Synthesis, crystal structure and hydrogenation properties of the ternary compounds LaNi4Mg and NdNi4Mg

Article abstract of DOI:10.1016/S0925-8388(02)00797-1

The title compounds have been prepared by induction melting and investigated with respect to structure and hydrogenation properties. Both crystallise with the cubic MgCu₄Sn type structure (space group F4?3m, LaNi₄Mg: a = 7.17-7.18 ?, NdNi₄Mg: a = 7.09875(1) ?) and absorb reversibly up to 4 hydrogen atoms per formula unit at 7-8 bar and ～50 °C. Synchrotron and neutron powder diffraction data on deuterated NdNi₄Mg indicate an orthorhombic lattice distortion (NdNi₄MgD_3.6, space group Pmn2₁, a = 5.0767(2), b = 5.4743(2), c = 7.3792(3) ?, ΔV/V=14.6%) and three almost fully occupied deuterium sites of which two are coordinated by a trigonal metal bipyramid ([A₂B₃] apices: A=2Nd, base: B=2Ni,Mg) and one is coordinated by a metal tetrahedron ([AB₃] A = Nd, B = 3Ni). The hydride is stable at room temperature under 1 bar hydrogen pressure, but desorbs rapidly at 80 °C under vacuum. Under air it decomposes by catalytic water formation.

Full text of DOI:10.1016/S0925-8388(02)00797-1

Journal of Alloys and Compounds 348 (2003) 129–137
L
www.elsevier.com/locate/jallcom
Synthesis, crystal structure and hydrogenation properties of the ternary
compounds LaNi4Mg and NdNi4Mg
1
*
´ ´
L. Guenee, V. Favre-Nicolin , K. Yvon
´
`
`
Laboratoire de Cristallographie, Universite de Geneve, 24 quai E. Ansermet, CH-1211 Geneve 4, Switzerland
Received 6 May 2002; accepted 17 May 2002
Abstract
The title compounds have been prepared by induction melting and investigated with respect to structure and hydrogenation properties.
¯
˚
˚
Both crystallise with the cubic MgCu4Sn type structure (space group F43m, LaNi4Mg: a 5 7.17–7.18 A, NdNi4Mg: a 5 7.09875(1) A)
and absorb reversibly up to 4 hydrogen atoms per formula unit at 7–8 bar and |50 8C. Synchrotron and neutron powder diffraction data
on deuterated NdNi4Mg indicate an orthorhombic lattice distortion (NdNi4MgD3.6, space group Pmn21, a 5 5.0767(2), b 5 5.4743(2),
˚
c 5 7.3792(3) A, DV/V5 14.6%) and three almost fully occupied deuterium sites of which two are coordinated by a trigonal metal
bipyramid ([A2B3] apices: A52Nd, base: B52Ni,Mg) and one is coordinated by a metal tetrahedron ([AB3] A5Nd, B53Ni). The
hydride is stable at room temperature under 1 bar hydrogen pressure, but desorbs rapidly at 80 8C under vacuum. Under air it decomposes
by catalytic water formation.
2002 Elsevier Science B.V. All rights reserved.
Keywords: Hydrogen absorbing materials; Metal hydrides; Gas–solid reactions; Crystal structure; Neutron diffraction
1. Introduction
determined. On the other hand, lanthanum based com-
pounds of composition La1 2 xMgxNi2 have been reported
to crystallise with a disordered cubic MgCu2 (C15) type
structure at low Mg contents (0 , x , 0.67 [5]) and with
the MgNi2 (C36) type structure at higher Mg contents
(x . 0.67). The composition (La0.5Mg0.5)Ni2 5LaMgNi4
was found to absorb up to 3.4 hydrogen atoms per formula
unit [5]. In view of these results we decided to investigate
LaNi4Mg and its neodymium analogue NdNi4Mg in more
detail.
