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Study of hydrogenation of Sm2Fe17−yGay by means of X-ray diffraction
Journal of Alloys and Compounds 305 (2000) 298–305
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Study of hydrogenation of Sm 2 Fe 172y Ga y by means of X-ray diffraction ¨ * A. Teresiak, M. Uhlemann, M. Kubis, B. Gebel, N. Mattern, K.-H. Muller ¨ Festkorper¨ Institut f ur und Werkstofforschung Dresden, 01171 Dresden, P.O. Box 270016, D-01171 Dresden, Germany Received 3 January 2000; accepted 26 January 2000
Abstract The hydrogenation process of Sm 2 Fe 172y Ga y ( y50–2) was studied. X-ray investigations show a decreasing hydrogen solubility in the intermetallic alloy with increasing Ga-content from 4.060.3 atoms per formula unit for Sm 2 Fe 17 to 2.8560.05 for Sm 2 Fe 15 Ga 2 . The larger Ga atoms reduce the size of the interstitial sites and thereby the maximum hydrogen concentration is decreased. The behaviour of the lattice parameters a and c with increasing Ga content points to a changed hydrogen distribution on the interstitial sites, becoming more statistical. In situ observations by means of high temperature X-ray diffraction show that the hydrogen absorption process is diffusion controlled. The hydrogen absorption starts at an annealing temperature of 120–1408C in all cases. The solubility of hydrogen decreases with increasing temperature. The hydrogen is completely desorbed above 3508C in all cases. The absorption / desorption process is reversible between room temperature and 4008C. Annealing at temperatures above 4008C leads to the decomposition of the Sm 2 Fe 17 phase, indicated by emerging of a-Fe. The formation of SmH x is established at 6008C. The decomposition temperature increases with increasing Ga-content. Up to 7508C, only Sm 2 Fe 17 is completely decomposed. 2000 Elsevier Science S.A. All rights reserved. Keywords: Sm–Fe–H-compounds; Structure investigations; Hydrogenation; In situ high temperature X-ray diffraction; Sm 2 Fe 17
1. Introduction The influence of hydrogen on rare earth (R)–transition metal (T)-compounds having the Th 2 Zn 17 type structure was investigated by several authors [1–6]. As in the case of the non-metals C and N, the introduction of H into interstitial sites results in improved magnetic properties e.g. the Curie temperature T c of R 2 Fe 17 increases by about 140–200 K [2,3,6,7]. Moreover, the reaction with hydrogen can be used to obtain a fine-grained microstructure by means of the hydrogenation–disproportionation–desorption–recombination (HDDR) process [8–10] and thereby to improve extrinsic magnetic properties as coercivity. In this case, the hydrogen and the R-metal form binary hydrides at the disproportionation step. Additionally, a partial substitution of Fe by other elements M like Ga, Al or Si increases the thermal stability of the Sm 2 T 17 compound with Th 2 Zn 17 -type structure [11–13]. In the Th 2 Zn 17 -type structure the hydrogen occupies interstitial sites, forming a solid solution. This leads to an expansion of the unit cell. There are geometrical restric*Corresponding author. Tel.: 149-351-4659-527; fax: 149-351-4659537. ¨ E-mail address: [email protected] (K.-H. Muller)
tions for the maximum hydrogen concentration. Generally it is accepted, that the H-atoms occupy the octahedral (9e) sites completely and the tetrahedral (18g) sites partially [2,3,14,15]. This behaviour is related to the size of the (18g) interstitial sites and the minimum H–H distance of ˚ [16]. A complete occupation of the 18g sites would 2.1 A result in a H–H distance smaller than this minimum. Thus, only 1 / 3 of these sites can be occupied [3,5,14,15]. As the unit cell contains three formula units, a maximum Hconcentration per formula unit (f.u.) of 5 H atoms is expected. For many R–T-compounds the interstitial hydrogen sites were determined by powder neutron diffraction and subsequent Rietveld refinement [14,15]. Hereby, hydrogen was replaced by deuterium. The maximum concentration of absorbed deuterium was confirmed to be 4–5 atoms / f.u. In studying hydrogenation processes it has also to be taken into account that the experimental conditions like temperature, pressure and particle sizes influence the diffusivity of hydrogen [17,18]. Most of the reported investigations on the hydrogen absorption / desorption in metal compounds were done by using a thermopiezic analyzer (TPA). The TPA-method is based on the measurement of the temperature dependence of the hydrogen pressure in a fixed volume containing the material under a hydrogen atmos-
0925-8388 / 00 / $ – see front matter 2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 00 )00743-X
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299
phere [3,6,19]. Important features of the absorption / desorption process and of the decomposition follow from such pressure–temperature curves. The aim of this work was to study the influence of the partial substitution of Fe by Ga in Sm 2 Fe 17 on the hydrogen absorption / desorption behaviour as well as on the decomposition of Sm 2 Fe 172y Ga y at higher temperatures. The hydrogen content determined by means of X-ray diffraction (XRD) from the unit cell volumes was compared with the results of hot extraction. It will be shown that the in situ high temperature XRD method enabled the direct observation of the gas–solid reaction.
2. Experimental Sm 2 Fe 172y Ga y ( y50, 0.5, 1, 2) was produced by arc melting, induction melting and subsequent homogenization at 10008C. Fine-grained powder (diameter #10 mm) was prepared by ball milling and sieving. The composition of the powder samples was determined by chemical methods. The hydrogenation was carried out isothermally in a temperature pressure analyzer (TPA) under hydrogen atmosphere with a constant volume of about 300 ml. Each Sm 2 Fe 172y Ga y starting powder-sample (about 200 mg) was hydrogenated at 1808C at a starting pressure of 40 kPa and 80 kPa. The heating rate was 5 K / min. The samples were hydrogenated for 1 and 3 h in order to check whether equilibrium conditions and a homogeneous hydrogen distribution are achieved. The lattice constants before and after hydrogenation were determined by XRD with internal Si standard by means of a PHILIPS vertical goniometer with secondary monochromator and CoK a radiation. In situ high temperature X-ray investigations of the hydrogenation process were performed by using a STOE transmission diffractometer with a high temperature chamber in DEBYE-SCHERRER optics. The measurements were carried out with CoK a1 radiation using a primary Ge-monochromator. A curved position sensitive detector with a 2Q -region of about 428 and an angular resolution of 0.158 FWHM was used. A closed microsystem with a volume of ¯2 ml was developed for the gas–solid reaction (Fig. 1): A commercial Debye-Scherrer quartz capillary with a length of 80 mm and an outer diameter of 0.7 mm was equipped with an additional quartz buffer-volume combined with a long ground finished cone (Fig. 1). The capillary was filled with the Sm 2 Fe 172y Ga y powder (about 30 mg) in a glove box (under Argon) and sealed by a 908 valve ‘‘Ventura’’ with a passage of 4 mm and a conical joint-socket. In this way, the capillary could be evacuated and subsequently filled with hydrogen with a pressure of 40 or 80 kPa. The quartz capillary was sealed by welding below the cone. In the high temperature attachment the capillary is surrounded with a graphite heater. The temperature was measured using a Pt / PtRh thermocouple. The heating rate was 5 K / min up to 3508C or 4008C, followed
Fig. 1. Equipment for high temperature XRD investigations: A commercial quartz-capillary for X-ray DEBYE-SCHERRER-technique is connected to a buffer-volume and conical joint-socket for evacuation and filling with hydrogen gas. Together with a graphite heater (not shown) this enables the in situ XRD investigation of hydrogenation processes.
by cooling to room temperature with a cooling rate of 20 K / min. The X-ray measurements were started after a holding time of 2 min at the chosen temperature. The exposure time per diagram was 10 min. The 2Q -region was chosen from 258 up to 678. After this procedure the samples were heated up to 8008C to investigate the decomposition of the samples. The hydrogen concentration of the powders after TPA and in situ high temperature treatment was determined using hot extraction by induction heating up to ¯10008C and complete decomposition of the compounds (LECO RH-402). For data analysis the Visual Xpow software package (Copyright STOE & CIE GmbH) and the PC-Rietveld plus software (PHILIPS) were used.
