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Influence of Al- and Cu-doping on the thermodynamic properties of the LaNiIn–H system
Journal of Alloys and Compounds 400 (2005) 184–187
Influence of Al- and Cu-doping on the thermodynamic properties of the LaNiIn–H system Masashi Sato a , R.V. Denys b,c , A.B. Riabov b , V.A. Yartys a,∗ b
a Institute for Energy Technology, Instituttveien 18, P.O. Box 40, Kjeller N-2027, Norway Physico-Mechanical Institute of the National Academy of Science of Ukraine, 5 Naukova St., Lviv 79601, Ukraine c The Studsvik Neutron Research Laboratory, Uppsala University, S-611 82 Nyk¨ oping, Sweden
Received 14 March 2005; accepted 31 March 2005 Available online 24 May 2005
Abstract The Pressure–Composition–Temperature (P–C–T) relations for the LaNiIn, LaNi0.95 Cu0.05 In and LaNiIn0.98 Al0.02 –H systems were measured by a volumetric Sieverts’ method at 398–423 K. All isotherms show plateau pressure regions indicating equilibria between two hydride phases. The replacements of Ni by Cu and In by Al affect the P–C–T diagrams, stability of the hydrides, homogeneity regions of the hydrides formed, slope of the isotherms and critical temperatures of the –␥ transition. In addition, the Cu-doping induces a significant hysteresis between the hydrogen absorption and desorption processes. The relative partial molar thermodynamic properties for the studied systems are: HH = −34.6 ± 2.1 kJ (molH )−1 , SH = −70.7 ± 3.6 J (K·molH )−1 for LaNiIn–H; HH = −34.1 ± 0.5 kJ (molH )−1 , SH = −74.9 ± 1.0 J (K·molH )−1 for LaNi0.95 Cu0.05 In–H; HH = −33.2 ± 0.8 kJ (molH )−1 , SH = −68.3 ± 1.2 J(K·molH )−1 for LaNiIn0.98 Al0.02 –H. © 2005 Elsevier B.V. All rights reserved. Keywords: Intermetallic compound; Metal hydride; Gas–solid reaction; Thermodynamic properties
1. Introduction Recently, the RENiIn-based hydrides (RE = La, Ce and Nd) [1] have attracted much attention due to the formation in their structures of unique close separations between the hy˚ drogen atoms, with shortest H· · ·H distances of 1.56–1.64 A. The structures of RENiInD1.2 compounds contain deuterium pairs located in trigonal bypiramids of RE3 Ni2 . The shrinking of the RE3 face of the RE3 Ni2 bypiramids, which separates the H· · ·H pair and strong covalent-like H Ni H bonding are responsible for the reduction of the repulsive interaction between these closest hydrogen atoms [1–3]. A study the contribution from the constituent elements in the RENiIn-related hydrides to the mechanism that leads to the formation of the short H· · ·H separations is of importance in order to better understand the reasons allowing a violation ˚ (the lowest limit for the H· · ·H of the well known “rule of 2 A” ∗
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 © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.03.057
separations) [4] applicable to metal–hydrogen systems. The rare earth atoms affect the thermodynamic properties of hydrogen in the RENiInH1.33 phase [5]. However, despite the fact that the hydrogen atoms are predominantly surrounded by the rare earth atoms in the tetrahedral RE3 Ni sites, the change in the thermodynamic properties over the rare earth series does not resemble the characteristic feature of the pure rare earth hydrides. The RE H bonding seems to be significantly weaker in the RENiInH1.33 compared to the REHx binary metal hydrides. In the case of the NdNiIn compound, a small (below 10%) level of substitution of Cu for Ni or Al for In does not significantly affect the H· · ·H separations [6]. The substituting elements, Cu and Al, prefer to occupy the 1b and 3f positions, respectively. Both these sites are not a part of the coordination polyhedra of the hydrogen atoms in the corresponding hydrides [6]. On the other hand, these substitutions induce pronounced changes in the thermodynamics of hydrogen in Nd(Ni1−x Cux )(In1−y Aly ) even at low substitution levels when x, y < 0.05 [7]. These data show that, from the
M. Sato et al. / Journal of Alloys and Compounds 400 (2005) 184–187
