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Interaction of hydrogen with the LaNi4.9In0.1, LaNi4.8In0.2 and LaNi4.8 alloys and their Nd analogues
Journal of Alloys and Compounds 420 (2006) 213–217
Interaction of hydrogen with the LaNi4.9In0.1, LaNi4.8In0.2 and LaNi4.8 alloys and their Nd analogues ˇ Blaˇzina ∗ A. Draˇsner, Z. – Boˇskovi´c, PO Box 180, 10002 Zagreb, Croatia Laboratory for Solid State Chemistry, Division of Materials Chemistry, Institute Ruder Received 4 October 2005; received in revised form 3 November 2005; accepted 4 November 2005 Available online 5 December 2005
Abstract The single phase nature of the alloys LaNi4.9 In0.1 , LaNi4.8 In0.2 , NdNi4.9 In0.1 , NdNi4.8 In0.2 of the systems LaNi5−x Inx and NdNi5−x Inx was confirmed by means of X-ray powder diffractometry. Nonstoichiometric alloys LaNi4.8 and NdNi4.8 were prepared and were also found to be good single phase materials. All these alloys crystallize with the same hexagonal structure of the CaCu5 type (space group P6/mmm) as do their prototypes LaNi5 and NdNi5 . In order to determine the interaction with hydrogen the alloys were exposed to hydrogen gas and the pressure composition desorption isotherms were measured. It was found that all alloys react readily and reversibly absorb large amounts of up to 6.54 hydrogen atoms per alloy formula unit. Generally the equilibrium pressure and the hydrogen capacity decrease with the decreasing nickel content. Presence of indium in the alloy acts in favour of these trends. Furthermore, the increasing content of indium in the alloy system drastically alters the slope and the pressure of the plateau observed at higher pressure of the two isotherm plateaux of the NdNi5 –hydrogen system. The final result is a merge of both plateaux into a single one for the hydrogen desorption isotherms of NdNi4.8 In0.2 . However, the isotherms of nonstoichiometric NdNi4.8 still exhibit two separated pressure plateau regions. The thermodynamic parameters of hydride formation, i.e., the entropy change, the enthalpy and the Gibbs free energy of formation have also been extracted for all alloy–hydrogen systems. © 2005 Elsevier B.V. All rights reserved. Keywords: Hydrogen absorbing materials; Metals and alloys; Gas–solid reactions
1. Introduction Since hydrogen was identified as promising fuel source for the future considerable interest raised in both fundamental and practical research of intermetallic compounds. This is because many intermetallic compounds react reversibly with hydrogen at moderate pressure and temperature and absorb large amounts of hydrogen and are therefore considered as potential materials for hydrogen storage purposes. Among intermetallic compounds those of the general composition AB5 have been extensively investigated. This is primarily due to LaNi5 which possesses exceptional hydrogen sorption properties and absorbs more than six hydrogen atoms per formula unit with a large pressure plateau at about 200 kPa at room temperature [1]. It is widely known that that partial substitution of either component in LaNi5 can drastically influence the thermodynamic parameters of the corresponding alloy–hydrogen system. Some examples where
∗
Corresponding author. Tel.: +385 1 4561084; fax: +385 1 4680084. ˇ Blaˇzina). E-mail address: [email protected] (Z.
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.11.003
elements of group IIIa and IVa play the role of substituents can be found in Refs. [2–8]. The LaNi5 based alloys are therefore among the most interesting materials, because partial substitution of components in LaNi5 can be used in tailoring materials for particular practical usage [9]. There are some published data on the LaNi5−x Inx system [10], where it was reported that LaNi4 In crystallizes with the ¯ cubic MgCu4 Sn type of structure (space group F 43m) but does not form hydrides up to 10 MPa. It was also reported [10], that addition of one atom of In into LaNi5 results in formation of LaNi5 In which crystallizes with the hexagonal CeNi5 Sn type of structure (space group P63 /mmc) closely related to the CaCu5 structure and forms hydrides with 1.8 H atoms per alloy formula unit at room temperature and 1 MPa of hydrogen. There are also some data available for the LaNix –hydrogen systems (x = 4.8–5.4) where it was reported that the hydrogen equilibrium pressure increases with the increased content of nickel [11]. A systematic study of the structural and hydrogen sorption properties of the LaNi5−x Inx alloys has not been performed so far. The results reported here represent the continuation of our studies on hydrogen sorption properties of RENi5 (RE = rare
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earth) compounds where nickel is partly replaced by other metals [12–16]. The hydrogen sorption properties of nonstoichiometric alloys LaNi4.8 and NdNi4.8 were studied for comparison with the alloys containing indium. 2. Experimental Sample alloys of the weight of 1.5–2.0 g were prepared by argon arc melting from commercial available lanthanum, neodymium, nickel (Johnson Matthey, UK, stated purity 3N) and indium (Koch-Light Laboratories, UK, stated purity 5N). Hydrogen used was of the purity 99.999% and was supplied by Messer Croatia Plin, Croatia. To ensure homogeneity the sample alloys were inverted and remelted several times. The weight loss of the material was checked and found to be negligible. Therefore it is assumed that the starting composition of the alloys is preserved. Good single phase alloys were obtained after annealing at 1500 K for 24 h. X-ray powder diffraction patterns were obtained with a Philips MPD 1880 diffractometer using nickel filtered Cu K␣ radiation. Line positions were corrected with respect to silicon powder (Koch-Light Laboratories, UK, 99.999%) used as an internal standard. The pressure composition isotherm (PCI) measurements were carried out in a stainless-steel apparatus of a Sieverts type and different temperatures and hydrogen pressures up to 5 MPa. Prior to PCI measurements the alloys were activated by heating under hydrogen (700 K, 5 MPa). After cooling, the absorbed hydrogen was removed by heating and evacuating. The procedure was repeated until the total amount of hydrogen released remained constant. The desorption PCI measurements were made by releasing small quantities of hydrogen from the reactor the alloys being previously completely saturated with hydrogen. The thermodynamic data of the alloy–hydrogen system were extracted from the PCI data applying the Van’t Hoff equation. The hydride composition was calculated from the pressure–temperature–volume data.
