R 2 Fe 17 type attracted much attention since it was discovered that the hard magnetic properties of these materials could be considerably improved by interstitial introduction of carbon or nitrogen [1–3]. Nitrogen leads to even more improvement. Thus, a special nitromagnetic direction in materials science has arisen [4]. To understand theoretically their magnetic properties (spontaneous magnetization formation, band structure, exchange and crystal field interactions) precise data on their crystal structure are needed. Previous investigations [2,5] have shown that the R 2 Fe 17 N x -type nitrides have basically the same crystal structure as the parent R 2 Fe 17 compounds. For example, 2:17 nitrides with R5Y, Dy, . . . , Lu have a hexagonal structure of the Th 2 Ni 17 -type (Fig. 1). The R-atoms occupy the 2b and 2d sites, the Fe-atoms the 4f-, 6g-, 12jand 12k-sites and the N-atoms the 6h, and probably, the 12j-sites [6]. On the other hand, the real crystal structure of R 2 Fe 17 type intermetallics does not belong to the ideal Th 2 Ni 17 type and should be considered as partially disordered [7,8]. In this structure about 30% of the R-atoms in 2b-sites are randomly replaced by dumbbells of Fe-atoms (4e-site, see Fig. 1). *Corresponding author. [email protected]
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0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII S0925-8388( 97 )00483-0
An attempt to apply such an approach to a Y 2 Fe 17 N x crystal structure interpretation has been made by Jaswal et al. [9]. We are not aware of any other reports where neutron diffraction patterns of R 2 Fe 17 N x with R±Y have been obtained and their crystal structure analysed, with the usage of a partially disordered Th 2 Ni 17 structure type model. The purpose of this work is the neutron diffraction study of the crystal structure of Er 2 Fe 17 nitride and the interpretation of the obtained data on the basis of the disordered Th 2 Ni 17 structure model.
2. Experimental details The initial R 2 Fe 17 alloy was prepared by placing stoichiometric quantities of Er (99.9) and Fe (99.99) in an aluminum crucible and melting these in an induction furnace, using a protective helium atmosphere. Extra erbium 2-at.% was added to compensate for vaporization losses. Subsequently, an ingot was annealed for several hours at 1223 K to reduce undesirable phases. The nitride was obtained by passing a purified nitrogen gas over finely ground Re 2 Fe 17 powder (10–30 mm) at a variable flow rate and atmospheric pressure at 773 K for 1–2 h. Neutron diffraction measurements were carried out in a high temperature chamber (helium protective atmosphere at (1 torr) with a D-7A type diffractometer installed on one horizontal channel of the IVV-2M Reactor. The neutron
40
V.I. Voronin et al. / Journal of Alloys and Compounds 266 (1998) 39 – 42
Fig. 2. Neutron diffraction pattern of Er 1.85 Fe 17.3 N 2.58 : points-experiment, line-calculation, at the bottom-difference.
Fig. 1. Crystal structures of ideal (a) and partially disordered (b) Th 2 Ni 17 type: Big circles denote Th-ions in positions 2b ( ), 2d ( ), 2c (s); medium circles-N-atoms in positions 6h ( ); small circles-Fe-atoms in positions 4e (s), 4f (d), 6g ( ), 12j ( , ) and 12k ( ).
˚ The diffraction patterns were wavelength was 1.515 A. analysed using a ‘‘Fullprof’’ computer program [10].
3. Results and discussion The nitride was heated up to 700 K and was kept at this temperature during the measurement to exclude the magnetic contribution to the diffraction pattern (the Curie point is 690 K). Its neutron diffraction pattern is shown in Fig. 2. The obtained neutron diffraction data were analysed in three stages. In the first stage, we applied the model of the ideal Th 2 Ni 17 structure. It was found that this approach could give a relatively good description (the R Breg -factor was about 9.5%), but a thorough inspection of the calculated and experimental patterns revealed evident divergence’s in some intervals (see Fig. 3). In the second stage, by using the model of a partially disordered Th 2 Ni 17 structure, we introduced three additional atomic positions: 2c for the Er-ion and 4e and 12jfor the Fe-ions. The introduction of 2c-site reflects the partial replacement of Fe-dumbbells in the 4f-site by Er-
Fig. 3. Fragments of the neutron diffraction pattern of Er 1.85 Fe 17.3 N 2.58 : points-experiment, lines-calculations based on models of ideal (bottom) and partially disordered (top) Th 2 Ni 17 -type structure.