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thermodynamic point of view, Ni and In play a very sensitive role in the formation of the phase containing short H· · ·H distances. This work is devoted to thermodynamic studies of the LaNiIn-based hydrides, with Ni and In partially replaced by the chemically related Cu and Al, respectively. In particular, the effect of substitutions on the thermodynamic properties of hydrogen in the corresponding hydrides has been studied by means of Pressure–Composition–Temperature measurements. 2. Experimental The alloys were prepared from the pure constituent metals with purity better than 99.9% by arc melting on a watercooled copper pad under argon atmosphere. The ingots were remelted several times and then homogenised by annealing in evacuated quartz tubes at 873 K for 4 weeks. The samples were quenched into a mixture of ice and water afterwards. Synchrotron powder X-ray diffraction data, collected at Swiss–Norwegian beam line at the European Synchrotron Radiation Facility, Grenoble, France, confirmed the formation of single-phase alloys with the hexagonal ZrNiAltype structures. The refined unit lattice parameters are: ˚ c = 4.050(1) A ˚ for LaNiIn; a = 7.60029(7) A, ˚ a = 7.5906(9) A, ˚ for LaNi0.95 Cu0.05 In; a = 7.58859(5) A, ˚ c = 4.05751(5) A ˚ for LaNiIn0.98 Al0.02 . c = 4.05277(4) A The Pressure–Composition–Temperature (P–C–T) isotherms were measured by a volumetric method on a Sieverts-type apparatus. The measurements were made at temperatures between 398 and 423 K. More detailed information concerning the technique applied is presented in [8].
Fig. 1. P–C–T diagrams of the LaNiIn–H system at 398, 410.5 and 423 K.
cause of the higher stability of the La-containing hydride compared to that in the chemically related NdNiIn–H system [7], higher measurement temperatures were employed. This, in turn, led to a narrowing of the plateau pressure regions for LaNiIn–H, because of the lowering of the H content in the hydrides on temperature increase. The thermal stability of the -RENiInH0.66 hydrides is markedly high [7]. Therefore, it was not possible to measure the isotherms showing equilibria for the transformation ␣-hydrogen solid solution ↔ -RENiInH0.66 under the mentioned pressure–temperature conditions employed in present work. Thus, the H concentration region below [H]/[LaNiIn] < 0.5 was not reached.
3.1. P–C–T relationships
3.1.2. LaNi0.95 Cu0.05 In–H system The P–C–T relations for the LaNi0.95 Cu0.05 In–H system are shown in Fig. 2. The plateau regions in the LaNi0.95 Cu0.05 In–H system cover broader ranges than those for the LaNiIn–H system for both absorption and desorption
3.1.1. LaNiIn–H system Fig. 1 shows the P–C–T plots for the LaNiIn–H system at 398, 410.5 and 423 K. A two-phase plateau region with phaseboundaries at [H]/[LaNiIn] = 0.6 and 0.8 at 398 K is clearly seen on the isotherms. These phase boundaries are identified as corresponding to the formation of the -LaNiInH0.66 and ␥-LaNiInH1.33 hydride phases, respectively. These two types of hydrides have been recently characterised by powder neutron diffraction [1]. In the structure of -LaNiInH0.66 , the hydrogen atoms statistically occupy one half of the available La3 Ni tetrahedral sites; in the ␥-LaNiInH1.33 hydride, the H atoms completely occupy these sites, which share triangular La3 faces. Because of that, H· · ·H pairs are lo˚ Overstoichiometry cated at a distance of only about 1.6 A. [H]/[LaNiIn] > 1.33 is achieved as a result of additional filling of the La3 NiIn2 octahedral interstices [9]. We note rather significant differences in the isotherms between the La- and Nd-containing RENiIn–H systems. Be-
Fig. 2. P–C–T diagrams of the LaNi0.98 Cu0.05 In–H system at 398, 410.5 and 423 K.
3. Results and discussion
186
M. Sato et al. / Journal of Alloys and Compounds 400 (2005) 184–187 Table 1 Thermodynamic properties of the LaNiIn-based hydrides (desorption process) Alloys
HH (kJ (molH )−1 )
SH (J (K·molH )−1 )
LaNiIn LaNi0.95 Cu0.05 In LaNiIn 0.98 Al0.02
−34.6 ± 2.1 −34.1 ± 0.5 −33.2 ± 0.8
−70.7 ± 3.6 −74.9 ± 1.0 −68.3 ± 1.2
tion is rather independent of the type of the RE element.The hysteresis during hydrogen absorption and desorption does not change, 341 J (molH )−1 at 398 K for the Al-containing hydride, compared to 343 J (molH )−1 for the LaNiIn–H system. This differs from the trend observed for the NdNiIn–H system [7]. Fig. 3. P–C–T diagrams of the LaNiIn0.98 Al0.02 –H system at 398, 410.5 and 423 K.
