Hydrogen absorption–desorption characteristics of the LaNi5Sn intermetallic compound

Journal of Alloys and Compounds 373 (2004) 161–166

Hydrogen absorption–desorption characteristics of the LaNi5Sn intermetallic compound Masashi Sato, Volodymyr A. Yartys∗ Institute for Energy Technology, Instituttveien 18, P.O. Box 40, Kjeller NO-2027, Norway Received 29 September 2003; received in revised form 16 October 2003; accepted 16 October 2003

Abstract The pressure–composition–temperature (P–C–T) relations in the LaNi5 Sn–H system were measured volumetrically at temperatures between 258 and 423 K. Two hydride phases, ␤-LaNi5 SnH2 and ␥-LaNi5 SnH3 are formed in this system in addition to the ␣-solid solution with a limiting H content of 0.3 H atoms per formula unit. The calculated work loss due to hysteresis is rather small, 112 J (mol H)−1 at 298 K. The partial molar enthalpy and entropy calculated for the formation of the ␤-LaNi5 SnH2 are −18.5 ± 0.8 kJ (mol H)−1 and −53.7 ± 2.3 J (K mol H)−1 , respectively. An advanced van der Waals lattice gas model was applied to fit the isotherms. The critical temperature of the LaNi5 SnH2 equals TC = 421 ± 17 K. © 2003 Elsevier B.V. All rights reserved. Keywords: Intermetallic compound; Rare earth compound; Metal hydride; Gas–solid reaction; Thermodynamic properties

1. Introduction Substitution of Sn for Ni in LaNi5 alloys reduces hysteresis and leads to remarkable improvements in cyclic stability, kinetics of hydrogen absorption and desorption and corrosion resistance making these alloys promising materials for Ni-metal hydride batteries [1–6]. Studies of the chemically related LaNiSnD2 showed that Sn is not a typical p-element and does not block occupation of the specific Sn-surrounded interstitial sites [7]. From these works, it is evident that Sn substitutions can be an attractive step towards advanced metal hydrides. Further studies are required to understand better the beneficial influence of Sn on the hydrogenation properties. This work was devoted to studies of the hydrogen interaction with the ternary intermetallic compound LaNi5 Sn. Its stoichiometry is close to the solid solution range LaNi5−x Snx (x < 0.4), so in accordance with the phase diagram of the ternary system La–Ni–Sn, LaNi5 Sn is in equilibrium with LaNi5−x Snx [8]. The LaNi5 Sn intermetallic compound adopts the hexagonal CeNi5 Sn type of structure (space group P63 /mmc), which

∗

Corresponding author. Tel.: +47-63-80-64-53; fax: +47-63-81-29-05. E-mail address: [email protected] (V.A. Yartys).

0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.10.027

is related to the CaCu5 type and is characterised by a relatively large unit cell (aLaNi5 Sn ∼ aLaNi5 ; cLaNi5 Sn ∼ 5cLaNi5 ). In CeNi5 Sn three different sites are collectively occupied by Ni and Sn and two pairs of sites are exclusively occupied by either Ce or Ni [9]. No studies of hydrogen interaction with LaNi5 Sn intermetallic compound have been reported so far. The aim of this work was to study the phase equilibria in the system LaNi5 Sn-H by measuring the pressure– composition–temperature (P–C–T) relations. The thermodynamic parameters were calculated and the critical temperature for the ␤-LaNi5 SnH2 hydride was evaluated on the basis of these diagrams.

2. Experimental 2.1. Sample preparation The LaNi5 Sn sample was prepared by arc melting in an Ar atmosphere using high purity elements with grade better than 99.9%. The alloy was sealed under vacuum in a quartz tube and annealed at 773 K for 4 weeks. Finally it was quenched into a mixture of ice and water. The formation of the LaNi5 Sn intermetallic compound crystallising with the CeNi5 Sn type was confirmed by powder X-ray diffraction with a Siemens D5000 diffractometer using Cu K␣1

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radiation. Small amounts of the LaNi5−x Snx intermetallic alloy were found as impurities. The refined unit cell parameters for LaNi5 Sn are a = 4.9661(9) Å; c = 19.977(5) Å. These values agree well with the reference data [10]. 2.2. P–C–T measurements P–C isotherms for H2 absorption and desorption were determined volumetrically using a Sieverts’ type apparatus. The hydrogenation–dehydrogenation cycles were performed several times as an activation treatment. The H2 gas used was of 99.999% grade purity. The measurements were made at temperatures between 258 and 423 K, and at H2 pressures in the range from 10−3 to 2.5 MPa. The data were considered as having reached the equilibrium state when the pressure changes became smaller than 10−5 MPa at pressures below 0.1 MPa, and below 10−4 MPa for the overall H2 pressures above 0.1 MPa.

