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Energies 2015, 8(4), 3198-3211; https://doi.org/10.3390/en8043198

Article
LaNi5-Assisted Hydrogenation of MgNi2 in the Hybrid Structures of La1.09Mg1.91Ni9D9.5 and La0.91Mg2.09Ni9D9.4
1
Institute for Energy Technology, P.O. Box 40, Kjeller NO 2027, Norway
2
Norwegian University of Science and Technology, Trondheim NO 7491, Norway
3
Queensland Micro- and Nanotechnology Centre, Griffith University, Nathan 4111, Brisbane, Australia
*
Author to whom correspondence should be addressed.
Academic Editor: Craig M. Jensen
Received: 26 February 2015 / Accepted: 13 April 2015 / Published: 21 April 2015

Abstract

:
This work focused on the high pressure PCT and in situ neutron powder diffraction studies of the LaMg2Ni9-H2 (D2) system at pressures up to 1,000 bar. LaMg2Ni9 alloy was prepared by a powder metallurgy route from the LaNi9 alloy precursor and Mg powder. Two La3−xMgxNi9 samples with slightly different La/Mg ratios were studied, La1.1Mg1.9Ni9 (sample 1) and La0.9Mg2.1Ni9 (sample 2). In situ neutron powder diffraction studies of the La1.09Mg1.91Ni9D9.5 (1) and La0.91Mg2.09Ni9D9.4 (2) deuterides were performed at 25 bar D2 (1) and 918 bar D2 (2). The hydrogenation properties of the (1) and (2) are dramatically different from those for LaNi3. The Mg-containing intermetallics reversibly form hydrides with ΔHdes = 24.0 kJ/molH2 and an equilibrium pressure of H2 desorption of 18 bar at 20 °C (La1.09Mg1.91Ni9). A pronounced hysteresis of H2 absorption and desorption, ~100 bar, is observed. The studies showed that LaNi5-assisted hydrogenation of MgNi2 in the LaMg2Ni9 hybrid structure takes place. In the La1.09Mg1.91Ni9D9.5 (1) and La0.91Mg2.09Ni9D9.4 (2) (a = 5.263/5.212; c = 25.803/25.71 Å) D atoms are accommodated in both Laves and CaCu5-type slabs. In the LaNi5 CaCu5-type layer, D atoms fill three types of interstices; a deformed octahedron [La2Ni4], and [La(Mg)2Ni2] and [Ni4] tetrahedra. The overall chemical compositions can be presented as LaNi5H5.6/5.0 + 2*MgNi2H1.95/2.2 showing that the hydrogenation of the MgNi2 slab proceeds at mild H2/D2 pressure of just 20 bar. A partial filling by D of the four types of the tetrahedral interstices in the MgNi2 slab takes place, including [MgNi3] and [Mg2Ni2] tetrahedra.
Keywords:
in situ studies; neutron powder diffraction; metal hydrides; lanthanum; magnesium