3.2. Thermodynamic properties
processes. The Cu-for-Ni substitution also leads to a substantial increase in equilibrium pressures of the –␥ transition. The P–C–T curves of the LaNi0.95 Cu0.05 In–H system reveal significantly different pressure hysteresis between hydrogen absorption and desorption processes compared to that for the LaNiIn–H system, Fig. 1. The calculated magnitude of the hysteresis (the midpoints on the plateaux were used), expressed by the thermodynamic expression 1/2RT ln(Pab /Pdes ), is much larger for the Cu-doped alloy: 2147 J (molH )−1 compared to 343 J (molH )−1 for the LaNiIn–H system at 398 K. Such a behaviour differs from the thermodynamic data for the NdNiIn-based hydrides [7], where Cu substitution causes minor changes in the plateau pressures and decreases the hysteresis effect. As derived from powder neutron diffraction studies [6], in RENi1−x Cux In Cu preferentially substitutes for Ni on the 1b site, which does not contribute to the coordination of the H atoms. Thus, since the 1b site is coordinated by a trigonal prism RE6 , the introduced Cu can affect the thermodynamic behaviour of hydrogen in the RENiIn compounds only indirectly, via interaction with the RE atoms. Obviously, the effect of Cu doping strongly depends on the particular rare-earth element. Even small substitutions induce sensitive changes in the Ni(Cu)–RE interactions, influencing, in turn, RE–H interactions and formation of the H· · ·H pairs.
The relative partial molar thermodynamic properties of those hydrides were determined from a thermodynamic equation:
3.1.3. LaNiIn0.98 Al0.02 –H system Fig. 3 shows the isotherms for the LaNiIn0.98 Al0.02 –H system. In this case a sharp sloping of the plateaux is observed. Such behaviour is similar to the data obtained earlier for the NdNiIn-based hydride [7]. Vajeeston et al. pointed out that strong bonding between Ni in 2c and In in 3f becomes weaker during the hydrogenation of the RENiIn intermetallics; the rearrangement of the electron density around Ni leads to the weakening of the Ni In bonding but, instead, allows the bonding between the Ni and H atoms [3]. It is evident that electron contribution from In(Al) to H occurs not through the RE atoms, and consequently the effect of Al substitu-
µH =
1 RT ln PH2 = HH − TSH 2
(1)
where µH is the relative partial molar chemical potential, HH is the relative partial molar enthalpy and SH is the relative partial molar entropy. The thermodynamic properties are summarised in Table 1. The enthalpy and entropy values for the Al- and Cu-substituted samples do not differ significantly from those for the LaNiIn–H system. This is opposite to the effect of substitution on the thermodynamic properties of the NdNiIn–H system [5], showing that the thermodynamic behaviours of the RENiIn-based hydrides strongly depend on the rare earth element. The logarithmic plots of absorption and desorption plateaux against reciprocal temperature are shown in an inset of Fig. 4. Since hysteresis disappears at temperatures above Tc , the point of intersection of two linear relations for the absorption and desorption processes indicates the critical temperature Tc [10]. The values of Tc obtained in such a way are, ∼448 K for LaNiIn, ∼446 K for LaNiIn0.98 Al0.02 and ∼590 K for LaNi0.95 Cu0.05 In. A significant increase of Tc is evident for the Cu-doped LaNiIn hydrides. Tc can be described by the relation Tc = −βWHH /4R, where β is the number of the available H sites and WHH is the H–H interaction energy. The ratio of Tc to Tc (Cu) for LaNiIn and LaNi0.95 Cu0.05 In hydrides, respectively, can be presented as Tc (Cu) β(Cu)WHH (Cu) = Tc βWHH
(2)
From our volumetric measurements at 298 K, the solubility limit for the LaNi0.95 Cu0.05 In–H system equals 1.3 at. H per formula unit. The stoichiometry limit of hydrogen content in LaNiIn has been previously reported to reach the higher value 1.63 at. H per LaNiIn [9] at room temperature.Thus, Eq. (2) can be rewritten using the values of Tc obtained from
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the Nd-based ones, this shows that these features are significantly affected by a particular rare earth metal. Furthermore, the complexity of the system is also reflected by its relative instability when even small amounts of the substituting elements cause marked modifications of the electronic structure leading to a change of the mechanism of hydrogen uptake.
