Hydrogenation of the Laves Phases of Gd (Mn, Al) 2, Tb (Mn, Al) 2 ...

Materials Science, Vol. 39, No. 6, 2003

HYDROGENATION OF THE LAVES PHASES OF Gd (Mn, Al) 2 , Tb (Mn, Al) 2 , AND Tb (Fe, Al) 2 COMPOUNDS O. V. M’yakush, R. V. Denys, I. V. Koval’chuk, Yu. V. Verbovyts’kyi, I. Yu. Zavalii, and B. Ya. Kotur

UDC 546.3-19′ 11

We study the formation of the AB2 Laves phases in RE–M–Al (where RE is G d or Tb and M is Fe or Mn) systems and determine their hydrogen-sorption properties. The crystal structures of the original compounds and their saturated hydrides are studied by the X-ray powder diffraction method. It is shown that these compounds absorb hydrogen at pressures of 0.1 –0.12 MPa without amorphization and their hydrogen-sorption ability decreases as the amount of aluminum in the compound increases.

In analyzing the interaction of hydrogen with AB2 intermetallic compounds based on rare-earth metals (RE) with the structure of the Laves phases, the attention of the researchers was mainly focused on the influence of hydrogen on the crystal structure and magnetic properties of these compounds and the possibility of using them as efficient accumulators of hydrogen [1]. The phenomenon of hydrogen-induced amorphization of these phases [2] and high levels of magnetostriction of REFe2 compounds [3] prove to be of especial interest for investigations. In numerous cases, the structural deformation of some hydrides based on the Laves phases is studied at elevated concentrations of hydrogen [4, 5]. In analyzing the hydrogenation of RE (Cu, Ni)2 chemically related solid solutions with different type of structure ( CeCu2 ), we established some interesting features of their behavior prior to amorphization depending on the degree of substitution of the B component [6] and the “unpredictable” transformation of their CeCu2-type structure into a Fe2 P-type structure after completion of the process of hydrogenation and subsequent holding of hydrides in air. The hydrides of the Ce (Mn, Al)2 compounds are characterized by the presence of intriguing hydrogen-induced phenomena, including the changes of the valent state of cerium and redistribution of Mn and Al atoms at lattice points [7]. However, the process of hydrogenation of Al-substituted quasibinary AB2 phases based on RE metals was never systematically investigated. In the present work, we study the formation of hydrides of AB2 compounds based on Gd and Tb in which Al partially substitutes the d-metals (i.e., Mn and Fe). Experimental Procedure The alloys were synthesized by the arc melting of compact metals (RE, Fe, Mn, and Al with a content of the base component of at least 99.8 at.%) on a water-cooled copper hearth in an atmosphere of argon. The phase-structural analysis of alloys and their hydrides was carried out according to the X-ray powder-diffraction data obtained by using DRON-3.0 and Phillips-PW1012 X-ray diffractometers in the Cu Kα-radiation. The lattice parameters of all intact alloys and their hydrides were computed with the help of the CSD software package [8] and the crystal structure of some selected hydrides was studied more precisely by the Rietveld method [9] with the help of the FULLPROF software package [10]. Franko Lviv National University, Lviv; Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, Lviv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 39, No. 6, pp. 77–80, November–December, 2003. Original article submitted June 21, 2003. 1068–820X/03/3906–0849 $25.00

© 2004

Plenum Publishing Corporation

849

850

O. V. M’YAKUSH, R. V. DENYS, I. V. KOVAL’CHUK, YU. V. VERBOVYTS’KYI, I. YU. Z AVALII,

AND

B. YA. KOTUR

Table 1. Crystallographic Parameters of the Intact Compounds and Their Saturated Hydrides

Compound

a, Å

c, Å

V, Å3

∆ V /V, %

H/M

Tb Fe1.7 Al0.3

7.4283(3)

—

409.9

—

—

Tb Fe1.7 Al0.3 H3.19

7.919(1)

—

496.7

21.2

1.06

Tb Fe1.55 Al0.45

7.4618(1)

—

415.5

—

—

Tb Fe1.55 Al0.45 H3.0

7.8943(2)

—

492.0

18.4

1.00

Tb Fe1.25 Al0.75

5.3435(4)

8.7170(7)

215.6

—

—

Tb Fe1.25 Al0.75 H2.40

5.5254(3)

9.1658(5)

242.3

11.4

0.80

Tb Fe1.1 Al0.9

5.3776(4)

8.7419(7)

218.9

—

—

Tb Fe1.1 Al0.9 H2.27

5.513(1)

9.124(3)

240.2

9.7

0.75

Gd Mn1.85 Al0.15

7.806(3)

—

475.6

—

—

Gd Mn1.85 Al0.15 H4.20

8.207(2)

—

552.8

16.2

1.4

Gd Mn1.50 Al0.50

7.838(2)

—

481.5

—

—

Gd Mn1.50 Al0.50 H3.45

8.069(2)

—

525.4

9.1

1.15

Tb Mn1.85 Al0.15

7.708(1)

