Chemical hydrogenation effects on R2Fe14B compounds with (R = Ce, Nd and Gd)

phys. stat. sol. (c) 1, No. 7, 1697–1700 (2004) / DOI 10.1002/pssc.200304447

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Chemical hydrogenation effects on R2Fe14B compounds (with R = Ce, Nd and Gd)

M. S. Ben Kraiem1, *, M. Ellouze1, A. Cheikh-Rouhou1, and Ph. L’Héritier2

1 Laboratoire de Physique des Matériaux, FSS, B. P. 802 – 3018, Sfax, Tunisie 2 Laboratoire des Matériaux et du Génie Physique (UMR 5628 CNRS) ENSPG, B. P. 46,

38402 Saint Martin d’Hères, France

Received 31 August 2003, accepted 31 December 2003 Published online 23 April 2004

PACS 61.10.Nz, 61.66.Dk, 75.30.Cr, 75.30.Kz, 75.50.Bb

The structural and magnetic properties of R2Fe14BHx compounds with R = Ce, Nd and Gd have been in-vestigated by X-ray diffraction (XRD) and magnetic measurements. XRD patterns show that all samples are single phase and crystallize in the quadratic structure. The hydrogen insertion increases the lattice con-stants a, c and the unit cell volume. The lattice parameters values of the hydrides obtained by chemical method are very close to those elaborated by the classical method. Thermomagnetic analysis shows that the Curie temperature of the hydrides are larger than that of the parent compounds. Hydrogen insertion leads to an increase of the saturated magnetization at room temperature.


1 Introduction

After many years of a great activity in the 1960s and the 1970s, a new start in the permanent magnet research field came from the discovery of the R2Fe14B intermetallic compounds [1, 2]. Due to its large economical consequences in the huge market of permanent magnets, a great deal of basic research was focused on Nd2Fe14B, which exhibits superior high-performance permanent-magnet properties over the earlier Sm–Co materials. This compound has a tetragonal crystal structure belonging to the P42/mnm space group. As described before, most of the new potential materials for permanent magnets are ternary alloys of rare earth, iron and metalloid atoms, such as C or B. These compounds usually offer several advantages in comparison with the original binary compounds: higher magnetization and higher Curie temperature. The increase in Curie temperature is also partly understood in terms of the Néel–Slater model, which correlates the exchange interaction with the interatomic 3dmetal–3dmetal distances. The inser-tion of light elements within the structure leads to larger (Fe–Fe) distances as already proposed [3–5]. As the new hard magnet materials are usually based on the iron-rich compounds such as Nd2Fe14B (82% Fe), the effects of the interstitial elements on the magnetic properties are now rather well understood. For example, in R2Fe14X with X = B or C, the interstitial element increases the number of charges within the plane surrounding the rare earth. In this paper, we report a chemical method of hydrogen atoms insertion in R2Fe14B compounds with R = Ce, Nd and Gd and we compare the structural and magnetic properties of hydrides obtained by chemical and classical method (under high pressure and heat treatment).

2 Experimental details

R2Fe14B samples were prepared from elements (R = Ce, Nd and Gd) using induction melting. Obtained alloys have undergone thermal homogenization proceedings. Annealing samples have been realized in a

* Corresponding author: e-mail: [email protected], Phone: ++ 216 98 656 912, Fax: ++ 216 74 274 437.

1698 M. S. Ben Kraiem et al.: Chemical hydrogenation effects on R2Fe14B compounds


bulb of quartz tube at T = 900 °C for 10 days. Finally the synthesized samples are rapidly quenched to room temperature in water. Our hydride samples (R2Fe14BHx) were obtained by a chemical method at room temperature, using an aqueous solution of NaBH4 according to the following reaction:

4 2 2 3 2M BH 3H O M H H BO 3,5H− −

+ + → − + +

where M and M–H are respectively the alloy and its hydride. After 24 h of reaction blends, the obtained compound was rinsed in alcohol then dried under air. Phase purity was checked by X-ray diffraction with iron radiation (λ = 1.9360 Å). Curie temperatures were obtained from thermomagnetic analysis using a home made Faraday-type balance. The saturated magnetization (Ms) was measured, at 300 K, using a vibrating sample magnetometer with an applied field up to 8 T.

3 Results and discussion

X-ray diffraction patterns show that all the hydrides conserve the quadratic structure of Nd2Fe14B with P42/mnm space group. The hydrogen insertion within the crystal lattice induces a significant increase of the unit cell volume of about 1.43 % for Ce2Fe14B, 2.73 % for Nd2Fe14B and 1.14 % for Gd2Fe14B. This feature is quite general when hydrogen goes into intermetallic compounds and has already been observed in other series of magnetic materials such as R2Fe17 [6–8], R2Fe17–xSix [9] and R2Fe16Ti [10] RFe11Ti [11]. We list in Table 1 the crystallographic and magnetic properties of the alloys and its hydrides. The unit cell volume of our hydrides are slightly smaller than those reported by l’Héritier et al. [12]. This behaviour may be probably due to the hydrogen content.