Acknowledgment This work was financially supported by the Research Council of Norway.
References Fig. 4. Variations of the equilibrium plateau pressures and estimated critical temperatures of LaNiIn-based hydrides on Cu- and Al-substitutions. Inset presents van’t Hoff plots of the absorption and desorption plateau regions for: LaNiIn () absorption, ( ) desorption at [H]/[LaNiIn] = 0.7; LaNi0.95 Cu0.05 In () absorption, (䊉) desorption at [H]/[LaNi0.95 Cu0.05 In] = 0.85; LaNiIn0.98 Al0.02 () absorption, () desorption at [H]/[LaNiIn0.98 Al0.02 ] = 0.65.
the dependences shown in the inset of Fig. 4, as WHH (Cu) = 1.66 WHH
(3)
Hence, the most significant effect of the substitution of Cu for Ni is a significant increase of WHH . However, since Cu substitution gives different modifications of the H storage properties for the La-containing compounds compared to
[1] V.A. Yartys, R.V. Denys, B.C. Hauback, H. Fjellv˚ag, I.I. Bulyk, A.B. Riabov, Y.M. Kalychak, J. Alloys Compd. 330–332 (2002) 132. [2] P. Ravindran, P. Vajeeston, R. Vidya, A. Kjekshus, H. Fjellv˚ag, Phys. Rev. Lett. 89 (2002) 106403. [3] P. Vajeeston, P. Ravindran, R. Vidya, A. Kjekshus, H. Fjellv˚ag, V.A. Yartys, Phys. Rev. B 67 (2002) 014101. [4] A.C. Switendick, Z. Phys. Chem., N.F. 117 (1979) 89. [5] M. Sato, R.V. Denys, A.B. Riabov, V.A. Yartys, J. Alloys Compd. 397 (1–2) (2005) 99–103. [6] A.B. Riabov, R.V. Denys, M. Sato, V.A. Yartys, R.G. Delaplane, J. Alloys Compd., in press. [7] M. Sato, R.V. Denys, A.B. Riabov, V.A. Yartys, J. Alloys Compd. 397 (1–2) (2005) 99–103. [8] M. Sato, V.A. Yartys, J. Alloys Compd. 373 (2004) 161. [9] R.V. Denys, A.B. Riabov, V.A. Yartys, B.C. Hauback, H. Brinks, J. Alloys Compd. 356–357 (2003) 65. [10] T. Kuji, Y. Matsumura, H. Uchida, T. Aizawa, J. Alloys Compd. 330–332 (2002) 718.