V.I. Voronin et al. / Journal of Alloys and Compounds 266 (1998) 39 – 42
ion (a reverse procedure to the introduction of 4e-site into ideal Th 2 Ni 17 structure). At this step we could not obtain realistic data of coordinates and site occupations for these positions. Therefore, we used the following procedure for their determination: the value of the 2b-site occupation number (n b ), deduced from the first refinement stage was taken as reference and the values of the coordinates and occupation numbers of the 2c-, 2d-, 4c-, 4e-, 6g-, 6h-, 12jand 12k-sites were determined. Subsequently, the n b -value was varied and the dependencies of the R Breg , R wp and R p -factors on the n b values were plotted (Fig. 4). As it is seen from Fig. 4, the minimum for all R-factors occurs at n b ¯0.77. The value of R Breg -factor for such n b is about to ¯7%. In the third refinement stage, the new position for N-atoms was introduced with coordinate numbers for x and y opposite to the first position. Its occupation by N-atoms is well possible, because in the planes with z50.25, 0.75, where the Fe-dumbbell is replaced randomly by an Er-ion, the N-atoms will have the same surroundings as in the main position. A standard minimization procedure with a variable value for the occupation factor of this additional position (n 6h ) was carried out. It was found that the values for all R-factors became lowest in the interval 0.02,n 6h ,0.07. The R-factor values became worse for higher n 6h .
41
Table 1 Structural parameters of Er 1.85 Fe 17.3 N 2.58 Atom
Site
x
y
z
Occupation, %
Er1 Fe1 Er2 Er3 Fe2 Fe3 Fe4a Fe4b Fe5 N1a N1b
2b 4e 2d 2c 4f 6g 12j 12j 12k 6h 6h
0 0 0.333 0.333 0.333 0.5 0.332(1) 0.285(3) 0.1673(7) 20.162(1) 0.172(14)
0 0 0.667 0.667 0.667 0 0.945(1) 0.988(4) 0.3347(7) 20.325(1) 0.344(14)
0.25 0.389(3) 0.75 0.25 0.1091(7) 0 0.25 0.25 0.9804(3) 0.25 0.25
77(3) 26(2) 100 8(2) 89(1) 100 80(2) 20(2) 100 82(1) 4(3)
The results of the structural refinement after the third stage are presented in Table 1. The temperature factors for all ions were taken the same and the values obtained were ˚ 2 )51.463(3). The lattice parameters are a5 equal to B(A ˚ c58.4896(7) A. ˚ The neutron diffraction 8.6193(5) A; pattern calculated with these parameters, as well as the difference between the observed and calculated intensity profiles are shown in Fig. 2. The values of R-factors are: 2 R exp 51.75, R p 52.64, R wp 53.42, Chi 53.83, R f 56.01, R Breg 56.22. Thus, this description of the observed neutron diffraction pattern for Er 2 Fe 17 nitride is the best one of all those possible. The investigated sample has the composition Er 1.85 Fe 17.3 N 2.58 . It reveals that the crystal structure of the nitride belongs to the partially disordered Th 2 Ni 17 structure type and the real ratio between Er and Fe-atoms in this interstitial compound is shifted from the ideal 2:17 ratio to a more Fe-rich ratio (2:18.7). The shift may be connected with the initial stoichiometry deviation in the parent Er 2 Fe 17 but may also be due to the nitrogenation. Future investigations will help to clarify this question.
Acknowledgements This work was partially supported by the program ‘‘Neutron Research of Matter’’, Grant No 96-104, No 96-305.
References
Fig. 4. Dependencies of agreement factors on 2b-site occupation number.
