- Email: [email protected]
Homogeneity range and crystal structure of Ni substituted Mg6(Pd,Ni) complex intermetallic compounds
Journal of Physics and Chemistry of Solids 71 (2010) 1259–1263
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Homogeneity range and crystal structure of Ni substituted Mg6(Pd,Ni) complex intermetallic compounds F. Cuevas a,n, J.F. Ferna´ndez b, J.R. Ares b, F. Leardini b, C. Sa´nchez b a b
CMTR/ICMPE/CNRS, UMR7182, 2-8 rue Henri Dunant, 94320 Thiais, Cedex, France ´noma de Madrid, 28049 Madrid, Spain Dpto. Fı´sica de Materiales, Facultad de Ciencias, Universidad Auto
a r t i c l e in fo
abstract
Article history: Received 5 March 2010 Received in revised form 7 May 2010 Accepted 10 May 2010
The solubility of nickel in Mg6Pd compound has been studied by chemical and structural methods. The solubility limit at 673 K attains 9 at% Ni, i.e. more than four times the value previously reported. Therefore, a re-determination of the ternary Mg–Pd–Ni phase diagram in the Mg-rich corner is desirable. Mg6(Pd,Ni) compounds have the same crystal structure as the binary Mg6Pd compound. The unit-cell composition at the solubility limit is Mg344(2)Pd12(1)Ni36(1) with 75% of Ni atoms substituting Pd ones. Ni substitution occurs on the three crystallographic sites exclusively occupied by Pd in Mg6Pd. The substitution is not fully random. Ni atoms have a slight preference for site with low coordination number. The results reported here enlarge the compositional range in which Mg6(Pd,Ni) compounds can be used as reversible hydrogen storage materials. & 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Intermetallic compounds C. X-ray diffraction D. Crystal structure D. Phase equilibria
1. Introduction Mg6Pd is a complex intermetallic compound. It crystallizes in the cubic F43m space group with lattice parameter a¼ 20.108 A˚ [1]. The unit-cell contains 396 atoms (Mg340Pd56). Beyond the fundamental interest of complex crystal structures [2,3], the high Mgcontent of this compound makes it interesting as a hydrogen storage medium [4–8]. Magnesium absorbs a large amount of hydrogen (7.6 wt% H, 108 gH2/l) but its hydride is too stable for room temperature applications. One strategy to circumvent this problem is to alloy Mg with last transition metals (such as Pd, Fe and Ni) having less affinity for hydrogen [9–11]. However, very few transition metals form stable intermetallic compounds with Mg [9]. The Mg–Pd system is a remarkable exception since at least eleven intermetallic compounds exist in its binary phase diagram [12,13]. The Mg6Pd phase is the Mg-richest one. Several groups have reported that the hydrogenation of Mg6Pd compound is reversible and occurs through three disproportionation reactions with a global hydrogen uptake of about 4 wt% H [7,8]. Unfortunately, the stability of the hydrogenation reaction does not differ much from that of the Mg/H2 system. Owing to the lower hydrogen affinity of nickel and to its lower cost, the substitution of palladium by nickel in Mg6Pd compound is attractive for hydrogen storage applications. Substitution is favored by the fact that Pd and Ni have the same electronic outer
n
Corresponding author. Tel.: + 33 1 49 78 12 25; fax: + 33 1 49 78 12 03. E-mail address: [email protected] (F. Cuevas).
0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2010.05.004
shell configuration. However, the Mg6Ni phase does not exist in the binary Mg–Ni phase diagram and therefore a limit for Ni substitution in Mg6Pd1 xNix compounds is anticipated. Published data on the ternary Mg–Ni–Pd phase diagram at 673 K indicate that the maximum Ni substitution in the Mg6Pd1 xNix phase (labeled as r) is as low as 2 at% Ni (x¼ 0.13) [14,15]. At higher Ni-content, Mg2Ni (labeled as b) and Mg (labeled as e) phases appear in equilibrium with the r-phase. However, we have recently proved that Mg6Pd0.5Ni0.5 compound with 7 at% Ni is thermodynamically stable at 660 K [16–18]. The scope of this paper is to determine the substitution limit of Pd by Ni in Mg6Pd1 xNix compounds and to accomplish a complete structural characterization of the whole homogeneity range.
2. Experimental Samples of nominal composition Mg6Pd1 xNix with x ¼0.5, 0.75, 0.875 and 1 were prepared by powder metallurgy starting from elemental powders of purity higher than 99.8% metal basis (Mg from Alfa Aesar with particle size PS o44 mm, Ni from Cerac PS o44 mm, Pd from CEA PS o100 mm). For each composition, elemental powders were mixed and cold pressed at 700 MPa in the form of pellets (typically 0.5 g in mass, 8 mm in diameter). The pellets were then placed in tantalum thimbles, which were introduced into stainless steel sample holders previously outgassed at 1173 K under secondary vacuum. Next, these holders were hermetically sealed by arc-welding to perform a thermal treatment of the pellets in a conventional resistance furnace.
1260
F. Cuevas et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1259–1263
All manipulations were done in an argon glove box (O2 o1 ppm, H2Oo30 ppm). The pellets were heated up to 1073 K for 12 h in order to promote sample melting (Mg6Pd melts at 973 K [19]), and then annealed at 673 K for one week to ensure chemical equilibrium. The sample holders were then cooled down to room temperature by water quenching. Sample composition was analyzed by electron probe microanalysis (EPMA) in a Cameca SX-100 instrument operated at 15 kV. To this aim, the samples were embedded in wood’s alloy and mechanically polished with methanol using SiC paper with lessening sizes (800, 1200 and 4000 mesh). Sample crystal structure was studied at room temperature by X-ray powder diffraction (XRPD). All Pd-containing samples exhibited brittle character. They were ground in an agate mortar and sieved to 63 mm. XRPD measurements were performed with a y–y diffractometer (Bruker AXS D8Advance equipped with backscattered rear graphite monochromator) using CuKa radiation. Data were recorded over the range 10–1001 by steps of 0.041 and analyzed by the Rietveld method using the Fullprof software [20]. As for the Pd-free sample, it exhibited a tough character. Therefore, a piece of it was mirror-like polished for performing XRD analysis in the same diffractometer. Because of the sample texture, XRD data of the Pd-free sample were only used for phase identification, indexation and determination of lattice parameters.