3. Results and discussion
3.1. Isothermal hydrogenation of Sm2 Fe172 y Gay The XRD patterns of the starting powders showed traces of additional phases e.g. SmFe 2 , SmFe 3 and a-Fe. GaSm was detected in the powders with the highest Ga-concentration. The content of the additional phases was less than 3 wt%. The influence of these phases on the hydrogenation of the Sm 2 Fe 172y Gay phase can be neglected. Furthermore, the formation of a Fe-hydride and a GaSm-
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Fig. 2. XRD patterns (Co–K a radiation) of Sm 2 Fe 17 hydrogenized in a temperature–pressure-analyzer (TPA) at TA 51808C: Hydrogenation for 1 h results in a partially and inhomogeneously hydrogenized phase, as indicated by splitting of the marked (arrows) peaks. Hydrogenation for 3 h results in the fully hydrogenized Sm 2 Fe 17 phase.
hydride is rather improbable because of the low values of their formation enthalpies [17,20].
3.1.1. Process parameters and determination of the hydrogen concentration The hydrogenation of the Sm 2 Fe 172y Ga y material was carried out under different experimental conditions to obtain a homogeneously hydrogenated powder with maximum hydrogen concentration. Fig. 2 shows the room temperature XRD patterns after hydrogenation of Sm 2 Fe 17 at 1808C for 1 h and 3 h. The peak splitting in the diagram of the 1 h annealed sample indicates an inhomogeneous hydrogen distribution due to a diffusion barrier on the grain boundaries. The estimated lattice constants point to a non-equilibrium state consisting of a hydrogenated phase ˚ and c512.537 A ˚ and a non-hydrogenated with a58.676 A ˚ and c512.443 A. ˚ Nitrogenated one with a58.555 A samples show a similar behaviour, but for higher annealing temperatures (above 4508C), as observations of the magnetic-domains patterns indicate [21]. After 3 h an equilibrium state is reached. The powder is homogeneously hydrogenated, with only one set of lattice constants being the same as mentioned above within the estimated error
limits. To achieve thermodynamic equilibrium for the interstitial hydrogen, all samples were hydrogenated for 3 h under the same experimental conditions and their amount was taken to be equal. The hydrogen content of the samples was determined after hydrogenation and cooling to room temperature using different methods. XRD measures the increase of the lattice constants, compared with the starting material, which is related to the solved hydrogen only. An average volume expansion of 3.6% and 2.5% was found for Sm 2 Fe 17 and Sm 2 Fe 15 Ga 2 , respectively (see Table 1). The hydrogen content was calculated from the increase of unit ˚ 3 per H-atom. This cell by assuming an expansion of 2.3 A is a typical value used for estimating the hydrogen concentration in R 2 T 17 structures [2,3,14,15,22]. In this way, about 4.060.3 H atoms / f.u. for Sm 2 Fe 17 H x and 2.86 0.1 H atoms / f.u. for Sm 2 Fe 15 Ga 2 H x , respectively, have been obtained. This means, that Sm 2 Fe 17 H x can be assumed to be nearly fully hydrogenated. On the other hand, not all of the interstitial sites in Sm 2 Fe 15 Ga 2 H x are occupied by hydrogen. The reason of this observation is not yet understood. Besides the size of the interstitial sites, probably a changed interactions between the hydrogen atoms and the atoms of the host lattice has to be taken into account as will be discussed later. In Fig. 3 the hydrogen content estimated from XRD investigations is compared with results obtained (i) from analysing the pressure-difference curves and (ii) from hot extraction. These values are higher than those calculated from the lattice expansion (¯15% at 40 kPa and ¯35% at 80 kPa). The reason for this difference is, that the last two methods detect also surface-adsorbed hydrogen atoms. This suggestion is confirmed by the observed increase of surface-adsorbed hydrogen with increasing reaction pressure. In order to exclude surface-adsorbed hydrogen, the results XRD measurements have been used in the following.
3.1.2. Structure of Sm2 Fe172 y Gay Hx Sm 2 Fe 172y Gay crystallizes in the Th 2 Zn 17 -type structure, whereby the Ga atoms occupy partially and statistically the 18h sites [23,24]. This means, that the Ga atoms always are involved in the formation of both interstitial
Table 1 Influence of the Ga concentration on hydrogen absorption of Sm 2 Fe 172y Ga y , hydrogenated at 1808C for 3 h at 40 kPa and 80 kPa using TPAa Composition
˚ 3] Volume VS [A
Volume after ˚ 3] hydrogenation VH [A
(VH 2VS ) /VS [%] 40 kPa
Sm 2 Fe 17 Sm 2 Fe 16.5 Ga 0.5 Sm 2 Fe 16 Ga 1 Sm 2 Fe 15 Ga 2
789 794 798 80461
40 kPa
80 kPa
816 817 821 823
81762 819 822 82461
3.560.1 2.9 2.8 2.5
Hydrogen atoms per formula unit 80 kPa 3.5560.25 3.1 2.9 2.4560.05
40 kPa
80 kPa
4.060.1 3.3 3.2 2.9
4.0560.25 3.6 3.5 2.8560.05
a VS is the unit cell volume of the non-hydrogenated compounds, VH is the unit cell volume after hydrogenation. The hydrogen content per formula unit ˚ 3 per hydrogen atom. was estimated assuming a volume of 2.3 A
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301
Fig. 3. Dependence of the hydrogen concentration x in Sm 2 Fe 172y Ga y H x on the Ga-content y after hydrogenation in a TPA at 1808C for 3 h, for two values of the hydrogen pressure pH2 , and cooling down to room temperature: The hydrogen concentration was determined i) from the lattice parameters using XRD (♦ pH2 580 kPa; j pH2 540 kPa), ii) using hot extraction (앳 80 kPa; h 40 kPa) and iii) from the pressure difference before and after hydrogenation (, 40 kPa; n 80 kPa).
sites (9e) and (18g). The octahedral site (9e) is favoured to ˚ in absorb the H atoms as the site radius is larger than 0.4 A the Sm 2 Fe 17 cell and the H–H distance is greater than 2.1 ˚ [16]. From this point of view a complete occupation of A the sites (9e) is possible. These sites are formed by 2 R (6c) atoms, 2 T (18h) and 2 T (18f ) atoms. The tetrahedral interstitial sites (18g) are formed by 2 R (6c) atoms and 2 T (18h) atoms. Besides the influence of the Ga-atoms on the size of the interstitial sites, also the electronegativity has to be considered. The electronegativity of the tetrahedral site (18g), formed by 2R and 2T has a higher value in comparison to the octahedral hole (2R and 4T), see also [2,20]. This fact could make the tetrahedral sites more attractive to be occupied by hydrogen. Fig. 4 shows the change of the lattice constants of the compounds Sm 2 Fe 172y Ga y caused by hydrogenation in dependence on the Ga content y up to 2 Ga-atoms / f.u. The c /a-ratio of the starting powders is about 1.45 in all cases. The lattice constants a and c at room temperature have been differently increased after hydrogenation. With increasing Ga concentration the lattice constants a and c increase. But the relative difference Da /a between the lattice parameters a of Sm 2 Fe 172y Ga y H x and Sm 2 Fe 172y Ga y decreases from about 1.3760.04% for y50 to about 0.7760.03% for y52. The corresponding Dc /c value increases continuously from 0.6860.10% for y50 to 0.866 0.01% for y52. The estimated error of the calcu˚ for the lattice lated lattice parameters is ¯60.001 A ˚ for c, respectively. constant a and ¯60.002 A In the Sm 2 Fe 17 H 4 cell all 9e sites are completely occupied by hydrogen. The residual H atoms are assumed to partially occupy 18g sites. The question is, why does the hydrogen concentration decrease with increasing Ga con-
Fig. 4. Dependence of the lattice constants a (a) and c (b) on the Ga concentration y before hydrogenation (Sm 2 Fe 172y Ga y , filled symbols) and after hydrogenation in the TPA at 1808C for 3 h under different hydrogen pressure (Sm 2 Fe 172y Ga y H x , open symbols).