1. Introduction

Despite significant differences in chemistry between La and Mg, magnesium forms a very extensive solid solution in the LaNi3 intermetallic alloy, crystallizing with a PuNi3 type trigonal structure. Up to 67% of La atoms can be replaced by Mg to form a LaMg2Ni9 intermetallic compound. The LaNi3 crystal structure is formed by a stacking of the LaNi5 (Haucke CaCu5 type) and MgNi2 (Laves type) slabs along the trigonal 00z axis (LaNi5 + 2MgNi2 = LaMg2Ni9). Studies of hydrogen absorption–desorption properties of the LaMg2Ni9 [1,2] have shown that it forms a hydride containing up to 1.2 wt% H (~0.8 H/M; LaMg2Ni9H9.6).
The building blocks of LaMg2Ni9—LaNi5 and MgNi2—are well characterized individually as hydride-forming intermetallic compounds. The thermodynamics and structural features of their interaction with hydrogen are quite different. At room temperature, LaNi5 forms a saturated LaNi5H6.7 hydride and shows a reversible interaction with hydrogen at hydrogen pressures slightly exceeding atmospheric pressure. Hydrogen atoms fill tetrahedral La2Ni2, LaNi3 and Ni4 sites in the hydride crystal structure [3].
In contrast, hydrogenation of the Laves phase MgNi2 compound is possible only at hydrogen pressures close to 30 kbar, while maintaining an interaction temperature of 300 °C. Formation of MgNi2H3 results in a complete rebuilding of the metal sublattice. Hydrogen atoms in the orthorhombic structure of trihydride fill two different sites, the Mg4Ni2 octahedra and the positions within the buckled Ni nets, consequently forming directional Ni-H bonds [4].
A gradual increase of Mg content in La3−xMgxNi9 is accompanied by a linear decrease of the volumes of the unit cells. Interestingly, a substantial contraction takes place not only for the (La,Mg)2Ni4 slabs, but also for Mg-free CaCu5-type LaNi5 slabs. Hydrogen interaction with the La3−xMgxNi9 alloys has been investigated by in situ synchrotron X-ray, neutron powder diffraction, theoretical modeling, electrochemical studies as metal hydride battery anode materials, rapid solidification and pressure–composition–temperature studies [1,2,5,6,7,8,9,10]. In the whole substitution range, La3xMgxNi9 alloys form intermetallic hydrides with H/M ratios ranging from 0.77 to 1.16. Magnesium influences structural features of the hydrogenation process and determines various aspects of the hydrogen interaction with intermetallics causing: (a) more than a 1,000-fold increase in the equilibrium pressures of hydrogen absorption and desorption for the Mg-rich LaMg2Ni9 as compared to the Mg-poor La2.3Mg0.7Ni9 and a substantial modification of the thermodynamics of the formation–decomposition of the hydrides; (b) an increase of the reversible hydrogen storage capacities following increase of Mg content in the La3xMgxNi9 to ~1.5 wt% H for La2MgNi9; (c) improvement of the resistance against hydrogen-induced amorphisation and disproportionation and (d) change of the mechanism of the hydrogenation from anisotropic to isotropic. Thus, optimisation of the magnesium content provides different possibilities for improving properties of the studied alloys as hydrogen storage and battery electrode materials. Studies of the thermodynamics and crystal chemistry of the RE2MgNi9H12−13 (RE = La and Nd) hydrides showed that La substitution by Pr or Nd causes destabilization of the formed hydrides without affecting their hydrogen storage capacities and leaves unchanged the most important features of their crystal structures [11].
Observed values of H capacities in the LaMg2Ni9-based hydride of 9.6 atoms H/f.u. cannot be explained by exclusive hydrogen insertion into the LaNi5 slabs, and requires H incorporation into the MgNi2 blocks of the structure to reach the experimentally observed H/M ratios. Thus, studies of the thermodynamics and crystal chemistry of La3−xMgxNi9-H2 systems are very interesting and important from the point of view of the effect of magnesium on the behaviours of the metal-hydrogen systems. The goal of the present study was to study two alloy compositions formed close to the limiting value of the magnesium solubility in LaNi3, LaMg2Ni9, by performing in situ neutron powder diffraction studies of the deuterated La0.91Mg2.09Ni9 and La1.09Mg1.91Ni9 and by studying the thermodynamics of the metal-hydrogen interactions by measurements of the PCT diagrams.