3. Results and discussion 3.1. P–C–T relationships and crystal structure analysis Fig. 1 shows the measured absorption and desorption P–C–T diagrams at temperatures between 258 and 423 K.

Fig. 1. Pressure–composition isotherms for the LaNi5 Sn–H system.

Fig. 2. The crystal structure of LaNi5 Sn. Two type of layers with LaNi5 and LaNi5 Sn2 stoichiometry are shown.

In the temperature range 273–373 K, clear plateau pressures could be reproducibly obtained between approximately [H]/[LaNi5 Sn] = 0.3 and 1.4, indicating the coexistence of the ␣-solid solution and the ␤-hydride LaNi5 SnH2 . The plateau pressure was around 0.01 MPa at room temperature, and the plateau shape was rather inclinatory compared to the AB5 type compounds. After reaching the saturation value 2.0 H atoms per formula unit LaNi5 Sn in the ␤-hydride, the isotherms show further increase of the H content, however, without formation of a plateau. The curves reach saturation in the vicinity of [H]/[LaNi5 Sn] = 3.0 where the ␥-hydride is formed. These observations show a drastic decrease of the H storage capacity when going from LaNi5 to LaNi5 Sn. The crystal structure of LaNi5 Sn can be presented as a stacking of two different types of slabs, with composition LaNi5 and LaNi5 Sn2 , along [0 0 1] (Fig. 2). Combination 2 LaNi5 + 2 LaNi5 Sn2 provides the overall stoichiometry 4 LaNi5 Sn / unit cell. During hydrogenation, one probable scenario for the formation of the interstitial hydride LaNi5 SnH3 is that the hydrogen atoms enter the LaNi5 layer only and that the H content is insignificant in the Sn-containing layer LaNi5 Sn2 . A reasonable suggestion is that in the hydrogen content for the LaNi5 -layer of the LaNi5 Sn is the same as for the corresponding binary intermetallic LaNi5 (6 H atoms per formula unit LaNi5 ). This provides an experimentally observed H concentration of 3.0 H atoms per formula unit LaNi5 Sn (2 LaNiH6 + 2 LaNi5 Sn2 H0 = 4 LaNi5 SnH3 ). Five types of tetrahedral interstices in the LaNi5 layers of LaNi5 Sn were identified as possible sites for the insertion of the hydrogen atoms. Two sites have a surrounding LaNi2 Sn

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163

Table 1 Possible interstices for the accommodation of hydrogen atoms inside the LaNi5 layer of the LaNi5 Sn structure (space group P63 /mmc (no. 194); a = 4.9661 Å; c = 19.977 Å) Interstice

Surrounding

x

y

z

ri , [Å]

na

Neighbouring interstices

24l

La(1)Ni(1)Ni(3)Ni(4)

0.3336

0.2367

0.2226

0.33

1/4

2×24l 1×12k1 1×12k2

12k1

La(1)Ni(1)2 Ni(3)

0.5479

0.4521

0.3088

0.31

1

2×24l 1×12k3 1×4f b

12k2

La(1)Ni(1)2 Ni(4)