3. Results and discussion 3.1. Sample composition Phase nature and their spatial distribution in the four studied samples were analyzed with the help of backscattered electron (BSE) images (Fig. 1) and elemental Mg, Ni and Pd map analysis (not shown here, see Electronic Annex 1 in the online version of this article). All samples exhibit a two-phase microstructure except for the sample with x¼ 0.875 where three phases are observed. At low Ni-content (x ¼0.5 and 0.75) the r-phase (in grey) is the dominant one and it coexists with secondary b-precipitates (in white in Fig.1). Then, a further increase in
Ni-content (x¼0.875) leads to the additional precipitation of secondary e-phase (in black). Finally, the sample without Pd, i.e. for x¼1, shows no r-phase and is composed of Mg(e) and Mg2Ni(b) phases in agreement with the Mg–Ni phase diagram. Phase composition results (determined by EPMA) are shown in Table 1. For each one of the Pd-containing samples, the analysis of the r-phase was performed on more than fifty randomly distributed points. The Ni to Pd ratio increases gradually with Ni substitution (x). One can notice that the Ni-content of the r-phase is lower than that of the nominal sample what can be attributed to the Mg2Ni(b) phase precipitation. For x¼0.5, the results of the r-phase analysis are less accurate due to the contribution of the finely dispersed Mg2Ni precipitates and to the spatial EPMA resolution ( 1 mm3). The given composition corresponds to the maximum content of Pd according to the EPMA analysis. Additionally, the Mg2Ni(b) precipitates’ composition was determined in the sample with x ¼0.75 since their sizes appear to be larger than the spatial EPMA resolution. The obtained composition was Mg66.6Ni31.5Pd1.9, what proves that the Pd solubility in Mg2Ni(b) phase is very low. 3.2. Structural characterization The diffraction patterns of the samples under study are displayed in Fig. 2. The diffraction peaks of the Pd-free sample (x ¼1) are assigned to Mg(e) and Mg2Ni(b) phases as expected from the Mg–Ni phase diagram and in agreement with EPMA results. In contrast, the diffraction patterns of Pd-containing samples are completely different. Except for a few diffraction peaks of minor intensity corresponding to e (for x ¼0.875) and b (for x¼0.75 and 0.5) phases, most of them belong to a different phase, which as established by EPMA analysis should be the Mg6(Pd,Ni)(r) phase. All diffraction patterns of Pd-containing samples were analyzed by the Rietveld method. The crystal structure of Mg(e) and Mg2Ni(b) phases can be easily implemented in the Rietveld refinement since, firstly, their structure is simple and well documented in the literature [21] and, secondly, the solubility of Pd and Ni in the e-phase or of Pd in the b-phase is negligible.
x = 0.75
x = 0.5
100 µm
100 µm
x=1
x = 0.875
100 µm
100 µm
Fig. 1. BSE Images of Mg6Pd1 xNix samples. White and black regions are identified as Mg2Ni(b) and Mg(e) phases, respectively. Grey regions are identified as the Mg6(Pd,Ni)(r) phase. For x¼ 0.5 and 0.75 black features are due to voids.
F. Cuevas et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1259–1263
1261
Table 1 Phase composition, content and crystal structure of Mg6Pd1 xNix samples. The standard deviations are given in parenthesis. Rietveld reliability factors of the XRPD refinement of Pd-containing samples are also given. Sample (x)
Nominal composition
Phase
Composition (EPMA)
S.G.
˚ Cell par. (A)
Composition (XRPD)
Content (wt%)
RB (%)
Rwp (%)
w2
0.5
Mg85.7Pd7.1Ni7.1
Mg6(Pd,Ni)(r) Mg2Ni(b)
Mg85.5Pd9.50Ni4.95 ( )
F4¯3m P6222
a¼ 20.1216(4) a¼ 5.2155(6) c¼ 13.251(2)
Mg87.0(9)Pd8.7(4)Ni4.2(4) ()
88(2) 12(1)
3.0 7.6
7.9
6.2
0.75
Mg85.7Pd3.6Ni10.7
Mg6(Pd,Ni)(r) Mg2Ni(b)
Mg86.5(1)Pd4.9(3)Ni8.6(3) Mg66.6Ni31.5Pd1.9
F4¯3m P6222
a¼ 20.1177(3) a¼ 5.2100(3) c¼ 13.255(1)
Mg87.5(6)Pd4.5(2)Ni7.9(2) ()
89(2) 11(1)
3.0 4.8
6.8
4.5
0.875
Mg85.7Pd1.8Ni12.5
Mg6(Pd,Ni)(r) Mg(e)
Mg87.5(6)Pd3.3(2)Ni9.2(4) ( )
F4¯3m P63/mmc
Mg87.6(7)Pd3.2(3)Ni9.2(3) ()
84(2) 16(1)
3.2 3.3
7.9
6.9
Mg2Ni(b)
( )
()
a¼ 20.1072(5) a¼ 3.2090(2) c¼ 5.2101(4) ( )
()
( )
()
Mg(e)
Mg
P63/mmc
()
( )
()
( )
()
Mg2Ni(b)
Mg66.5(2)Ni33.5(2)
P6222
a¼ 3.2099(2) c¼ 5.2083(4) a¼ 5.2077(3) c¼ 13.255(2)
()
( )
()
1
Mg85.7Ni14.3
Fig. 2. X-ray diffraction patterns of Mg6Pd1 xNix samples. Diffraction peaks related to Mg2Ni(b) and Mg(e) phases are marked by open and filled symbols, respectively. Non-marked peaks belong to the Mg6(Pd,Ni)(r) phase.
On the contrary, the structural model for the refinement of the Mg6(Pd,Ni)(r) phase is more complex and deserves a more detailed explanation. To start with, the structural model proposed by Samson et al. for the binary Mg6Pd compound was used but allowing for partial occupation of Ni atoms in Pd sites (i.e. atom nos. 10. 11, 12 and 13 in Ref. [1]). In this model, one should take into consideration that site 12 can be simultaneously occupied by Mg, Ni and Pd atoms that prevents determining the fractional atomic occupancy on this site from a unique diffraction data set. In a previous work, we circumvented this problem by constraining the chemical composition of the crystallographic unit-cell to that of EPMA results [17]. Such a work was done with a single phase Mg6.1Pd0.5Ni0.4 compound elaborated by induction melting. Owing to the multiphase character of the present samples, EPMA results of the Mg6(Pd,Ni) phase are not confident enough for the crystallographic determination (especially for x¼ 0.5) and therefore another strategy had to be adopted. In previous work, we have shown that, due to steric effects, Ni occupation of site 12 is very low (site occupation factor SOF¼ 0.04) and therefore only Mg and Pd atoms are expected to significantly occupy that site. Thus, we now assume that site 12 is only occupied by Mg and Pd atoms, the ratio of which can be unequivocally determined from one diffraction pattern.