2. Experimental

La1.09Mg1.91Ni9 and La0.91Mg2.09Ni9 alloys were prepared by a powder metallurgy route from LaNi5 alloy precursor, Mg and Ni. Initial metals La, Mg and Ni with high purity exceeding 99.9% were used in the synthesis. LaNi5 precursor was prepared by arc melting of a stoichiometric 1:5 mixture of La and Ni.
The powder mixture LaNi5 + Mg + Ni was ball milled under protective atmosphere of argon gas in a SPEX 8000D mill for 8 h. After the milling process, the mixture was placed into a tantalum crucible and then annealed in Ar atmosphere in the sealed stainless steel containers at 600–1000 °C. Two samples with a slightly different stoichiometry were prepared. Their stoichiometric compositions were: sample 1: La1.09(1)Mg1.91(1)Ni9; sample 2: La0.91(1)Mg2.09(1)Ni9.
The first sample was annealed at 800 °C for 8 h and then at 600 °C for 8 h. The second sample was annealed at 1000 °C for 2 h and, later, at 800 °C for 12 h. The samples were quenched into a mixture of water and ice after the annealing. A small excess of Mg (5 wt%) was introduced into the initial mixtures to compensate for its sublimation at high temperatures.
The homogeneity of the prepared samples was characterized by XRD. Laboratory powder X-ray diffraction data were collected with a Siemens D5000 diffractometer (Oslo, Norway) equipped with a Ge primary monochromator giving Cu Kα1 radiation. Initial phase-structural analysis was performed by X-ray powder diffraction using a Bruker D8 Advance diffractometer (Kjeller, Norway) with Cu-Kα radiation. High-resolution SR XRD data were collected at the Swiss-Norwegian Beamlines (SNBL, BM01B) at ESRF, Grenoble, France. A monochromatic beam with λ = 0.5009(1) Å was provided by a double Si monochromator. A 2θ angular range of 1°–50.5° was scanned with a detector bank consisting of six scintillation detectors mounted in series with 1.1° separation. The data were binned to the step size Δ2θ = 0.003°. The instrumental contribution to the line broadening was evaluated by refining the profile parameters for a standard Si sample.
In situ neutron powder diffraction studies were performed at HRPT diffractometer, SINQ, PSI, Switzerland using a wavelength of λ = 1.494 Å. The deuteride of sample 1 was synthesized at 25 bar D2 and −30 °C (Peq. for absorption ~20 bar); it was synthesized and studied by NPD using a thin walled stainless steel sample cell (6 mm OD). The deuteride of sample 2 was synthesized at 950 bar D2 and measured at 912 bar D2 at room temperature. The experimental setup for the in situ NPD study consisted of a high-pressure Sieverts’ manometric hydrogenator connected to a high-pressure sample cell made of a null matrix coherent scattering alloy (Zr–Ti) with a thin stainless steel inner liner.
Powder diffraction data were analysed by the Rietveld whole-profile refinement method using the General Structure Analysis System (GSAS) [12] and FULLPROF [13] software packages. Pressure-composition-temperature isotherms were measured at −40, −20, 0 and 20 °C.