[1] H. Sun, H. Bo-ping, L. Hong-shou, J.M.D. Coey, Solid State Comm. 74 (1990) 727. [2] M.D. Coey, H. Sun, Y. Otani, A new family of rare earth iron nitrides, VI International Symposium on Magnetic Anisotropy and Coercivity, in: Rare Earth Transition Metal Alloys, Proceeding Book, Pitsburg, 1990, p. 36. [3] J.M.D. Coey, H. Sun, J. Magn. Magn. Mater. 87 (1990) L251. [4] Proceeding of the International Conference on Nitromagnetics, J. Alloys and Comp., 222 (1995).
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V.I. Voronin et al. / Journal of Alloys and Compounds 266 (1998) 39 – 42
[5] K.H.J. Buschow, R. Coehoorn, D.B. de Mooij, K. de Waard, T.H. Jacobs, J. Magn. Magn. Mater. 92 (1990) L35. [6] R.M. Ibberson, O. Moze, T.H. Jacobs, K.H.J. Buschow, J. Phys. Condens. Mater. 3 (1991) 1219. [7] D. Givord, R. Lemaire, J.M. Moreau, E. Roudaut, J. Less-Comm. Met. 29 (1972) 361.
[8] O. Moze, R. Caciuffo, B. Gillon, G. Calestani, F.E. Kayzel, J.M.M. Franse, Phys. Rev. B 50 (1994) 9293. [9] S.S. Jaswal, W.B. Yellon, G.C. Hadjipanayis, Y.Z. Wang, Phys. Rev. Lett. 67 (1991) 644. [10] J. Rodrigues-Carvajal, Physica B 192 (1993) 55.
L
Crystal structure of Er 2 Fe 17 nitride a b b a, V.I. Voronin , A.V. Zinin , N.V. Kudrevatykh , A.N. Pirogov * a
Ural Branch of Russian Academy of Sciences, Institute of Metal Physics, Russian Academy of Sciences, S. Kovalevskaya St. 18, 620219 Ekaterinburg, Russia b Institute of Physics and Applied Mathematics at Ural State University, 620083 Ekaterinburg, Russia Received 30 July 1997; received in revised form 3 September 1997
Abstract The crystal structure of the Er 1.85 Fe 17.3 N 2.58 compound, which belongs to the family of R 2 Fe 17 N 3 -type nitrides has been studied by neutron diffraction. It was found that the structure of this compound should be described as a partially disordered Th 2 Ni 17 -type. Its peculiar feature is a random substitution of Fe-atoms (‘‘dumbbells’’) for Er atoms. It leads to a deviation from the precise 2:17 stoichiometry to a more Fe-rich ratio (up to 2:19). Nitrogen atoms in this nitride occupy mainly only one type of interstitial 6h-sites. 1998 Elsevier Science S.A. Keywords: Rare earth compounds nitrides; Disordered structure; Neutron diffraction
1. Introduction In the early 90s rare earth compounds of the R 2 Fe 17 type attracted much attention since it was discovered that the hard magnetic properties of these materials could be considerably improved by interstitial introduction of carbon or nitrogen [1–3]. Nitrogen leads to even more improvement. Thus, a special nitromagnetic direction in materials science has arisen [4]. To understand theoretically their magnetic properties (spontaneous magnetization formation, band structure, exchange and crystal field interactions) precise data on their crystal structure are needed. Previous investigations [2,5] have shown that the R 2 Fe 17 N x -type nitrides have basically the same crystal structure as the parent R 2 Fe 17 compounds. For example, 2:17 nitrides with R5Y, Dy, . . . , Lu have a hexagonal structure of the Th 2 Ni 17 -type (Fig. 1). The R-atoms occupy the 2b and 2d sites, the Fe-atoms the 4f-, 6g-, 12jand 12k-sites and the N-atoms the 6h, and probably, the 12j-sites [6]. On the other hand, the real crystal structure of R 2 Fe 17 type intermetallics does not belong to the ideal Th 2 Ni 17 type and should be considered as partially disordered [7,8]. In this structure about 30% of the R-atoms in 2b-sites are randomly replaced by dumbbells of Fe-atoms (4e-site, see Fig. 1). *Corresponding author. [email protected]
Fax:
17
3432
740003;
e-mail:
0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII S0925-8388( 97 )00483-0
An attempt to apply such an approach to a Y 2 Fe 17 N x crystal structure interpretation has been made by Jaswal et al. [9]. We are not aware of any other reports where neutron diffraction patterns of R 2 Fe 17 N x with R±Y have been obtained and their crystal structure analysed, with the usage of a partially disordered Th 2 Ni 17 structure type model. The purpose of this work is the neutron diffraction study of the crystal structure of Er 2 Fe 17 nitride and the interpretation of the obtained data on the basis of the disordered Th 2 Ni 17 structure model.