An additional complication arises from the possible variation of Mg-content within the homogeneity domain of the r-phase, which extends from 85 to 87.4 at% Mg in the binary Mg–Pd system [13]. This is accommodated by point defects whose nature and extent depend on the Mg/Pd ratio as described by Makongo et al. [13]. The precipitation of Mg2Ni phase in the studied samples suggests that the obtained Mg6(Pd,Ni) phases stand on the Mg poor side of the homogeneity domain. In this case, site 14 is reported to be fully occupied by Mg whereas constitutional vacancies should appear at site 5. The implemented model allows for this possible configuration. General results on the structural analysis of the studied samples are gathered in Table 1. Mg6(Pd,Ni)(r) phase is the major phase ( 484 wt%) in all Pd-containing samples. The lattice parameter of this phase gradually decreases with Ni-content (from 20.1216 to 20.1072 A˚ on going from x¼ 0.5 to 0.875), which is attributed to the smaller atomic radius of Ni as compared to ˚ rPd ¼ 1.37 A). ˚ In agreement with these that of Pd (rNi ¼1.24 A, results, the metastable Mg6Ni compound is reported to have the smallest lattice parameter: 19.987 A˚ [22]. As for secondary phases, Mg2Ni(b) is detected for x¼0.5 and 0.75 samples, and Mg(e) for x¼0.875. Contrary to EPMA analysis, Mg2Ni(b) phase is not detected by XRPD for x ¼0.875 sample as a result of its low amount (see the few white precipitates in Fig. 1). The lattice parameters of the b-phase in the Pd-containing samples are close to that of the Pd-free sample, which confirms the low solubility of Pd in the b-phase. As for the Mg(e) phase, very similar lattice parameters are observed for Pd-free and Pd-containing (x ¼0.875) samples. The performed Rietveld analysis also provides detailed information on the dependence of the crystal structure of the r-phase with the nickel substitution. As an example, Fig. 3 shows the graphic output of the Rietveld refinement analysis for the x¼0.875 sample. The structural data for the r-phase are gathered in Table 2. These results are particularly interesting since they concern the crystal structure of the r-phase with maximum nickel content. Fit residuals of the r-phase are rather low (RB ¼3.2%) confirming the refinement quality. Owing to the low accuracy of XRD diffraction in the determination of thermal displacement factors (B), a unique B value was refined for all Mg sites and another one for the remaining substituted sites. The refined positional parameters and, thereby, the interatomic distances are quite close to those reported by Samson [1] for Mg6Pd compound. Constitutional vacancies (7%) occur on site 5. Substitution of Pd by Ni occurs on sites 10, 11 and 13.
1262
F. Cuevas et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1259–1263
Fig. 3. XRPD Rietveld analysis of Mg6Pd1 xNix sample with x ¼0.875. Experimental data (circles), calculated diffraction patterns (continuous line), diffraction line positions (vertical bars) and difference curve at the same scale (below) are given.
Table 2 Structural parameters of the r-Mg6(Pd,Ni) phase for x ¼ 0.875 sample. The standard deviations are given in parenthesis. Site no.
Atom
Wyckoff
x
1 2 3 4 5 6 7 8 9 10
Mg Mg Mg Mg Mg Mg Mg Mg Mg Pd Ni Pd Ni Pd Mg Pd Ni Mg
48(h) 48(h) 48(h) 48(h) 48(h) 24(f) 24(f) 24(g) 16(e) 16(e)
0.143 0.096 0.150 0.056 0.199 0.103 0.382 0.066 0.301 0.167
16(e)
11 12 13 14
B (A˚ 2)
SOF
0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 1.4
1 1 1 1 0.93 1 1 1 1 0.33 0.67 0.20 0.80 0.03 0.97 0.23 0.77 1
y
z
x x x x x 0 0 ¼ x x
0.035 0.273 0.527 0.768 0.911 0 0 ¼ x x
0.407 (1)
x
x
1.4 (3)
16(e)
0.674 (2)
x
x
1.4 (3)
16(e)
0.901 (1)
x
x
1.4 (3)
4(d)
¼
¼
3
0.9 (3)
(1) (1) (1) (1) (1) (2) (2) (2) (2) (1)
/4
(2) (2) (2) (2) (2)
(3) (3) (3) (3) (3) (3) (3) (3) (3) (3)
(4)
(4) (4) (4) (4) (2) (2) (4) (4)
˚ RB ¼ 3.2%, Rwp ¼7.9%. w2 ¼ 6.9. S.G. F43 m, a¼ 20.1072 (5) A. No. of independent reflections: 288. No. of intensity variables: 25.
The substitution is not random and site 12 is almost fully occupied by Mg atoms since the Pd occupancy is practically negligible (SOF ¼0.0370.02). The unit-cell composition of the r-phase is Mg344(2)Pd12(1)Ni36(1) in the x ¼0.875 sample. The formula unit (f.u.) of the Mg6(Pd,Ni)(r) phase with maximum nickel content can be written as Mg6.14(4)Pd0.21(2)Ni0.64(2) for Z ¼56 f.u. in the unit cell. The r-phase crystal structure at lower Ni concentrations (x ¼0.5 and 0.75 samples) is very similar to that previously described (x ¼0.875). For instance, very similar Mg vacancy concentrations are found on site 5 (6% and 8% for x ¼ 0.5 and 0.75, respectively), which confirm that the r-phase composition corresponds to the Mg-poor limit for all studied samples. The main change on the structural results between the different samples obviously concerns the occupation factors on sites 10, 11 and 13 due to their different Ni contents. The evolution of Ni occupancy of these sites (determined by XRPD) as a function of the overall Pd by Ni substitution in the r-phase (determined by EPMA) is displayed in Fig. 4. As a general trend, Ni occupancy on the three sites linearly increases with substitution. However, Ni site occupancy is not fully random. Ni shows some slight preference for the site with the smallest coordination number (site 11, CN10) probably due to its smaller atomic radius [17]. As for the SOF of Mg and Pd atoms at site 12, the Pd occupation
Fig. 4. Dependence of nickel SOF on the overall nickel substitution (determined by EPMA) in the Mg6(Pd,Ni)(r) phase. The hypothetical random occupancy is indicated by a dashed line.
gradually decreases with the Ni substitution (0.2, 0.05 and 0.03 for x ¼ 0.5, 0.75 and 0.875, respectively). This suggests that the instability of the r-phase with respect to the two phase b + e mixture at high Ni-content is driven by the full occupancy of site 12 by Mg atoms. The chemical composition of the r-phase for all samples as determined from the Rietveld refinement is gathered in Table 1. The results are in close agreement with EPMA analysis.
3.3. Implications for the Mg–Ni–Pd ternary diagram The previously reported Mg–Ni–Pd phase diagram at 673 K is displayed in Fig. 5a [14,15]. Taking into consideration the results obtained in this work (experimental points in Fig. 5b), the ternary diagram should be drastically modified as concerns the homogeneity range of the Mg6(Pd,Ni)(r) phase. The location of b, r and e phases in the ternary diagram indicates that the threephase sample (x¼0.875) shows the maximum solubility of Ni in the r-phase. The obtained Ni solubility limit (9 at% Ni) is more than four times that of the previously reported value. As a consequence, the two-phase (b + r) and (r + e) domains are expanded at the expense of the three-phase (b + r + e) domain. Last modifications need to be confirmed by a systematic re-determination of the Mg–Ni–Pd diagram at the Mg-rich corner.