tent (cf. Fig. 3 and Table 1). Does the distribution of H atoms change with y? In Sm 2 Fe 15 Ga 2 H 2.8 (x,3) incompletely occupied 9e sites and unoccupied tetrahedral 18g sites are expected. This results in an anisotropic expansion of the lattice. In all cases reported so far, the lattice constant a changes stronger than c for Sm 2 Fe 17 H x and other R 2 Fe 17 H x compounds [2,5,21]. Similar results were observed introducing nitrogen or carbon into Sm 2 Fe 15 Ga 2 (Table 2). The larger non-metal atoms N, C occupy the 9e sites in planes z50, 1 / 3 and 2 / 3. As expected, the lattice expansion is anisotropic also for not completely occupied 9e sites. This is different from the results obtained in this work for the Sm 2 Fe 172y Ga y H x compounds. Therefore, also for x,3 an occupation of the 18g site is suggested. A similar tendency was found in [25]. But in this case, the samples were not fully hydrogenated (see also Table 2). The Ga atom is larger than the Fe atom. This should result in a decrease of the size of the interstitial site compared with that of the Sm 2 Fe 17 parent cell. The values for the octahedral site volume were estimated from the ˚ 3 for distances between the involved atoms to be 0.36 A 3 ˚ for Sm 2 Fe 15 Ga 2 . As the larger C Sm 2 Fe 17 and 0.34 A and N atoms are capable to occupy the (9e) site completely, it is assumed that the lower H concentration in the
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302
Table 2 Relative change of the lattice constants, Da /a and Dc /c, for Sm 2 Fe 172y Ga y X x (X5N, C, H) with different non-metals X and different values of the Ga content y, isothermally hydrogenated in the TPAa Da /a [%]
Dc /c [%]
(Dc /c) /(Da /a)
Reference
Sm 2 Fe 17 N 2 Sm 2 Fe 17 N 3 Sm 2 Fe 17 N 3 Sm 2 Fe 17 C 0.6 Sm 2 Fe 17 C 3 Sm 2 Fe 15 Ga 2 C 3
2.03 2.14 2.22 0.63 2.3 1.79
1.48 1.70 1.73 0.16 1.13 0.48
0.72 0.79 0.78 0.26 0.5 0.22
[26] [27] [22] [22] [23] [23]
Sm 2 Fe 17 H 5 Sm 2 Fe 17 H 2
1.50 1.17
0.86 0.51
0.57 0.44
[22] [6]
Sm 2 Fe 16 Ga 1 H x Sm 2 Fe 15 Ga 2 H x
0.74 0.66
0.49 0.49
0.66 0.68
[25] [25]
Sm 2 Fe 17 H 3.8 – 4.3 Sm 2 Fe 16.5 Ga 0.5 H 3.3 – 3.6 Sm 2 Fe 16 Ga 1 H 3..2 – 3.5 Sm 2 Fe 15 Ga 2 H 2.8 – 2.9
1.3860.05 1.1460.03 1.0060.01 0.7760.03
0.6860.10 0.6960.05 0.8460.04 0.8760.01
0.49 0.61 0.84 1.14
Present work Present work Present work Present work
a
The results are compared with data from literature.
Sm 2 Fe 15 Ga 2 H x cell is caused by changed interactions between the H and Ga atoms. If the interstitial sites partially formed by Ga atoms should be less attractive for H atoms, these would occupy preferentially such octahedral and tetrahedral sites formed without participation of Ga atoms, before filling the octahedral sites completely. This would explain the nearly isotropic volume expansion for high Ga and low H contents (Da /a¯Dc /c) during hydrogenation.
by means of isothermal treatment using TPA. As to be expected, hot extraction is sensitive also to surface-adsorbed hydrogen. The error was estimated comparing results of different measurements. Additional errors due to
3.2. High temperature X-ray investigations The aim of the high temperature X-ray investigation was (i) the determination of the thermal expansion coefficients and (ii) in situ investigation of the hydrogenation process. In order to separate the influence of the hydrogen absorption on the thermal expansion corresponding experiments were performed on the starting powder. Expansion along the a and c direction was determined by means of XRD. Heating was carried out under the same conditions but in argon atmosphere. Measurements were performed up to 8008C. The thermal expansion is different along aand c-direction. The linear approximation in the range up to 4008C (shown in Fig. 5) gives the linear thermal expansion coefficients aa 51.138310 25 / K (aa 50.9463 10 25 / K) and ac 51.827310 25 / K (ac 51.177310 25 / K) for Sm 2 Fe 17 (Sm 2 Fe 15 Ga 2 ), respectively. These values show that the thermal expansion as well as its anisotropy are lowered with increasing Ga-content. After in situ hydrogenation at temperatures up to 4008C the hydrogen concentration of the samples was determined by means of hot extraction. These values are again larger than those determined from the lattice expansion. They are in the same scale as the results for materials hydrogenated
Fig. 5. Thermal expansion of Sm 2 Fe 17 and Sm 2 Fe 15 Ga 2 . The lattice constant a (a) and c (b) have been measured under Ar atmosphere in the temperature range from 208C to 4508C. The expansion is a nearly linear function of temperature characterized by the temperature coefficients aa and ac of the lattice constants.
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variation of sample volume and to the of capillary volume are negligible.
3.2.1. Hydrogenation of Sm2 Fe17 and high temperature XRD Fig. 6 shows the in situ XRD patterns measured during hydrogenation of a Sm 2 Fe 17 sample, taken at various selected annealing temperatures TA . The first pattern presents the XRD diagram of the Sm 2 Fe 17 phase at room temperature before heating. The next two diagrams demonstrate the Sm 2 Fe 17 phase after the beginning of the hydrogen absorption. They reveal the coexistence of two phases at TA 51608C, a hydrogenated and a non-hydrogenated one, resulting in a peak-splitting. At 1808C a peak broadening is found, pointing to an inhomogeneously hydrogenated phase. The next XRD diagram shows the Sm 2 Fe 17 H x phase during the desorption process at 3008C. The Sm 2 Fe 17 phase decomposes when annealing is done above 4508C. This can be seen by the emerging a-Fe peak. It is expected, that the hydrogen and the released Sm form a hydride-like material already at TA 54508C. However, in the X-ray pattern a SmH x -phase emerges only at TA 5 6008C with very broad peaks, suggesting a very small grain size and an inhomogeneous chemical composition. At TA 56008C the FWHM of the peaks of the main phase are broadened too. This indicates increasing lattice strains due to volume changes during phase decomposition, because of preferred hydrogen adsorption on small-angle grain boundaries. These strains lead to cracking of the particles into smaller ones. The compound decomposes gradually at 4508C,TA ,6008C. Above 6508C the Sm 2 Fe 17 phase is completely decomposed (not shown here). The absorption / desorption behaviour of hydrogen is also reflected in the behaviour of the lattice constants,
Fig. 6. X-ray patterns of the Sm 2 Fe 17 H x powder recorded during the in situ high temperature investigation (Co–K a1 radiation). The appearance and position of characteristic peaks is related to the absorption and desorption of hydrogen. The decomposition of the phase is apparent by emerging of a-Fe (s) and SmH x (3).
303
which were determined from the diffraction angles during heating the samples. The estimated error for the lattice constants, determined from such X-ray diagrams, is ˚ for a and ¯0.007 A ˚ for c in all cases. These ¯0.002 A relatively large errors are due to the short measuring time. In Fig. 7 the change of the lattice constant a for a Sm 2 Fe 17 sample during heat treatment is shown as an example for such an in situ measurement. The arrows indicate the direction of the temperature variation. The marked onset of the decomposition of Sm 2 Fe 17 is given by emerging of a-Fe and SmH x peaks. During the first heating the hydrogen-gas must, at first, dissociate at the powder surface. The absorption starts at 120–1408C and is characterized by a strong increase of a. As in Fig. 6, the two-phase-region is observed up to 1608C and the lattice constant a of both phases is observed. At 1608C the greatest average volume expansion of the unit cell is achieved. For TA .1608C only the hydrogenated phase can be detected, still with a strong broadening of the reflections. Then, the desorption starts caused by the predomination of thermal activation [3,17,20]. At about 4008C the hydrogen is completely desorbed. Above this temperature the a(T )-curve becomes nearly that of Sm 2 Fe 17 . When cooling down, lattice parameters increase due to increased hydrogen solubility at lower temperatures. The cooling of the samples down to room temperature was performed at the same rate of 5 K / min. Below 1408C the lattice constant increases furthermore slightly. Repeating the annealing cycle showed the reversibility of this behaviour. No twophase region is observed during the second cycle. Now the powder surface is already in a state allowing an easy absorption of hydrogen. After repeating the annealing procedure the samples were annealed up to 8008C to
Fig. 7. Lattice constant a of Sm 2 Fe 17 vs. annealing temperature, measured during hydrogenation in the X-ray high temperature chamber. First run: RT→3008C (m and j)→RT (m), second run: RT→3008C→ RT (h), third run: annealing up to 6008C (s). The figure shows the two-phase-region around 1508C (m, j) in the first run, the desorption behaviour, the reversibility of the annealing process and the decomposition Sm 2 Fe 17 H x →a-Fe1SmH x . For comparison the thermal expansion of Sm 2 Fe 17 (앳) is shown.