3. Results and discussion Ternary alloys of the composition LaNi5−x Inx and NdNi5−x Inx were prepared and their phase equilibrium was studied. The results of X-ray powder diffraction analysis revealed the single phase nature only for LaNi4.9 In0.1 and LaNi4.8 In0.2 , and their neodymium analogues. For all the above-mentioned alloys it was determined that they crystallize with the same crystal structure as do the binary prototypes LaNi5 and NdNi5 (hexagonal CaCu5 type of structure; space group P6/mmm). The large ˚ and nickel difference in size in atomic radius of indium (1.66 A) ˚ seems to be the main reason for the substitution being (1.24 A) limited Spto only x = 0.2. Beyond this composition significant presence of another phase (LaNi4 In) was found. Partial replacement of nickel by indium results in an increase of lattice parameters and cell volumes (Table 1) due to the difference in atomic size of indium and nickel. It should be noted that the values of parameter c change more rapidly than the values of parameter a. This suggests which of the two available crystallographic positions of nickel in the crystal structure are involved in the substitution process. It is well known that in the LaNi5 and NdNi5 structures there exist two distinguished layers of atoms. The basal layer (z = 0) with Ni atoms (sites 2(c)) and La (Nd) atoms (sites 1(a)), and the intermediate layer (z = 1/2) containing Ni atoms only (sites 3(g)). The observed trends in values of parameters c and a indicate that replacement of nickel with indium take place within the intermediate layer at the z = 1/2 plane rather than within the basal layer or both available layers. In related systems LaNi5−x Mx and NdNi5−x Mx (M = Si, Ge,
Table 1 Cell parameters and cell volumes for the LaNi5 , LaNi4.9 In0.1 , LaNi4.8 In0.2 , LaNi4.8 alloys and their neodymium analogues Composition
˚ a (A)
˚ c (A)
c/a
˚ 3) v (A
LaNi5 a LaNi4.9 In0.1 LaNi4.8 In.0.2 LaNi4.8
5.016 5.034 5.051 5.008
3.986 4.008 4.028 3.982
0.795 0.796 0.797 0.795
86.85 87.96 88.99 86.49
NdNi5 b NdNi4.9 In0.1 NdNi4.8 In0.2 NdNi4.8
4.953 4.971 4.986 4.947
3.974 3.995 4.014 3.971
0.802 0.804 0.805 0.803
84.43 85.49 86.42 84.16
a b
From Ref. [23]. From Ref. [16].
Al or Ga) substitution without changing the crystal structure is possible up to x = 2 and nickel atoms at 3(g) are involved in the substitution process [3,7,8,15–17]. When the large Sn atom ˚ plays the role of the substituent the substitution process (1.62 A) is limited only up to x = 0.5. While for Nd containing systems there are no available data about the crystallographic positions of Sn atoms, in La containing systems it was assumed that both available nickel sites are involved in the substitution process. In the later case it was also proposed that the Ni/Sn substitution is accompanied with distortion (displacement) of neighbouring La and Ni atoms [18]. However recently published data on powder neutron diffraction and single crystal X-ray diffraction of LaNi5−x Snx (0.2 ≤ x ≤ 0.5) confirmed the preferential substitution of Ni at 3(g) sites [19]. Due to the fact that binary LaNi5 and NdNi5 alloy phases exhibit a quite large homogeneity region at 1500 K [11,20], as well as for comparison purposes with the alloys containing indium, nonstoichiometric LaNi4.8 and NdNi4.8 were also prepared. Using X-ray diffraction it was confirmed that these single phase materials crystallize with the hexagonal structure of the CaCu5 type but as a result of nickel deficiency the unit cell parameters are smaller than those of stoichiometric binary compounds (Table 1). In order to determine the thermodynamic properties of the alloy–hydrogen systems single phase alloys, i.e., LaNi4.9 In0.1 , LaNi4.8 In0.2 , LaNi4.8 , and their neodymium analogues, were crushed and pulverized into powder which was then exposed to hydrogen gas at different temperatures and pressures. The alloys were easily activated and were found to absorb remarkable amounts of up to 6.54 hydrogen atoms per alloy formula unit. Figs. 1 and 2 illustrate the results of desorption PCI measurements. A brief analysis of the PC isotherms indicates that the reaction with hydrogen of lanthanum and neodymium containing alloys is somewhat different. While the isotherms of alloys containing lanthanum exhibit wide and mostly flat plateaux extending over more than 5 H atoms per alloy formula unit, most of the alloys containing neodymium exhibit two pressure plateau regions. In the alloy–hydrogen systems with lanthanum the hydrogen equilibrium pressure and the hydrogen capacity decrease as the nickel content in the system decreases. Addition of
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215
Fig. 1. Desorption PCI for the (a) LaNi4.9 In0.1 –hydrogen, (b) LaNi4.8 In0.2 –hydrogen and (c) LaNi4.8 –hydrogen system.