2. Experimental details The initial R 2 Fe 17 alloy was prepared by placing stoichiometric quantities of Er (99.9) and Fe (99.99) in an aluminum crucible and melting these in an induction furnace, using a protective helium atmosphere. Extra erbium 2-at.% was added to compensate for vaporization losses. Subsequently, an ingot was annealed for several hours at 1223 K to reduce undesirable phases. The nitride was obtained by passing a purified nitrogen gas over finely ground Re 2 Fe 17 powder (10–30 mm) at a variable flow rate and atmospheric pressure at 773 K for 1–2 h. Neutron diffraction measurements were carried out in a high temperature chamber (helium protective atmosphere at (1 torr) with a D-7A type diffractometer installed on one horizontal channel of the IVV-2M Reactor. The neutron
40
V.I. Voronin et al. / Journal of Alloys and Compounds 266 (1998) 39 – 42
Fig. 2. Neutron diffraction pattern of Er 1.85 Fe 17.3 N 2.58 : points-experiment, line-calculation, at the bottom-difference.
Fig. 1. Crystal structures of ideal (a) and partially disordered (b) Th 2 Ni 17 type: Big circles denote Th-ions in positions 2b ( ), 2d ( ), 2c (s); medium circles-N-atoms in positions 6h ( ); small circles-Fe-atoms in positions 4e (s), 4f (d), 6g ( ), 12j ( , ) and 12k ( ).
˚ The diffraction patterns were wavelength was 1.515 A. analysed using a ‘‘Fullprof’’ computer program [10].
3. Results and discussion The nitride was heated up to 700 K and was kept at this temperature during the measurement to exclude the magnetic contribution to the diffraction pattern (the Curie point is 690 K). Its neutron diffraction pattern is shown in Fig. 2. The obtained neutron diffraction data were analysed in three stages. In the first stage, we applied the model of the ideal Th 2 Ni 17 structure. It was found that this approach could give a relatively good description (the R Breg -factor was about 9.5%), but a thorough inspection of the calculated and experimental patterns revealed evident divergence’s in some intervals (see Fig. 3). In the second stage, by using the model of a partially disordered Th 2 Ni 17 structure, we introduced three additional atomic positions: 2c for the Er-ion and 4e and 12jfor the Fe-ions. The introduction of 2c-site reflects the partial replacement of Fe-dumbbells in the 4f-site by Er-
Fig. 3. Fragments of the neutron diffraction pattern of Er 1.85 Fe 17.3 N 2.58 : points-experiment, lines-calculations based on models of ideal (bottom) and partially disordered (top) Th 2 Ni 17 -type structure.
V.I. Voronin et al. / Journal of Alloys and Compounds 266 (1998) 39 – 42
ion (a reverse procedure to the introduction of 4e-site into ideal Th 2 Ni 17 structure). At this step we could not obtain realistic data of coordinates and site occupations for these positions. Therefore, we used the following procedure for their determination: the value of the 2b-site occupation number (n b ), deduced from the first refinement stage was taken as reference and the values of the coordinates and occupation numbers of the 2c-, 2d-, 4c-, 4e-, 6g-, 6h-, 12jand 12k-sites were determined. Subsequently, the n b -value was varied and the dependencies of the R Breg , R wp and R p -factors on the n b values were plotted (Fig. 4). As it is seen from Fig. 4, the minimum for all R-factors occurs at n b ¯0.77. The value of R Breg -factor for such n b is about to ¯7%. In the third refinement stage, the new position for N-atoms was introduced with coordinate numbers for x and y opposite to the first position. Its occupation by N-atoms is well possible, because in the planes with z50.25, 0.75, where the Fe-dumbbell is replaced randomly by an Er-ion, the N-atoms will have the same surroundings as in the main position. A standard minimization procedure with a variable value for the occupation factor of this additional position (n 6h ) was carried out. It was found that the values for all R-factors became lowest in the interval 0.02,n 6h ,0.07. The R-factor values became worse for higher n 6h .