4. Conclusion The substitution limit of Pd by Ni in the Mg6Pd phase is 9 at% Ni at 673 K, i.e. more than four times the value previously reported [14,15]. We have shown that the unit-cell of the r-phase structure shrinks slightly with Ni substitution due to the smaller atomic radius of Ni compared to Pd. The Ni substitution is accommodated by gradual replacement of Pd by Ni atoms on the three crystallographic sites exclusively occupied by Pd atoms in the complex binary Mg6Pd compound. However, the substitution is not fully random since Ni atoms exhibit a slight preference for sites with low coordination number. At the Ni substitution limit, the cell composition is Mg344(2)Pd12(1)Ni36(1) and site 12, which is randomly occupied by Mg and Pd atoms in the binary compound, is exclusively occupied by Mg atoms. The large extension of the
F. Cuevas et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1259–1263
0
0
50
50
10
10
40
) (at % 30
%)
30
Ni
30
20
) at%
(at %
( Pd
(at
30
20
Pd
20
)
40
20
ρ
ρ 40
40
Mg (at%)
0 0
80
90
ρ+ε ε β+ρ+ε
β 70
0
60
ε
10
β+ρ
50
90
80
70
60
50
β
10 ρ+ε
50
50
β+ρ+ε
10 0
β+ρ
10
Ni
1263
Mg (at%)
Fig. 5. Ternary Mg–Ni–Pd phase diagram at the Mg corner at 673 K: (a) as reported by Kolesnichenko et al. [14], (b) this work. Filled dots: measured compositions. Thin line: tie-line between b and r phases for sample with x¼ 0.75. Thick-line: tie-triangle of b, r and e phases.
Ni-substitution in the Mg6Pd phase gives more room for the study of Mg6Pd1 xNix compounds as hydrogen storage materials. Acknowledgements We thank N.L.-Do for technical assistance, E. Leroy for EPMA measurements and J.-M. Joubert for fruitful discussions and comments. Several authors thank the Spanish Minister of Education and Science, MEC, for financial support under contract no. MAT2008-06547-C02-01. References [1] S. Samson, Complex cubic A6B compounds. II. The crystal structure of Mg6Pd, Acta Cryst. B28 (1972) 936–945. [2] K. Urban, M. Feuerbacher, Structurally complex alloy phases, J. Non-Cryst. Solids 334-335 (2004) 143–150. [3] R.F. Berger, S. Lee, J. Johnson, B. Nebgen, F. Sha, J.Q. Xu, The mystery of perpendicular fivefold axes and the fourth dimension in intermetallic structures, Chem.-Eur. J. 14 (2008) 3908–3930. [4] Y. Kume, A. Weiss, On the interaction of hydrogen with the intermetallic phase Mg6Pd, J. Less-Common Met. 136 (1987) 51–54. [5] T. Yamada, J. Yin, K. Tanaka, Hydrogen storage properties and phase structures of Mg-rich Mg–Pd, Mg–Nd and Mg–Pd–Nd alloys, Mater. Trans 42 (2001) 2415–2421. [6] C. Zlotea, Y. Andersson, Microstructural modifications induced by hydrogen absorption in Mg5Ga2 and Mg6Pd, Acta Mater. 54 (2006) 5559–5564. [7] N. Takeichi, K. Tanaka, H. Tanaka, T.T. Ueda, Y. Kamiya, M. Tsukahara, H. Miyamura, S. Kikuvhi, Hydrogen storage poperties of Mg/Cu and Mg/Pd laminate composites and metallographic structure, J. Alloys Compd. 446-447 (2007) 543–548. [8] J. Huot, A. Yonkeu, J. Dufour, Rietveld analysis of neutron powder diffraction of Mg6Pd alloy at various hydriding stages, J. Alloys Compd. 475 (2009) 168–172.
[9] S. Orimo, H. Fujii, Materials science of Mg–Ni-based new hydrides, Appl. Phys. A72 (2001) 167–186. [10] B. Sakintuna, F. Lamari-Darkrim, M. Hirscher, Metal hydride materials for solid hydrogen storage: a review, Int. J. Hydrogen Energy 32 (2007) 1121–1140. [11] I. P. Jain, C. Lal, A. Jain, Hydrogen storage in Mg: a most promising material, Int. J. Hydrogen Energy 35 (2010) 5133–5144. [12] C. Wannek, B. Harbrecht, Structure and thermal stability of the new intermetallics MgPd2, MgPd3, and Mg3Pd5 and the kinetics of the iodinecatalyzed formation of MgPd2, J, Solid State Chem. 159 (2004) 113–120. [13] J.P.A Makongo, Y. Prots, U. Burkhardt, R. Niewa, C. Kudla, G. Kreiner, A case study of complex metallic alloy phases: structure and disorder phenomena of Mg–Pd compounds, Philos. Mag. 86 (2006) 427–433. [14] V.E. Kolesnichenko, V.V. Karonik, A.V. Ivanishchev, Phase-diagram of the Mg–Ni–Pd system in the magnesium-rich range, Russ. Metall. 4 (1981) 207–210. [15] K.P. Gupta, The Mg–Ni–Pd (magnesium–nickel–palladium) system, J. Phase Equilib. Diffus. 25 (2004) 191–194. [16] J.F. Ferna´ndez, J.R. Ares, F. Cuevas, J. Bodega, F. Leardini, C. Sa´nchez, A thermodynamic study of the hydrogenation of the pseudo-binary Mg6Pd0.5Ni0.5 intermetallic compound, Intermetallics 18 (2010) 233–241. [17] F. Cuevas, J.F. Ferna´ndez, J.R. Ares, F. Leardini, M. Latroche, Crystal structure and hydrogenation properties of pseudo-binary Mg6Pd0.5Ni0.5 complex metallic alloy, J. Solid State Chem. 182 (2009) 2890–2896. [18] J. F. Ferna´ndez, F. Cuevas, F. Leardini, J. Bodega, J. R. Ares, G. Garce´s, P. Pe´rez, C. Sa´nchez, A new pseudo-binary Mg6Pd0.5Ni0.5 intermetallic compound stabilised by Pd for hydrogen storage, J. Alloys Compd. 495 (2010) 663–666. [19] P Villars, M. Berndt, K Brandenburg, K. Cenzual, J Daams, F Hulliger, T. Massalski, H. Okamoto, K. Osaki, A Prince, H. Putz, S. Iwata, PAULING FILE Binaries Edition, Materials ParkASM International, Ohio, 2002. [20] J. Rodrı´guez-Carvajal, Fullprof: a program for Rietveld refinement and pattern matching analysis, Physica B 192 (1993) 55–69. [21] P. Villars, K. Cenzual, Pearson’s Crystal Data: Crystal Structure Database for Inorganic Compounds, Release 2009/10, Materials ParkASM International, Ohio, USA, 2009. [22] A. Kamegawa, Y. Goto, R. Kataoka, H. Takamura, M. Okada, High-pressure synthesis of novel compounds in an Mg–Ni system, Renew. Energy 33 (2008) 221–225.