indium into the system acts in the same way decreasing further the equilibrium pressure and the hydrogen capacity. The result is a higher hydrogen equilibrium pressure and hydrogen capacity for the LaNi4.8 –hydrogen system than for the LaNi4.8 In0.2 –hydrogen system. The slope of the isotherms for the LaNi4.9 In0.1 –hydrogen and LaNi4.8 In0.2 –hydrogen systems increases with temperature and/or the indium concentration. As a consequence of addition of indium into the system, the flat plateaux observed for the LaNi4.8 –hydrogen system show quite declined slopes in the LaNi4.8 In0.2 –hydrogen system. We wish to draw attention to the observation that the plateau splitting accompanied by a sloping of the plateau sometimes observed for the LaNi5 –hydrogen system and attributed to internal stresses of the alloy [21], was not observed for the nonstoichiometric LaNi4.8 –hydrogen system. Contrary to the isotherms of the alloy–hydrogen systems with lanthanum, the isotherms of the alloy–hydrogen systems with neodymium generally exhibit two plateau regions which are, depending on the alloy composition are more or less separated. The equilibrium pressure of the first plateau for the nonstoichiometric NdNi4.8 is lower than the corresponding values for binary NdNi5 (Fig. 2). The slope and the equilibrium pressure of the second plateau for NdNi4.8 is much more affected by temperature than the second plateau for the binary NdNi5 . Addition of indium
into the alloy further decreases the equilibrium pressure and the slope of the first plateau. The equilibrium pressure and the slope of the second plateau is also drastically influenced by the amount of indium so that this plateau almost vanishes at the alloy composition NdNi4.8 In0.2 . The final result is a merger of both plateaux into a single one which extends now over up to 5.7 hydrogen atoms per alloy formula (the 253 K isotherm of NdNi4.8 In0.2 ). With the decreasing nickel content in the system the hydrogen equilibrium pressure of the first plateau decreases as does the hydrogen capacity. Indium additionally decreases the equilibrium pressure and the hydrogen capacity of the alloy–hydrogen system. It should be noted that during the course of the study of the related NdNi5−x Snx –hydrogen and NdNi5−x Gax –hydrogen systems [16,22] the plateau splitting were also reported for the NdNi5 –hydrogen system. The two plateaux are strongly and unequally altered during replacement of nickel with gallium, indium or tin. The effect of the element which replaces nickel to suppress the second plateau increases in the direction Ga → In → Sn. The single plateau which is the result of merging of the two plateaux was therefore observed for the alloys beyond the compositions NdNi4.9 Ga0.2 , NdNi4.8 In0.2 and NdNi4.8 Sn0.2 within each respective series. The thermodynamic parameters, the enthalpy, H, and the entropy change, S, were extracted from the PCI curves
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Fig. 2. Desorption PCI for the (a) NdNi5 –hydrogen, (b) NdNi4.9 In0.1 –hydrogen, (c) NdNi4.8 In0.2 –hydrogen and (d) NdNi4.8 –hydrogen system.
at different temperatures and at the ratio of three hydrogen atoms per alloy formula unit. Only the values extracted from the first plateau are given for the alloy–hydrogen systems with neodymium. A least square fit of the Van’t Hoff equation ln peq = H/RT − S/R where peq is the plateau pressure, R the universal gas constant, T is the temperature was applied (Figs. 3 and 4). The Gibbs free energy of formation (G) was calculated according to G = H − TS. The corresponding values for entropy, enthalpy change and Gibbs
free energy of formation at room temperature are given in Table 2. The Gibbs free energy of formation in all alloys decreases (becomes more negative) as the nickel content decreases. Addition of indium acts in favour of the observed trends. This indicates that the relative stability of the alloy–hydrogen system increases with the decreasing nickel content and/or the increasing indium content since it is known that the reaction is spontaneous in the direction of decreasing free energy. This is
Fig. 3. Van’t Hoff plots of the desorption PCI for the lanthanum containing alloys as shown in Fig. 1.