—

457.9

—

—

Tb Mn1.85 Al0.15 H4.26

8.164(8)

—

544.1

18.8

1.42

Tb Mn1.5 Al0.5

7.761(1)

—

467.5

—

—

8.053(4)

—

522.2

11.7

1.17

Tb Mn1.5 Al0.5 H3.51

The characteristics of absorption of hydrogen by alloys were determined by using the standard manometric method in a constant volume [11]. The hydrogenation curves were plotted for a constant pressure of hydrogen (100 kPa) according to the experimental data by the formula X = 1 – exp [– K0 ( P – Pd) ( τ – τi )n ] , Xmax where X and Xmax are, respectively, the current and maximum concentrations of hydrogen in hydride, K0 is the constant of reaction of hydrogenation, P and Pd are, respectively, the current and equilibrium levels of pressure, τi is the induction period, and n is the order of reaction [12]. Results and Discussion Depending on the degree of substitution of the transition metal M with aluminum, the Laves phases in RE– M–Al systems are characterized by the Mg Cu2- or Mg Zn2-type structures [13].

HYDROGENATION OF THE LAVES PHASES OF GD (MN, AL) 2 , TB (MN, AL) 2 ,

(a)

AND

TB (FE, AL) 2 COMPOUNDS

851

(b)

Fig. 1. Crystal structures of the Laves phases: (a) cubic (Mg Cu2 -type), (b) hexagonal (Mg Zn2 -type).

(a)

(b)

Fig. 2. Dependences of the concentration of hydrogen on the time of hydrogenation for Gd Mn2 – x Alx , Tb Mn2 – x Alx (a), and Tb Fe2 – x Alx alloys (b) recalculated for a pressure of 100 kPa.

The data of crystallochemical analyses demonstrate (Fig. 1) that these structures contain three types of tetrahedral voids, namely A2 B2 , AB3 , and B4 . In the phases based on RE and transition metals, atoms of the first type (RE) occupy positions A and atoms of the second type occupy positions B. The analysis of the literature data shows that, in the structure of the Laves phases, atoms of hydrogen first occupy voids with the largest number of RE atoms in the neighborhood, namely, A2 B2 and then AB3 [14]. In the previous works, it was established that, in hydrides of these compounds, the symmetry of the metal matrix of the intact phases is preserved for low concentrations of hydrogen. At the same time, as the concentration of hydrogen increases, one may observe structural deformation associated with the process of ordering of the hydrogen sublattice [4, 5]. We prepared the AB2 Laves phases with M g Cu2- and M g Zn2-type structures in RE–Mn–Al and RE– Fe–Al systems and studied these phases by the X-ray powder-diffraction method. Hydrides were synthesized at room temperature under a pressure of hydrogen of 120 kPa in an autoclave after preliminary activation of samples in a vacuum at 350–400˚C. For all samples ( Gd Mn2 – x Alx , Tb Mn2 – x Alx , and Tb Fe2 – x Alx ), the process of hydride formation is accompanied by a significant increase in the volume of unit cell without changes in its symmetry (Table 1). As the aluminum content of quasibinary compounds increases, their hydrogen-sorption ability and the rate of absorption of hydrogen gradually decrease (Fig. 2). The hydrogen content of hydrides per one atom of the metal (H / M) is presented in Table 1. It easy to see that, as the amount of aluminum increases to one atom per formula unit, the hydrogen-sorption ability of the compound decreases by 0.5–0.72 H / M.

852


(a)

AND

B. YA. KOTUR

(b)

Fig. 3. Experimental ( • ) and theoretical (–) X-ray diffraction profiles of the Tb Fe1.55 Al0.45 H3.0 (a) and Tb Fe1.25 Al0.75 H2.41 (b) hydrides. The difference between these profiles is presented at the bottom of the figure. The reflections of the constituent phases are marked vertically downward: (a) Tb Fe1.55 Al0.45 H3.0 and Tb 2 O3 , (b) Tb Fe1.25 Al0.75 H2.4 , Tb (Fe, Al)2 H x (MgCu2 -type), and Tb2 O3 .

Table 2. Crystallochemical Parameters of Hydrides Tb Fe1.55 Al0.45 H3.0 Fd 3m; a = 7.8943(2) Å, V = 491.97(2) Å3 Atom

x

y

z

B

Tb

1 /8

1 /8

1 /8

1.6(1)

1

1 /2

1 /2

1 /2

1.1(1)

M

Tb Fe1.25 Al0.75 H2.40 P63 / mmc; a = 5.5254(4) Å, c = 9.1658(6) Å, V = 242.34(2) Å3 Atom

x

y

z

B

Tb

1 /3

2 /3

0.0642(2)

1.31(8)

1

0

0

0

0.6(2)

1

0.8351(8)

2x

1 /4

1.2(1)

M1 M2

Comment: 1. M = 0.775Fe + 0.225Al, M1 = M2 = 0.625Fe + 0.375Al. The X-ray diffraction profiles of the Tb Fe1.55 Al0.45 H3.0 (Mg Cu2-type) and Tb Fe1.25 Al0.75 H2.40 (MgZn2 type) hydrides are presented in Fig. 3. The results of more precise analysis of their crystal structure performed by the Rietveld method are presented in Table 2. The comparison of the literature data for hydrides of the unsubstituted Laves phases with our data reveals the changes in the volume of unit cell per hydrogen atom ( ∆ V /at.H ) in the process of hydrogenation depicted in Fig. 4.