3. Results and Discussion

3.1. XRD Characterization of the Initial Intermetallic Alloys La0.91Mg2.09Ni9 and La1.09Mg1.91Ni9

XRD characterization of two studied alloys La0.91Mg2.09Ni9 and La1.09Mg1.91Ni9 showed that they both contain PuNi3 trigonal La3−xMgxNi9 as the main phase constituents (80% for sample 1 and 75% for sample 2). The common secondary constituent was identified as a LaNi5 binary intermetallic. Furthermore, sample 1 contained an admixture of the MgNi2 Laves-type intermetallic phase, while sample 2 contained a cubic MgNi3 intermetallic compound recently also observed during the studies of the MgNi2-H2 system [4]. MgNi3 compound (sp.gr. Pm 3 ¯ m; a = 3.7185(5) Å) has an AlCu3-type structure and earlier it was synthesized by high-energy ball milling of a mixture of Mg and Ni metals [14]. We assume that in present study MgNi3 was synthesised already during the reactive ball milling and remained stable during the consecutive annealing at 1,000 and 800 °C. As an example, Figure 1 shows an excellent fit of the experimental X-ray powder diffraction pattern collected for the sample 2, La0.91Mg2.09Ni9.
Figure 1. XRD pattern of La0.91Mg2.09Ni9 (sample 2) (Cu-Kα1 radiation).
Figure 1. XRD pattern of La0.91Mg2.09Ni9 (sample 2) (Cu-Kα1 radiation).
Energies 08 03198 g001
Crystallographic data for the studied intermetallic samples obtained from the refinements of the XRD pattern are listed in Table 1.
Table 1. Crystal structure data for the La0.91Mg2.09Ni9 and La1.09Mg1.91Ni9 alloys from Rietveld refinements of the X-ray diffraction data. PuNi3 type of structure, space group R 3 ¯ m.
Table 1. Crystal structure data for the La0.91Mg2.09Ni9 and La1.09Mg1.91Ni9 alloys from Rietveld refinements of the X-ray diffraction data. PuNi3 type of structure, space group R 3 ¯ m.
AlloySample 1Sample 2
Source of experimental dataSR XRD collected at BM01B, SNBL using a wavelength λ = 0.5009(1) ÅSiemens D5000 diffractometer, Cu Kα1 radiation
Composition of AB3 phaseLa1.09(1)Mg1.91(1)Ni9La0.91(1)Mg2.09(1)Ni9
Unit cell parameters:
a (Å)4.94024(8)4.8986(1)
c (Å)23.8188(4)23.957(1)
V3)503.44(1)497.86(2)
Atomic parameters:
La1/Mg1 in 3a (0, 0, 0)
Uiso×100 (Å2)0.43(5)2.1(2)
nMg, (nLa = 1–nMg)0.0(–)0.09(1)
La2/Mg2 in 6c (0, 0, z)
z0.1453(3)0.1471(6)
Uiso×100 (Å2)1.2(3)0.5(3)
nMg, (nRE = 1–nMg)0.954(5)1.0(–)
Ni1 in 3b (0, 0, ½)
Uiso × 100 (Å2)
0.7(1)0.8(3)
Ni2 in 6c (0, 0, z)
z0.3335(2)0.3334(4)
Uiso×100 (Å2)0.13(8)1.8(3)
Ni3 in 18h (x, –x, z)
x0.5009(3)0.5014(6)
z0.08529(8)0.0854(2)
Uiso × 100 (Å2)0.57(5)1.4(2)
R-factors of refinements
Rp8.97.4
Rwp11.99.6
χ22.02.1
Impurity phasesLaNi5 7.8(2) wt%
MgNi2 12.0(2) wt%
LaNi5 20.5(2) wt%
MgNi3 4.2(3) wt%
The crystallographic characteristics of LaNi3 change significantly on Mg → La substitution; a decrease in the unit cell parameters takes place from a = 5.0842(2); c = 25.106(1) Å (LaNi3) to a = 4.8986(1) (sample 2)-4.94024(8) (sample 1); c = 23.8188(4) Å (sample 1)-23.957(1) (sample 2). Furthermore, comparison of the data shows that the studied intermetallic samples exhibit significant differences in the volumes of the unit cells and c/a ratios. A shrinkage along [001] appears to be more pronounced (Δc/c, −5.1%) as compared to Δa/a, −3.7%. The overall volume contraction is quite significant reaching 10.5%–11.5%. The measured dimensions of the unit cells well agree with the data reported for the stoichiometric LaMg2Ni9 alloy studied by single crystal XRD (a = 4.9241, c = 23.875 Å; V = 501.3 Å3 [15]), which shows intermediate values of a, c and V being in between the values for the samples 1 and 2, as it could be expected from comparison of their chemical compositions.
Refined volumes of the unit cells correlate with their chemical compositions and Mg/La ratios. Indeed, sample 1, La1.09(1)Mg1.91(1)Ni9 with a larger unit cell has a higher content of lanthanum, while for sample 2, La0.91(1)Mg2.09(1)Ni9 with a smaller unit cell, the content of lanthanum becomes smaller than 1 atom/f.u., and the content of Mg reaches overstoichiometric compositions with more than 2 Mg atoms/f.u. (La,Mg)3Ni9.
Comparison of the data presented in Table 1 with crystallographic data for the (La,Mg)3Ni9 intermetallics studied in [1] shows a linear dependence between the decrease of the unit cell volumes and the content of Mg in the alloys.
We note a very interesting feature of the crystal structure of La0.91(1)Mg2.09(1)Ni9 where a partial substitution of La by Mg takes place within the CaCu5 type layer in the position 6c. This contrasts with the behaviour of the alloys in the La-Mg-Ni system with compositions close to LaNi5. In the latter case studies of phase equilibria showed no dissolution of an appreciable amount of Mg in LaNi5 [16]. Thus, the present study demonstrates that the situation with Mg solubility in the LaNi5 slabs of the LaNi3 structure becomes different in the sample 2 La0.91(1)Mg2.09(1)Ni9. Here LaNi5, when influenced by the MgNi2 slabs of the hybrid structure, becomes capable of forming solid solutions of such a type with experimentally refined composition of La0.95Mg0.05Ni5. Thus, La0.91(1)Mg2.09(1)Ni9 should be considered as the first reported case where a CaCu5 type layer accommodates Mg atoms allowing a Mg content of 2.09 at./f.u. (La,Mg)3Ni9. Consequently, the limits of Mg solubility in LaNi3 are not confined to LaMg2Ni9 and extend to the composition La0.91Mg2.09Ni9.