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observe the decomposition of the compound into Smhydride and a-Fe. The volume fraction of the Sm 2 Fe 17 phase decreases with temperature, but small amounts are observed up to 6008C. The lattice constants of Sm 2 Fe 17 at TA .4008C are found to be identical under argon atmosphere and hydrogen atmosphere. Thus it can be concluded, that the lattice expansion for TA .4008C is only related to thermal effects and not to renewed insertion of hydrogen into the lattice. The decomposition of the main phase Sm 2 Fe 17 starts already at 4508C (visible by the emerging of an a-Fe peak in the X-ray pattern, cf. Fig. 6). In Fig. 7 the expansion of the residual Sm 2 Fe 17 phase is shown. It decomposes completely above 6508C.
3.2.2. Hydrogenation of Sm2 Fe172 y Gay and high temperature XRD Sm 2 Fe 172y Ga y compounds with 0.5#y#2 show a similar absorption behaviour during in situ high temperature investigations. However, a decreasing diffusion rate was found with increasing Ga content y. Therefore, no two-phase region is observed for the investigated Gacontaining compounds. The diffraction peaks are slightly broadened between 1408C and 1808C for y50.5 and 1, and between 1408C to 2208C for y52, pointing to an inhomogeneous hydrogen-distribution in the material in this temperature range. Table 3 summarizes the obtained results. The maximum hydrogen concentration during the first absorption process decreases with increasing Ga content and is achieved only at higher annealing temperatures, TA,max 51808C for y50.5 and 1 and at TA,max 5 2008C for y52. The hydrogen desorption is completed at 4008C in all cases. The development of the lattice constants a and c during the absorption / desorption process of Sm 2 Fe 15 Ga 2 is shown in Fig. 8. During cooling the hydrogen content increases continuously down to about
Fig. 8. Development of the lattice parameters a (a), c (b) and of the unit cell volume VH (T ) (c) of Sm 2 Fe 12 Ga 2 H x during the hydrogen absorption / desorption process in the X-ray high temperature chamber. The arrows indicate the increase / decrease of the temperature. In (c) VS (T ) is the unit cell volume of Sm 2 Fe 17 .
Table 3 Influence of the Ga concentration on hydrogen absorption of Sm 2 Fe 172y Ga y at 40 kPa, using in situ high temperature XRD measurements; TA,max , the temperature for which the maximum hydrogen concentration during the first annealing run is obtained a Composition Starting temperature of absorption [8C] TA,max [8C] (VH 2VS ) /VS [%] at TA,max ˚ 3] VS [A ˚ 3] VH [A (VH 2VS ) /VS [%] at room temperature H / f.u. (XRD) H / f.u. (hot extraction) starting temperature of decomposition [8C] a
Sm 2 Fe 17
Sm 2 Fe 16.5 Ga 0.5
Sm 2 Fe 16 Ga 1
Sm 2 Fe 15 Ga 2
120 160 1.5
120 180 1.5
120 180 1.0
120 200 0.89
789 80262 1.6 60.1
794 80661 1.560.1
798 808 1.3
803 82262 2.260.1
1.860.1 2.1
1.760.1 3.3
1.5 2.2
2.660.1 3.8
.450
.500
.500
.550
VS is the unit cell volume of the non-hydrogenated compounds, VH is the volume of hydrogenated unit cell at room temperature after one annealing ˚ 3 per hydrogen atom and cycle. The hydrogen content per formula unit (H / f.u.) was (i) estimated from the XRD data assuming a volume increase of 2.3 A (ii) by hot extraction measurements.
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1808C. Between 1808C and room temperature a increases less and c more strongly. The volume expansion increases continuously down to room temperature and amounts to more than 2% (Fig. 8c), which corresponds to about 2.6 H atoms / f.u. The values of Da /a¯0.75 60.05% and Dc /c ¯0.706 0.02% are of the same order as for isothermally hydrogenated samples. Heat treatment of other samples show the same behaviour. After the second absorption / desorption cycle a further annealing up to 8008C was performed. When the desorption process is completed at about 4008C the further increase of lattice parameters is only caused by the thermal expansion as in the Sm 2 Fe 17 samples. It is well known, that the thermal stability of Sm 2 Fe 172y Ga y is increased by Ga [18]. Therefore Sm 2 Fe 172y Ga y decomposes at higher temperatures depending on the Ga concentration, or the decomposition is prevented at all [28]. During heating up to 8008C the decomposition starts at TA ¯5008C and is completed above 6508C for compounds with y50.5 and 1, as shown by the appearance a weak a-Fe peak. The decomposition of Sm 2 Fe 15 Ga 2 starts at TA .5508C. This material ( y52) is still incompletely decomposed at TA 5 7508C. The decomposition of Sm 2 Fe 172y Ga y compounds annealed under Ar atmosphere starts above 5508C only.
4. Summary The lattice parameters obtained from XRD give direct information on the hydrogen solved in the Sm 2 Fe 172y Ga y H x phases. The solid solution obtained after hydrogenation is stable at room temperature. The reversible behaviour of the compounds in a hydrogen atmosphere in the in situ high temperature XRD measurements indicates that equilibrium states are realized. The isothermal reactions are determined by the free energy of formation which includes the configuration part given by the occupancy of the different interstitial sites of the lattice. The reduction of hydrogen solubility with increasing Ga content is correlated to the changes in the behaviour of the lattice parameters a and c. The interactions between the hydrogen and the different metal atoms are decisive for the hydrogen distribution on the interstitial sites. The investigations showed a decreasing Da /a and increasing Dc /c by hydrogenation for increasing Ga concentration at the chosen process parameters pressure, temperature and time. This suggests a more statistical distribution of hydrogen on the octahedral 9e- and the tetrahedral 18g-sites. To clarify this in more detail, the investigation of the thermal desorption will be combined with mass spectrometry in future work.
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Acknowledgements The work was supported by DFG (MA1531-3) and BMBF (13N7409 / 4).
References [1] C.N. Christodoulou, T. Takeshita, J. Alloys Comp. 194 (1993) 113. [2] D. Fruchart, M. Bacmann, P. de Rango et al., J. Alloys Comp. 253–254 (1997) 121. [3] S. Wirth, R. Skomski, J.M.D. Coey, Phys. Rev. B 55 (1997) 5700. [4] J.L. Soubeyroux, D. Fruchart, O. Isnard, S. Miraglia, E. Tomey, J. Alloys Comp. 219 (1995) 16. [5] O. Isnard, S. Miraglia, D. Fruchart, J. Deportes, J. Magn. Magn. Mat. 103 (1992) 157. [6] W. Xiang-Zhong, K. Donnelly, J.M.D. Coey, J. Mat. Sci. 23 (1988) 329. [7] O. Isnard, Thesis, J. Fourier University, Grenoble, 1992. [8] C.N. Christodoulou, T. Takeshita, J. Alloys Comp. 196 (1993) 155. ¨ [9] P.A.P. Wendhausen, B. Gebel, N.M. Dempsey, K.H. Muller, J.M.D. Coey, in: Proceedings of the 13th International Workshop on REMagnets and their Applications, Birmingham, 1994, p. 831. [10] O. Gutfleisch, J.C. Clarke, A.C. Neiva, S.A. Sinan, I.R. Harris, J. Alloys Comp. 233 (1996) 216. [11] B.G. Shen, F.W. Wang, L.S. Kong, L. Cao, W.S. Zhan, J. Appl. Phys. 75 (1994) 6253. [12] B.G. Shen, L.S. Kong, F.W. Wang, L. Cao, Appl. Phys. Lett. 63 (1993) 2288. [13] Z.H. Cheng, B.G. Shen, F.W. Wang, J.X. Zhang, H.Y. Gong, J.G. Zhao, J. Phys.: Condensed Matter 6 (1994) L185. [14] O. Isnard, S. Miraglia, J.L. Soubeyroux, D. Fruchart, J. Less-Comm. Met. 162 (1990) 273. [15] O. Isnard, S. Miraglia, J.L. Soubeyroux, D. Fruchart, J. Deportes, K.H.J. Buschow, J. Phys.: Condensed Matter 5 (1993) 5481. [16] D.G. Westlake, J. Less-Comm. Met. 90 (1983) 251. [17] Y. Fukai, The Metal–Hydrogen System, Basic Bulk Properties, Springer-Verlag, Berlin, Heidelberg, 1993. [18] K.H.J. Buschow, IEEE Trans. Magn. 30 (1994) 565. [19] D.H. Ryan, J.M.D. Coey, J. Phys. E 19 (1986) 693. [20] K.H.J. Buschow, P.C.P. Bouten, A.R. Miedema, Rep. Prog. Phys. 45 (1982) 937. ¨ [21] K.-H. Muller, L. Cao, N.M. Dempsey, P.A.P. Wendhausen, J. Appl. Phys. 79 (1996) 5045. [22] O. Isnard, S. Miraglia, M. Guillot, D. Fruchart, K.H.J. Buschow, J. Appl. Phys. 76 (1994) 6035. [23] N. Mattern, A. Handstein, L. Cao et al., Phys. Stat. Sol. (a) 164 (1997) 649. [24] Z. Hu, W.B. Yelon, S. Mishra et al., J. Appl. Phys. 76 (1994) 443. [25] R.A. Dunlap, Z.W. Wang, Can. J. Phys. 54 (1993) 574. [26] C.-J. Yang, W.-Y. Lee, S.-D. D Choi, Mat. Sci. Eng. A181 /A182 (1994) 1010. [27] A. Teresiak, B. Gebel, A. Handstein, N. Mattern, H. Klose, K.-H. ¨ Muller, Fresenius J. Anal. Chem. 361 (1998) 674. ¨ [28] M. Kubis, L. Cao, A. Handstein, B. Gebel, K.-H. Muller, L. Schultz, J. Appl. Phys. 81 (1997) 6485.