Influence of Al- and Cu-doping on the thermodynamic properties of the LaNiIn–H system Masashi Sato a , R.V. Denys b,c , A.B. Riabov b , V.A. Yartys a,∗ b
a Institute for Energy Technology, Instituttveien 18, P.O. Box 40, Kjeller N-2027, Norway Physico-Mechanical Institute of the National Academy of Science of Ukraine, 5 Naukova St., Lviv 79601, Ukraine c The Studsvik Neutron Research Laboratory, Uppsala University, S-611 82 Nyk¨ oping, Sweden
Received 14 March 2005; accepted 31 March 2005 Available online 24 May 2005
Abstract The Pressure–Composition–Temperature (P–C–T) relations for the LaNiIn, LaNi0.95 Cu0.05 In and LaNiIn0.98 Al0.02 –H systems were measured by a volumetric Sieverts’ method at 398–423 K. All isotherms show plateau pressure regions indicating equilibria between two hydride phases. The replacements of Ni by Cu and In by Al affect the P–C–T diagrams, stability of the hydrides, homogeneity regions of the hydrides formed, slope of the isotherms and critical temperatures of the –␥ transition. In addition, the Cu-doping induces a significant hysteresis between the hydrogen absorption and desorption processes. The relative partial molar thermodynamic properties for the studied systems are: HH = −34.6 ± 2.1 kJ (molH )−1 , SH = −70.7 ± 3.6 J (K·molH )−1 for LaNiIn–H; HH = −34.1 ± 0.5 kJ (molH )−1 , SH = −74.9 ± 1.0 J (K·molH )−1 for LaNi0.95 Cu0.05 In–H; HH = −33.2 ± 0.8 kJ (molH )−1 , SH = −68.3 ± 1.2 J(K·molH )−1 for LaNiIn0.98 Al0.02 –H. © 2005 Elsevier B.V. All rights reserved. Keywords: Intermetallic compound; Metal hydride; Gas–solid reaction; Thermodynamic properties
1. Introduction Recently, the RENiIn-based hydrides (RE = La, Ce and Nd) [1] have attracted much attention due to the formation in their structures of unique close separations between the hy˚ drogen atoms, with shortest H· · ·H distances of 1.56–1.64 A. The structures of RENiInD1.2 compounds contain deuterium pairs located in trigonal bypiramids of RE3 Ni2 . The shrinking of the RE3 face of the RE3 Ni2 bypiramids, which separates the H· · ·H pair and strong covalent-like H Ni H bonding are responsible for the reduction of the repulsive interaction between these closest hydrogen atoms [1–3]. A study the contribution from the constituent elements in the RENiIn-related hydrides to the mechanism that leads to the formation of the short H· · ·H separations is of importance in order to better understand the reasons allowing a violation ˚ (the lowest limit for the H· · ·H of the well known “rule of 2 A” ∗
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 © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.03.057
separations) [4] applicable to metal–hydrogen systems. The rare earth atoms affect the thermodynamic properties of hydrogen in the RENiInH1.33 phase [5]. However, despite the fact that the hydrogen atoms are predominantly surrounded by the rare earth atoms in the tetrahedral RE3 Ni sites, the change in the thermodynamic properties over the rare earth series does not resemble the characteristic feature of the pure rare earth hydrides. The RE H bonding seems to be significantly weaker in the RENiInH1.33 compared to the REHx binary metal hydrides. In the case of the NdNiIn compound, a small (below 10%) level of substitution of Cu for Ni or Al for In does not significantly affect the H· · ·H separations [6]. The substituting elements, Cu and Al, prefer to occupy the 1b and 3f positions, respectively. Both these sites are not a part of the coordination polyhedra of the hydrogen atoms in the corresponding hydrides [6]. On the other hand, these substitutions induce pronounced changes in the thermodynamics of hydrogen in Nd(Ni1−x Cux )(In1−y Aly ) even at low substitution levels when x, y < 0.05 [7]. These data show that, from the
M. Sato et al. / Journal of Alloys and Compounds 400 (2005) 184–187
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thermodynamic point of view, Ni and In play a very sensitive role in the formation of the phase containing short H· · ·H distances. This work is devoted to thermodynamic studies of the LaNiIn-based hydrides, with Ni and In partially replaced by the chemically related Cu and Al, respectively. In particular, the effect of substitutions on the thermodynamic properties of hydrogen in the corresponding hydrides has been studied by means of Pressure–Composition–Temperature measurements. 2. Experimental The alloys were prepared from the pure constituent metals with purity better than 99.9% by arc melting on a watercooled copper pad under argon atmosphere. The ingots were remelted several times and then homogenised by annealing in evacuated quartz tubes at 873 K for 4 weeks. The samples were quenched into a mixture of ice and water afterwards. Synchrotron powder X-ray diffraction data, collected at Swiss–Norwegian beam line at the European Synchrotron Radiation Facility, Grenoble, France, confirmed the formation of single-phase alloys with the hexagonal ZrNiAltype structures. The refined unit lattice parameters are: ˚ c = 4.050(1) A ˚ for LaNiIn; a = 7.60029(7) A, ˚ a = 7.5906(9) A, ˚ for LaNi0.95 Cu0.05 In; a = 7.58859(5) A, ˚ c = 4.05751(5) A ˚ for LaNiIn0.98 Al0.02 . c = 4.05277(4) A The Pressure–Composition–Temperature (P–C–T) isotherms were measured by a volumetric method on a Sieverts-type apparatus. The measurements were made at temperatures between 398 and 423 K. More detailed information concerning the technique applied is presented in [8].
