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The hydrides of YFe2 and GdFe2
Journal
of the Less-Common
86 (1982)
Metals,
Ll - L4
Ll
Letter
The hydrides
of YFe, and GdFe,
HENRY
A. KIERSTEAD
Argonne
National
(Received
Laboratory,
9700
South
Cass Avenue,
Argonne,
IL 60439
(U.S.A.)
May 11,1982)
1. Introduction It is well known that the hydrides of intermetallic compounds of rare earth metals with transition metals are thermodynamically unstable with respect to decomposition into the rare earth hydride and the transition metal, e.g. ErFe,H,
+ ErH, + 2Fe
However, because of the low rate of metal atom diffusion at temperatures near room temperature, many of these compounds can be subjected to hundreds of hydriding-dehydriding cycles without decomposition. For example, Schlapbach et al. [l] have reported that they cycled LaNi, 100 times with only 1% decomposition. In contrast, Busch et al. [2] have reported that La,Nis decomposes into LaI-Ia and LaNi, during the first hydrogenation. Ce,Nia behaves similarly. Decomposition during initial hydriding has also been reported in rare earth-magnesium compounds [ 31. Not much is known about the stability of the hydrides of the Laves phase compounds. Shaltiel [4] listed 42 ABs compounds which absorb substantial quantities of hydrogen. In most cases these hydrides have not been very thoroughly investigated. It is possible that the nine compounds which were reported to absorb only about three hydrogen atoms per mole of intermetallic compound had decomposed to the rare earth trihydride and the transition metal, whereas those which absorbed four or more hydrogen atoms had resisted decomposition at least in the initial hydriding. Cohen et al. [ 51 have reported that EuRh,, which initially absorbs five hydrogen atoms, has only 10% of its reversible hydrogen absorbing capacity remaining after five absorption-desorption cycles. We have reported isotherms of ErFe, [6] and TmFea [7] hydrides which show no evidence of decomposition after five hydrogenationdehydrogenation cycles. In contrast, we have only with difficulty prepared hydrides of YFe, and GdFe,, and they decomposed after one desorption isotherm. We report on these experiments here. 0022-5088/82/0000-0000/$02.75
0 Elsevier
Sequoia/Printed
in The Netherlands
L2
2. Experimental procedure 2.1. YFe, The YFe, sample was prepared from 99.9% pure yttrium and iron by repeated arc melting in an argon atmosphere. In order to obtain a sample of YFe, free from YFe, impurity it was found necessary to use a 5% excess of yttrium. The sample was annealed at 850 “C for 20 days. Debye-Scherrer X-ray patterns showed single-phase cubic Laves phase (Cl5 type) crystals with a lattice constant of 7.351 f 0.01 A. On exposure to hydrogen at 10000 Torr and 0 “C it absorbed only 3.05 hydrogen atoms per yttrium atom. On desorption at 300 “C the hydrogen concentration was reduced to 2.07 hydrogen atoms per yttrium atom at a pressure of 10.38 Torr. There seems little doubt that the sample had decomposed to YHs and iron during the initial hydriding and the YHs was converted to YHs during the 300 “C desorption. Another sample of the YFe, was activated by heating to 300 “C in a vacuum for 1 h and in 50 Torr of hydrogen for 1 h. It was then evacuated for 1 h at 300 “C and cooled to 20 “C. With this pretreatment the sample was able to react with hydrogen at 400 Torr and 20 ‘C, taking up 3.52 hydrogen atoms per mole of YFe, at a final pressure of 92.8 Torr. This more gentle hydrogenation is believed to diminish local heating resulting from the large heat of absorption. Finally the sample was exposed to hydrogen at 10000 Torr and came to equilibrium at a pressure of 8195 Torr and a hydrogen concentration x of 4.32 hydrogen atoms per mole of YFea. A 20 “C desorption isotherm taken on this sample is shown by the square symbols in Fig. 1. Because of the low equilibrium pressures below x = 3.3 it was necessary to heat the sample to various temperatures below 100 “C!to drive out the hydrogen, but the sample was cooled to 20 “C to measure the equilibrium pressure. For the same reason it was impossible to measure the desorption isotherm below x = 2.0. At the end of the desorption
H/YFe2
Fig. 1. Desorption
isotherms
of YFe2
at 20 “C: 0, first desorption;
0, second
desorption.