The search for new hydrogen storage compounds based
on the CaCu5 type structure has led us to examine ternary
R–Ni–Mg systems (R5Rare earths). A recent study of
structure stability maps suggests [1] that compositions
close to RNi4Mg are outside the stability range of the
CaCu5 type structure and inside the range of the MgCu4Sn
type structure. The latter can be considered as ordered
ternary derivative of the AuBe5 type structure (space group
¯
F43m) [2] in which Mg occupies the Au site (4a: 0 0 0,
etc.) and Sn one of the Be sites (4c: 1/4 1/4 1/4, etc.).
Known representatives containing R and Ni include cerium
based CeNi4Mg [3] and yttrium based MgYNi4 [4]. While
the former compound was not investigated with respect to
hydrogen absorption, the latter was found to absorb 3.6
hydrogen atoms per formula unit (H/f.u.) at ambient
temperature and 30 bars hydrogen pressure [4]. The
structure of the resulting hydride, however, was not
2. Experimental
2.1. Synthesis
The lanthanum compound was prepared by pressing La
rods (Johnson Matthey, 99.99%) and Mg (Strem Chemi-
cals, 99.8%) and Ni powders (Johnson Matthey, 5N) at the
nominal composition LaMg1.108Ni4.06 into pellets that
were placed into a silver crucible and melted in an
induction furnace under argon (6N). The as-cast alloys
generally contained three phases (MgNi2, LaNi2 and
elemental Ni) that were not well crystallised. In order to
increase sample homogeneity and minimise evaporation
*
Corresponding author. Tel.: 141-22-702-6231; fax: 141-22-702-
6864.
E-mail address: klaus.yvon@cryst.unige.ch (K. Yvon).
1Present address: European Synchrotron Radiation Facility ESRF, BP
220, F-38043 Grenoble Cedex, France.
0925-8388/02/$ – see front matter
PII: S0925-8388(02)00797-1
2002 Elsevier Science B.V. All rights reserved.

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L. Guenee et al. / Journal of Alloys and Compounds 348 (2003) 129–137
130
˚
losses magnesium was first pre-reacted with nickel to the
binary metal compound Mg2Ni, and pellets of powder
mixtures of La, Mg2Ni and Ni were then melted at various
nominal compositions. The resulting weight losses did not
exceed 2%. As-cast samples of nominal composition
La1.08MgNi4 consisted of a majority phase whose cubic
parameters a 5 7.18 and 7.17 A. However, the quality of
the data was insufficient to derive reliable Mg/Ni ratios for
either one of the phases and thus prove their different
chemical compositions. For the neodymium sample,
˚
synchrotron data were collected at l 5 0.50012 A within
the angular range 2u 5 3.028–33.268 in steps of 0.0048.
The sample contained two phases of which the principal
one could be attributed to a NdNi4Mg phase crystallising
with the MgCu4Sn type structure and the minor one to a
NdNi3 phase crystallising with the PuNi3 type structure.
The structures were refined by using the program Fullprof
˚
cell parameter (a 5 7.17 A) was within the previously
reported range for the C15 type solid solution
La1 2 xMgxNi2. However, the presence of 200, 420, 600
and 640 diffraction planes in the X-ray patterns were not
¯
consistent with this structure type (space group Fd3m,
h00: h 5 4n and hk0: h 1 k 5 4n) and rather indicated a
MgCu4Sn type structure (space group F43m). Further-
more, the samples contained two minority phases, one
[7] and by varying 21 parameters (NdNi4Mg: 6
¯
structural15 profile1scale factor; PuNi3: 9 structural, 1
scale). For NdNi4Mg a small substitution of Ni by Mg (site
4c) allowed to improve the agreement indices, in particular
RBragg. The patterns are represented in Fig. 1 and refine-
ment results are summarised in Table 1.
