Hydrogen absorption in R2Fe17 alloys (R=rare earth metals) thermodynamics, structural and magnetic properties

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ALLOYS AND COM~UNDS ELSEVIER

Journal of Alloys and Compounds 257 (1997) 150-155

Hydrogen absorption in R2Fe17 alloys (R=rare earth metals) thermodynamics, structural and magnetic properties O. Isnard a, S. Miraglia a, D. Fruchart a'*, E. Akiba b, K. Nomura b "Laboratoire de Cristallographie du C.N.R.S., associd ~ t 'Universitg J. Fourier, BP 166, 38042 Grenoble Cedex 9, France bNational Institute for Materials and Chemical Research, 1-1 Higashi Tsukuba, Ibaraki 305, Japan Received I1 December 1996; received in revised form 23 December I996

Abstract Hydrogen absorption effects in the RzFelT series of alloys have been established from pressure-composition (pc) isotherms, crystal structure determination and magnetic measurements. If no plateau pressure has been observed in the (pc)-isotherms, such curves have been found to be dependent on the nature of the R element. The results agree with a progressive but successive filling of the octahedrat 2R-4Fe and the tetrahedral 2R-2Fe sites. Most of the modifications observed in the magnetic properties can be correlated with changes in selected Fe-Fe distances. © 1997 Elsevier Science S.A. Keywords: Crystal structure; Magnetic properties; Metal hydrides; Rare earth intermetallics; Rare earth-iron alloys

1. Introduction The R2Fel7 series of alloys with R=rare earth metal, exhibit potential high magnet properties when fight interstitial elements such as X = C , N are inserted in the metal lattice. Improvements simultaneously occur on Curie temperature, magnetisation and anisotropy field parameters. Hydrogen insertion can be achieved up to x=5H/f.u., Since two interstitial sites (3-octahedral O and 2-tetrahedral T sites) are concerned. Otherwise, only the Curie temperature and magnetisation are improved contrarily to the case of the most electronegative X = C and N (up to x=3X/f.u. in the O sites). If these last elements can favourably act on the magnetocrystalline anisotropy parameters, then this is contrary to hydrogen which has been found to decrease the anisotropy field as well as the electric field gradient [1]. However, hydrogen insertion being easy to continuously monitor, the changes in the intrinsic parameters can be analyzed in terms of the structural modifications induced versus the amount of H inserted.

3N5 purity grades, for iron and the R elements, respectively. To measure the pressure-composition (pc) isotherms, a conventional constant volume apparatus was used. In order to check the crystalline quality of the samples both before and after hydrogenation, X-ray powder diffraction apparatuses were used, as a Guinier-Hfigg camera (AveKa 1) and a Bragg-Brentano type goniometer equipped with a backscattering graphite monochromator (AcoK~). The neutron diffraction experiments and results have been already presented in detail in [2], and only selected trends will be analyzed hereafter. A Faraday type of torque has been used in order to measure the Curie temperature behaviour of the samples, versus the hydrogen content. For this purpose, the samples have been sealed in small silica tubes, with a restricted internal dead-volume. By so doing, hydrogen desorption is markedly minimised on heating, and in a first approximation, the hydrogen composition of the sample is estimated to be constant.

2. Experimental The alloys were HF melted by using the so-called cold crucible method. The starting elements were of 4N and *Corresponding author. 0925-8388/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0925-8388(97)00002-9

3. Hydrogen uptake The hydrogen absorption isotherms have been measured for the first time on NdaFe17 and reported by Rupp et al. [3,4], in the range of 10 to 104pa and at 470 and 570 K