L,3
isotherm hydrogen was admitted at 390 Torr and x increased to 3.02 at an equilibrium pressure of 346 Torr. The sample would not take more hydrogen even at 9140 Torr. The 20 “C desorption isotherm measured after this second hydriding is shown by the circles in Fig. 1. It is what would be expected for the desorption isotherm of YHs which has an equilibrium pressure of only 10 Torr at 250 “C [8]. 2.2. GdFe, The GdFe, sample was prepared from 99.9% pure gadolinium and iron by repeated arc melting in an argon atmosphere. It was annealed at 900 “C for 10 days. Debye-Scherrer X-ray patterns showed single-phase cubic Laves phase (Cl5 type) crystals with a lattice constant of 7.403 A. The material would not absorb hydrogen at 1 atm; at 10000 Torr it absorbed hydrogen only to x = 3.4. Very little hydrogen was given up on desorption. The 57Fe Mijssbauer spectrum of this sample, removed at x = 2.91, showed only the hyperfine field characteristic of metallic iron. Debye-Scherrer X-ray patterns showed lines of metallic iron and GdHs. The intensities of the GdH, lines relative to those of the iron lines accounted for only about 20% of the gadolinium. No GdHs lines were seen. Two broad lines which could not be classified may be due to some other hydride in microcrystalline form. Another sample of GdFe, was activated by heating to 200 “C in a vacuum, exposing it to 130 Torr of hydrogen at 200 “C for 16 h and evacuating again at 200 “C. It was then successfully hydrided at 20 “C and 2 atm pressure up to 3c = 4.2 (equilibrium pressure 653 Torr). Finally, it was exposed to 10000 Torr of hydrogen to bring it up to x = 4.73 at an equilibrium pressure of 8716 Torr. The 20 “C desorption isotherm shown in Fig. 2 was then measured. As with YFe, it was necessary to heat the sample (but not above 100 “C) to drive off hydrogen in measuring the lower part of the desorption curve.
H/GdFe2 Fig. 2. Desorption
isotherms
of GdFez.
L4
When the sample was rehydrided at 20 “C it could be brought only to x = 4.20 at 4120 Torr compared with 4.70 at that pressure on the first hydriding cycle. This suggests that 30% of the GdFe, had already decomposed to GdHs and iron. The 80 “C desorption curve shown in Fig. 2 was then measured. The end of the desorption curve at x = 0.83 is consistent with the hypothesis that at that point 30% of the gadolinium was in the form of GdHs and the rest in the form of GdFea containing very little hydrogen. When the sample was hydrided a third time, it absorbed hydrogen only to x = 2.81. The third desorption isotherm resembled the second desorption isotherm of YFe, in Fig. 1. 3. Discussion These studies make it clear that, while GdFe, and YFe, hydrides can be prepared, they are very unstable. The nature of the decomposition products is still obscure. While the iron precipitates as crystalline iron and some crystalline rare earth dihydride forms, the bulk of the rare earth and hydrogen are apparently left in an amorphous state containing somewhat more than three hydrogen atoms per rare earth atom, which is nevertheless quite stable with respect to absorbing or desorbing hydrogen.
1 L. Schlapbach, A. Seiler, F. Stucki, P. Ziircher, P. Fischer and J. Schefer, 2. Phys. Chem. (Frankfurt am Main), I1 7 (1979) 205. 2 G. Busch, L. Schlapbach and Th. von Waldkirch, J. Less-Common Met., 60 (1978) 83. 3 B. Darriet, M. Pezat, A. Hbika and P. Hagenmueller, Znt. J. Hydrogen Energy, 5 (1980) 173. 4 D. Shaltiel, J. Less-Common Met., 62 (1978) 407. 5 R. L. Cohen, K. W. West and K. H. J. Buschow, Solid State Commun., 25 (1978) 293. 6 H. A. Kierstead, J. Less-Common Met., 70 (1980) 199. 7 H. A. Kierstead, J. Less-Common Met., 85 (1982) 213. 8 L. N. Yannopoulos, R. K. Edwards and P. G. Wahlbeck, J. Phys. Chem., 69 (1965) 2515.
of the Less-Common
86 (1982)
Metals,
Ll - L4
Ll
Letter
The hydrides
of YFe, and GdFe,
HENRY
A. KIERSTEAD
Argonne
National
(Received
Laboratory,
9700
South
Cass Avenue,
Argonne,
IL 60439
(U.S.A.)