0.7631

0.8815

0.3097

0.32

1

2×24l 1×12k4 1×4eb

12k3

La(1)Ni(1)2 Sn

0.4791

0.5209

0.3394

0.31

1

1×12k1 2×12k4 1×12k5 b

12k4

La(1)Ni(1)2 Sn

0.6282

0.8141

0.3394

0.31

1

1×12k2 2×12k3 1×12k6 b

LaNi5 Sn was considered as isostructural with CeNi5 Sn [9]. 2 La(1) in 2c: 1/3, 2/3, 1/4; 2 La(2) in 2a: 0, 0, 0; 12 Ni(1) in 12k: 0.831, 2x, 0.1458; 11.64 Ni + 0.36 Sn; 4 Ni(2) in 4f: 1/3, 2/3, 0.5425; 3.48 Ni + 0.52 Sn; 2 Ni(3) in 2d: 1/3, 2/3, 3/4; 2.00 Ni + 0 Sn; 2 Ni(4) in 2b: 0, 0, 1/4; 2.00 Ni + 0 Sn; 4Sn in 4f: 1/3, 2/3, 0.0873; 3.32 Sn + 0.68 Ni. a Maximum possible occupation number. b Interstices 4f, 4e, 12k and 12k are located in the LaNi Sn layer. 5 6 5 2

(12k3 and 12k4 ) and three other are formed by one La and three nickel atoms (24l, 12k1 and 12k2 ). The radii of these interstices were calculated using the rigid ball model, with rLa = 1.877 Å, rNi = 1.246 Å and rSn = 1.623 Å [11] (Table 1). A possible simultaneous occupation of different sites has been analysed using the suggestion that the H atoms do not simultaneously occupy interstices that share common faces. Two possible models have been identified: Model I Model II

6H in 24l + 6H in 12k3 or in 12k4 6H in 24l + 3H in 12k1 + 3H in 12k2

These two models both provide a storage capacity of 3.0 H atoms per formula unit of LaNi5 Sn. Model I is identical to the model of 3 at.H in 6m (La2 Ni2 ) + 3 at.H in 12n (LaNi3 ) for the LaNi5 -based hydride. Model II is equivalent to 3 at.H in 12n + 3 at.H in 12o (both sites have LaNi3 surrounding) of the LaNi5 -based hydride. Studies of the absorption and desorption processes showed a hysteresis effect. The loss of work corresponding to hysteresis can be expressed as 1/2 RT ln(P ab /P des ) where Pab and Pdes are the plateau pressure for absorption and desorption processes, respectively. Luo et al. observed that introduction of Sn decreases the hysteresis from ∼308 J (mol H)−1 for LaNi5 to ∼199 J (mol H)−1 in the LaNi4.8 Sn0.2 at room temperature [3,4]. The calculated hysteresis in the LaNi5 Sn–H system, 112 J (mol H)−1 at 298 K, is significantly smaller than for the LaNi5−x Snx –H systems. This may indicate that the introduction of Sn reduces markedly the hysteresis effects and shows a beneficial intrinsic role of Sn. It seems that the effect of Sn is very

specific since in the chemically similar system LaNi5 In–H the hysteresis is significantly higher (770 J (mol H)−1 at 292 K, according to the data presented in [12]) and even exceeds the values for the LaNi5 –H system. 3.2. Thermodynamic properties The relative partial molar enthalpy HH and entropy SH change for H2 absorption and desorption were calculated from the van’t Hoff plots based on the measured P–C–T relations. The calculated value of HH and SH during the formation of the LaNi5 SnH2 phase are equal to −18.5 ± 0.8 kJ (mol H)−1 and −53.7 ± 2.3 J (K mol H)−1 , respectively. The obtained experimentally thermodynamic parameters and reference data are summarised in Table 2. With increasing Sn content in LaNi5−x Snx , the enthalpy change per mole H decreases in parallel with an increase of the stability of the hydrides. Further, when going to LaNi5 Sn, the stability of the hydride decreases. However, the entropy value changes Table 2 Thermodynamic properties for hydrogen absorption in LaNi5−x Snx (x = 0 − 0.4) and LaNi5 Sn Alloys

HH [kJ (mol H)−1 ]

SH [J (K mol H)−1 ]

Reference

LaNi5 LaNi4.8 Sn0.2 LaNi4.7 Sn0.3 LaNi4.6 Sn0.4 LaNi5 Sn

−15.9 −17.0 −18.3 −19.0 −18.5 ± 0.8

−54.0 −54.0 −56.3 −54.3 −53.7 ± 2.3

[13] [4] [8] [4] This work

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Fig. 3. The change in the relative partial molar enthalpy HH during H2 -absorption as a function of H concentration for the LaNi5 Sn–H system.