L
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Study of hydrogenation of Sm 2 Fe 172y Ga y by means of X-ray diffraction ¨ * A. Teresiak, M. Uhlemann, M. Kubis, B. Gebel, N. Mattern, K.-H. Muller ¨ Festkorper¨ Institut f ur und Werkstofforschung Dresden, 01171 Dresden, P.O. Box 270016, D-01171 Dresden, Germany Received 3 January 2000; accepted 26 January 2000
Abstract The hydrogenation process of Sm 2 Fe 172y Ga y ( y50–2) was studied. X-ray investigations show a decreasing hydrogen solubility in the intermetallic alloy with increasing Ga-content from 4.060.3 atoms per formula unit for Sm 2 Fe 17 to 2.8560.05 for Sm 2 Fe 15 Ga 2 . The larger Ga atoms reduce the size of the interstitial sites and thereby the maximum hydrogen concentration is decreased. The behaviour of the lattice parameters a and c with increasing Ga content points to a changed hydrogen distribution on the interstitial sites, becoming more statistical. In situ observations by means of high temperature X-ray diffraction show that the hydrogen absorption process is diffusion controlled. The hydrogen absorption starts at an annealing temperature of 120–1408C in all cases. The solubility of hydrogen decreases with increasing temperature. The hydrogen is completely desorbed above 3508C in all cases. The absorption / desorption process is reversible between room temperature and 4008C. Annealing at temperatures above 4008C leads to the decomposition of the Sm 2 Fe 17 phase, indicated by emerging of a-Fe. The formation of SmH x is established at 6008C. The decomposition temperature increases with increasing Ga-content. Up to 7508C, only Sm 2 Fe 17 is completely decomposed. 2000 Elsevier Science S.A. All rights reserved. Keywords: Sm–Fe–H-compounds; Structure investigations; Hydrogenation; In situ high temperature X-ray diffraction; Sm 2 Fe 17
1. Introduction The influence of hydrogen on rare earth (R)–transition metal (T)-compounds having the Th 2 Zn 17 type structure was investigated by several authors [1–6]. As in the case of the non-metals C and N, the introduction of H into interstitial sites results in improved magnetic properties e.g. the Curie temperature T c of R 2 Fe 17 increases by about 140–200 K [2,3,6,7]. Moreover, the reaction with hydrogen can be used to obtain a fine-grained microstructure by means of the hydrogenation–disproportionation–desorption–recombination (HDDR) process [8–10] and thereby to improve extrinsic magnetic properties as coercivity. In this case, the hydrogen and the R-metal form binary hydrides at the disproportionation step. Additionally, a partial substitution of Fe by other elements M like Ga, Al or Si increases the thermal stability of the Sm 2 T 17 compound with Th 2 Zn 17 -type structure [11–13]. In the Th 2 Zn 17 -type structure the hydrogen occupies interstitial sites, forming a solid solution. This leads to an expansion of the unit cell. There are geometrical restric*Corresponding author. Tel.: 149-351-4659-527; fax: 149-351-4659537. ¨ E-mail address: [email protected] (K.-H. Muller)
tions for the maximum hydrogen concentration. Generally it is accepted, that the H-atoms occupy the octahedral (9e) sites completely and the tetrahedral (18g) sites partially [2,3,14,15]. This behaviour is related to the size of the (18g) interstitial sites and the minimum H–H distance of ˚ [16]. A complete occupation of the 18g sites would 2.1 A result in a H–H distance smaller than this minimum. Thus, only 1 / 3 of these sites can be occupied [3,5,14,15]. As the unit cell contains three formula units, a maximum Hconcentration per formula unit (f.u.) of 5 H atoms is expected. For many R–T-compounds the interstitial hydrogen sites were determined by powder neutron diffraction and subsequent Rietveld refinement [14,15]. Hereby, hydrogen was replaced by deuterium. The maximum concentration of absorbed deuterium was confirmed to be 4–5 atoms / f.u. In studying hydrogenation processes it has also to be taken into account that the experimental conditions like temperature, pressure and particle sizes influence the diffusivity of hydrogen [17,18]. Most of the reported investigations on the hydrogen absorption / desorption in metal compounds were done by using a thermopiezic analyzer (TPA). The TPA-method is based on the measurement of the temperature dependence of the hydrogen pressure in a fixed volume containing the material under a hydrogen atmos-
0925-8388 / 00 / $ – see front matter 2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 00 )00743-X
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299
phere [3,6,19]. Important features of the absorption / desorption process and of the decomposition follow from such pressure–temperature curves. The aim of this work was to study the influence of the partial substitution of Fe by Ga in Sm 2 Fe 17 on the hydrogen absorption / desorption behaviour as well as on the decomposition of Sm 2 Fe 172y Ga y at higher temperatures. The hydrogen content determined by means of X-ray diffraction (XRD) from the unit cell volumes was compared with the results of hot extraction. It will be shown that the in situ high temperature XRD method enabled the direct observation of the gas–solid reaction.
2. Experimental Sm 2 Fe 172y Ga y ( y50, 0.5, 1, 2) was produced by arc melting, induction melting and subsequent homogenization at 10008C. Fine-grained powder (diameter #10 mm) was prepared by ball milling and sieving. The composition of the powder samples was determined by chemical methods. The hydrogenation was carried out isothermally in a temperature pressure analyzer (TPA) under hydrogen atmosphere with a constant volume of about 300 ml. Each Sm 2 Fe 172y Ga y starting powder-sample (about 200 mg) was hydrogenated at 1808C at a starting pressure of 40 kPa and 80 kPa. The heating rate was 5 K / min. The samples were hydrogenated for 1 and 3 h in order to check whether equilibrium conditions and a homogeneous hydrogen distribution are achieved. The lattice constants before and after hydrogenation were determined by XRD with internal Si standard by means of a PHILIPS vertical goniometer with secondary monochromator and CoK a radiation. In situ high temperature X-ray investigations of the hydrogenation process were performed by using a STOE transmission diffractometer with a high temperature chamber in DEBYE-SCHERRER optics. The measurements were carried out with CoK a1 radiation using a primary Ge-monochromator. A curved position sensitive detector with a 2Q -region of about 428 and an angular resolution of 0.158 FWHM was used. A closed microsystem with a volume of ¯2 ml was developed for the gas–solid reaction (Fig. 1): A commercial Debye-Scherrer quartz capillary with a length of 80 mm and an outer diameter of 0.7 mm was equipped with an additional quartz buffer-volume combined with a long ground finished cone (Fig. 1). The capillary was filled with the Sm 2 Fe 172y Ga y powder (about 30 mg) in a glove box (under Argon) and sealed by a 908 valve ‘‘Ventura’’ with a passage of 4 mm and a conical joint-socket. In this way, the capillary could be evacuated and subsequently filled with hydrogen with a pressure of 40 or 80 kPa. The quartz capillary was sealed by welding below the cone. In the high temperature attachment the capillary is surrounded with a graphite heater. The temperature was measured using a Pt / PtRh thermocouple. The heating rate was 5 K / min up to 3508C or 4008C, followed
Fig. 1. Equipment for high temperature XRD investigations: A commercial quartz-capillary for X-ray DEBYE-SCHERRER-technique is connected to a buffer-volume and conical joint-socket for evacuation and filling with hydrogen gas. Together with a graphite heater (not shown) this enables the in situ XRD investigation of hydrogenation processes.