Fig. 1. P–C–T diagrams of the LaNiIn–H system at 398, 410.5 and 423 K.
cause of the higher stability of the La-containing hydride compared to that in the chemically related NdNiIn–H system [7], higher measurement temperatures were employed. This, in turn, led to a narrowing of the plateau pressure regions for LaNiIn–H, because of the lowering of the H content in the hydrides on temperature increase. The thermal stability of the -RENiInH0.66 hydrides is markedly high [7]. Therefore, it was not possible to measure the isotherms showing equilibria for the transformation ␣-hydrogen solid solution ↔ -RENiInH0.66 under the mentioned pressure–temperature conditions employed in present work. Thus, the H concentration region below [H]/[LaNiIn] < 0.5 was not reached.
3.1. P–C–T relationships
3.1.2. LaNi0.95 Cu0.05 In–H system The P–C–T relations for the LaNi0.95 Cu0.05 In–H system are shown in Fig. 2. The plateau regions in the LaNi0.95 Cu0.05 In–H system cover broader ranges than those for the LaNiIn–H system for both absorption and desorption
3.1.1. LaNiIn–H system Fig. 1 shows the P–C–T plots for the LaNiIn–H system at 398, 410.5 and 423 K. A two-phase plateau region with phaseboundaries at [H]/[LaNiIn] = 0.6 and 0.8 at 398 K is clearly seen on the isotherms. These phase boundaries are identified as corresponding to the formation of the -LaNiInH0.66 and ␥-LaNiInH1.33 hydride phases, respectively. These two types of hydrides have been recently characterised by powder neutron diffraction [1]. In the structure of -LaNiInH0.66 , the hydrogen atoms statistically occupy one half of the available La3 Ni tetrahedral sites; in the ␥-LaNiInH1.33 hydride, the H atoms completely occupy these sites, which share triangular La3 faces. Because of that, H· · ·H pairs are lo˚ Overstoichiometry cated at a distance of only about 1.6 A. [H]/[LaNiIn] > 1.33 is achieved as a result of additional filling of the La3 NiIn2 octahedral interstices [9]. We note rather significant differences in the isotherms between the La- and Nd-containing RENiIn–H systems. Be-
Fig. 2. P–C–T diagrams of the LaNi0.98 Cu0.05 In–H system at 398, 410.5 and 423 K.
3. Results and discussion
186
M. Sato et al. / Journal of Alloys and Compounds 400 (2005) 184–187 Table 1 Thermodynamic properties of the LaNiIn-based hydrides (desorption process) Alloys
HH (kJ (molH )−1 )
SH (J (K·molH )−1 )
LaNiIn LaNi0.95 Cu0.05 In LaNiIn 0.98 Al0.02
−34.6 ± 2.1 −34.1 ± 0.5 −33.2 ± 0.8
−70.7 ± 3.6 −74.9 ± 1.0 −68.3 ± 1.2
tion is rather independent of the type of the RE element.The hysteresis during hydrogen absorption and desorption does not change, 341 J (molH )−1 at 398 K for the Al-containing hydride, compared to 343 J (molH )−1 for the LaNiIn–H system. This differs from the trend observed for the NdNiIn–H system [7]. Fig. 3. P–C–T diagrams of the LaNiIn0.98 Al0.02 –H system at 398, 410.5 and 423 K.
3.2. Thermodynamic properties
processes. The Cu-for-Ni substitution also leads to a substantial increase in equilibrium pressures of the –␥ transition. The P–C–T curves of the LaNi0.95 Cu0.05 In–H system reveal significantly different pressure hysteresis between hydrogen absorption and desorption processes compared to that for the LaNiIn–H system, Fig. 1. The calculated magnitude of the hysteresis (the midpoints on the plateaux were used), expressed by the thermodynamic expression 1/2RT ln(Pab /Pdes ), is much larger for the Cu-doped alloy: 2147 J (molH )−1 compared to 343 J (molH )−1 for the LaNiIn–H system at 398 K. Such a behaviour differs from the thermodynamic data for the NdNiIn-based hydrides [7], where Cu substitution causes minor changes in the plateau pressures and decreases the hysteresis effect. As derived from powder neutron diffraction studies [6], in RENi1−x Cux In Cu preferentially substitutes for Ni on the 1b site, which does not contribute to the coordination of the H atoms. Thus, since the 1b site is coordinated by a trigonal prism RE6 , the introduced Cu can affect the thermodynamic behaviour of hydrogen in the RENiIn compounds only indirectly, via interaction with the RE atoms. Obviously, the effect of Cu doping strongly depends on the particular rare-earth element. Even small substitutions induce sensitive changes in the Ni(Cu)–RE interactions, influencing, in turn, RE–H interactions and formation of the H· · ·H pairs.