3.2. Thermodynamics of the (La,Mg)3Ni9—H2 systems

The hydrogenation/deuteration properties of the prepared La1±0.1Mg2±0.1Ni9 intermetallics appear to be dramatically different from those for LaNi3. While LaNi3 is prone to the hydrogen-induced disproportionation, the Mg-containing intermetallics reversibly form hydrides with ΔHdes = 24.0 kJ/molH2and equilibrium pressure of H2 desorption of 20 bar at room temperature for La1.09Mg1.91Ni9 (see Figure 2). A pronounced hysteresis of H2 absorption and desorption is evidenced by a high value of H2 absorption pressure, more than 100 bar higher than that for desorption.
For La2MgNi9 [6] at room temperature the values of plateau pressures are 0.05 and 0.1 bar for hydrogen desorption and absorption, respectively, ΔHdes = 35.9 kJ/molH2. Equilibrium pressure of hydrogen desorption for La0.91Mg2.09Ni9 is by more than 1000 times higher than that for La2MgNi9.
Figure 2. Room temperature isotherms of hydrogen absorption and desorption (a); and van’t Hoff plots (b) for La1.09Mg1.91Ni9-based hydride. At room temperature equilibrium pressure of hydrogen absorption is ~120 bar D2, while for the desorption Peq. equals to ~20 bar D2.
Figure 2. Room temperature isotherms of hydrogen absorption and desorption (a); and van’t Hoff plots (b) for La1.09Mg1.91Ni9-based hydride. At room temperature equilibrium pressure of hydrogen absorption is ~120 bar D2, while for the desorption Peq. equals to ~20 bar D2.
Energies 08 03198 g002