by cooling to room temperature with a cooling rate of 20 K / min. The X-ray measurements were started after a holding time of 2 min at the chosen temperature. The exposure time per diagram was 10 min. The 2Q -region was chosen from 258 up to 678. After this procedure the samples were heated up to 8008C to investigate the decomposition of the samples. The hydrogen concentration of the powders after TPA and in situ high temperature treatment was determined using hot extraction by induction heating up to ¯10008C and complete decomposition of the compounds (LECO RH-402). For data analysis the Visual Xpow software package (Copyright STOE & CIE GmbH) and the PC-Rietveld plus software (PHILIPS) were used.
3. Results and discussion
3.1. Isothermal hydrogenation of Sm2 Fe172 y Gay The XRD patterns of the starting powders showed traces of additional phases e.g. SmFe 2 , SmFe 3 and a-Fe. GaSm was detected in the powders with the highest Ga-concentration. The content of the additional phases was less than 3 wt%. The influence of these phases on the hydrogenation of the Sm 2 Fe 172y Gay phase can be neglected. Furthermore, the formation of a Fe-hydride and a GaSm-
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300
Fig. 2. XRD patterns (Co–K a radiation) of Sm 2 Fe 17 hydrogenized in a temperature–pressure-analyzer (TPA) at TA 51808C: Hydrogenation for 1 h results in a partially and inhomogeneously hydrogenized phase, as indicated by splitting of the marked (arrows) peaks. Hydrogenation for 3 h results in the fully hydrogenized Sm 2 Fe 17 phase.
hydride is rather improbable because of the low values of their formation enthalpies [17,20].
3.1.1. Process parameters and determination of the hydrogen concentration The hydrogenation of the Sm 2 Fe 172y Ga y material was carried out under different experimental conditions to obtain a homogeneously hydrogenated powder with maximum hydrogen concentration. Fig. 2 shows the room temperature XRD patterns after hydrogenation of Sm 2 Fe 17 at 1808C for 1 h and 3 h. The peak splitting in the diagram of the 1 h annealed sample indicates an inhomogeneous hydrogen distribution due to a diffusion barrier on the grain boundaries. The estimated lattice constants point to a non-equilibrium state consisting of a hydrogenated phase ˚ and c512.537 A ˚ and a non-hydrogenated with a58.676 A ˚ and c512.443 A. ˚ Nitrogenated one with a58.555 A samples show a similar behaviour, but for higher annealing temperatures (above 4508C), as observations of the magnetic-domains patterns indicate [21]. After 3 h an equilibrium state is reached. The powder is homogeneously hydrogenated, with only one set of lattice constants being the same as mentioned above within the estimated error
limits. To achieve thermodynamic equilibrium for the interstitial hydrogen, all samples were hydrogenated for 3 h under the same experimental conditions and their amount was taken to be equal. The hydrogen content of the samples was determined after hydrogenation and cooling to room temperature using different methods. XRD measures the increase of the lattice constants, compared with the starting material, which is related to the solved hydrogen only. An average volume expansion of 3.6% and 2.5% was found for Sm 2 Fe 17 and Sm 2 Fe 15 Ga 2 , respectively (see Table 1). The hydrogen content was calculated from the increase of unit ˚ 3 per H-atom. This cell by assuming an expansion of 2.3 A is a typical value used for estimating the hydrogen concentration in R 2 T 17 structures [2,3,14,15,22]. In this way, about 4.060.3 H atoms / f.u. for Sm 2 Fe 17 H x and 2.86 0.1 H atoms / f.u. for Sm 2 Fe 15 Ga 2 H x , respectively, have been obtained. This means, that Sm 2 Fe 17 H x can be assumed to be nearly fully hydrogenated. On the other hand, not all of the interstitial sites in Sm 2 Fe 15 Ga 2 H x are occupied by hydrogen. The reason of this observation is not yet understood. Besides the size of the interstitial sites, probably a changed interactions between the hydrogen atoms and the atoms of the host lattice has to be taken into account as will be discussed later. In Fig. 3 the hydrogen content estimated from XRD investigations is compared with results obtained (i) from analysing the pressure-difference curves and (ii) from hot extraction. These values are higher than those calculated from the lattice expansion (¯15% at 40 kPa and ¯35% at 80 kPa). The reason for this difference is, that the last two methods detect also surface-adsorbed hydrogen atoms. This suggestion is confirmed by the observed increase of surface-adsorbed hydrogen with increasing reaction pressure. In order to exclude surface-adsorbed hydrogen, the results XRD measurements have been used in the following.
3.1.2. Structure of Sm2 Fe172 y Gay Hx Sm 2 Fe 172y Gay crystallizes in the Th 2 Zn 17 -type structure, whereby the Ga atoms occupy partially and statistically the 18h sites [23,24]. This means, that the Ga atoms always are involved in the formation of both interstitial
Table 1 Influence of the Ga concentration on hydrogen absorption of Sm 2 Fe 172y Ga y , hydrogenated at 1808C for 3 h at 40 kPa and 80 kPa using TPAa Composition
˚ 3] Volume VS [A
Volume after ˚ 3] hydrogenation VH [A
(VH 2VS ) /VS [%] 40 kPa
Sm 2 Fe 17 Sm 2 Fe 16.5 Ga 0.5 Sm 2 Fe 16 Ga 1 Sm 2 Fe 15 Ga 2
789 794 798 80461
40 kPa
80 kPa
816 817 821 823
81762 819 822 82461
3.560.1 2.9 2.8 2.5
Hydrogen atoms per formula unit 80 kPa 3.5560.25 3.1 2.9 2.4560.05
40 kPa
80 kPa
4.060.1 3.3 3.2 2.9
4.0560.25 3.6 3.5 2.8560.05
a VS is the unit cell volume of the non-hydrogenated compounds, VH is the unit cell volume after hydrogenation. The hydrogen content per formula unit ˚ 3 per hydrogen atom. was estimated assuming a volume of 2.3 A
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301
Fig. 3. Dependence of the hydrogen concentration x in Sm 2 Fe 172y Ga y H x on the Ga-content y after hydrogenation in a TPA at 1808C for 3 h, for two values of the hydrogen pressure pH2 , and cooling down to room temperature: The hydrogen concentration was determined i) from the lattice parameters using XRD (♦ pH2 580 kPa; j pH2 540 kPa), ii) using hot extraction (앳 80 kPa; h 40 kPa) and iii) from the pressure difference before and after hydrogenation (, 40 kPa; n 80 kPa).