having the hexagonal La2Ni7 type structure (a55.038
˚
c524.35 A) and an unidentified phase that could be
indexed on an orthorhombic C-centred lattice with cell
˚
parameters a54.21, b510.27 and c58.34 A. In order to
detect a possible deviation of the majority phase from the
ideal metal ratio Ni/Mg54 samples of composition
LaNi3Mg2 (Ni/Mg,4) and LaNi4.5Mg0.5 (Ni/Mg.4)
were prepared. The former was found to consist mainly of
the cubic LaNi4Mg phase and showed diffraction line
broadening and splitting. The concentration of the non-
identified orthorhombic phase was higher than in the
La1.08MgNi4 sample. The LaNi4.5Mg0.5 sample contained
LaNi4Mg, LaNi5 (CaCu5 type structure) and elemental Ni.
None of the lanthanum alloys were annealed. For the
neodymium compound compressed powder mixtures of Nd
2.3. Hydrogen absorption and desorption
Due to the relatively large concentration of secondary
phases in the lanthanum samples the hydrogen absorption
properties were mainly studied on the neodymium sam-
ples. One of the latter ([1, nominal composition NdNi4Mg
annealed at 800 8C for 24 h) was powdered in a glove box
and introduced into a microbalance (Hiden Analytical) that
was evacuated for a few hours under preliminary vacuum
(1023 bar). Hydrogenation was carried out under a pres-
sure of 7 bars at 50 8C for 24 h. After a few minutes the
sample started to absorb slowly but steadily as shown in
Fig. 2. After about 3 h the weight increase levelled off at a
value corresponding to about 3.6 H/f.u. (value corrected
for the presence of |26 wt.% hydrogen inert NdNi3 phase,
see ch. 2.4). A pressure–composition-isotherm ( p–c–T)
obtained under the same conditions on part of another
sample ([3, nominal composition Nd1.038Mg1.087Ni4, an-
nealed at 780 8C for 28 h, 18% secondary NdNi3 phase) is
shown in Fig. 3. The NdNi4Mg phase absorbs up to 4
H/f.u. (value corrected for the presence of hydrogen inert
NdNi3 phase). One notices a relatively narrow (|2 H/f.u)
and ill-defined plateau region at p(H2)|1bar, followed by
a relatively wide (2H/u.f.) single-phase domain at higher
pressures. Thus the hydrogenation properties of NdNi4Mg
are rather different from those of LaNi5 type hydrides for
which long plateaus and narrow single phase domains were
found [8]. The absorption/desorption kinetics is relatively
slow, thus precluding p–c–T measurements. After the
experiment, the sample was investigated by X-ray diffrac-
tion in air, and then replaced into the thermobalance and
evacuated at 80 8C for a few hours. The patterns indicated
almost complete loss of hydrogen and recovery of the
initial alloy structure. Interestingly, after the hydrogenated
samples were exposed to air in a glass container, water
droplets appeared after a few hours on the surface of the
container walls. No such effects occurred if the hydride
was kept in the glove box. Finally, one sample ([2,
¨
(Riedel–de Haen, .99.9%), Mg2Ni and Ni of various
nominal compositions were melted in an induction furnace
and annealed for up to 33 h at 780–800 8C in quartz tubes
filled by 0.3 bar argon (6N). The weight losses did not
exceed 3%. All samples contained a majority phase having
the MgCu4Sn type structure and a minority phase (up to
25%) of binary NdNi3 having the PuNi3 type structure [6].
Samples of nominal composition NdNi4Mg ([1),
Nd1.008Ni4Mg ([2) and Nd1.038Ni4Mg1.087 ([3) were
used for hydrogen absorption, synchrotron diffraction and
pressure-composition–isotherm/neutron diffraction mea-
surements, respectively.