O. Isnard et al. / Journal of Alloys and Compounds 257 (1997) 150-155

respectively. No evidence of a plateau pressure was found in this relatively low pressure range, that corresponds to a restricted part of the M - H phase diagram, considering the expected maximum hydrogen uptake of 5H/f.u. [5]. At temperatures checked by Rupp, but in the range of pressure 104 to 5.106pa, we have undertaken new measurements (Fig. la). Close to 104Pa, our data are in good agreement with the previous results. However, a small shift between the curves should indicate that a few free Nd (<--2% in mass) may be present in our Nd2Fe17 sample, see part 3.1 in [2], or that of equivalent weak amounts of non reacting free iron in the samples of Refs. [3,4]. No more here, a plateau equilibrium pressure is pointed out, but the (pc) curves exhibit different slopes, roughly of 1 MPa/H for c<-0.5, and of 1 kPa/H for 0.4<-c<-4. Moreover, the (pc) absorption curve recorded at 470 K indicates a final uptake close to 4.5H/f.u. for pressure up to 5.106 Pa, that is more than the filling level of the octahedral site only (3H/f.u.) [5]. The H-uptake is not markedly reduced at T=570 K, evidencing the possible existence of a plateau temperature in the range of the highest pressure. We have performed similar measurements on the R=Ce, Pr and Ho R2Fet7 compounds (Fig. lb). Clear differences appear between these compounds in their absorption isotherms that could be related to the nature of R (atom size, tri- or intermediate valence state). However, the desorption isotherms measured after a step of 12 h at constant temperature and constant pressure (570 K and 4.106 Pa), start similarly and indicate a limit composition close to 5H/f.u. This is exactly the limit predicted earlier and measured by using neutron diffraction [5]. In any case, this type of result does not at all allows us to point out in the solid solution like behaviour, some trends related to the respective filling of the two different types of sites. In the 0<-x--<3 range of composition, Fig. lb reveals that the stability of the ternary hydrides depends on the volume unit cell of the alloy. It decreases from Pr, Nd, Ce (anomalous) to Ho, in good agreement with the maximum hydrogen uptake Hmax. curve reported in Fig. 1 of Ref. [6]. The in situ desorption experiment performed by neutron diffraction under normal hydrogen pressure, clearly evidence a two sites behaviour from the refinement of the respective occupation numbers (Fig. 2a) [2]. A straight correlation can be established with the behaviours of the cell parameters (Fig. 2b). The octahedral (O) and tetrahedral (T) sites earlier pointed out [5] both exhibit plateau temperature stability up to 470 and 370 K respectively. However, hydrogen releases from these sites over a rather large range of temperature, up to 650 and 500 K for the two active sites, respectively. This should be understood by taking into consideration that both the two types of sites are multisites, 3 for (O) and 2 only for (T), existing in two types of hexagonal Fe rings and pairs of R atoms, respectively. The

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been applied to predict the restricted distribution onto the (T) sites [5,9]. Hence, the insertion sites do not share any common triangular faces, but 3 (or 2) hydrogen atoms may be bonded to the same R atom, for the O (or T) sites. So, it is possible that thermal activation creating a local vacancy

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O. lsnard et al. / Journal of Alloys and Compounds 257 (1997) 150-155

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temperature. It is represented in Fig. 5 as the best fit to the data. Three types of variation are observed. First a thermal expansion up to 440 K during which no appreciable desorption of hydrogen is observed (Fig. 2a). A similar trend occurs for the highest temperature (660-750 K) where the compound was found to be entirely desorbed, tn the intermediate temperature range, a marked drop of the F e l - F e 1 distance can be correlated to the hydrogen desorption process. From this behaviour, and referring to the room temperature dumbbell distance in Nd2Fe17H=5 and in Nd2Fe17 respectively, correction of the thermal cell expansion has been applied. The hypothetical variation of the F e t - F e ~ distance versus the thermally induced desorption process of hydrogen only, is plotted on Fig. 5. Then using the hydrogen content (x) variation with temperature (Fig. 2a), the behaviour of the dumbbell distance versus x = H / f . u , was estimated. If the distance increases continuously (Fig. 6), two regimes of variation can be distinguished, for 0--x->5, the rate is ~0.1 ]~/H. A limitation of the distance expansion rate is clearly visible for 2.5---x--<3.5.

5. T h e C u r i e t e m p e r a t u r e magnetisation

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0 lsnard et al / Journal of Alloys and Compounds 257 (1997) 150-155

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f.u. (Fig. 7). This trend was observed on all our different R-containing samples; thus we consider that the slight deviation to a linear expansion detected by Rupp et al. 2.46

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O. lsnard et al. / Jo,~rnat of Alloys and Compounds 25;7 (1997) 150-155

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compared to the relative change in exchange energy versus the F e i - F e ~ distance related to the reference distance of 2.5 A in a-Fe. The best fit has been observed to a power law expression [2 G - F~/(2G_Fe-dFeI_F~ 1)]~ when J3~3. This could be interpreted in terms of indirect exchange of Ruderman-Kittel type with Tc ~ 12 cos 2kFR/(kFR) 3 [12]. Hence, the change in the dumbbell distance dF~_ve ~ seems to determine most of the increase of Curie temperature in the R2Fe I7-H systems.

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Fig. 8. Behaviour of the Fe~-Fe~ (dumbbell) distance versus x the hydrogen content, in NdzFe~TH~ (squares and fitting line); behaviour of the reduced mean exchange energy JF~-F~ versus x the hydrogen content as deduced from Eq. (1), compared to the relative distance expansion [2G.FJ(2G.r~--dr~l_~i)] 3 (diamonds). Both sets of data are plotted in normalised units. ly increases with the hydrogen content (Fig. 8). So, (JF~-Fe) appears to increase as well when the O and then the T sites are progressively filled. The former site is the only concerned in the parent carbides and nitrides [11], but in these cases the cell volume expansion is markedly larger than for hydrides.