The relative partial molar thermodynamic properties of those hydrides were determined from a thermodynamic equation:
3.1.3. LaNiIn0.98 Al0.02 –H system Fig. 3 shows the isotherms for the LaNiIn0.98 Al0.02 –H system. In this case a sharp sloping of the plateaux is observed. Such behaviour is similar to the data obtained earlier for the NdNiIn-based hydride [7]. Vajeeston et al. pointed out that strong bonding between Ni in 2c and In in 3f becomes weaker during the hydrogenation of the RENiIn intermetallics; the rearrangement of the electron density around Ni leads to the weakening of the Ni In bonding but, instead, allows the bonding between the Ni and H atoms [3]. It is evident that electron contribution from In(Al) to H occurs not through the RE atoms, and consequently the effect of Al substitu-
µH =
1 RT ln PH2 = HH − TSH 2
(1)
where µH is the relative partial molar chemical potential, HH is the relative partial molar enthalpy and SH is the relative partial molar entropy. The thermodynamic properties are summarised in Table 1. The enthalpy and entropy values for the Al- and Cu-substituted samples do not differ significantly from those for the LaNiIn–H system. This is opposite to the effect of substitution on the thermodynamic properties of the NdNiIn–H system [5], showing that the thermodynamic behaviours of the RENiIn-based hydrides strongly depend on the rare earth element. The logarithmic plots of absorption and desorption plateaux against reciprocal temperature are shown in an inset of Fig. 4. Since hysteresis disappears at temperatures above Tc , the point of intersection of two linear relations for the absorption and desorption processes indicates the critical temperature Tc [10]. The values of Tc obtained in such a way are, ∼448 K for LaNiIn, ∼446 K for LaNiIn0.98 Al0.02 and ∼590 K for LaNi0.95 Cu0.05 In. A significant increase of Tc is evident for the Cu-doped LaNiIn hydrides. Tc can be described by the relation Tc = −βWHH /4R, where β is the number of the available H sites and WHH is the H–H interaction energy. The ratio of Tc to Tc (Cu) for LaNiIn and LaNi0.95 Cu0.05 In hydrides, respectively, can be presented as Tc (Cu) β(Cu)WHH (Cu) = Tc βWHH
(2)
From our volumetric measurements at 298 K, the solubility limit for the LaNi0.95 Cu0.05 In–H system equals 1.3 at. H per formula unit. The stoichiometry limit of hydrogen content in LaNiIn has been previously reported to reach the higher value 1.63 at. H per LaNiIn [9] at room temperature.Thus, Eq. (2) can be rewritten using the values of Tc obtained from
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the Nd-based ones, this shows that these features are significantly affected by a particular rare earth metal. Furthermore, the complexity of the system is also reflected by its relative instability when even small amounts of the substituting elements cause marked modifications of the electronic structure leading to a change of the mechanism of hydrogen uptake.
Acknowledgment This work was financially supported by the Research Council of Norway.
References Fig. 4. Variations of the equilibrium plateau pressures and estimated critical temperatures of LaNiIn-based hydrides on Cu- and Al-substitutions. Inset presents van’t Hoff plots of the absorption and desorption plateau regions for: LaNiIn () absorption, ( ) desorption at [H]/[LaNiIn] = 0.7; LaNi0.95 Cu0.05 In () absorption, (䊉) desorption at [H]/[LaNi0.95 Cu0.05 In] = 0.85; LaNiIn0.98 Al0.02 () absorption, () desorption at [H]/[LaNiIn0.98 Al0.02 ] = 0.65.
the dependences shown in the inset of Fig. 4, as WHH (Cu) = 1.66 WHH
(3)
Hence, the most significant effect of the substitution of Cu for Ni is a significant increase of WHH . However, since Cu substitution gives different modifications of the H storage properties for the La-containing compounds compared to
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