HYDROGENATION OF THE LAVES PHASES OF GD (MN, AL) 2 , TB (MN, AL) 2 ,

AND

TB (FE, AL) 2 COMPOUNDS

853

Fig. 4. Hydrogen-induced changes in the volume of unit cell (per hydrogen atom) for the Laves phases [TbFe2 – x Alx (1), TbMn2 – x Alx (2), and Gd Mn2 – x Alx (3)].

The indicated specific volume significantly decreases as the aluminum content of the compound increases. Although this phenomenon can be explained by the expansion of the crystal lattice of the original compounds caused by the replacement of d-metals with Al (which, in turn, decreases the hydrogen-induced changes in the volume of unit cell), it requires additional investigations. CONCLUSIONS Under the analyzed conditions of hydrogenation, the Gd Mn2 – x Alx , Tb Mn2 – x Alx , and Tb Fe2 – x Alx aluminum-substituted Laves phases absorb hydrogen without amorphization and, moreover all hydrides preserve the structure of the original metal matrix. As the aluminum content increases, the hydrogen-sorption ability decreases for Gd(Tb) Mn2 – x Alx from 1.433 (x = 0) to 1.15–1.17 H / M (x = 0.5) and for Tb Fe2 – x Alx from ∼ 1.27 (x = 0) to 0.75 H / M (x = 0.9). REFERENCES 1. V. Paul–Boncour and A. Percheron–Guegan, “The influence of hydrogen on the magnetic properties of intermetallic compounds: YFe2–D2 system as an example,” J. Alloys Comp., 293–295, 237–242 (1999). 2. M. Dilixiati, K. Kanda, K. Ishikawa, and K. Aoki, “Hydrogen-induced amorphization in C15 Laves phases RFe2 ,” J. Alloys Comp., 337, 128–135 (2002). 3. K. Itoh, K. Kanda, K. Aoki, et al., “X-ray and neutron diffraction studies of atomic scale structure of crystalline and amorphous Tb Fe2 D x ,” J. Alloys Comp., 348, 167–172 (2003). 4. H. Figiel, A. Budziak, J. Zukrowski, et al., “Structural and magnetic properties of Tb Mn2 H x hydrides,” J. Alloys Comp., 335, 48–58 (2002). 5. J. Przevoznik, V. Paul–Boncour, M. Latroshe, and A. Percheron–Guegan, “X-ray diffraction and extended X-ray absorption finestructure study of RMn2 hydrides (R = Y, G d, or Dy),” J. Alloys Comp., 232, 107–118 (1996). 6. I. Yu. Zavaliy, R. Černý, V. N. Verbetsky, et al., “Interaction of hydrogen with RE Cu2 and RE (Cu, Ni) 2 intermetallic compounds (RE = Y, Pr, Dy, Ho),” J. Alloys Comp., 358, 146–151 (2003). 7. Y. E. Filinchuk, D. Sheptyakov, G. Hilsher, and K. Yvon, “Hydrogenation induced valence change and metal atom site exchange at room temperature in the C14-type substructure of Ce Mn1.8 Al0.2 H4.4 ,” J. Alloys Comp., 356-357, 673–678 (2003). 8. L. G. Akselrud, Yu. N. Grin, P. Yu. Zavalij, V. K. Pecharsky, et al, “Use of the CSD program package for structure determination,” in: Proc. of the 2nd Europ. Conf. on Powder Diffraction, Netherlands (1992), p. 41. 9. R. Young (editor), The Rietveld Method, Oxford University Press, Oxford (2000).

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10. J. Rodriguez–Carvajal, FULLPROF Version 0.2, LLB, Saclay (1998). 11. V. A. Yartys’, I. Yu. Zavalii, and M. V. Lotots’kii, “Absorbents of low-pressure hydrogen based on Zr–V and Zr–V–Fe alloys modified by oxide admixtures,” Koord. Khim., 18, No. 4, 409–423 (1992). 12. I. Yu. Zavaliy, A. B. Ryabov, and V. A. Yartys, “Hydrogen absorption and phase structural characteristics of oxygen-containing Zr–V alloys substituted by Hf, Ti, Nb, Fe,” J. Alloys Comp., 219, 34–37 (1995). 13. E. I. Gladyshevs’kyi and O. I. Bodak, Crystal Chemistry of Intermetallic Compounds of Rare-Earth Metals [in Ukrainian], Vyshcha Shkola, Lviv (1982). 14. V. A. Yartys’, V. V. Burnasheva, and K. N. Semenenko, “Structural chemistry of hydrides of intermetallic compounds,” Usp. Khimii, 52, No. 4, 529–562 (1983).