May 11,1982)
1. Introduction It is well known that the hydrides of intermetallic compounds of rare earth metals with transition metals are thermodynamically unstable with respect to decomposition into the rare earth hydride and the transition metal, e.g. ErFe,H,
+ ErH, + 2Fe
However, because of the low rate of metal atom diffusion at temperatures near room temperature, many of these compounds can be subjected to hundreds of hydriding-dehydriding cycles without decomposition. For example, Schlapbach et al. [l] have reported that they cycled LaNi, 100 times with only 1% decomposition. In contrast, Busch et al. [2] have reported that La,Nis decomposes into LaI-Ia and LaNi, during the first hydrogenation. Ce,Nia behaves similarly. Decomposition during initial hydriding has also been reported in rare earth-magnesium compounds [ 31. Not much is known about the stability of the hydrides of the Laves phase compounds. Shaltiel [4] listed 42 ABs compounds which absorb substantial quantities of hydrogen. In most cases these hydrides have not been very thoroughly investigated. It is possible that the nine compounds which were reported to absorb only about three hydrogen atoms per mole of intermetallic compound had decomposed to the rare earth trihydride and the transition metal, whereas those which absorbed four or more hydrogen atoms had resisted decomposition at least in the initial hydriding. Cohen et al. [ 51 have reported that EuRh,, which initially absorbs five hydrogen atoms, has only 10% of its reversible hydrogen absorbing capacity remaining after five absorption-desorption cycles. We have reported isotherms of ErFe, [6] and TmFea [7] hydrides which show no evidence of decomposition after five hydrogenationdehydrogenation cycles. In contrast, we have only with difficulty prepared hydrides of YFe, and GdFe,, and they decomposed after one desorption isotherm. We report on these experiments here. 0022-5088/82/0000-0000/$02.75
0 Elsevier
Sequoia/Printed
in The Netherlands
L2
2. Experimental procedure 2.1. YFe, The YFe, sample was prepared from 99.9% pure yttrium and iron by repeated arc melting in an argon atmosphere. In order to obtain a sample of YFe, free from YFe, impurity it was found necessary to use a 5% excess of yttrium. The sample was annealed at 850 “C for 20 days. Debye-Scherrer X-ray patterns showed single-phase cubic Laves phase (Cl5 type) crystals with a lattice constant of 7.351 f 0.01 A. On exposure to hydrogen at 10000 Torr and 0 “C it absorbed only 3.05 hydrogen atoms per yttrium atom. On desorption at 300 “C the hydrogen concentration was reduced to 2.07 hydrogen atoms per yttrium atom at a pressure of 10.38 Torr. There seems little doubt that the sample had decomposed to YHs and iron during the initial hydriding and the YHs was converted to YHs during the 300 “C desorption. Another sample of the YFe, was activated by heating to 300 “C in a vacuum for 1 h and in 50 Torr of hydrogen for 1 h. It was then evacuated for 1 h at 300 “C and cooled to 20 “C. With this pretreatment the sample was able to react with hydrogen at 400 Torr and 20 ‘C, taking up 3.52 hydrogen atoms per mole of YFe, at a final pressure of 92.8 Torr. This more gentle hydrogenation is believed to diminish local heating resulting from the large heat of absorption. Finally the sample was exposed to hydrogen at 10000 Torr and came to equilibrium at a pressure of 8195 Torr and a hydrogen concentration x of 4.32 hydrogen atoms per mole of YFea. A 20 “C desorption isotherm taken on this sample is shown by the square symbols in Fig. 1. Because of the low equilibrium pressures below x = 3.3 it was necessary to heat the sample to various temperatures below 100 “C!to drive out the hydrogen, but the sample was cooled to 20 “C to measure the equilibrium pressure. For the same reason it was impossible to measure the desorption isotherm below x = 2.0. At the end of the desorption
H/YFe2
Fig. 1. Desorption
isotherms
of YFe2
at 20 “C: 0, first desorption;
0, second
desorption.