2.2. Structure analysis of alloys
The alloy samples LaNi3Mg2 and NdNi4Mg ([2) were
investigated by synchrotron powder diffraction (ESRF,
Grenoble, SNBL, capillaries of 0.3 mm diameter). For the
˚
lanthanum sample, the pattern (l 5 0.85022 A) confirmed
the presence of two phases, one having the MgCu4Sn type
structure and thus the likely composition LaNi4Mg (La, Ni
and Mg on Mg, Cu and Sn sites, respectively), and the
other a hitherto unknown orthorhombic C-centred struc-
ture. The diffraction lines of the LaNi4Mg phase were split
and had to be modelled during structure refinement by the
assumption of two MgCu4Sn type phases having the cell

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L. Guenee et al. / Journal of Alloys and Compounds 348 (2003) 129–137
131
˚
Fig. 1. Observed (points), calculated (line) and difference (bottom line) synchrotron powder diffraction pattern of NdNi4Mg (sample [2, l 5 0.50012 A).
Vertical bars indicate Bragg position of contributing phases NdNi4Mg (74%, top) and NdNi3 (26%, bottom).
nominal composition Nd1.008Ni4Mg, 25% secondary
NdNi3 phase) was hydrogenated in an autoclave at 7 bar
and 50 8C during a few hours for the synchrotron diffrac-
tion study, and part of another sample ([3) was deuterated
under 7.5 bars at 50 8C for the neutron diffraction study.
As for the lanthanum compound, hydrogenation experi-
ments were performed on a sample of nominal com-
position La1.08Ni4Mg. The multiphase sample (containing
the secondary phases La2Ni7 and the non-identified ortho-
rhombic C-centred phase) was placed into the microbal-
ance and exposed to hydrogen of 8 bars pressure at 53 8C.
The observed weight increase was found to correspond to a
hydrogen uptake of |5 H/f.u. However, in view of the
presence of hydrogen absorbing secondary phases this
value is presumably lower (see Section 2.4).
2.4. Structure analysis of hydrides (deuterides)
Preliminary Guinier X-ray diagrams of the hydrogenated
neodymium samples revealed significant structural changes
of the metal atom substructure. One sample ([2) was
re-measured by synchrotron radiation (ESRF, SNBL, l 5
˚
0.50012 A, capillary [ 0.3 mm). Although it partially
desorbed during the measurement the pattern of the main
hydride phase could be indexed on an orthorhombic lattice
having the cell parameters a55.0788(2), b55.4887(2),
˚
c57.3846(2) A. The cell parameter relationships with/Œth]2e,
bortho ¯ acub / 2, cortho ¯ acub; Vortho 51/2 Vcub, and the
relative volume increase during hydrogenation is DV/V5
15.1%. The systematically absent reflections suggested
non-centrosymetric space group Pmn21. Centrosymmetric
Pmnm was rejected because it precludes a continuous
hydrogen free cubic compound are aortho ¯ acub
Table 1
]
Œ
Structure refinement results on synchrotron powder diffraction data for
NdNi4Mg
Phase 1: NdNi4Mg (MgCu4Sn type structure, refined content |74 wt.%)
space group, F43m, a 5 7.09875(1) A
Atoms
¯
˚
2
˚
Site
x
y
z
B (A )
Occupancy
(non-reconstructive)
transition
from
intermetallic
Nd
4a
4c
4c
0
1/4
1/4
0.62376(6)
0
0
0.34(2)
0.5(1)
BMg
1.0
Mg
Ni1
Ni2
1/4
1/4
x
1/4
1/4
x
0.912(7)
1–Mg
1.0
NdNi4Mg to the hydride. The metal atom substructure was
solved ab initio by the newly developed computer program
FOX [9]. However, due to the partial desorption of the
sample during the measurement a satisfactory structure
refinement on these data was not possible. Neutron diffrac-
tion data on a deuterated sample ([3) were collected on
the powder diffractometer HRPT at SINQ (PSI, Villigen,
16e
0.50(2)
R
Bragg 51.92%, Nrefa526, 12 variables
Phase 2: NdNi3 (PuNi3 type structure, refined content |26 wt.%)
¯
˚
space group R3m, a 5 4.9918(2), c 5 24.334(1) A
Bragg 512.1%, Naref 5133, 10 variables
R
˚
high resolution mode, l 5 1.494 A, scattering range
Rp 513.7%, Rwp 512.8%, Chi253.11
2.95 , 2Q , 162.908, step size D2Q 5 0.058, vanadium
container). The deuterium substructure was determined by
FOX by introducing 7 deuterium atoms per unit cell at
Sample [2, two phases, e.s.d.’s in parentheses.
a Nref: number of observed reflections.

