Phase relations in the Fe-rich region of the Nd–Fe–V system at 1323 K

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Journal of Alloys and Compounds 283 (1999) 203–207

Phase relations in the Fe-rich region of the Nd–Fe–V system at 1323 K G.Y. Huo a

a ,b ,c ,

*, Z.Y. Qiao b , G.H. Rao a , X.L. Chen a , J.K. Liang a ,d , F. Huang a

Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, PO Box 603, Beijing 100080, People’ s Republic of China b Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, People’ s Republic of China c Department of Chemistry, Hebei University, Baoding 071002, People’ s Republic of China d International Centre for Materials Physics, Academia Sinica, Shenyang 110015, People’ s Republic of China Received 6 July 1998; received in revised form 22 September 1998

Abstract Phase relations in the Fe-rich region of the Nd–Fe–V system at 1323 K have been investigated by means of powder X-ray diffraction. In the investigated composition region, two ternary phases: Nd(Fe 12xVx ) 12 (x50.125–0.167) and Nd 3 (Fe 12xVx ) 29 (x50.066–0.095), and one pseudobinary phase: Nd 2 (Fe 12xVx ) 17 (x50.000–0.067) are identified. The dependence of the cell parameters on x in Nd(Fe 12xVx ) 12 , Nd 3 (Fe 12xVx ) 29 and Nd 2 (Fe 12xVx ) 17 are presented. Also given are the dependences of the Curie temperatures on the V content (x) in Nd(Fe 12xVx ) 12 and Nd 2 (Fe 12xVx ) 17 . The preferential occupation of V in Nd(Fe 12xVx ) 12 and Nd 2 (Fe 12xVx ) 17 is deduced from the slope of a and c with V content.  1999 Published by Elsevier Science S.A. Keywords: Ternary neodymium compounds; Nd–Fe–V compounds; Crystal structure; Solid solubilities; Phase equilibria

1. Introduction The discovery of the high-performance permanent magnet, Nd 2 Fe 14 B, has brought about an intensive exploration of novel magnetic intermetallic compounds in rare earth (R)–iron (Fe)–M systems, where M is an element other than R and Fe and usually introduced for stabilizing intermetallic phases. In many R–Fe–M systems, the intermetallic compounds, especially their nitrides, with the tetragonal ThMn 12 -type structure exhibit excellent inherent magnetic properties that are comparable to those of Nd 2 Fe 14 B, and therefore, are potential candidates for permanent magnets. In preparation of the Nd(Fe 12x Ti x ) 12 phase, Collocott et al. [1] confirmed the existence of a new Fe-rich phase: Nd 2 (Fe 12x Ti x ) 19 , in the Nd–Fe–Ti system. Subsequent X-ray [2] and neutron [3] diffraction experiments indicated that this compound crystallized in a monoclinic structure (space group P21 /c) derived from the CaCu 5 -type structure, and that the correct stoichiometry of the compound should be Nd 3 (Fe 12x Ti x ) 29 . The magnetic properties of the R 3 (Fe 12x Ti x ) 29 compounds are excellent. For example, the Curie temperature is between 400 and 560 K, the saturation magnetization is in the range 95–150 Am 2 / kg, and the anisotropy field is between 2 and 17 T. It *Corresponding author.

is well known that the crystal structure of the R 2 Fe 17 and the R(Fe 12x M x ) 12 compounds are derivatives of the RT 5 compounds with CaCu 5 -type structure. They can be obtained by replacement of a fraction, d, of the R atoms in RT 5 by pairs of transition metal T–T (dumb bell). This process may be illustrated by the following relation: R 12d (2T)d T 5 → RT z

d 5 (z 2 5) /(z 1 2)

with (i) d 51 / 3 corresponding to the 2:17 structure; (ii) d 51 / 2 corresponding to the 1:12 structure, and (iii) d 52 / 5 corresponding to the 3:29 structure. However, R 3 (Fe 12xVx ) 29 is very difficult to obtain in single-phase form, since these are high temperature intermetallic phases existing in narrow temperature and composition ranges. So far, few papers have been reported on phase relations in the Fe-rich region above 1323 K. In this paper we report the phase relations in the Fe-rich region of the Nd–Fe–V system at 1323 K.