L,3
isotherm hydrogen was admitted at 390 Torr and x increased to 3.02 at an equilibrium pressure of 346 Torr. The sample would not take more hydrogen even at 9140 Torr. The 20 “C desorption isotherm measured after this second hydriding is shown by the circles in Fig. 1. It is what would be expected for the desorption isotherm of YHs which has an equilibrium pressure of only 10 Torr at 250 “C [8]. 2.2. GdFe, The GdFe, sample was prepared from 99.9% pure gadolinium and iron by repeated arc melting in an argon atmosphere. It was annealed at 900 “C for 10 days. Debye-Scherrer X-ray patterns showed single-phase cubic Laves phase (Cl5 type) crystals with a lattice constant of 7.403 A. The material would not absorb hydrogen at 1 atm; at 10000 Torr it absorbed hydrogen only to x = 3.4. Very little hydrogen was given up on desorption. The 57Fe Mijssbauer spectrum of this sample, removed at x = 2.91, showed only the hyperfine field characteristic of metallic iron. Debye-Scherrer X-ray patterns showed lines of metallic iron and GdHs. The intensities of the GdH, lines relative to those of the iron lines accounted for only about 20% of the gadolinium. No GdHs lines were seen. Two broad lines which could not be classified may be due to some other hydride in microcrystalline form. Another sample of GdFe, was activated by heating to 200 “C in a vacuum, exposing it to 130 Torr of hydrogen at 200 “C for 16 h and evacuating again at 200 “C. It was then successfully hydrided at 20 “C and 2 atm pressure up to 3c = 4.2 (equilibrium pressure 653 Torr). Finally, it was exposed to 10000 Torr of hydrogen to bring it up to x = 4.73 at an equilibrium pressure of 8716 Torr. The 20 “C desorption isotherm shown in Fig. 2 was then measured. As with YFe, it was necessary to heat the sample (but not above 100 “C) to drive off hydrogen in measuring the lower part of the desorption curve.
H/GdFe2 Fig. 2. Desorption
isotherms
of GdFez.
L4
When the sample was rehydrided at 20 “C it could be brought only to x = 4.20 at 4120 Torr compared with 4.70 at that pressure on the first hydriding cycle. This suggests that 30% of the GdFe, had already decomposed to GdHs and iron. The 80 “C desorption curve shown in Fig. 2 was then measured. The end of the desorption curve at x = 0.83 is consistent with the hypothesis that at that point 30% of the gadolinium was in the form of GdHs and the rest in the form of GdFea containing very little hydrogen. When the sample was hydrided a third time, it absorbed hydrogen only to x = 2.81. The third desorption isotherm resembled the second desorption isotherm of YFe, in Fig. 1. 3. Discussion These studies make it clear that, while GdFe, and YFe, hydrides can be prepared, they are very unstable. The nature of the decomposition products is still obscure. While the iron precipitates as crystalline iron and some crystalline rare earth dihydride forms, the bulk of the rare earth and hydrogen are apparently left in an amorphous state containing somewhat more than three hydrogen atoms per rare earth atom, which is nevertheless quite stable with respect to absorbing or desorbing hydrogen.
1 L. Schlapbach, A. Seiler, F. Stucki, P. Ziircher, P. Fischer and J. Schefer, 2. Phys. Chem. (Frankfurt am Main), I1 7 (1979) 205. 2 G. Busch, L. Schlapbach and Th. von Waldkirch, J. Less-Common Met., 60 (1978) 83. 3 B. Darriet, M. Pezat, A. Hbika and P. Hagenmueller, Znt. J. Hydrogen Energy, 5 (1980) 173. 4 D. Shaltiel, J. Less-Common Met., 62 (1978) 407. 5 R. L. Cohen, K. W. West and K. H. J. Buschow, Solid State Commun., 25 (1978) 293. 6 H. A. Kierstead, J. Less-Common Met., 70 (1980) 199. 7 H. A. Kierstead, J. Less-Common Met., 85 (1982) 213. 8 L. N. Yannopoulos, R. K. Edwards and P. G. Wahlbeck, J. Phys. Chem., 69 (1965) 2515.