Fig. 4. Van’t Hoff plots of the desorption PCI for the neodymium containing alloys as shown in Fig. 2.
ˇ Blaˇzina / Journal of Alloys and Compounds 420 (2006) 213–217 A. Draˇsner, Z.
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Table 2 The thermodynamic parameters H, S, G and the hydrogen equilibrium pressure for the alloy–hydrogen systems at 3H atoms/alloy formula unit together with the maximum hydrogen capacity under 3 MPa at 293 K Composition
H (kJ/mol H2 )
S (J/mol H2 )
G (kJ/mol H2 , at 293 K)
LaNi5 a LaNi4.9 In0.1 LaNi4.8 In0.2 LaNi4.8
−30.12 −32.94 −35.11 −35.16
−109.20 −112.23 −116.38 −120.54
+1.876 −0.057 −1.011 +0.158
180 102 67 110
6.70 6.54 6.17 6.50
NdNi5 b NdNi4.9 In0.1 NdNi4.8 In0.2 NdNi4.8
−24.65 −26.98 −30.98 −26.04
−106.42 −113.06 −124.70 −109.73
+6.531 +6.146 +5.557 +6.111
1427 1243 1016 1225
5.85 5.49 4.92 4.52
a b
peq (kPa, at 293 K)
n
From Ref. [3]. From Ref. [16].
clearly reflected in the observed decreasing equilibrium pressure with the decreasing amount of nickel and/or increasing indium content (Figs. 1 and 2), which should also be regarded as a measure of the stability of the alloy–hydrogen system. 4. Conclusion The structural study of the LaNi5−x Inx and NdNi5−x Inx alloys revealed their single phase nature only up to the composition LaNi4.8 In0.2 and NdNi4.8 In0.2 , respectively. Good single phase alloys of the nonstoichiometric composition LaNi4.8 and NdNi4.8 were also prepared. All the above-mentioned alloys are of the hexagonal symmetry and crystallize with the CaCu5 type of structure. The alloys react readily and reversibly with hydrogen and absorb remarkable amounts of up to 6.54 hydrogen atoms per alloy formula unit (the 293 K desorption isotherm of the LaNi4.9 In0.1 –hydrogen system). Generally, the hydrogen capacity and the equilibrium pressure of the alloy–hydrogen system are drastically influenced by the amount of nickel and/or indium in the alloy system. Pressure composition desorption isotherms of the alloy–hydrogen systems with lanthanum exhibit wide pressure plateau regions. The hydrogen capacity and the equilibrium pressure decreases as the nickel content in the system decreases. Addition of indium into the alloy acts in favour of the above-mentioned trends. In contrast, the isotherms for the alloy–hydrogen systems with neodymium exhibit two pressure plateau regions. The equilibrium pressure and the slope of the second plateau is drastically influenced by the amount of indium so that this plateau vanishes and for NdNi4.8 In0.2 merges with first plateau into a single one. From the thermodynamic point of view and high hydrogen content the LaNi4.9 In0.1 and LaNi4.8 alloys could be regarded as potential materials for practical usage as hydrogen storage alloys.
References [1] J.H.N. Van Vucht, F.A. Kuijpers, H.C.A.M. Bruning, Philips Res. Rep. 25 (1970) 133–146. [2] H.H. Van Mal, K.H.J. Buschow, A.R. Miedema, J. Less-Common Met. 35 (1974) 65–76. [3] M.H. Mendelsohn, D.M. Gruen, A.E. Dwight, Nature 269 (1977) 45–47. [4] T. Takeshita, S.K. Malik, W.E. Wallace, J. Solid State Chem. 23 (1978) 271–274. [5] F. Meli, A. Z¨uttel, L. Schlapbach, J. Alloys Comp. 190 (1992) 17–24. [6] S. Luo, W. Luo, J.D. Clewley, T.B. Flangan, L.A. Wade, J. Alloys Comp. 231 (1995) 467–472. [7] C. Witham, R.C. Bowman Jr., B. Fultz, J. Alloys Comp. 253–254 (1997) 574–578. [8] S. Luo, T.B. Flanagan, R.C. Bowman Jr., J. Alloys Comp. 330–332 (2002) 531–535. [9] G. Sandrock, S. Suda, L. Schlapbach, in: L. Schlapbach (Ed.), Hydrogen in Intermetallic Compounds, vol. II, Springer-Verlag, Berlin, 1992, pp. 197–258. [10] I.I. Bulyk, Int. J. Hydrogen Energy 24 (1999) 927–932. [11] K.H.J. Buschow, H.H. Van Mal, J. Less-Common Met. 29 (1972) 203–210. ˇ ˇ Blaˇzina, J. Phys. Condens. Matter 10 (1998) [12] B. Sorgi´ c, A. Draˇsner, Z. 4777–4783. ˇ ˇ Blaˇzina, J. Alloys Comp. 221 (1995) 169–173. [13] B. Sorgi´ c, A. Draˇsner, Z. ˇ Blaˇzina, A. Draˇsner, J. Phys. Condens. Matter 