´ ´
L. Guenee et al. / Journal of Alloys and Compounds 348 (2003) 129–137
132
Fig. 2. Mass increase (mg) versus hydrogen uptake (H/f.u.) of NdNi4Mg sample [1 in microbalance (T550 8C, 7 bars hydrogen pressure, initial sample
weight 42.55 mg, total weight variation Dm 5 287.6 mg, maximum hydrogen content 3.6 H/f.u., values corrected for the presence of 25 wt.% hydrogen
inert NdNi3 phase).
random positions and fixing the metal atom substructure in
space group Pmn21. Three independent D sites with
practically full occupancy were found at sites 4b (x, y, z
etc.) and 2a (0, y, z etc.). Structure refinement of the
two-phase sample was performed by Fullprof [7] by
varying a total of 46 parameters of which 28 were atomic
parameters of the NdNi4MgD3.6 phase. No deuterium sites
were identified in the secondary NdNi3 phase as expected
from its cell parameters that remained practically un-
changed after deuteration. Results are summarised in Table
2, and the diffraction patterns are represented in Fig. 4. As
to the hydrogenated lanthanum samples, Guinier X-ray
diagrams revealed an orthorhombic lattice distortion of the
LaNi4MgHx phase (estimated cell parameters a55.12,
˚
b55.54, c57.47 A; relative volume expansion DV/V|
15%) similar to that of the neodymium analogue, thus
suggesting similar amounts of absorbed hydrogen (|3.6
H/u.f).
Fig. 3. Pressure–composition isotherm of NdNi4Mg sample [3 (T550 8C, composition corrected for 18 wt.% hydrogen inert NdNi3 phase; maximum
hydrogen content 4 H/f.u.).

´ ´
L. Guenee et al. / Journal of Alloys and Compounds 348 (2003) 129–137
133
Table 2
Structure refinement results on neutron powder diffraction data for the deuteride NdNi4MgD3.6
Phase 1: NdNi4MgD3.6 (refined content |83 wt.%)
space group Pmn21, a55.0767(2), b55.4743(2), c57.3792(3) A, DV/V 5 14.6%
a
˚
2
˚
Atoms
Site
x
y
z
B (A )
Occupancy
Nd
2a
2a
2a
2a
4b
4b
2a
2a
0
0
0
0
0.302(1)
0.813(2)
0.4519(7)
0.9935(7)
0.2270(4)
0.5089(7)
0.716(1)
0.9458(9)
0.0
0.91(9)
2.3 (2)
0.68(5)
0.65(5)
0.42(3)
1.26(7)
2.1(1)
1.0
1.0
1.0
1.0
Mg
Ni1
Ni2
Ni3
D1
0.223(1)
0.6233(8)
0.6053(8)
0.3811(6)
0.7544(9)
0.513(1)
0.8245(9)
0.7505(7)
0.7462(7)
0
0
1.0
0.921(9)
0.891(1)
0.899(1)
D2
D3
0.4(1)
R
Bragg 53.8%, Nref 5296, 32 variables
Phase 2: NdNi3, (PuNi3 type structure, refined content |17 wt.%)
¯
˚
space group R3m, a 5 4.995(2), c 5 24.19(1) A
Bragg 54.9%, Nrefb5195, 14 variables
R
Rp 58.3%, Rwp 59.0%, Chi2 59.01
Sample [3, 2 phases, e.s.d.’s in parentheses.
a Relative volume increase during deuteration (Vcub 5 2Vortho).
b Nref: number of observed reflections.