3.3. In situ NPD studies

In situ neutron powder diffraction studies of the La1±0.1Mg2±0.1Ni9D9.4−9.5 deuterides were performed at the Spallation Neutron Source SINQ accommodated at Paul Scherrer Institute (Villigen, Switzerland). Two samples, La1.09Mg1.91Ni9D9.5(3) (sample 1) and La0.9Mg2.1Ni9D9.4(6) (sample 2) were synthesised and studied under different conditions.
For the synthesis of La1.09Mg1.91Ni9D9.5, a 6 mm diameter stainless steel autoclave with a wall thickness of 0.2 mm was used. The synthesis was performed by saturating activated samples with deuterium gas (25 bar) at a sub–zero temperature of −30 °C. This was done in order to decrease the equilibrium pressure of hydrogen absorption-desorption in the La1.09Mg1.91Ni9—D2 system. The alloy absorbed deuterium to reach a composition La1.09Mg1.91Ni9D9.5 and was measured at 25 °C and deuterium pressure of 25 bar.
The second sample, La0.9Mg2.1Ni9D9.4, was synthesized at high pressure deuterium gas of 950 bar D2. The studied sample was placed inside a TiZr sample cell with a stainless steel liner, which was used as a sample holder during the in situ NPD experiments (see Figure 3). The pressure during the NPD measurements performed at 20 °C was set to 912 bar D2. No preliminary activation was applied prior to the synthesis.
Figure 3. High pressure synthesis setup for the in situ NPD measurements at pressures up to 1000 bar D2.
Figure 3. High pressure synthesis setup for the in situ NPD measurements at pressures up to 1000 bar D2.
Energies 08 03198 g003
For the La1.09Mg1.91Ni9D9.5(5) sample (No.1) at the highest applied deuterium pressure of 25 bar D2, the deuteration resulted in the formation of a two-phase mixture of the α-solid solution of deuterium in the alloy and a corresponding β-deuteride. Such a mixture of the phase constituents was observed after allowing a deuteration time of ~20 h at interaction temperature of −30° C. Since applied temperature-pressure conditions were rather close to the equilibrium ones (see Figure 2), the transformation was slow and was not completed on the time scale of the measurements performed. The second sample with a slightly higher content of magnesium, La0.91Mg2.09Ni9D9.3(7) was saturated by deuterium at deuterium pressure of 950 bar and was equilibrated at 912 bar D2 and 25 °C. Analysis of the diffraction pattern showed an excellent fit between the experimental data and calculated NPD profiles (Figure 4) and indicated a completeness of the transformation of the α-solid solution into the β-deuteride.
The results of the refinements of the NPD data for La1.09Mg1.91Ni9D9.5(5) and for La0.91Mg2.09Ni9D9.4(6) are summarized in Table 2. The data show a formation of very similar structures, with only minor differences in the occupancies of the specific D-sites of five various types. These sites are shown in Figure 5 and include four types of tetrahedral and one tetragonal bipyramid.
A partial filling by D atoms of the four types of the tetrahedral interstices takes place inside the MgNi2 slab; these include two types of the [MgNi3] (18h and 6c) tetrahedra and two types of the [Mg2Ni2] (36i and 18h) interstitial sites.
In addition, similar to the other studied La3−xMgxNi9-based deuterides, the remaining 5.0 or 5.6 at. D/f.u. form a standard hydrogen sublattice within the LaNi5 slab which are statistically distributed within the four types of the interstices; hydrogen atoms partially occupy [La2Ni4] octahedra, three types of [Ni4] tetrahedra, and two types of the [LaMgNi2] sites.
Figure 4. NPD pattern of La0.9Mg2.1Ni9D9.4(6) (912 bar D2, 298 K). Note that the most significant contributions to the difference intensities are coming from the sample cell. Rp = 2.4%, Rwp = 3.2; χ2 = 6.0.
Figure 4. NPD pattern of La0.9Mg2.1Ni9D9.4(6) (912 bar D2, 298 K). Note that the most significant contributions to the difference intensities are coming from the sample cell. Rp = 2.4%, Rwp = 3.2; χ2 = 6.0.
Energies 08 03198 g004
Table 2. Crystal structure data for the deuterated La1±0.1Mg2±0.1Ni9 alloys (PuNi3 type, sp.gr. R 3 ¯ m) from the Rietveld refinements of in situ neutron diffraction data.