7 (1995) 7209–7215. [14] Z. ˇ Blaˇzina, J. Alloys Comp. 359 (2003) 180–185. [15] A. Draˇsner, Z. ˇ Blaˇzina, J. Alloys Comp. 381 (2004) 188–191. [16] A. Draˇsner, Z. [17] J.L. Bobet, S. Pechev, B. Chevalier, B. Darriet, J. Alloys Comp. 267 (1998) 136–141. [18] J.S. Cantrell, T.A. Beiter, R.C. Bowman Jr., J. Alloys Comp. 207/208 (1994) 372–376. [19] J.M. Joubert, M. Latroche, R. Cerny, R.C. Bowman Jr., A. PercheronGuegan, K. Yvon, J. Alloys Comp. 293–295 (1999) 124–129. [20] T.B. Masalski (Ed.), Binary Alloy Phase Diagrams, 2nd ed., American Society for Metals, 1987, p. 1713. [21] S. Luo, J.D. Clewley, T.B. Flangan, R.C. Bowman Jr., J.S. Cantrell, J. Alloys Comp. 253–254 (1997) 226–231. [22] Y. Takaguchi, K. Tanaka, J. Alloys Comp. 297 (2000) 73–80. [23] A.E. Dwight, Trans. Am. Soc. Met. 53 (1961) 479–500.
Interaction of hydrogen with the LaNi4.9In0.1, LaNi4.8In0.2 and LaNi4.8 alloys and their Nd analogues ˇ Blaˇzina ∗ A. Draˇsner, Z. – Boˇskovi´c, PO Box 180, 10002 Zagreb, Croatia Laboratory for Solid State Chemistry, Division of Materials Chemistry, Institute Ruder Received 4 October 2005; received in revised form 3 November 2005; accepted 4 November 2005 Available online 5 December 2005
Abstract The single phase nature of the alloys LaNi4.9 In0.1 , LaNi4.8 In0.2 , NdNi4.9 In0.1 , NdNi4.8 In0.2 of the systems LaNi5−x Inx and NdNi5−x Inx was confirmed by means of X-ray powder diffractometry. Nonstoichiometric alloys LaNi4.8 and NdNi4.8 were prepared and were also found to be good single phase materials. All these alloys crystallize with the same hexagonal structure of the CaCu5 type (space group P6/mmm) as do their prototypes LaNi5 and NdNi5 . In order to determine the interaction with hydrogen the alloys were exposed to hydrogen gas and the pressure composition desorption isotherms were measured. It was found that all alloys react readily and reversibly absorb large amounts of up to 6.54 hydrogen atoms per alloy formula unit. Generally the equilibrium pressure and the hydrogen capacity decrease with the decreasing nickel content. Presence of indium in the alloy acts in favour of these trends. Furthermore, the increasing content of indium in the alloy system drastically alters the slope and the pressure of the plateau observed at higher pressure of the two isotherm plateaux of the NdNi5 –hydrogen system. The final result is a merge of both plateaux into a single one for the hydrogen desorption isotherms of NdNi4.8 In0.2 . However, the isotherms of nonstoichiometric NdNi4.8 still exhibit two separated pressure plateau regions. The thermodynamic parameters of hydride formation, i.e., the entropy change, the enthalpy and the Gibbs free energy of formation have also been extracted for all alloy–hydrogen systems. © 2005 Elsevier B.V. All rights reserved. Keywords: Hydrogen absorbing materials; Metals and alloys; Gas–solid reactions
1. Introduction Since hydrogen was identified as promising fuel source for the future considerable interest raised in both fundamental and practical research of intermetallic compounds. This is because many intermetallic compounds react reversibly with hydrogen at moderate pressure and temperature and absorb large amounts of hydrogen and are therefore considered as potential materials for hydrogen storage purposes. Among intermetallic compounds those of the general composition AB5 have been extensively investigated. This is primarily due to LaNi5 which possesses exceptional hydrogen sorption properties and absorbs more than six hydrogen atoms per formula unit with a large pressure plateau at about 200 kPa at room temperature [1]. It is widely known that that partial substitution of either component in LaNi5 can drastically influence the thermodynamic parameters of the corresponding alloy–hydrogen system. Some examples where
∗
Corresponding author. Tel.: +385 1 4561084; fax: +385 1 4680084. ˇ Blaˇzina). E-mail address: [email protected] (Z.