3. Discussion
stability maps [1]. Interestingly, the lanthanum compound
lies within the compositional range of the La1 2 xMgxNi2
series (0 , x , 0.67) for which a disordered C15 type
structure has been reported, and its cell parameter
3.1. Structure of intermetallic compounds
˚
(LaNi4Mg: 7.17–7.18 A) is significantly larger than that of
The structure analysis confirms that both LaNi4Mg and
NdNi4Mg crystallise with ordered MgCu4Sn type struc-
tures. Although they can be formally derived from the
cubic AuBe5 type structure, their crystal chemistry and
structural analogy between Mg and La(Nd) sites suggest
that they should rather be considered as derivatives of the
cubic C15 type structure, in agreement with structure
the C15 type structure having the same composition
˚
(La0.5Mg0.5Ni2: a 5 7.149 A [5]). This polymorphism may
be attributed to different sample preparation conditions.
Small deviations from RNi4Mg stoichiometry are possible
in view of the range of cell parameters found for the
˚
lanthanum compound (7.17–7.18 A) and the slight occupa-
˚
Fig. 4. Observed (points), calculated (line) and difference (bottom line) neutron diffraction pattern of NdNi4MgD3.6 (sample [3, l 5 1.494 A). Vertical
bars indicate Bragg positions of contributing phases NdNi4MgD3.6 (83%, top) and NdNi3 (17%, bottom).

´ ´
L. Guenee et al. / Journal of Alloys and Compounds 348 (2003) 129–137
134
tional disorder found for the neodymium compound (site
4c is occupied 91% by magnesium and 9% by nickel).
Given their relatively high atom coordination numbers
(CN516 for R and Mg, CN512 for Ni) the structures can
be considered as topologically close packed. The inter-
atomic distances (Table 3) are in the usual range.
Table 3
Comparison between metal–metal distances in cubic NdNi4Mg and orthorhombic NdNi4MgD3.6
¯
NdNi4Mg
F43m
NdNi4MgD3.6
Central atom
Pmn21
Central
atom
CNa
16
Coordinated
atoms
Distanceb
(A)
CNa
16
Coordinated
atoms
Distance
˚
˚
(A)
Nd
Mg
Ni
12 Ni
4 Mg
2.9471(4)
3.0750(0)
Nd
1 Ni1
2 Ni3
2 Ni1
2 Ni2
2 Ni3
1 Mg
1 Mg
2 Ni3
2 Mg
1 Ni2
2 Ni3
2 Ni3
2 Ni2
1 Ni2
2 Ni1
1 Nd
2.899 (6)
3.007 (5)
3.015 (4)
3.108 (4)
3.111 (4)
3.14 (1)
3.25 (1)
3.281 (6)
3.319 (6)
3.366 (6)
2.835 (9)
2.844 (5)
2.884 (5)
2.99 (1)
3.016 (6)
3.14 (1)
3.25 (1)
3.319 (6)
3.55 (1)
3.64 (1)
2.513 (6)
2.513 (5)
2.886 (6)
2.968 (5)
2.899 (6)
3.015 (4)
3.016 (6)
3.55 (1)
2.444 (6)
2.513 (5)
2.686 (6)
2.968 (5)
2.884 (5)
2.99 (1)
3.108 (4)
3.366 (6)
2.444 (6)
2.513 (6)
2.533 (5)
2.543 (5)
2.686 (6)
2.886 (6)
2.844 (9)
2.835 (9)
3.007 (6)
3.111 (4)
3.281 (6)
3.64 (1)
16
12
12 Ni
4 Nd
2.9418(4)
3.0750(0)
3 Nic
3 Nic
3 Nd
3 Mg
2.4860(6)
2.5360(6)
2.9471(4)
2.9418(4)
Mg
16
1 Nd
2 Nd
1 Ni1
2 Ni3
2 Ni3c
1 Ni2c
2 Ni3d
1 Ni2d
1 Nd
Ni1
Ni2
Ni3
12
12
12
2 Nd
2 Mg
1 Mg
2 Ni3c
1 Ni1c
2 Ni3d
1 Ni1d
2 Mg
1 Mg
2 Nd
1 Nd
1 Ni2c
1 Ni1c
1 Ni3c
1 Ni3d
1 Ni2d
1 Ni1d
1 Mg
1 Mg
1 Nd
1 Nd
1 Nd
1 Mg
a CN: coordination number.