Table 2. Crystal structure data for the deuterated La1±0.1Mg2±0.1Ni9 alloys (PuNi3 type, sp.gr. R 3 ¯ m) from the Rietveld refinements of in situ neutron diffraction data.
DeuterideLa1.09Mg1.91Ni9D9.5(5)La0.91Mg2.09Ni9D9.4(6)
Conditions25 bar at 25 °C (prepared at −30 °C)912 bar at 25 °C
Unit cell parameters:
a (Å)5.263(1)5.212(1)
c (Å)25.803(9)25.71(1)
V3)618.9(3)604.8(3)
Unit cell parameters:
Δa/a (%)6.56.4
Δc/c (%)8.37.3
ΔV/V (%)23.021.6
ΔV/V[LaNi5] (%)20.420.7
ΔV/V[MgNi2] (%)25.422.2
Atomic parameters:
La1/Mg1 in 3a (0, 0, 0)
nMg, (nLa = 1–nMg)
0.0(–)0.09(–)
La2/Mg2 in 6c (0, 0, z)
z
Uiso × 100 (Å2)
nMg, (nRE = 1–nMg)
1.0(–)
0.95(–)
1.0(–)
1.0(–)
Ni1 in 3b (0, 0, ½)
Uiso × 100 (Å2)
1.0(–)1.0(–)
Ni2 in 6c (0, 0, z)
z
Uiso × 100 (Å2)
0.3279(7)
1.0(–)
0.3220(6)
1.0(–)
Ni3 in 18h (x, –x, z)
x
z
Uiso × 100 (Å2)
0.498(1)
0.0871(4)
1.0(–)
0.506(1)
0.0859(3)
1.0(–)
D1 in 18h (x, –x, z)
x
z
n
0.484(4)
0.023(1)
0.33(1)
0.496(3)
0.023(1)
0.31(2)
D2 in 6c (0, 0, z)
z
n
0.390(1)
0.50(3)
0.385(1)
0.58(3)
D4’ in 18h (x, –x, z)
x
z
n
0.814(3)
0.0626(9)
0.43(2)
0.792(2)
0.051(1)
0.33(3)
D5’ in 18h (x, –x, z)
x
z
n
0.201(2)
0.120(1)
0.45(2)
0.192(3)
0.123(1)
0.35(2)
D6 in 18h (x, –x, z)
x
z
n
0.819(4)
0.117(1)
0.20(2)
0.819(4)
0.117(1)
0.39(2)
Uiso × 100 (Å2) for D1-D62.0(–)2.0(–)
Atomic parameters:
D distribution in the structure
LaNi5
2 MgNi2
5.6(3)
3.9(2)
5.0(4)
4.4(2)
Shortest Metal—Hydrogen distances, Å
La…D
Mg…D
Ni…D
2.34(3)
1.97(3)
1.56(3)
2.29(2)
1.93(2)
1.53(2)
R-factors of refinements
Rp
Rwp
χ2
2.7
3.4
5.0
2.4
3.2
6.0
Secondary constituentsα-solid solution La0.9Mg2.1Ni9D0.9. Sp.gr. R 3 ¯ m; a = 4.9459(2); c = 23.842(2) Å; V = 505.10(4). 0.3 D in D3 18h (0.15, 0.3, 0.085) and 0.6 D in D4 18h (0.3, 0.15, 0.085); 35.7(2) wt% LaNi5D7; Sp.gr. P63mc; a = 5.438(3), c= 8.598(5) Å; V = 220.3(2) Å3; 4.6(3) wt%. Atomic structure was taken from [3]. MgNi2; MgNi2 structure type; Sp.gr. P63/mmc; a = 4.8356(4), c = 15.850(3) Å; V = 320.97(5) Å3; 12.4(2) wt%. Atomic structure was taken from [4]. Sample holder: stainless steel; Sp.gr. Fm   3 ¯ m ; a = 3.598 Å.LaNi5D7; Sp.gr. P63mc; a = 5.430(1), c = 8.606(4) Å; V = 219.8(2) Å3; 21.5(5) wt%. Atomic structure was taken from [3]. MgNi3; AuCu3 structure type; Sp.gr. Pm 3 ¯ m; a = 3.7185 Å; 1 Mg in 1a: 0, 0, 0; 3 Ni in 3c: 1/2, 1/2, 0; 3.7(2) wt%. Sample holder: zero matrix TiZr alloy with Fe liner. The peaks from Fe liner are only observed. Sp.gr. Fm   3 ¯ m ; a = 3.5949(1) Å.
From the refinements of the NPD data we conclude that the overall chemical compositions La1.09Mg1.91Ni9D9.5/La0.91Mg2.09Ni9D9.4can be presented as LaNi5H5.6/LaNi5H5.0 + 2*MgNi2H1.95/MgNi2H2.2. Thus, in the hybrid La1±0.1Mg2±0.1Ni9 structure, a LaNi5-assisted hydrogenation of the MgNi2 slab proceeds at rather mild H2/D2 pressure conditions; the equilibrium D2 desorption pressure is just 20 bar D2. In contrast, the parent MgNi2 intermetallic remains inert with respect to hydrogenation even at much higher hydrogen pressures as well as the conditions applied in the present study of 912 bar D2 for sample 2.
Figure 5. Crystal structure of La1±0.1Mg2±0.1Ni9D9.4−9.5 and types of the filled interstices.
Figure 5. Crystal structure of La1±0.1Mg2±0.1Ni9D9.4−9.5 and types of the filled interstices.
Energies 08 03198 g005
The shortest Me–D distances in the studied deuterides are listed in Table 2 and are within the regular values for the La–H, Mg–H and Ni–H distances in the structures of the metal and intermetallic hydrides.
The data of the present study clearly shows an influence of the LaNi5 and MgNi2 layers in the hybrid La1±0.1Mg2±0.1Ni9 structures on the hydrogenation of the other buildings blocks of the structure. MgNi2 slabs accommodate hydrogen up to a composition MgNi2H2.2 at much lower pressures as compared to those required to form a hydride by the pure MgNi2 intermetallic. In contrast, the LaNi5 block absorbs 5.0–5.6 at.H/f.u., which is quite close to the maximum hydrogenation capacity of the title intermetallic alloy, LaNi5H7; however, hydrogen desorption from the LaNi5H5.0/5.6 block proceeds much easier, at significantly higher pressures of H2/D2 as compared to the individual LaNi5H7 hydride—as a result of influence of the MgNi2 slab.