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Homogeneity range and crystal structure of Ni substituted Mg6(Pd,Ni) complex intermetallic compounds F. Cuevas a,n, J.F. Ferna´ndez b, J.R. Ares b, F. Leardini b, C. Sa´nchez b a b
CMTR/ICMPE/CNRS, UMR7182, 2-8 rue Henri Dunant, 94320 Thiais, Cedex, France ´noma de Madrid, 28049 Madrid, Spain Dpto. Fı´sica de Materiales, Facultad de Ciencias, Universidad Auto
a r t i c l e in fo
abstract
Article history: Received 5 March 2010 Received in revised form 7 May 2010 Accepted 10 May 2010
The solubility of nickel in Mg6Pd compound has been studied by chemical and structural methods. The solubility limit at 673 K attains 9 at% Ni, i.e. more than four times the value previously reported. Therefore, a re-determination of the ternary Mg–Pd–Ni phase diagram in the Mg-rich corner is desirable. Mg6(Pd,Ni) compounds have the same crystal structure as the binary Mg6Pd compound. The unit-cell composition at the solubility limit is Mg344(2)Pd12(1)Ni36(1) with 75% of Ni atoms substituting Pd ones. Ni substitution occurs on the three crystallographic sites exclusively occupied by Pd in Mg6Pd. The substitution is not fully random. Ni atoms have a slight preference for site with low coordination number. The results reported here enlarge the compositional range in which Mg6(Pd,Ni) compounds can be used as reversible hydrogen storage materials. & 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Intermetallic compounds C. X-ray diffraction D. Crystal structure D. Phase equilibria
1. Introduction Mg6Pd is a complex intermetallic compound. It crystallizes in the cubic F43m space group with lattice parameter a¼ 20.108 A˚ [1]. The unit-cell contains 396 atoms (Mg340Pd56). Beyond the fundamental interest of complex crystal structures [2,3], the high Mgcontent of this compound makes it interesting as a hydrogen storage medium [4–8]. Magnesium absorbs a large amount of hydrogen (7.6 wt% H, 108 gH2/l) but its hydride is too stable for room temperature applications. One strategy to circumvent this problem is to alloy Mg with last transition metals (such as Pd, Fe and Ni) having less affinity for hydrogen [9–11]. However, very few transition metals form stable intermetallic compounds with Mg [9]. The Mg–Pd system is a remarkable exception since at least eleven intermetallic compounds exist in its binary phase diagram [12,13]. The Mg6Pd phase is the Mg-richest one. Several groups have reported that the hydrogenation of Mg6Pd compound is reversible and occurs through three disproportionation reactions with a global hydrogen uptake of about 4 wt% H [7,8]. Unfortunately, the stability of the hydrogenation reaction does not differ much from that of the Mg/H2 system. Owing to the lower hydrogen affinity of nickel and to its lower cost, the substitution of palladium by nickel in Mg6Pd compound is attractive for hydrogen storage applications. Substitution is favored by the fact that Pd and Ni have the same electronic outer
n
Corresponding author. Tel.: + 33 1 49 78 12 25; fax: + 33 1 49 78 12 03. E-mail address: [email protected] (F. Cuevas).
0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2010.05.004
shell configuration. However, the Mg6Ni phase does not exist in the binary Mg–Ni phase diagram and therefore a limit for Ni substitution in Mg6Pd1 xNix compounds is anticipated. Published data on the ternary Mg–Ni–Pd phase diagram at 673 K indicate that the maximum Ni substitution in the Mg6Pd1 xNix phase (labeled as r) is as low as 2 at% Ni (x¼ 0.13) [14,15]. At higher Ni-content, Mg2Ni (labeled as b) and Mg (labeled as e) phases appear in equilibrium with the r-phase. However, we have recently proved that Mg6Pd0.5Ni0.5 compound with 7 at% Ni is thermodynamically stable at 660 K [16–18]. The scope of this paper is to determine the substitution limit of Pd by Ni in Mg6Pd1 xNix compounds and to accomplish a complete structural characterization of the whole homogeneity range.
2. Experimental Samples of nominal composition Mg6Pd1 xNix with x ¼0.5, 0.75, 0.875 and 1 were prepared by powder metallurgy starting from elemental powders of purity higher than 99.8% metal basis (Mg from Alfa Aesar with particle size PS o44 mm, Ni from Cerac PS o44 mm, Pd from CEA PS o100 mm). For each composition, elemental powders were mixed and cold pressed at 700 MPa in the form of pellets (typically 0.5 g in mass, 8 mm in diameter). The pellets were then placed in tantalum thimbles, which were introduced into stainless steel sample holders previously outgassed at 1173 K under secondary vacuum. Next, these holders were hermetically sealed by arc-welding to perform a thermal treatment of the pellets in a conventional resistance furnace.
1260
F. Cuevas et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1259–1263
All manipulations were done in an argon glove box (O2 o1 ppm, H2Oo30 ppm). The pellets were heated up to 1073 K for 12 h in order to promote sample melting (Mg6Pd melts at 973 K [19]), and then annealed at 673 K for one week to ensure chemical equilibrium. The sample holders were then cooled down to room temperature by water quenching. Sample composition was analyzed by electron probe microanalysis (EPMA) in a Cameca SX-100 instrument operated at 15 kV. To this aim, the samples were embedded in wood’s alloy and mechanically polished with methanol using SiC paper with lessening sizes (800, 1200 and 4000 mesh). Sample crystal structure was studied at room temperature by X-ray powder diffraction (XRPD). All Pd-containing samples exhibited brittle character. They were ground in an agate mortar and sieved to 63 mm. XRPD measurements were performed with a y–y diffractometer (Bruker AXS D8Advance equipped with backscattered rear graphite monochromator) using CuKa radiation. Data were recorded over the range 10–1001 by steps of 0.041 and analyzed by the Rietveld method using the Fullprof software [20]. As for the Pd-free sample, it exhibited a tough character. Therefore, a piece of it was mirror-like polished for performing XRD analysis in the same diffractometer. Because of the sample texture, XRD data of the Pd-free sample were only used for phase identification, indexation and determination of lattice parameters.