sites (9e) and (18g). The octahedral site (9e) is favoured to ˚ in absorb the H atoms as the site radius is larger than 0.4 A the Sm 2 Fe 17 cell and the H–H distance is greater than 2.1 ˚ [16]. From this point of view a complete occupation of A the sites (9e) is possible. These sites are formed by 2 R (6c) atoms, 2 T (18h) and 2 T (18f ) atoms. The tetrahedral interstitial sites (18g) are formed by 2 R (6c) atoms and 2 T (18h) atoms. Besides the influence of the Ga-atoms on the size of the interstitial sites, also the electronegativity has to be considered. The electronegativity of the tetrahedral site (18g), formed by 2R and 2T has a higher value in comparison to the octahedral hole (2R and 4T), see also [2,20]. This fact could make the tetrahedral sites more attractive to be occupied by hydrogen. Fig. 4 shows the change of the lattice constants of the compounds Sm 2 Fe 172y Ga y caused by hydrogenation in dependence on the Ga content y up to 2 Ga-atoms / f.u. The c /a-ratio of the starting powders is about 1.45 in all cases. The lattice constants a and c at room temperature have been differently increased after hydrogenation. With increasing Ga concentration the lattice constants a and c increase. But the relative difference Da /a between the lattice parameters a of Sm 2 Fe 172y Ga y H x and Sm 2 Fe 172y Ga y decreases from about 1.3760.04% for y50 to about 0.7760.03% for y52. The corresponding Dc /c value increases continuously from 0.6860.10% for y50 to 0.866 0.01% for y52. The estimated error of the calcu˚ for the lattice lated lattice parameters is ¯60.001 A ˚ for c, respectively. constant a and ¯60.002 A In the Sm 2 Fe 17 H 4 cell all 9e sites are completely occupied by hydrogen. The residual H atoms are assumed to partially occupy 18g sites. The question is, why does the hydrogen concentration decrease with increasing Ga con-
Fig. 4. Dependence of the lattice constants a (a) and c (b) on the Ga concentration y before hydrogenation (Sm 2 Fe 172y Ga y , filled symbols) and after hydrogenation in the TPA at 1808C for 3 h under different hydrogen pressure (Sm 2 Fe 172y Ga y H x , open symbols).
tent (cf. Fig. 3 and Table 1). Does the distribution of H atoms change with y? In Sm 2 Fe 15 Ga 2 H 2.8 (x,3) incompletely occupied 9e sites and unoccupied tetrahedral 18g sites are expected. This results in an anisotropic expansion of the lattice. In all cases reported so far, the lattice constant a changes stronger than c for Sm 2 Fe 17 H x and other R 2 Fe 17 H x compounds [2,5,21]. Similar results were observed introducing nitrogen or carbon into Sm 2 Fe 15 Ga 2 (Table 2). The larger non-metal atoms N, C occupy the 9e sites in planes z50, 1 / 3 and 2 / 3. As expected, the lattice expansion is anisotropic also for not completely occupied 9e sites. This is different from the results obtained in this work for the Sm 2 Fe 172y Ga y H x compounds. Therefore, also for x,3 an occupation of the 18g site is suggested. A similar tendency was found in [25]. But in this case, the samples were not fully hydrogenated (see also Table 2). The Ga atom is larger than the Fe atom. This should result in a decrease of the size of the interstitial site compared with that of the Sm 2 Fe 17 parent cell. The values for the octahedral site volume were estimated from the ˚ 3 for distances between the involved atoms to be 0.36 A 3 ˚ for Sm 2 Fe 15 Ga 2 . As the larger C Sm 2 Fe 17 and 0.34 A and N atoms are capable to occupy the (9e) site completely, it is assumed that the lower H concentration in the
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302
Table 2 Relative change of the lattice constants, Da /a and Dc /c, for Sm 2 Fe 172y Ga y X x (X5N, C, H) with different non-metals X and different values of the Ga content y, isothermally hydrogenated in the TPAa Da /a [%]
Dc /c [%]
(Dc /c) /(Da /a)
Reference
Sm 2 Fe 17 N 2 Sm 2 Fe 17 N 3 Sm 2 Fe 17 N 3 Sm 2 Fe 17 C 0.6 Sm 2 Fe 17 C 3 Sm 2 Fe 15 Ga 2 C 3
2.03 2.14 2.22 0.63 2.3 1.79
1.48 1.70 1.73 0.16 1.13 0.48
0.72 0.79 0.78 0.26 0.5 0.22
[26] [27] [22] [22] [23] [23]
Sm 2 Fe 17 H 5 Sm 2 Fe 17 H 2
1.50 1.17
0.86 0.51
0.57 0.44
[22] [6]
Sm 2 Fe 16 Ga 1 H x Sm 2 Fe 15 Ga 2 H x
0.74 0.66
0.49 0.49
0.66 0.68
[25] [25]
Sm 2 Fe 17 H 3.8 – 4.3 Sm 2 Fe 16.5 Ga 0.5 H 3.3 – 3.6 Sm 2 Fe 16 Ga 1 H 3..2 – 3.5 Sm 2 Fe 15 Ga 2 H 2.8 – 2.9
1.3860.05 1.1460.03 1.0060.01 0.7760.03
0.6860.10 0.6960.05 0.8460.04 0.8760.01
0.49 0.61 0.84 1.14
Present work Present work Present work Present work
a
The results are compared with data from literature.
Sm 2 Fe 15 Ga 2 H x cell is caused by changed interactions between the H and Ga atoms. If the interstitial sites partially formed by Ga atoms should be less attractive for H atoms, these would occupy preferentially such octahedral and tetrahedral sites formed without participation of Ga atoms, before filling the octahedral sites completely. This would explain the nearly isotropic volume expansion for high Ga and low H contents (Da /a¯Dc /c) during hydrogenation.
by means of isothermal treatment using TPA. As to be expected, hot extraction is sensitive also to surface-adsorbed hydrogen. The error was estimated comparing results of different measurements. Additional errors due to
3.2. High temperature X-ray investigations The aim of the high temperature X-ray investigation was (i) the determination of the thermal expansion coefficients and (ii) in situ investigation of the hydrogenation process. In order to separate the influence of the hydrogen absorption on the thermal expansion corresponding experiments were performed on the starting powder. Expansion along the a and c direction was determined by means of XRD. Heating was carried out under the same conditions but in argon atmosphere. Measurements were performed up to 8008C. The thermal expansion is different along aand c-direction. The linear approximation in the range up to 4008C (shown in Fig. 5) gives the linear thermal expansion coefficients aa 51.138310 25 / K (aa 50.9463 10 25 / K) and ac 51.827310 25 / K (ac 51.177310 25 / K) for Sm 2 Fe 17 (Sm 2 Fe 15 Ga 2 ), respectively. These values show that the thermal expansion as well as its anisotropy are lowered with increasing Ga-content. After in situ hydrogenation at temperatures up to 4008C the hydrogen concentration of the samples was determined by means of hot extraction. These values are again larger than those determined from the lattice expansion. They are in the same scale as the results for materials hydrogenated
Fig. 5. Thermal expansion of Sm 2 Fe 17 and Sm 2 Fe 15 Ga 2 . The lattice constant a (a) and c (b) have been measured under Ar atmosphere in the temperature range from 208C to 4508C. The expansion is a nearly linear function of temperature characterized by the temperature coefficients aa and ac of the lattice constants.
A. Teresiak et al. / Journal of Alloys and Compounds 305 (2000) 298 – 305
variation of sample volume and to the of capillary volume are negligible.
3.2.1. Hydrogenation of Sm2 Fe17 and high temperature XRD Fig. 6 shows the in situ XRD patterns measured during hydrogenation of a Sm 2 Fe 17 sample, taken at various selected annealing temperatures TA . The first pattern presents the XRD diagram of the Sm 2 Fe 17 phase at room temperature before heating. The next two diagrams demonstrate the Sm 2 Fe 17 phase after the beginning of the hydrogen absorption. They reveal the coexistence of two phases at TA 51608C, a hydrogenated and a non-hydrogenated one, resulting in a peak-splitting. At 1808C a peak broadening is found, pointing to an inhomogeneously hydrogenated phase. The next XRD diagram shows the Sm 2 Fe 17 H x phase during the desorption process at 3008C. The Sm 2 Fe 17 phase decomposes when annealing is done above 4508C. This can be seen by the emerging a-Fe peak. It is expected, that the hydrogen and the released Sm form a hydride-like material already at TA 54508C. However, in the X-ray pattern a SmH x -phase emerges only at TA 5 6008C with very broad peaks, suggesting a very small grain size and an inhomogeneous chemical composition. At TA 56008C the FWHM of the peaks of the main phase are broadened too. This indicates increasing lattice strains due to volume changes during phase decomposition, because of preferred hydrogen adsorption on small-angle grain boundaries. These strains lead to cracking of the particles into smaller ones. The compound decomposes gradually at 4508C,TA ,6008C. Above 6508C the Sm 2 Fe 17 phase is completely decomposed (not shown here). The absorption / desorption behaviour of hydrogen is also reflected in the behaviour of the lattice constants,
Fig. 6. X-ray patterns of the Sm 2 Fe 17 H x powder recorded during the in situ high temperature investigation (Co–K a1 radiation). The appearance and position of characteristic peaks is related to the absorption and desorption of hydrogen. The decomposition of the phase is apparent by emerging of a-Fe (s) and SmH x (3).