Fig. 4. The change in the relative partial molar entropy SH in the H2 absorption as a function of H concentration for the LaNi5 Sn–H system.

for the LaNi5 Sn–H system are similar to the values observed for the LaNi5−x Snx –H system. Decrease of the stability of the LaNi5 Sn-based hydride may be related to the absence of La2 Ni2 interstitial sites in its structure (the most attractive sites for hydrogen in the AB5 -based hydrides), in contrast to the LaNi5 and LaNi5−x Snx intermetallics. The change of HH and SH as a function of H content are shown in Figs. 3 and 4, respectively. The relative partial molar enthalpy changes in LaNi5 SnHx , HH , pass through a broad minimum at x = 0.5 − 0.8 and increase rapidly at lower and at higher H concentrations. Similar spline-like curve in the enthalpy changes was earlier reported for the Nb–H system by Kuji and Oates [14]. Such a behaviour may be attributed to the repulsive hydrogen–hydrogen (H–H) interactions between the close distant neighbours at [H]/[LaNi5 Sn] > 0.5 − 0.8. Following an increase of the H content, the H–H interactions deviate positively from the contributions coming from the attractive interactions (Fig. 3). At H concentrations higher than [H]/[LaNi5 Sn] > 0.8, the H–H interactions are totally dominated by the repulsive contributions. The change in the enthalpy values becomes discontinuous at [H]/[LaNi5 Sn] ∼ 0.8 and 1.4, where the jumps in the dependencies are observed. Similarly, such a drastic change in the variations of the entropy could be observed at the same two H concentrations. Such a behaviour can be associated with the onset of occupancy of the new, extra hydrogen site in the crystal structure taking place in the vicinity of 0.8 and 1.4 H atoms per formula unit LaNi5 Sn. Previ-

Fig. 5. The change in the relative partial molar excess entropy SHXE during the H2 absorption as a function of H concentration for the LaNi5 Sn–H system. The configurational entropy change SC is shown by solid lines. The inset presents the derivative of the excess entropy vs. H contents with two peaks at 0.8 and 1.4 H atoms per formula unit.

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ously a similar behaviour was observed in the Sm2 Fe17 –H system where a two-site H occupancy was linked with the observed nonmonotonic change of the thermodynamic properties [15]. However, despite a complete reproducibility of this anomaly in different absorption-desorption cycles, more precise measurements are required to confirm a significance of these modulations. In order to explain the rapid change in the thermodynamic behaviour at [H]/[LaNi5 Sn] = 0.8 and 1.4, the relative partial molar excess entropy of H was calculated according to SHXE = SH −SC where SC = −R ln [r/(β−r)] is the configuration entropy of H atoms in LaNi5 Sn and β is the limiting value of r = [H]/[LaNi5 Sn] from crystal structure considerations when assuming a one-site occupancy for the H atoms. Since the crystal structure data for LaNi5 Sn-based hydrides are presently not available, the excess entropies were calculated for the most simple cases under the assumption that β adopts the values 1 (limiting H content 1 at.H/LaNi5 Sn), 2 (2 at.H/LaNi5 Sn) and 3 (3 at.H/LaNi5 Sn). The calculated dependencies shown in Fig. 5 exhibit jump-like changes at r = 0.8 and r = 1.4 only for β = 2 and 3 (see inset of Fig. 5). This means that the entropy changes are not controlled by configurational terms and that at least two types of interstices should be occupied by H atoms in the LaNi5 Sn-based hydrides at H/LaNi5 Sn > 0.8. The interpretation of the drastic changes in the entropy term is rather difficult, since the excess entropy SHXE con-

Fig. 6. Calculated isotherms for the LaNi5 Sn–H system. The dotted line shows the miscibility gap.

165

sists of contributions from the vibration, the lattice changes and the electron entropy terms. However, the electron entropy terms cannot play a dominant role because of the relatively low temperatures applied in present study. In addition, the thermodynamic properties cannot be significantly influenced by the crystal structure changes since our studies indicated only slight volume expansion accompanying the hydrogenation. Because of that, we expect that the main contributions to SHXE are coming from the vibrational entropy as H atoms seem to have a multiple-site occupancy in the hydride. More detailed information on the hydrogen coordination in the metal sublattice will be received during the structural studies of LaNi5 SnH3 which are presently in progress. 3.3. Partial phase diagram An advanced lattice–gas model proposed by Lototsky et al. [16] was applied to the measured P–C–T curves in order to construct the partial phase diagram of this system. The equilibrium pressure P can be expressed as
 1 θ P 27 Tc θ = ln ln − θ+ , 2 P0.5 1−θ 4 T 1−θ where θ is the fraction of the occupied sites with the H content fitted for two regions, namely for 0 < [H]/[LaNi5 Sn] <

Fig. 7. Calculated partial phase diagram for the LaNi5 Sn–H system. The dotted line outlines the miscibility gap.