3. Results and discussion 3.1. Sample composition Phase nature and their spatial distribution in the four studied samples were analyzed with the help of backscattered electron (BSE) images (Fig. 1) and elemental Mg, Ni and Pd map analysis (not shown here, see Electronic Annex 1 in the online version of this article). All samples exhibit a two-phase microstructure except for the sample with x¼ 0.875 where three phases are observed. At low Ni-content (x ¼0.5 and 0.75) the r-phase (in grey) is the dominant one and it coexists with secondary b-precipitates (in white in Fig.1). Then, a further increase in
Ni-content (x¼0.875) leads to the additional precipitation of secondary e-phase (in black). Finally, the sample without Pd, i.e. for x¼1, shows no r-phase and is composed of Mg(e) and Mg2Ni(b) phases in agreement with the Mg–Ni phase diagram. Phase composition results (determined by EPMA) are shown in Table 1. For each one of the Pd-containing samples, the analysis of the r-phase was performed on more than fifty randomly distributed points. The Ni to Pd ratio increases gradually with Ni substitution (x). One can notice that the Ni-content of the r-phase is lower than that of the nominal sample what can be attributed to the Mg2Ni(b) phase precipitation. For x¼0.5, the results of the r-phase analysis are less accurate due to the contribution of the finely dispersed Mg2Ni precipitates and to the spatial EPMA resolution ( 1 mm3). The given composition corresponds to the maximum content of Pd according to the EPMA analysis. Additionally, the Mg2Ni(b) precipitates’ composition was determined in the sample with x ¼0.75 since their sizes appear to be larger than the spatial EPMA resolution. The obtained composition was Mg66.6Ni31.5Pd1.9, what proves that the Pd solubility in Mg2Ni(b) phase is very low. 3.2. Structural characterization The diffraction patterns of the samples under study are displayed in Fig. 2. The diffraction peaks of the Pd-free sample (x ¼1) are assigned to Mg(e) and Mg2Ni(b) phases as expected from the Mg–Ni phase diagram and in agreement with EPMA results. In contrast, the diffraction patterns of Pd-containing samples are completely different. Except for a few diffraction peaks of minor intensity corresponding to e (for x ¼0.875) and b (for x¼0.75 and 0.5) phases, most of them belong to a different phase, which as established by EPMA analysis should be the Mg6(Pd,Ni)(r) phase. All diffraction patterns of Pd-containing samples were analyzed by the Rietveld method. The crystal structure of Mg(e) and Mg2Ni(b) phases can be easily implemented in the Rietveld refinement since, firstly, their structure is simple and well documented in the literature [21] and, secondly, the solubility of Pd and Ni in the e-phase or of Pd in the b-phase is negligible.
x = 0.75
x = 0.5
100 µm
100 µm
x=1
x = 0.875
100 µm
100 µm
Fig. 1. BSE Images of Mg6Pd1 xNix samples. White and black regions are identified as Mg2Ni(b) and Mg(e) phases, respectively. Grey regions are identified as the Mg6(Pd,Ni)(r) phase. For x¼ 0.5 and 0.75 black features are due to voids.
F. Cuevas et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1259–1263
1261
Table 1 Phase composition, content and crystal structure of Mg6Pd1 xNix samples. The standard deviations are given in parenthesis. Rietveld reliability factors of the XRPD refinement of Pd-containing samples are also given. Sample (x)
Nominal composition
Phase
Composition (EPMA)
S.G.
˚ Cell par. (A)
Composition (XRPD)
Content (wt%)
RB (%)
Rwp (%)
w2
0.5
Mg85.7Pd7.1Ni7.1
Mg6(Pd,Ni)(r) Mg2Ni(b)
Mg85.5Pd9.50Ni4.95 ( )
F4¯3m P6222
a¼ 20.1216(4) a¼ 5.2155(6) c¼ 13.251(2)
Mg87.0(9)Pd8.7(4)Ni4.2(4) ()
88(2) 12(1)
3.0 7.6
7.9
6.2
0.75
Mg85.7Pd3.6Ni10.7
Mg6(Pd,Ni)(r) Mg2Ni(b)
Mg86.5(1)Pd4.9(3)Ni8.6(3) Mg66.6Ni31.5Pd1.9
F4¯3m P6222
a¼ 20.1177(3) a¼ 5.2100(3) c¼ 13.255(1)
Mg87.5(6)Pd4.5(2)Ni7.9(2) ()
89(2) 11(1)
3.0 4.8
6.8
4.5
0.875
Mg85.7Pd1.8Ni12.5
Mg6(Pd,Ni)(r) Mg(e)
Mg87.5(6)Pd3.3(2)Ni9.2(4) ( )
F4¯3m P63/mmc
Mg87.6(7)Pd3.2(3)Ni9.2(3) ()
84(2) 16(1)
3.2 3.3
7.9
6.9
Mg2Ni(b)
( )
()
a¼ 20.1072(5) a¼ 3.2090(2) c¼ 5.2101(4) ( )
()
( )
()
Mg(e)
Mg
P63/mmc
()
( )
()
( )
()
Mg2Ni(b)
Mg66.5(2)Ni33.5(2)
P6222
a¼ 3.2099(2) c¼ 5.2083(4) a¼ 5.2077(3) c¼ 13.255(2)
()
( )
()
1
Mg85.7Ni14.3
Fig. 2. X-ray diffraction patterns of Mg6Pd1 xNix samples. Diffraction peaks related to Mg2Ni(b) and Mg(e) phases are marked by open and filled symbols, respectively. Non-marked peaks belong to the Mg6(Pd,Ni)(r) phase.
On the contrary, the structural model for the refinement of the Mg6(Pd,Ni)(r) phase is more complex and deserves a more detailed explanation. To start with, the structural model proposed by Samson et al. for the binary Mg6Pd compound was used but allowing for partial occupation of Ni atoms in Pd sites (i.e. atom nos. 10. 11, 12 and 13 in Ref. [1]). In this model, one should take into consideration that site 12 can be simultaneously occupied by Mg, Ni and Pd atoms that prevents determining the fractional atomic occupancy on this site from a unique diffraction data set. In a previous work, we circumvented this problem by constraining the chemical composition of the crystallographic unit-cell to that of EPMA results [17]. Such a work was done with a single phase Mg6.1Pd0.5Ni0.4 compound elaborated by induction melting. Owing to the multiphase character of the present samples, EPMA results of the Mg6(Pd,Ni) phase are not confident enough for the crystallographic determination (especially for x¼ 0.5) and therefore another strategy had to be adopted. In previous work, we have shown that, due to steric effects, Ni occupation of site 12 is very low (site occupation factor SOF¼ 0.04) and therefore only Mg and Pd atoms are expected to significantly occupy that site. Thus, we now assume that site 12 is only occupied by Mg and Pd atoms, the ratio of which can be unequivocally determined from one diffraction pattern.