b
˚
Calculated with a 5 7.10215(2) A.
c Uncapped Ni4 tetrahedra.
d Ni4 tetrahedra capped by D atoms (see text).

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L. Guenee et al. / Journal of Alloys and Compounds 348 (2003) 129–137
135
3.2. Hydrogen absorption
NdNi4Mg and LaNi4Mg absorb about the same quantity
of hydrogen (NdNi4Mg sample [1: 0.60 H/metal atom,
sample [3: 0.67 H/metal atom) as isostructural YNi4Mg
under similar conditions (0.6 H/metal atom [4]), but less
than C15 type La0.5Mg0.5Ni2 (1.1 H/metal atom at room
temperature and P510 bars [5]). Similar to the latter,
NdNi4Mg and LaNi4Mg have relatively slow kinetics and
do not tend to decompose and/or become amorphous
during hydrogenation. However, in contrast to
La0.5Mg0.5Ni2 that tends to segregate into LaH3, MgH2
and Ni during desorption at 80 8C, no segregation was
observed for NdNi4Mg during desorption at 80 8C under
vacuum during a few hours. In fact, the compound released
almost quantitatively its hydrogen and returned to the
initial cubic MgCu4Sn-type structure without a significant
change in cell parameter.
3.3. Structure of deuteride
Fig. 5. Bridging deuterium ligands in [Ni4D4] unit, viewed approximate-
ly along the mirror plane (point group symmetry m).
The refined deuterium content of NdNi4MgDx (x5
3.63(2)) is in good agreement with that determined for the
hydride on the microbalance (sample [1: x53.6, [3:
x54). During deuteration the metal atom substructure
undergoes important changes as shown by the symmetry
reduction from cubic to orthorhombic, the strongly aniso-
tropic lattice expansion (Db/b 5 9.3%, Dc/c 5 4.0%, Da/
a 5 1.2%) and the unequal lengthening of the Ni–Ni bonds
(see below). The three deuterium sites are nearly fully
occupied thus suggesting the upper phase limit to be near
the composition NdNi4MgD4. The metal coordinations of
the D sites are represented in Fig. 5 and the corresponding
bond distances are summarised in Table 4. Two sites (D1,
D2) are coordinated by trigonal metal bipyramids ([A2B3]
apices: A52Nd, base: B52Ni,Mg) with bond distances
Table 4
Metal–deuterium bond distances, D–D contact distances and interstitial
hole size (Rsite) in NdNi4MgD3.6, e.s.d.’s in parentheses
a
b
˚
˚
Atom
Site
Coordinated
atoms
Distance (A)
Rsite (A)
Nd
2a
1D3
2D1
2D1
2D2
2.340(7)
2.482(6)
2.496(6)
2.5422(6)
Mg
Ni1
Ni2
Ni3
D1
2a
2a
2a
4b
4b
2D1
1D2
2.174(9)
2.21(1)
2D1
1D2
1.641(6)
1.66(1)
˚
˚
D–Nd52.48–2.54 A, D–Mg52.17, 2.21 A, D–Ni51.64–
˚
1D3
1D2
1.639(9)
1.66(1)
1.72 A, and one (D3) is coordinated by a metal tetrahedron
([AB3] A5Nd, B53Ni) with bond distances D–Nd52.34
˚
(Nd) and D–Ni51.64 A. The corresponding radii of the
1D3
1D1
1.639(5)
1.722(6)
metal interstices (Rsite) as calculated within the hard sphere
˚
approximation are larger than 0.43 A and thus consistent
Nd
Nd
Mg
Ni3
Ni1
2.482(6)
2.496(6)
2.174(9)
1.722(6)
1.641(6)
0.47
0.47
with Westlake’s model [10]. Although the displacement
amplitudes of the deuterium atoms in the trigonal