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.11.003
elements of group IIIa and IVa play the role of substituents can be found in Refs. [2–8]. The LaNi5 based alloys are therefore among the most interesting materials, because partial substitution of components in LaNi5 can be used in tailoring materials for particular practical usage [9]. There are some published data on the LaNi5−x Inx system [10], where it was reported that LaNi4 In crystallizes with the ¯ cubic MgCu4 Sn type of structure (space group F 43m) but does not form hydrides up to 10 MPa. It was also reported [10], that addition of one atom of In into LaNi5 results in formation of LaNi5 In which crystallizes with the hexagonal CeNi5 Sn type of structure (space group P63 /mmc) closely related to the CaCu5 structure and forms hydrides with 1.8 H atoms per alloy formula unit at room temperature and 1 MPa of hydrogen. There are also some data available for the LaNix –hydrogen systems (x = 4.8–5.4) where it was reported that the hydrogen equilibrium pressure increases with the increased content of nickel [11]. A systematic study of the structural and hydrogen sorption properties of the LaNi5−x Inx alloys has not been performed so far. The results reported here represent the continuation of our studies on hydrogen sorption properties of RENi5 (RE = rare
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ˇ Blaˇzina / Journal of Alloys and Compounds 420 (2006) 213–217 A. Draˇsner, Z.
earth) compounds where nickel is partly replaced by other metals [12–16]. The hydrogen sorption properties of nonstoichiometric alloys LaNi4.8 and NdNi4.8 were studied for comparison with the alloys containing indium. 2. Experimental Sample alloys of the weight of 1.5–2.0 g were prepared by argon arc melting from commercial available lanthanum, neodymium, nickel (Johnson Matthey, UK, stated purity 3N) and indium (Koch-Light Laboratories, UK, stated purity 5N). Hydrogen used was of the purity 99.999% and was supplied by Messer Croatia Plin, Croatia. To ensure homogeneity the sample alloys were inverted and remelted several times. The weight loss of the material was checked and found to be negligible. Therefore it is assumed that the starting composition of the alloys is preserved. Good single phase alloys were obtained after annealing at 1500 K for 24 h. X-ray powder diffraction patterns were obtained with a Philips MPD 1880 diffractometer using nickel filtered Cu K␣ radiation. Line positions were corrected with respect to silicon powder (Koch-Light Laboratories, UK, 99.999%) used as an internal standard. The pressure composition isotherm (PCI) measurements were carried out in a stainless-steel apparatus of a Sieverts type and different temperatures and hydrogen pressures up to 5 MPa. Prior to PCI measurements the alloys were activated by heating under hydrogen (700 K, 5 MPa). After cooling, the absorbed hydrogen was removed by heating and evacuating. The procedure was repeated until the total amount of hydrogen released remained constant. The desorption PCI measurements were made by releasing small quantities of hydrogen from the reactor the alloys being previously completely saturated with hydrogen. The thermodynamic data of the alloy–hydrogen system were extracted from the PCI data applying the Van’t Hoff equation. The hydride composition was calculated from the pressure–temperature–volume data.
3. Results and discussion Ternary alloys of the composition LaNi5−x Inx and NdNi5−x Inx were prepared and their phase equilibrium was studied. The results of X-ray powder diffraction analysis revealed the single phase nature only for LaNi4.9 In0.1 and LaNi4.8 In0.2 , and their neodymium analogues. For all the above-mentioned alloys it was determined that they crystallize with the same crystal structure as do the binary prototypes LaNi5 and NdNi5 (hexagonal CaCu5 type of structure; space group P6/mmm). The large ˚ and nickel difference in size in atomic radius of indium (1.66 A) ˚ seems to be the main reason for the substitution being (1.24 A) limited Spto only x = 0.2. Beyond this composition significant presence of another phase (LaNi4 In) was found. Partial replacement of nickel by indium results in an increase of lattice parameters and cell volumes (Table 1) due to the difference in atomic size of indium and nickel. It should be noted that the values of parameter c change more rapidly than the values of parameter a. This suggests which of the two available crystallographic positions of nickel in the crystal structure are involved in the substitution process. It is well known that in the LaNi5 and NdNi5 structures there exist two distinguished layers of atoms. The basal layer (z = 0) with Ni atoms (sites 2(c)) and La (Nd) atoms (sites 1(a)), and the intermediate layer (z = 1/2) containing Ni atoms only (sites 3(g)). The observed trends in values of parameters c and a indicate that replacement of nickel with indium take place within the intermediate layer at the z = 1/2 plane rather than within the basal layer or both available layers. In related systems LaNi5−x Mx and NdNi5−x Mx (M = Si, Ge,
Table 1 Cell parameters and cell volumes for the LaNi5 , LaNi4.9 In0.1 , LaNi4.8 In0.2 , LaNi4.8 alloys and their neodymium analogues Composition
˚ a (A)
˚ c (A)
c/a
˚ 3) v (A
LaNi5 a LaNi4.9 In0.1 LaNi4.8 In.0.2 LaNi4.8
5.016 5.034 5.051 5.008
3.986 4.008 4.028 3.982
0.795 0.796 0.797 0.795
86.85 87.96 88.99 86.49
NdNi5 b NdNi4.9 In0.1 NdNi4.8 In0.2 NdNi4.8
4.953 4.971 4.986 4.947
3.974 3.995 4.014 3.971
0.802 0.804 0.805 0.803
84.43 85.49 86.42 84.16
a b
From Ref. [23]. From Ref. [16].