An additional complication arises from the possible variation of Mg-content within the homogeneity domain of the r-phase, which extends from 85 to 87.4 at% Mg in the binary Mg–Pd system [13]. This is accommodated by point defects whose nature and extent depend on the Mg/Pd ratio as described by Makongo et al. [13]. The precipitation of Mg2Ni phase in the studied samples suggests that the obtained Mg6(Pd,Ni) phases stand on the Mg poor side of the homogeneity domain. In this case, site 14 is reported to be fully occupied by Mg whereas constitutional vacancies should appear at site 5. The implemented model allows for this possible configuration. General results on the structural analysis of the studied samples are gathered in Table 1. Mg6(Pd,Ni)(r) phase is the major phase ( 484 wt%) in all Pd-containing samples. The lattice parameter of this phase gradually decreases with Ni-content (from 20.1216 to 20.1072 A˚ on going from x¼ 0.5 to 0.875), which is attributed to the smaller atomic radius of Ni as compared to ˚ rPd ¼ 1.37 A). ˚ In agreement with these that of Pd (rNi ¼1.24 A, results, the metastable Mg6Ni compound is reported to have the smallest lattice parameter: 19.987 A˚ [22]. As for secondary phases, Mg2Ni(b) is detected for x¼0.5 and 0.75 samples, and Mg(e) for x¼0.875. Contrary to EPMA analysis, Mg2Ni(b) phase is not detected by XRPD for x ¼0.875 sample as a result of its low amount (see the few white precipitates in Fig. 1). The lattice parameters of the b-phase in the Pd-containing samples are close to that of the Pd-free sample, which confirms the low solubility of Pd in the b-phase. As for the Mg(e) phase, very similar lattice parameters are observed for Pd-free and Pd-containing (x ¼0.875) samples. The performed Rietveld analysis also provides detailed information on the dependence of the crystal structure of the r-phase with the nickel substitution. As an example, Fig. 3 shows the graphic output of the Rietveld refinement analysis for the x¼0.875 sample. The structural data for the r-phase are gathered in Table 2. These results are particularly interesting since they concern the crystal structure of the r-phase with maximum nickel content. Fit residuals of the r-phase are rather low (RB ¼3.2%) confirming the refinement quality. Owing to the low accuracy of XRD diffraction in the determination of thermal displacement factors (B), a unique B value was refined for all Mg sites and another one for the remaining substituted sites. The refined positional parameters and, thereby, the interatomic distances are quite close to those reported by Samson [1] for Mg6Pd compound. Constitutional vacancies (7%) occur on site 5. Substitution of Pd by Ni occurs on sites 10, 11 and 13.
1262
F. Cuevas et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1259–1263
Fig. 3. XRPD Rietveld analysis of Mg6Pd1 xNix sample with x ¼0.875. Experimental data (circles), calculated diffraction patterns (continuous line), diffraction line positions (vertical bars) and difference curve at the same scale (below) are given.
Table 2 Structural parameters of the r-Mg6(Pd,Ni) phase for x ¼ 0.875 sample. The standard deviations are given in parenthesis. Site no.
Atom
Wyckoff
x
1 2 3 4 5 6 7 8 9 10
Mg Mg Mg Mg Mg Mg Mg Mg Mg Pd Ni Pd Ni Pd Mg Pd Ni Mg
48(h) 48(h) 48(h) 48(h) 48(h) 24(f) 24(f) 24(g) 16(e) 16(e)
0.143 0.096 0.150 0.056 0.199 0.103 0.382 0.066 0.301 0.167
16(e)
11 12 13 14
B (A˚ 2)
SOF
0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 1.4
1 1 1 1 0.93 1 1 1 1 0.33 0.67 0.20 0.80 0.03 0.97 0.23 0.77 1
y
z
x x x x x 0 0 ¼ x x
0.035 0.273 0.527 0.768 0.911 0 0 ¼ x x
0.407 (1)
x
x
1.4 (3)
16(e)
0.674 (2)
x
x
1.4 (3)
16(e)
0.901 (1)
x
x
1.4 (3)
4(d)
¼
¼
3
0.9 (3)
(1) (1) (1) (1) (1) (2) (2) (2) (2) (1)
/4
(2) (2) (2) (2) (2)
(3) (3) (3) (3) (3) (3) (3) (3) (3) (3)
(4)
(4) (4) (4) (4) (2) (2) (4) (4)
˚ RB ¼ 3.2%, Rwp ¼7.9%. w2 ¼ 6.9. S.G. F43 m, a¼ 20.1072 (5) A. No. of independent reflections: 288. No. of intensity variables: 25.
The substitution is not random and site 12 is almost fully occupied by Mg atoms since the Pd occupancy is practically negligible (SOF ¼0.0370.02). The unit-cell composition of the r-phase is Mg344(2)Pd12(1)Ni36(1) in the x ¼0.875 sample. The formula unit (f.u.) of the Mg6(Pd,Ni)(r) phase with maximum nickel content can be written as Mg6.14(4)Pd0.21(2)Ni0.64(2) for Z ¼56 f.u. in the unit cell. The r-phase crystal structure at lower Ni concentrations (x ¼0.5 and 0.75 samples) is very similar to that previously described (x ¼0.875). For instance, very similar Mg vacancy concentrations are found on site 5 (6% and 8% for x ¼ 0.5 and 0.75, respectively), which confirm that the r-phase composition corresponds to the Mg-poor limit for all studied samples. The main change on the structural results between the different samples obviously concerns the occupation factors on sites 10, 11 and 13 due to their different Ni contents. The evolution of Ni occupancy of these sites (determined by XRPD) as a function of the overall Pd by Ni substitution in the r-phase (determined by EPMA) is displayed in Fig. 4. As a general trend, Ni occupancy on the three sites linearly increases with substitution. However, Ni site occupancy is not fully random. Ni shows some slight preference for the site with the smallest coordination number (site 11, CN10) probably due to its smaller atomic radius [17]. As for the SOF of Mg and Pd atoms at site 12, the Pd occupation
Fig. 4. Dependence of nickel SOF on the overall nickel substitution (determined by EPMA) in the Mg6(Pd,Ni)(r) phase. The hypothetical random occupancy is indicated by a dashed line.
gradually decreases with the Ni substitution (0.2, 0.05 and 0.03 for x ¼ 0.5, 0.75 and 0.875, respectively). This suggests that the instability of the r-phase with respect to the two phase b + e mixture at high Ni-content is driven by the full occupancy of site 12 by Mg atoms. The chemical composition of the r-phase for all samples as determined from the Rietveld refinement is gathered in Table 1. The results are in close agreement with EPMA analysis.
3.3. Implications for the Mg–Ni–Pd ternary diagram The previously reported Mg–Ni–Pd phase diagram at 673 K is displayed in Fig. 5a [14,15]. Taking into consideration the results obtained in this work (experimental points in Fig. 5b), the ternary diagram should be drastically modified as concerns the homogeneity range of the Mg6(Pd,Ni)(r) phase. The location of b, r and e phases in the ternary diagram indicates that the threephase sample (x¼0.875) shows the maximum solubility of Ni in the r-phase. The obtained Ni solubility limit (9 at% Ni) is more than four times that of the previously reported value. As a consequence, the two-phase (b + r) and (r + e) domains are expanded at the expense of the three-phase (b + r + e) domain. Last modifications need to be confirmed by a systematic re-determination of the Mg–Ni–Pd diagram at the Mg-rich corner.