41
Table 1 Structural parameters of Er 1.85 Fe 17.3 N 2.58 Atom
Site
x
y
z
Occupation, %
Er1 Fe1 Er2 Er3 Fe2 Fe3 Fe4a Fe4b Fe5 N1a N1b
2b 4e 2d 2c 4f 6g 12j 12j 12k 6h 6h
0 0 0.333 0.333 0.333 0.5 0.332(1) 0.285(3) 0.1673(7) 20.162(1) 0.172(14)
0 0 0.667 0.667 0.667 0 0.945(1) 0.988(4) 0.3347(7) 20.325(1) 0.344(14)
0.25 0.389(3) 0.75 0.25 0.1091(7) 0 0.25 0.25 0.9804(3) 0.25 0.25
77(3) 26(2) 100 8(2) 89(1) 100 80(2) 20(2) 100 82(1) 4(3)
The results of the structural refinement after the third stage are presented in Table 1. The temperature factors for all ions were taken the same and the values obtained were ˚ 2 )51.463(3). The lattice parameters are a5 equal to B(A ˚ c58.4896(7) A. ˚ The neutron diffraction 8.6193(5) A; pattern calculated with these parameters, as well as the difference between the observed and calculated intensity profiles are shown in Fig. 2. The values of R-factors are: 2 R exp 51.75, R p 52.64, R wp 53.42, Chi 53.83, R f 56.01, R Breg 56.22. Thus, this description of the observed neutron diffraction pattern for Er 2 Fe 17 nitride is the best one of all those possible. The investigated sample has the composition Er 1.85 Fe 17.3 N 2.58 . It reveals that the crystal structure of the nitride belongs to the partially disordered Th 2 Ni 17 structure type and the real ratio between Er and Fe-atoms in this interstitial compound is shifted from the ideal 2:17 ratio to a more Fe-rich ratio (2:18.7). The shift may be connected with the initial stoichiometry deviation in the parent Er 2 Fe 17 but may also be due to the nitrogenation. Future investigations will help to clarify this question.
Acknowledgements This work was partially supported by the program ‘‘Neutron Research of Matter’’, Grant No 96-104, No 96-305.
References
Fig. 4. Dependencies of agreement factors on 2b-site occupation number.
[1] H. Sun, H. Bo-ping, L. Hong-shou, J.M.D. Coey, Solid State Comm. 74 (1990) 727. [2] M.D. Coey, H. Sun, Y. Otani, A new family of rare earth iron nitrides, VI International Symposium on Magnetic Anisotropy and Coercivity, in: Rare Earth Transition Metal Alloys, Proceeding Book, Pitsburg, 1990, p. 36. [3] J.M.D. Coey, H. Sun, J. Magn. Magn. Mater. 87 (1990) L251. [4] Proceeding of the International Conference on Nitromagnetics, J. Alloys and Comp., 222 (1995).
42
V.I. Voronin et al. / Journal of Alloys and Compounds 266 (1998) 39 – 42
[5] K.H.J. Buschow, R. Coehoorn, D.B. de Mooij, K. de Waard, T.H. Jacobs, J. Magn. Magn. Mater. 92 (1990) L35. [6] R.M. Ibberson, O. Moze, T.H. Jacobs, K.H.J. Buschow, J. Phys. Condens. Mater. 3 (1991) 1219. [7] D. Givord, R. Lemaire, J.M. Moreau, E. Roudaut, J. Less-Comm. Met. 29 (1972) 361.
[8] O. Moze, R. Caciuffo, B. Gillon, G. Calestani, F.E. Kayzel, J.M.M. Franse, Phys. Rev. B 50 (1994) 9293. [9] S.S. Jaswal, W.B. Yellon, G.C. Hadjipanayis, Y.Z. Wang, Phys. Rev. Lett. 67 (1991) 644. [10] J. Rodrigues-Carvajal, Physica B 192 (1993) 55.