bipyramids (D1, D2) are somewhat larger than those in the
tetrahedra (D3), they suggest triangular ([MgNi2] type)
rather than tetrahedral ([NdMgNi2] type) nearest neigh-
bour environment within the bipyramids. An alternative
way to rationalise the structure is to consider the deuterium
atoms as bridging ligands of Ni4 tetrahedra. As shown in
Fig. 6, this corresponds to the presence of [Ni4D4] units in
which one face (Ni3–Ni3–Ni2) and three edges (two
Ni1–Ni3, one Ni1–Ni2) are capped by D3, D1 and D2
atoms, respectively. As expected, the capped Ni4 tetra-
D2
2a
2a
2 Nd
Mg
Ni1
Ni2
2.5422(6)
2.21(1)
1.66(1)
1.66(1)
D3
Nd
Ni2
2 Ni3
2.340(7)
1.639(9)
1.639(5)
0.43
a
˚
D–Ni cut-off distance ,3.0 A.
b Calculated from atomic radii RNd 5 1.821 A, RMg 5 1.602 A, RNi 5
1.246 A [11].
˚
˚
˚
˚
hedra (Ni–Ni52.54–2.97 A) are considerably expanded

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L. Guenee et al. / Journal of Alloys and Compounds 348 (2003) 129–137
136
Fig. 6. Metal coordinations of deuterium sites in orthorhombic NdNi4MgD3.6 viewed along two perpendicular directions. For clarity, only one
representative of each site is drawn.
˚
(by up to 0.5 A) compared to the uncapped ones (Ni–Ni5
C15 type La1 2 xMgxNi2 hydrides but its mechanism is
unknown.
˚
2.44–2.51 A, see Table 3) that have about the same size as
those in the deuterium free cubic compound NdNi4Mg
˚
(Ni–Ni52.49–2.54 A). The D–D distances are all longer
˚
than 2.5 A and thus indicative for repulsive D–D interac-
Acknowledgements
tions.
The authors thank D. Sheptyakov (PSI,Villigen) for help
with collecting the neutron diffraction data on HRPT at
SINQ, and the staff of the Swiss Norwegian beamline at
ESRF (Grenoble) for help with collecting the synchrotron
data. Thanks are also due to R. Cerny (Geneva) for useful
discussions. This work was supported by the Swiss Nation-
al Science Foundation and the Swiss Federal Office of
Energy.
4. Conclusions
LaNi4Mg and NdNi4Mg absorb reversibly up to 4
hydrogen atoms per formula unit at 7 bars and 50 8C
without amorphisation or segregation into binary hydrides.
Although their hydrogen density (NdNi4MgH4: 0.99 wt.%,
65.05 g H2 l
21) and absorption/desorption kinetics are
somewhat lower than that of CaCu5 type hydrides, their
p–c–T isotherms are in a useful range for hydrogen
storage applications. Hydrogenation leads to an ortho-
rhombic distortion of the cubic MgCu4Sn type metal host
structure in which hydrogen occupies one tetrahedral
[AB3] (A5Nd, B53Ni) and two triangular bipyramidal
coordinated [A2B3] metal sites (A52Nd, base: B5
2Ni,Mg) in a nearly ordered configuration. In cubic C15
type hydrides of similar composition, hydrogen is dis-
ordered exclusively over tetrahedral sites of type [A2B2].
Finally, NdNi4MgHx and presumably also LaNi4MgHx
desorb hydrogen rapidly in air by catalytic water forma-
tion. A similar phenomenon was already observed for the
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