Al or Ga) substitution without changing the crystal structure is possible up to x = 2 and nickel atoms at 3(g) are involved in the substitution process [3,7,8,15–17]. When the large Sn atom ˚ plays the role of the substituent the substitution process (1.62 A) is limited only up to x = 0.5. While for Nd containing systems there are no available data about the crystallographic positions of Sn atoms, in La containing systems it was assumed that both available nickel sites are involved in the substitution process. In the later case it was also proposed that the Ni/Sn substitution is accompanied with distortion (displacement) of neighbouring La and Ni atoms [18]. However recently published data on powder neutron diffraction and single crystal X-ray diffraction of LaNi5−x Snx (0.2 ≤ x ≤ 0.5) confirmed the preferential substitution of Ni at 3(g) sites [19]. Due to the fact that binary LaNi5 and NdNi5 alloy phases exhibit a quite large homogeneity region at 1500 K [11,20], as well as for comparison purposes with the alloys containing indium, nonstoichiometric LaNi4.8 and NdNi4.8 were also prepared. Using X-ray diffraction it was confirmed that these single phase materials crystallize with the hexagonal structure of the CaCu5 type but as a result of nickel deficiency the unit cell parameters are smaller than those of stoichiometric binary compounds (Table 1). In order to determine the thermodynamic properties of the alloy–hydrogen systems single phase alloys, i.e., LaNi4.9 In0.1 , LaNi4.8 In0.2 , LaNi4.8 , and their neodymium analogues, were crushed and pulverized into powder which was then exposed to hydrogen gas at different temperatures and pressures. The alloys were easily activated and were found to absorb remarkable amounts of up to 6.54 hydrogen atoms per alloy formula unit. Figs. 1 and 2 illustrate the results of desorption PCI measurements. A brief analysis of the PC isotherms indicates that the reaction with hydrogen of lanthanum and neodymium containing alloys is somewhat different. While the isotherms of alloys containing lanthanum exhibit wide and mostly flat plateaux extending over more than 5 H atoms per alloy formula unit, most of the alloys containing neodymium exhibit two pressure plateau regions. In the alloy–hydrogen systems with lanthanum the hydrogen equilibrium pressure and the hydrogen capacity decrease as the nickel content in the system decreases. Addition of
ˇ Blaˇzina / Journal of Alloys and Compounds 420 (2006) 213–217 A. Draˇsner, Z.
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Fig. 1. Desorption PCI for the (a) LaNi4.9 In0.1 –hydrogen, (b) LaNi4.8 In0.2 –hydrogen and (c) LaNi4.8 –hydrogen system.
indium into the system acts in the same way decreasing further the equilibrium pressure and the hydrogen capacity. The result is a higher hydrogen equilibrium pressure and hydrogen capacity for the LaNi4.8 –hydrogen system than for the LaNi4.8 In0.2 –hydrogen system. The slope of the isotherms for the LaNi4.9 In0.1 –hydrogen and LaNi4.8 In0.2 –hydrogen systems increases with temperature and/or the indium concentration. As a consequence of addition of indium into the system, the flat plateaux observed for the LaNi4.8 –hydrogen system show quite declined slopes in the LaNi4.8 In0.2 –hydrogen system. We wish to draw attention to the observation that the plateau splitting accompanied by a sloping of the plateau sometimes observed for the LaNi5 –hydrogen system and attributed to internal stresses of the alloy [21], was not observed for the nonstoichiometric LaNi4.8 –hydrogen system. Contrary to the isotherms of the alloy–hydrogen systems with lanthanum, the isotherms of the alloy–hydrogen systems with neodymium generally exhibit two plateau regions which are, depending on the alloy composition are more or less separated. The equilibrium pressure of the first plateau for the nonstoichiometric NdNi4.8 is lower than the corresponding values for binary NdNi5 (Fig. 2). The slope and the equilibrium pressure of the second plateau for NdNi4.8 is much more affected by temperature than the second plateau for the binary NdNi5 . Addition of indium
into the alloy further decreases the equilibrium pressure and the slope of the first plateau. The equilibrium pressure and the slope of the second plateau is also drastically influenced by the amount of indium so that this plateau almost vanishes at the alloy composition NdNi4.8 In0.2 . The final result is a merger of both plateaux into a single one which extends now over up to 5.7 hydrogen atoms per alloy formula (the 253 K isotherm of NdNi4.8 In0.2 ). With the decreasing nickel content in the system the hydrogen equilibrium pressure of the first plateau decreases as does the hydrogen capacity. Indium additionally decreases the equilibrium pressure and the hydrogen capacity of the alloy–hydrogen system. It should be noted that during the course of the study of the related NdNi5−x Snx –hydrogen and NdNi5−x Gax –hydrogen systems [16,22] the plateau splitting were also reported for the NdNi5 –hydrogen system. The two plateaux are strongly and unequally altered during replacement of nickel with gallium, indium or tin. The effect of the element which replaces nickel to suppress the second plateau increases in the direction Ga → In → Sn. The single plateau which is the result of merging of the two plateaux was therefore observed for the alloys beyond the compositions NdNi4.9 Ga0.2 , NdNi4.8 In0.2 and NdNi4.8 Sn0.2 within each respective series. The thermodynamic parameters, the enthalpy, H, and the entropy change, S, were extracted from the PCI curves
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Fig. 2. Desorption PCI for the (a) NdNi5 –hydrogen, (b) NdNi4.9 In0.1 –hydrogen, (c) NdNi4.8 In0.2 –hydrogen and (d) NdNi4.8 –hydrogen system.