2. Experimental Samples weighing 5.0 g were prepared by arc-melting appropriate amounts of high purity ($99.9%) metals: Nd, Fe, and V, under high purity argon atmosphere. The

0925-8388 / 99 / $ – see front matter  1999 Published by Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )00868-8

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samples were remelted several times in order to achieve full homogenization. More than 70 samples with different compositions were prepared. Weight loss of the samples during arc-melting is less than 0.5%. Each sample was wrapped with tantalum foil, encapsulated in an evacuated quartz tube (air pressure less than 10 Pa), and annealed at 1323 K for more than 3 days, followed by quenching in ice-water. Samples were characterized by X-ray powder diffraction (XRD) using a four-layer monochromatic focusing transmission Guinier–de Wolff camera with CoKa radiation. Some samples were then characterized by X-ray diffraction using CuKa radiation with graphite monochrometer on a Rigaku model D/ max-2400 X-ray diffractometer or on a Philips X9 Pert model X-ray diffractometer.

3. Results and discussion Samples quenched from 1323 K to ice-water were examined by X-ray powder diffraction at room temperature. Based on the X-ray diffraction patterns of more than 70 samples, part of the phase diagram of the ternary

system Nd–Fe–V was constructed and is shown in Fig. 1. In the Fe-rich corner there exist two ternary phases: Nd(Fe 12xVx ) 12 (x50.125–0.167), Nd 3 (Fe 12xVx ) 29 (x5 0.066–0.095), and one pseudobinary phase: Nd 2 (Fe 12xVx ) 17 (x50.000–0.067). In the investigated composition region, six three-phase regions and six twophase regions were identified as shown in Fig. 1. Nd 2 Fe 17 and Nd 2 (Fe 12xVx ) 17 crystallize in the rhombohedral Th 2 Zn 17 -type structure. The homogeneity range of the Nd 2 (Fe 12xVx ) 17 solid solution extends up to x5 0.067. It is slightly larger than that of Nd 2 (Fe 12x Ti x ) 17 (x50.000–0.042) and Gd 2 (Fe 12x Ti x ) 17 (x50.000–0.051) reported by Margarian et al. [4] and Huo et al. [5] because the radius of V is smaller than that of Ti and the radius of Nd is larger than that of Gd. The lattice constant of Nd 2 (Fe 12xVx ) 17 are listed in Table 1. The concentration dependence of the lattice parameters a and c is shown in Fig. 2. The lattice parameters increase with increasing V content in this solid solution. This can be attributed to the metallic radius of V that is slightly larger than that of Fe. It can be calculated that the slopes of the lattice parameter, a and c, with the V content is 0.003 and 0.002 nm / V-atom, respectively. We can see that the slope of lattice parameter

Fig. 1. Fe-rich corner of the Nd–Fe–V phase relations at 1323 K. (A) Nd(Fe 12xVx ) 12 , (B) Nd 3 (Fe 12xVx ) 29 and (C) Nd 2 (Fe 12xVx ) 17 ). This phase diagram consists of compounds mentioned above and they construct six three-phase regions: (1) Nd 2 (Fe 0.933 V0.0667 ) 17 1Fe1Nd 3 (Fe 0.934 V0.066 ) 29 ; (6) Nd 3 (Fe 0.905 V0.095 ) 29 1Fe1Nd(Fe 0.875 V0.125 ) 12 ; (9) Nd 2 (Fe 0.933 V0.067 ) 17 1Nd(?)1Nd 3 (Fe 0.905 V0.095 ) 29 ; (10) Nd 3 (Fe 0.905 V0.095 ) 29 1Nd(?)1 Nd(Fe 0.833 V0.167 ) 12 ; (11) Nd(Fe 0.833 V0.167 ) 12 1Nd(?)1FeV(?); (12) FeV(?)1Nd(Fe 0.833 V0.167 ) 12 1Fe; and six two-phase regions: (2) Nd(Fe 12xVx ) 12 1Fe; (3) Nd 2 (Fe 12xVx ) 17 1Nd 3 (Fe 12xVx ) 29 ; (4) Nd(Fe 12xVx ) 12 1Nd 3 (Fe 12xVx ) 29 ; (5) Nd 3 (Fe 12xVx ) 29 1Fe; (7) Nd 2 (Fe 12xVx ) 17 1Fe; (8) Nd 2 (Fe 12xVx ) 17 1Nd(?).

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Table 1 The lattice parameters of the compounds Nd 2 (Fe 12xVx ) 17 , Nd(Fe 12xVx ) 12 and Nd 3 (Fe 12xVx ) 29 Phase