6. Discussion: local effects and distances Precise neutron diffraction experiments performed on a series of R2Fe~v hydrides have revealed that the most important change by far in the F e - F e distances concerns the dumbbell distance [9]. It is more than the mean cell parameter expansion that generally concerns all the interatomic distances (e.g. all the F e - F e distances). From the in situ neutron diffraction experiments, we have determined the dF~_F~ a (dumbbell) distance variation versus the hydrogen content. After correction of the thermal expansion of the parameter, we have determined the normalised (to room temperature) dF~,_F~, expansion rate versus x, the hydrogen content (Fig. 6). So we can proceed with a discussion based on the variations with the hydrogen content of both selected crystalline and magnetic parameters. It appears that in the RzFeI7H;, series, the increase in Curie temperature versus x results in: • for 0-
7. Conclusion A continuous and progressive filling scheme of the interstitial sites is found from the measurements of the total hydrogen uptake from (pc) isotherm experiments as well as from refinements of neutron diffraction experiments. The pressure isotherms curves fairly resemble those of the o~-solid solution systems found for the storage materials (e.g. LaNis) in the corresponding range of values of the H / M ratio. However, anisotropic expansion effects (unit cell parameters), local atom displacements, magnetisation measurements, in situ neutron diffraction a n a l y s i s . . , reveal that the octahedral O site is thermodynamically more stable than the tetrahedral T one. A phenomenological scheme has been used in order to describe the increase of the Curie temperature T c, on the basis of indirect exchange couplings [12] varying with selected distances. Local considerations seem of first importance for the local magnetic moments, and then on magnetisation. This corroborates the atomic model point of view on the Fe magnetism [13]. Saturation magnetisation M s varies much quickly when x > 3 than for x < 3 , i.e. the mean increase of the iron atomic volume (WS volume, [13]) is much larger when tetrahedral sites are charged. This corresponds to a fast c cell parameter expansion. Exchange interactions appear to be much more improved during the filling of the octahedral site, i.e. when the dFd_Fe ~ short distance (lying along the c-axis!) is markedly expanded. In spite of their typicat solid solution behaviour, the R2FeIT-H systems reveal magnetic behaviours that are particularly sensitive to local characteristics.

Acknowledgments A MITI grant was given to one of us (D.F.) during a stage at NIMC (Tsukuba) and it has permitted the achievement of this work.

References [1] O. Isnard, P. Vuiltet, A. Blaise, J.E Sanchez S. Miraglia and D. Fruchart, I. Magn. Magn. Mat., 131 (1993) 83.

O. Isnard et al. / Journal of Alloys and Compounds 257 (1997) i 5 0 - 1 5 5

[2] O. Isnard, J.L. Soubeyroux, S. MiragIia, D. Fruchart, L.M, Garcia and J. Bartolom~, Physica B, 180-181 (1992) 629. [3] B. Rupp and G. Wiesinger, J. Magn. Magn. Mat., 7I (1988) 269. [4] B. Rupp, A. Resnik, D. Shalthiel and P. Rogl, J, Mat. Sci,, 23 (1988) 2133. [5] O. Isnard, S. Miraglia, D. Frucbart, J-L. Soubeyroux and A. Stergiou, J. Less- Comm. Met., 162 (1990) 273. [6] D. Fruchart, O. Isnard, S. Miraglia and J-L. Soubeyroux, J. Alloys Comp,, 23I (I995) 188. [7] A.C. Switendick, in G. Alefeld (ed.), Hydrogen in Metals I, Springer Verlag, Berlin, 1978, Chap. 5.

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[8] D.G. Westlake, J, Less-Comm. Met., 90 (1983) 251 and 91 (1983) 275. [9] O. Isnard, Thesis, University of Grenoble (1992). [10] O. Isnard, S, Miraglia, D. Fmclqart, C. Giorgetti, S. P]zzini, E. Dartyge and G. Krill, Phys. Rev. B, 49(22) (1994) 15692. [11] S. Miraglia, J.L. Soubeyroux, C. Kolbeck, O. Isnard, D. Fruchart and M. Ouillot, J. Less Comm. Met., 171 (t99I) 51. [12] M,A. Ruderman, C. Kittel, Phys. Rev., 96 (1954) 99. [13] O. Isnard and D. Fruchart, J. AlLoys Comp., 205 (1994). 1.