at different temperatures and at the ratio of three hydrogen atoms per alloy formula unit. Only the values extracted from the first plateau are given for the alloy–hydrogen systems with neodymium. A least square fit of the Van’t Hoff equation ln peq = H/RT − S/R where peq is the plateau pressure, R the universal gas constant, T is the temperature was applied (Figs. 3 and 4). The Gibbs free energy of formation (G) was calculated according to G = H − TS. The corresponding values for entropy, enthalpy change and Gibbs
free energy of formation at room temperature are given in Table 2. The Gibbs free energy of formation in all alloys decreases (becomes more negative) as the nickel content decreases. Addition of indium acts in favour of the observed trends. This indicates that the relative stability of the alloy–hydrogen system increases with the decreasing nickel content and/or the increasing indium content since it is known that the reaction is spontaneous in the direction of decreasing free energy. This is
Fig. 3. Van’t Hoff plots of the desorption PCI for the lanthanum containing alloys as shown in Fig. 1.
Fig. 4. Van’t Hoff plots of the desorption PCI for the neodymium containing alloys as shown in Fig. 2.
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Table 2 The thermodynamic parameters H, S, G and the hydrogen equilibrium pressure for the alloy–hydrogen systems at 3H atoms/alloy formula unit together with the maximum hydrogen capacity under 3 MPa at 293 K Composition
H (kJ/mol H2 )
S (J/mol H2 )
G (kJ/mol H2 , at 293 K)
LaNi5 a LaNi4.9 In0.1 LaNi4.8 In0.2 LaNi4.8
−30.12 −32.94 −35.11 −35.16
−109.20 −112.23 −116.38 −120.54
+1.876 −0.057 −1.011 +0.158
180 102 67 110
6.70 6.54 6.17 6.50
NdNi5 b NdNi4.9 In0.1 NdNi4.8 In0.2 NdNi4.8
−24.65 −26.98 −30.98 −26.04
−106.42 −113.06 −124.70 −109.73
+6.531 +6.146 +5.557 +6.111
1427 1243 1016 1225
5.85 5.49 4.92 4.52
a b
peq (kPa, at 293 K)
n
From Ref. [3]. From Ref. [16].
clearly reflected in the observed decreasing equilibrium pressure with the decreasing amount of nickel and/or increasing indium content (Figs. 1 and 2), which should also be regarded as a measure of the stability of the alloy–hydrogen system. 4. Conclusion The structural study of the LaNi5−x Inx and NdNi5−x Inx alloys revealed their single phase nature only up to the composition LaNi4.8 In0.2 and NdNi4.8 In0.2 , respectively. Good single phase alloys of the nonstoichiometric composition LaNi4.8 and NdNi4.8 were also prepared. All the above-mentioned alloys are of the hexagonal symmetry and crystallize with the CaCu5 type of structure. The alloys react readily and reversibly with hydrogen and absorb remarkable amounts of up to 6.54 hydrogen atoms per alloy formula unit (the 293 K desorption isotherm of the LaNi4.9 In0.1 –hydrogen system). Generally, the hydrogen capacity and the equilibrium pressure of the alloy–hydrogen system are drastically influenced by the amount of nickel and/or indium in the alloy system. Pressure composition desorption isotherms of the alloy–hydrogen systems with lanthanum exhibit wide pressure plateau regions. The hydrogen capacity and the equilibrium pressure decreases as the nickel content in the system decreases. Addition of indium into the alloy acts in favour of the above-mentioned trends. In contrast, the isotherms for the alloy–hydrogen systems with neodymium exhibit two pressure plateau regions. The equilibrium pressure and the slope of the second plateau is drastically influenced by the amount of indium so that this plateau vanishes and for NdNi4.8 In0.2 merges with first plateau into a single one. From the thermodynamic point of view and high hydrogen content the LaNi4.9 In0.1 and LaNi4.8 alloys could be regarded as potential materials for practical usage as hydrogen storage alloys.
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