Table 1 Crystallographic and magnetic data of R2Fe14B compounds before and after hydrogen insertion.

Samples a ( Å ) c ( Å ) c/a V (Å3 ) ∆V/V (%) Tc (K) ∆Tc/Tc (%) Ms (emu/g)

Ce2Fe14B 8.7573 12.1113 1.3830 804.38 - 441 - 139.7 Ce2Fe14BHx

* 8.8020 12.1625 1.3817 816.05 1.43 535 17.6 152.5 Ce2Fe14BH3.8

1 8.7881 12.2624 1.3953 820.15 1.92 546 19.2 - Nd2Fe14B 8.8049 12.2056 1.3862 819.48 - 588 - 110.9 Nd2Fe14BHx

* 8.8980 12.2915 1.3813 842.79 2.73 667 11.8 118.3 Nd2Fe14BH4.3

1 8.9166 12.3656 1.3868 851.42 3.75 667 11.8 - Gd2Fe14B 8.7841 12.0799 1.3752 807.21 - 666 - 76.9 Gd2Fe14BHx* 8.8120 12.1424 1.3779 816.55 1.14 711 6.3 84.5 Gd2Fe14BH3.5

1 8.8687 12.2041 1.3760 831.30 1.77 712 6.4 -

* Hydrides elaborated by chemical method. 1 Hydrides elaborated by classical method [12]. Thermomagnetic analysis shows that all our hydrides exhibit a paramagnetic to ferromagnetic transition with decreasing temperature. Our synthesized hydrides present a higher Curie temperature compared to their parent alloys (Fig. 1). Such effect was already observed in the hydrides elaborated by classical method. In our hydrides, the Curie temperature values are similar to those obtained by gas–solid reac-tion. This effect confirm that the hydrogen content inserted by the chemical method is similar to that obtained by the classical method. The Tc increase can be attributed to the greater average Fe–Fe intera-tomic distances [13–15], which reinforces the positive character of the exchange interactions. Magnetization (M) measurements versus magnetic applied field up to 8 T at 300 K were performed to determine the saturated magnetization (Ms) of the samples before and after hydrogen insertion. We plot in Fig. 2 the magnetization evolution versus applied field at 300 K. The saturated magnetization has been obtained by polynomial extrapolation of M versus 1/H2. The other major change upon hydrogen insertion is an increase of the saturated magnetization at room temperature as reported in Table 1. The highest value obtained is for the Ce2Fe14BHx sample. The increase of Ms is about 9.9 % for Gd2Fe14B, 9.2 % for Ce2Fe14B and 6.7 % for Nd2Fe14B.

phys. stat. sol. (c) 1, No. 7 (2004) / www.pss-c.com 1699


4 0 0

5 0 0

6 0 0

7 0 0

8 0 0

R2Fe

1 4B

R2Fe

1 4B H

x

TC

(K)

R are -ea rt h a tom

G dN dC e

Fig. 1 Evolution of the Curie temperature before and after hydrogen insertion in R2Fe14B.

0

20

40

60

80

100

120

140

160

0 1 2 3 4 5 6 7 8

Gd2Fe

14BH

y

Gd2Fe

14B

Nd2Fe

14B

Nd2Fe

14BH

y

Ce2Fe

14BHy

Ce2Fe

14B

Ma

gn

eti

zati

on

(e.

m.u

/g)

H (Tesla)

Fig. 2 Magnetization versus applied field at 300 K of R2Fe14B with R = Ce, Nd and Gd before and after hydrogen insertion.

The insertion of hydrogen in the structure increases the distance between magnetic atoms and conse-quently the positive interaction between Fe–Fe and R–Fe atoms [16, 17]. In Ce2Fe14B, there are two ferromagnetic atoms Ce and Fe, which are parallely coupled in this system, this is why the saturated magnetization in this sample is higher than in the other samples. However, in the Gd2Fe14B sample the magnetic moments are coupled antiparallely, which reduced the value of the magnetization compared to the other samples.

4 Conclusion

We have successfully inserted hydrogen atoms by chemical method in R2Fe14B compounds with R = Nd, Ce and Gd. The hydrogen insertion leads to an increase of the lattice parameters, a and c, and the unit cell volume. The Curie temperatures, as well as the saturated magnetization, increase also after hydrogen insertion and the obtained values are higher than those obtained before hydrogen insertion and almost similar to those obtained by solid–gas reaction.

1700 M. S. Ben Kraiem et al.: Chemical hydrogenation effects on R2Fe14B compounds


Acknowledgements This study has been supported by the Tunisian Ministry of High Education, Scientific Re-search and Technology and by the CMCU collaboration (01/F-1127).

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Chemical hydrogenation effects on R2Fe14B compounds with (R = Ce, Nd and Gd)