303
which were determined from the diffraction angles during heating the samples. The estimated error for the lattice constants, determined from such X-ray diagrams, is ˚ for a and ¯0.007 A ˚ for c in all cases. These ¯0.002 A relatively large errors are due to the short measuring time. In Fig. 7 the change of the lattice constant a for a Sm 2 Fe 17 sample during heat treatment is shown as an example for such an in situ measurement. The arrows indicate the direction of the temperature variation. The marked onset of the decomposition of Sm 2 Fe 17 is given by emerging of a-Fe and SmH x peaks. During the first heating the hydrogen-gas must, at first, dissociate at the powder surface. The absorption starts at 120–1408C and is characterized by a strong increase of a. As in Fig. 6, the two-phase-region is observed up to 1608C and the lattice constant a of both phases is observed. At 1608C the greatest average volume expansion of the unit cell is achieved. For TA .1608C only the hydrogenated phase can be detected, still with a strong broadening of the reflections. Then, the desorption starts caused by the predomination of thermal activation [3,17,20]. At about 4008C the hydrogen is completely desorbed. Above this temperature the a(T )-curve becomes nearly that of Sm 2 Fe 17 . When cooling down, lattice parameters increase due to increased hydrogen solubility at lower temperatures. The cooling of the samples down to room temperature was performed at the same rate of 5 K / min. Below 1408C the lattice constant increases furthermore slightly. Repeating the annealing cycle showed the reversibility of this behaviour. No twophase region is observed during the second cycle. Now the powder surface is already in a state allowing an easy absorption of hydrogen. After repeating the annealing procedure the samples were annealed up to 8008C to
Fig. 7. Lattice constant a of Sm 2 Fe 17 vs. annealing temperature, measured during hydrogenation in the X-ray high temperature chamber. First run: RT→3008C (m and j)→RT (m), second run: RT→3008C→ RT (h), third run: annealing up to 6008C (s). The figure shows the two-phase-region around 1508C (m, j) in the first run, the desorption behaviour, the reversibility of the annealing process and the decomposition Sm 2 Fe 17 H x →a-Fe1SmH x . For comparison the thermal expansion of Sm 2 Fe 17 (앳) is shown.
304
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observe the decomposition of the compound into Smhydride and a-Fe. The volume fraction of the Sm 2 Fe 17 phase decreases with temperature, but small amounts are observed up to 6008C. The lattice constants of Sm 2 Fe 17 at TA .4008C are found to be identical under argon atmosphere and hydrogen atmosphere. Thus it can be concluded, that the lattice expansion for TA .4008C is only related to thermal effects and not to renewed insertion of hydrogen into the lattice. The decomposition of the main phase Sm 2 Fe 17 starts already at 4508C (visible by the emerging of an a-Fe peak in the X-ray pattern, cf. Fig. 6). In Fig. 7 the expansion of the residual Sm 2 Fe 17 phase is shown. It decomposes completely above 6508C.
3.2.2. Hydrogenation of Sm2 Fe172 y Gay and high temperature XRD Sm 2 Fe 172y Ga y compounds with 0.5#y#2 show a similar absorption behaviour during in situ high temperature investigations. However, a decreasing diffusion rate was found with increasing Ga content y. Therefore, no two-phase region is observed for the investigated Gacontaining compounds. The diffraction peaks are slightly broadened between 1408C and 1808C for y50.5 and 1, and between 1408C to 2208C for y52, pointing to an inhomogeneous hydrogen-distribution in the material in this temperature range. Table 3 summarizes the obtained results. The maximum hydrogen concentration during the first absorption process decreases with increasing Ga content and is achieved only at higher annealing temperatures, TA,max 51808C for y50.5 and 1 and at TA,max 5 2008C for y52. The hydrogen desorption is completed at 4008C in all cases. The development of the lattice constants a and c during the absorption / desorption process of Sm 2 Fe 15 Ga 2 is shown in Fig. 8. During cooling the hydrogen content increases continuously down to about
Fig. 8. Development of the lattice parameters a (a), c (b) and of the unit cell volume VH (T ) (c) of Sm 2 Fe 12 Ga 2 H x during the hydrogen absorption / desorption process in the X-ray high temperature chamber. The arrows indicate the increase / decrease of the temperature. In (c) VS (T ) is the unit cell volume of Sm 2 Fe 17 .
Table 3 Influence of the Ga concentration on hydrogen absorption of Sm 2 Fe 172y Ga y at 40 kPa, using in situ high temperature XRD measurements; TA,max , the temperature for which the maximum hydrogen concentration during the first annealing run is obtained a Composition Starting temperature of absorption [8C] TA,max [8C] (VH 2VS ) /VS [%] at TA,max ˚ 3] VS [A ˚ 3] VH [A (VH 2VS ) /VS [%] at room temperature H / f.u. (XRD) H / f.u. (hot extraction) starting temperature of decomposition [8C] a
Sm 2 Fe 17
Sm 2 Fe 16.5 Ga 0.5
Sm 2 Fe 16 Ga 1
Sm 2 Fe 15 Ga 2
120 160 1.5
120 180 1.5
120 180 1.0
120 200 0.89
789 80262 1.6 60.1
794 80661 1.560.1
798 808 1.3
803 82262 2.260.1
1.860.1 2.1
1.760.1 3.3
1.5 2.2
2.660.1 3.8
.450
.500
.500
.550
VS is the unit cell volume of the non-hydrogenated compounds, VH is the volume of hydrogenated unit cell at room temperature after one annealing ˚ 3 per hydrogen atom and cycle. The hydrogen content per formula unit (H / f.u.) was (i) estimated from the XRD data assuming a volume increase of 2.3 A (ii) by hot extraction measurements.
A. Teresiak et al. / Journal of Alloys and Compounds 305 (2000) 298 – 305
1808C. Between 1808C and room temperature a increases less and c more strongly. The volume expansion increases continuously down to room temperature and amounts to more than 2% (Fig. 8c), which corresponds to about 2.6 H atoms / f.u. The values of Da /a¯0.75 60.05% and Dc /c ¯0.706 0.02% are of the same order as for isothermally hydrogenated samples. Heat treatment of other samples show the same behaviour. After the second absorption / desorption cycle a further annealing up to 8008C was performed. When the desorption process is completed at about 4008C the further increase of lattice parameters is only caused by the thermal expansion as in the Sm 2 Fe 17 samples. It is well known, that the thermal stability of Sm 2 Fe 172y Ga y is increased by Ga [18]. Therefore Sm 2 Fe 172y Ga y decomposes at higher temperatures depending on the Ga concentration, or the decomposition is prevented at all [28]. During heating up to 8008C the decomposition starts at TA ¯5008C and is completed above 6508C for compounds with y50.5 and 1, as shown by the appearance a weak a-Fe peak. The decomposition of Sm 2 Fe 15 Ga 2 starts at TA .5508C. This material ( y52) is still incompletely decomposed at TA 5 7508C. The decomposition of Sm 2 Fe 172y Ga y compounds annealed under Ar atmosphere starts above 5508C only.
4. Summary The lattice parameters obtained from XRD give direct information on the hydrogen solved in the Sm 2 Fe 172y Ga y H x phases. The solid solution obtained after hydrogenation is stable at room temperature. The reversible behaviour of the compounds in a hydrogen atmosphere in the in situ high temperature XRD measurements indicates that equilibrium states are realized. The isothermal reactions are determined by the free energy of formation which includes the configuration part given by the occupancy of the different interstitial sites of the lattice. The reduction of hydrogen solubility with increasing Ga content is correlated to the changes in the behaviour of the lattice parameters a and c. The interactions between the hydrogen and the different metal atoms are decisive for the hydrogen distribution on the interstitial sites. The investigations showed a decreasing Da /a and increasing Dc /c by hydrogenation for increasing Ga concentration at the chosen process parameters pressure, temperature and time. This suggests a more statistical distribution of hydrogen on the octahedral 9e- and the tetrahedral 18g-sites. To clarify this in more detail, the investigation of the thermal desorption will be combined with mass spectrometry in future work.
305
Acknowledgements The work was supported by DFG (MA1531-3) and BMBF (13N7409 / 4).
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