4. Conclusions

LaNi5-assisted hydrogenation of MgNi2 is observed in the LaMg2Ni9 hybrid structure. Formation of LaMg2Ni9D9.5 proceeds via an isotropic expansion of the trigonal unit cell. D atoms are accommodated in both Laves and CaCu5-type slabs H atoms filling interstitial sites in both LaNi5 and MgNi2 structural fragments.
Limits of Mg solubility in LaNi3 are not confined to LaMg2Ni9 and extend to the composition La0.91Mg2.09Ni9 with a refined composition of the CaCu5-type block of La0.95Mg0.05Ni5.
Within the LaNi5 CaCu5-type layer, D atoms fill three types of interstices; a deformed octahedron [La2Ni4], and two types of tetrahedra, [LaNi3] and [Ni4], to yield LaNi5D5−5.6 composition. D distribution is very similar to that in the individual β-LaNi5D7 deuteride.
In the MgNi2 slab hydrogen atoms fill two types of tetrahedra, [Mg2Ni2] and [MgNi3]. The hydrogen sublattice formed is unique and is not formed in the studied structures of the Laves-type intermetallic hydrides.
A significant mutual influence of the LaNi5 and MgNi2 slabs causes a dramatic altering of their hydrogenation behaviours leading to:
(a)
significant decrease of the stability of the LaNi5-type hydride;
(b)
much easier hydrogenation of the MgNi2 slabs compared to the parent intermetallic compound;
(c)
increased hysteresis.

Acknowledgments

This work received support from the Research Council of Norway (project 223084 NOVELMAG “NOVEL MAGNESIUM BASED NANOMATERIALS FOR ADVANCED RECHARGEABLE BATTERIES”) and is a part of the activities within the IEA Task32 “Hydrogen Based Energy Storage”. The skillful assistance from the staff of the Swiss-Norwegian Beam Lines during the experimental studies at ESRF is gratefully acknowledged. Denis Sheptyakov (PSI, Switzerland) is sincerely thanked for the collaboration in the neutron powder diffraction experiments at HRPT, PSI (experiments 20090547, 20101348 and 20110612).

Author Contributions

All authors contributed extensively to the work presented in this paper. Volodymyr A. Yartys supervised the project. Evan MacA. Gray and Colin J. Webb designed and built the high pressure 1000 bar rig. Roman V. Denys, Volodymyr A. Yartys and Colin J. Webb jointly performed the in situ neutron powder diffraction experiments, while Roman V. Denys analyzed the NPD data. Roman V. Denys and Evan MacA. Gray measured the PCT diagrams. Volodymyr A. Yartys wrote the paper. All authors discussed the results and commented on the manuscript at all stages.

Conflicts of Interest

The authors declare no conflict of interest.

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