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1.4 (␤) and 1.4 < [H]/[LaNi5 Sn] < 2.4 (␥). P0.5 is the pressure at θ = 0.5. Because the plateau width for ␥-LaNi5 SnH3 was higher than the pressure range applied in the current work, the critical temperature of the ␥-phase was not determined. By fitting the measured P–C–T relations, the critical temperature of the LaNi5 SnH2 hydride was estimated as Tc = 421 ± 17 K (Fig. 6). Based on the calculated isotherms, we have constructed a partial phase diagram for the system LaNi5 Sn–H (Fig. 7).

4. Conclusions The P–C–T relations in the LaNi5 Sn–H system were measured and showed the existence of two hydride phases containing two (␤-hydride) and three (␥-hydride) atoms H per formula unit of LaNi5 Sn. The differences in the stoichiometry and the thermodynamic behaviour of the LaNi5 Sn-based hydrides compared to LaNi5−x Snx H6−y seem to be associated with the crystal structure changes when going from the CaCu5 -type to the CeNi5 Sn-type compounds. The great advantage of the Sn-containing compounds lies in reducing the hysteresis effects during H2 absorption and desorption.

Acknowledgements This study was financially supported by the Research Council of Norway. The authors sincerely thank Dr. M.V.

Lototsky, Institute for Energy Technology, for the helpful discussions. References [1] R.C. Bowman Jr., C.H. Luo, C.C. Ahn, C.K. Witham, B. Fultz, J. Alloys Compd. 217 (1995) 185. [2] M.H. Mendelsohn, D.M. Gruen, A.E. Dwight, Mater. Res. Bull. 13 (1978) 1221. [3] S. Luo, J.D. Clewley, T.B. Flanagan, R.C. Bowman Jr., L.A. Wade, J. Alloys Compd. 267 (1998) 171. [4] S. Luo, T.B. Flanagan, R.C. Bowman Jr., J. Alloys Compd. 330–332 (2002) 531. [5] T. Vogt, J.J. Reilly, J.R. Johnson, G.D. Adzic, J. McBreen, Mater. Sci. Forum 315–317 (1999) 94. [6] B.V. Ratnakumar, C. Witham, R.C. Bowman Jr., A. Hightower, B. Fultz, J. Electrochem. Soc. 143 (8) (1996) 2578. [7] V.A. Yartys, T. Olavesen, B.C. Hauback, H. Fjellvåg, H.W. Brinks, J. Alloys Compd. 330–332 (2002) 141. [8] O.Yu. Khyzhun, M.V. Lototsky, A.B. Riabov, C. Rosenkilde, V.A. Yartys, S. Jørgensen, R.V. Denys, J. Alloys Compd. 356–357 (2003) 773. [9] O. Moze, W. Kokelmann, E. Brück, K.H.J. Buschow, J. Alloys Compd. 279 (1998) 110. [10] R.V. Skolozdra, in: K.A. Gschneidner, Jr., L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earths, vol. 24, 1997, p. 436. [11] W.B. Pearson, The Crystal Chemistry and Physics of Metals and Alloys, Wiley, New York, 1972. [12] I.I. Bulyk, Int. J. Hydrogen Energy 24 (1999) 927. [13] H.H. van Mal, K.H.J. Buschow, A.R. Miedema, J. Less-Common Met. 35 (1974) 65. [14] T. Kuji, W.A. Oates, J. Less-Common Met. 102 (1984) 251. [15] T. Kuji, H. Uchida, K. Kinoshita, M. Yamamuro, A. Komatsu, J. Alloys Compd. 330–332 (2002) 197. [16] M.V. Lototsky, V.A. Yartys, V.S. Marinin, N.M. Lototsky, J. Alloys Compd. 356–357 (2003) 27.