Nd 2 (Fe 12xVx ) 17

Nd(Fe 12xVx ) 12

Nd 3 (Fe 12xVx ) 29

V content (x) a (nm) b (nm) c (nm) V (nm 3 ) b0

0.000–0.067 0.8555–0.8666 0.8555–0.8666 1.2440–1.2505 0.7884–0.8133 —

0.125–0.167 0.85568–0.85608 0.85568–0.85608 0.47660–0.47686 0.34896–0.34948 —

0.066–0.095 1.0633–1.0650 0.8500–0.8590 0.9757–0.9777 0.8755–0.8868 96.9–97.5

a with V content is larger than that of c, which would agree with the preference of V to occupy the 18f site [6]. It can be obtained from the X-ray diffraction patterns that other phases except 2:17 phase exists for V contents (x) larger than 0.067. The dependence of the Curie temperatures of Nd 2 (Fe 12xVx ) 17 on V content (x) is shown in Fig. 3. The Curie temperatures increase with V content because the Fe–Fe coupling in Nd 2 (Fe 12xVx ) 17 is increased due to the increase in volume. Fig. 4 shows the X-ray pattern of an

Fig. 2. Dependence of the lattice parameters of the Nd 2 (Fe 12xVx ) 17 on the V content (x).

Fig. 3. Relation of the Curie temperatures of the Nd 2 (Fe 12xVx ) 17 with V content (x).

aligned sample (x50.029 in the Nd 2 (Fe 12xVx ) 17 ) system, indicating that the magnetic anisotropy is of the planar type. The Nd(Fe 12xVx ) 12 compounds crystallize in the tetragonal ThMn 12 -type structure. The solid solution range of Nd(Fe 12xVx ) 12 is from x50.125–0.167. The lattice parameters of the Nd(Fe 12xVx ) 12 compounds are listed in Table 1. The dependence of the lattice parameters on the V content is shown in Fig. 5. It can be seen that the lattice parameters increase with increasing V content because the radius of V is larger than that of Fe. The rate of expansion of the lattice parameters with V content is smaller than that of the 2:17 compounds (see below) since the Fe–Fe spacing in the 1:12 compound is larger than that of the 2:17 compound. The slopes of the lattice parameters, a and c, with V content are 0.0004 and 0.0003 nm / V-atom, respectively. It can be inferred from the slopes of a and c with V content that the V atoms prefer to occupy the 8i site. This result agrees with reported data [7,8]. We determined the Curie temperatures of the Nd(Fe 12xVx ) 12 compounds using a vibrating sample magnetometer (VSM). The Curie temperatures of the Nd(Fe 12xVx ) 12 compounds decrease with increasing V content, which is agreement other R(Fe, M) 12 compounds for which it was reported that the

Fig. 4. X-ray pattern of the aligned sample Nd 2 (Fe 0.971 V0.029 ) 17 .

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other phases for V contents (x) beyond the range from 0.125–0.167. The Nd 3 (Fe 12xVx ) 29 compounds crystallize in a monoclinic lattice (space group P21 /c) and the lattice constants are listed in Table 1. Fig. 7 shows the dependence of the lattice constants of the Nd 3 (Fe 12 xVx ) 29 compounds on V content (x). The lattice constants a, b, and c increase with x. The slopes of the lattice parameters a, b, and c with V content are 0.0010, 0.0054, and 0.0012 nm / V-atom, respectively. The homogeneity range of Nd 3 (Fe 12xVx ) 29 is from x50.066–0.095. These values are larger than that in

Fig. 5. Dependence of the lattice parameters of the Nd(Fe 12xVx ) 12 on the V content (x).

Curie temperatures decreases with M concentration. The relation between the Curie temperatures of Nd(Fe 12xVx ) 12 and V content (x) is shown in Fig. 6. The X-ray diffraction patterns show that there are mixtures of Nd(Fe 12xVx ) 12 and

Fig. 6. Relation of the Curie temperature of the Nd(Fe 12xVx ) 12 with V content (x).

Fig. 7. Dependence of the lattice parameters of the Nd 3 (Fe 12xVx ) 29 on the V content (x).

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Gd 3 (Fe 12x Ti x ) 29 (x50.011–0.034). This means that it needs more of the stabilizing element than Gd 3 (Fe 12x Ti x ) 29 . On the one hand, the rare-earth metallic radius of Nd is larger than that of Gd, on other hand, the metallic radius of V is smaller than that of Ti. In conclusion, three compounds are observed in the investigated composition region, including two ternary phases: Nd(Fe 12xVx ) 12 with x50.125–0.167 and Nd 3 (Fe 12xVx ) 29 with x50.066–0.095, one pseudobinary phase: Nd 2 (Fe 12xVx ) 17 with x50.000–0.067. The dependence of the lattice parameters on V concentration in Nd 2 (Fe 12xVx ) 17 , Nd(Fe 12xVx ) 12 and Nd 3 (Fe 12xVx ) 29 is presented. The relations between the Curie temperatures and V contents in Nd 2 (Fe 12xVx ) 17 and Nd(Fe 12xVx ) 12 are reported.

Acknowledgements The project is supported by the National Natural Science Foundation of China

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