4. Conclusion The substitution limit of Pd by Ni in the Mg6Pd phase is 9 at% Ni at 673 K, i.e. more than four times the value previously reported [14,15]. We have shown that the unit-cell of the r-phase structure shrinks slightly with Ni substitution due to the smaller atomic radius of Ni compared to Pd. The Ni substitution is accommodated by gradual replacement of Pd by Ni atoms on the three crystallographic sites exclusively occupied by Pd atoms in the complex binary Mg6Pd compound. However, the substitution is not fully random since Ni atoms exhibit a slight preference for sites with low coordination number. At the Ni substitution limit, the cell composition is Mg344(2)Pd12(1)Ni36(1) and site 12, which is randomly occupied by Mg and Pd atoms in the binary compound, is exclusively occupied by Mg atoms. The large extension of the
F. Cuevas et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1259–1263
0
0
50
50
10
10
40
) (at % 30
%)
30
Ni
30
20
) at%
(at %
( Pd
(at
30
20
Pd
20
)
40
20
ρ
ρ 40
40
Mg (at%)
0 0
80
90
ρ+ε ε β+ρ+ε
β 70
0
60
ε
10
β+ρ
50
90
80
70
60
50
β
10 ρ+ε
50
50
β+ρ+ε
10 0
β+ρ
10
Ni
1263
Mg (at%)
Fig. 5. Ternary Mg–Ni–Pd phase diagram at the Mg corner at 673 K: (a) as reported by Kolesnichenko et al. [14], (b) this work. Filled dots: measured compositions. Thin line: tie-line between b and r phases for sample with x¼ 0.75. Thick-line: tie-triangle of b, r and e phases.
Ni-substitution in the Mg6Pd phase gives more room for the study of Mg6Pd1 xNix compounds as hydrogen storage materials. Acknowledgements We thank N.L.-Do for technical assistance, E. Leroy for EPMA measurements and J.-M. Joubert for fruitful discussions and comments. Several authors thank the Spanish Minister of Education and Science, MEC, for financial support under contract no. MAT2008-06547-C02-01. References [1] S. Samson, Complex cubic A6B compounds. II. The crystal structure of Mg6Pd, Acta Cryst. B28 (1972) 936–945. [2] K. Urban, M. Feuerbacher, Structurally complex alloy phases, J. Non-Cryst. Solids 334-335 (2004) 143–150. [3] R.F. Berger, S. Lee, J. Johnson, B. Nebgen, F. Sha, J.Q. Xu, The mystery of perpendicular fivefold axes and the fourth dimension in intermetallic structures, Chem.-Eur. J. 14 (2008) 3908–3930. [4] Y. Kume, A. Weiss, On the interaction of hydrogen with the intermetallic phase Mg6Pd, J. Less-Common Met. 136 (1987) 51–54. [5] T. Yamada, J. Yin, K. Tanaka, Hydrogen storage properties and phase structures of Mg-rich Mg–Pd, Mg–Nd and Mg–Pd–Nd alloys, Mater. Trans 42 (2001) 2415–2421. [6] C. Zlotea, Y. Andersson, Microstructural modifications induced by hydrogen absorption in Mg5Ga2 and Mg6Pd, Acta Mater. 54 (2006) 5559–5564. [7] N. Takeichi, K. Tanaka, H. Tanaka, T.T. Ueda, Y. Kamiya, M. Tsukahara, H. Miyamura, S. Kikuvhi, Hydrogen storage poperties of Mg/Cu and Mg/Pd laminate composites and metallographic structure, J. Alloys Compd. 446-447 (2007) 543–548. [8] J. Huot, A. Yonkeu, J. Dufour, Rietveld analysis of neutron powder diffraction of Mg6Pd alloy at various hydriding stages, J. Alloys Compd. 475 (2009) 168–172.
[9] S. Orimo, H. Fujii, Materials science of Mg–Ni-based new hydrides, Appl. Phys. A72 (2001) 167–186. [10] B. Sakintuna, F. Lamari-Darkrim, M. Hirscher, Metal hydride materials for solid hydrogen storage: a review, Int. J. Hydrogen Energy 32 (2007) 1121–1140. [11] I. P. Jain, C. Lal, A. Jain, Hydrogen storage in Mg: a most promising material, Int. J. Hydrogen Energy 35 (2010) 5133–5144. [12] C. Wannek, B. Harbrecht, Structure and thermal stability of the new intermetallics MgPd2, MgPd3, and Mg3Pd5 and the kinetics of the iodinecatalyzed formation of MgPd2, J, Solid State Chem. 159 (2004) 113–120. [13] J.P.A Makongo, Y. Prots, U. Burkhardt, R. Niewa, C. Kudla, G. Kreiner, A case study of complex metallic alloy phases: structure and disorder phenomena of Mg–Pd compounds, Philos. Mag. 86 (2006) 427–433. [14] V.E. Kolesnichenko, V.V. Karonik, A.V. Ivanishchev, Phase-diagram of the Mg–Ni–Pd system in the magnesium-rich range, Russ. Metall. 4 (1981) 207–210. [15] K.P. Gupta, The Mg–Ni–Pd (magnesium–nickel–palladium) system, J. Phase Equilib. Diffus. 25 (2004) 191–194. [16] J.F. Ferna´ndez, J.R. Ares, F. Cuevas, J. Bodega, F. Leardini, C. Sa´nchez, A thermodynamic study of the hydrogenation of the pseudo-binary Mg6Pd0.5Ni0.5 intermetallic compound, Intermetallics 18 (2010) 233–241. [17] F. Cuevas, J.F. Ferna´ndez, J.R. Ares, F. Leardini, M. Latroche, Crystal structure and hydrogenation properties of pseudo-binary Mg6Pd0.5Ni0.5 complex metallic alloy, J. Solid State Chem. 182 (2009) 2890–2896. [18] J. F. Ferna´ndez, F. Cuevas, F. Leardini, J. Bodega, J. R. Ares, G. Garce´s, P. Pe´rez, C. Sa´nchez, A new pseudo-binary Mg6Pd0.5Ni0.5 intermetallic compound stabilised by Pd for hydrogen storage, J. Alloys Compd. 495 (2010) 663–666. [19] P Villars, M. Berndt, K Brandenburg, K. Cenzual, J Daams, F Hulliger, T. Massalski, H. Okamoto, K. Osaki, A Prince, H. Putz, S. Iwata, PAULING FILE Binaries Edition, Materials ParkASM International, Ohio, 2002. [20] J. Rodrı´guez-Carvajal, Fullprof: a program for Rietveld refinement and pattern matching analysis, Physica B 192 (1993) 55–69. [21] P. Villars, K. Cenzual, Pearson’s Crystal Data: Crystal Structure Database for Inorganic Compounds, Release 2009/10, Materials ParkASM International, Ohio, USA, 2009. [22] A. Kamegawa, Y. Goto, R. Kataoka, H. Takamura, M. Okada, High-pressure synthesis of novel compounds in an Mg–Ni system, Renew. Energy 33 (2008) 221–225.