Article
pubs.acs.org/JPCC
Structure and Deuterium Desorption from Ca3Mg2Ni13 Deuteride: A
Neutron Diffraction Study
Qingan Zhang,† Dalin Sun,*,‡ Junxian Zhang,§ Michel Latroche,§ Liuzhang Ouyang,∥ and Min Zhu*,∥
†
School of Materials Science and Engineering, Anhui University of Technology, Maanshan 243002, China
Department of Materials Science, Fudan University, Shanghai 200433, China
§
Institut de Chimie et des Matériaux de Paris Est, CMTR, UMR 7182, CNRS-UPEC, Thiais 94320, France
∥
School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, China
‡
S Supporting Information
*
ABSTRACT: The Ca3Mg2Ni13 unit cell can be viewed as the
stacking of three blocks along the c axis. Each block is
composed of two sub-blocks; one sub-block contains one layer
of a [CaMgNi4] unit, and the other sub-block consists of one
layer of a [CaMgNi4] unit and one layer of a [CaNi5] unit. To
understand the deuterium release from the Ca3Mg2Ni13
deuteride, crystal structures of Ca 3 Mg 2 Ni 1 3 D 1 5 . 6 ,
Ca3Mg2Ni13D5.9, and Ca3Mg2Ni13D0.3 corresponding to before,
during, and after deuterium desorption were determined by
neutron diffraction. In Ca3Mg2Ni13D15.6, D atoms occupy
interstitial sites within [CaNi5] and [CaMgNi4] units as well as
sites at two unit borders. Upon deuterium desorption, the D atoms located at unit borders are released first. Then D atoms
located within [CaNi5] and [CaMgNi4] units are simultaneously released which leads to the coexistence of Ca3Mg2Ni13D5.9 and a
deuterium-poor solid solution phase. With further desorption, Ca3Mg2Ni13D5.9 transforms into Ca3Mg2Ni13D0.3 where D atoms
reside in [CaNi5] units only.
system.32 Indeed, ternary compounds such as La2MgNi9 (m =
1, n = 1), La3MgNi14 (m = 1, n = 2), and La4MgNi19 (m = 1, n =
3) have been synthesized experimentally, and crystal structures
of these compounds have the same structure as the
corresponding binary structures, LaNi3, La2Ni7, and La5Ni19,
respectively.23 Similarly, in the Pr−Mg−Ni system, the
Pr1.5Mg0.5Ni7 (m = 1, n = 2) and Pr3.75Mg1.25Ni19 (m = 1, n
= 3) compounds derived from binary Pr2Ni7 and Pr5Ni19
demonstrate much improved thermodynamic properties for
hydrogen storage.33 Note that only compounds with m = 1
have been reported for these ternary systems.
Recently, we found a new compound, Ca3Mg2Ni13 (m = 2, n
= 1), in the Ca−Mg−Ni system;34 however, its corresponding
binary compound Ca5Ni13 does not exist in the Ca−Ni system.
To our knowledge, Ca3Mg2Ni13 is the first compound reported
to date with m = 2. X-ray diffraction (XRD) studies show that
Ca3Mg2Ni13 crystallizes in a space group of R3̅m with cell
parameters: a = 4.978(2) Å and c = 36.180(2) Å and Z = 3. The
Ca3Mg2Ni13 crystal structure is illustrated in Figure 1 and
shows stacking along the c axis of three blocks (A, B, C). Each
block consists of two sub-blocks (I, II) composed of a layer of
[CaMgNi4] for the first block and a layer of [CaMgNi4] and
1. INTRODUCTION
Compared to highly H-containing chemical hydrides such as
NaAlH4, LiBH4, and BH3NH3 and their derivatives,1−6
advantages of using hydride-forming intermetallics as hydrogen
storage materials are a simpler chemical process, better
reversibility, higher hydrogen purity, and a lower working
temperature.7 However, to meet the latest demands for
practical onboard application, the development of new
intermetallics with higher hydrogen capacity is imperative,
and this goal can be achieved by the substitution of light metals
for heavy metals. For instance, ternary X−Mg−Ni (X = Ca, Y,
and rare earths) compounds have been developed from
corresponding binary X−Ni compounds.8−11 Like binary X−
Ni compounds, ternary X−Mg−Ni compounds have layered
structures where [XMgNi4] and [XNi5] units stack along the caxis alternatively according to different patterns.12−20 To reflect
the stacking structure, the chemical formula for this series of
compounds can be expressed as: m[XMgNi4]·n[XNi5]; i.e.,
Xn+mMgmNi5n+4m where m and n represent numbers of
[XMgNi4] and [XNi5] layers, respectively.21−23
As a prototype of X−Mg−Ni systems, the La−Mg−Ni
system has attracted extensive attention which is motivated for
both fundamental interests and potential applications.24−31
Using density functional theory calculations, Crivello et al.
predicted the structural stability of various compounds with m
= 1 and n = 1, 2, 3 that may be formed in the La−Mg−Ni
© 2014 American Chemical Society
Received: December 18, 2013
Revised: February 10, 2014
Published: February 16, 2014
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ensure good homogeneity and was then annealed at 550 °C for
2 days and subsequently at 850 °C for 3 days under an argon
atmosphere. The as-received sample was crushed mechanically
into powders of 38 μm in a glovebox under a dry argon
atmosphere and was characterized by XRD to identify phase
components. XRD was conducted using a Rigaku D/Max
2500VL/PC diffractometer with Cu Kα radiation at 50 kV and
150 mA.
The XRD pattern of the prepared Ca3Mg2Ni13 sample was
refined by the Rietveld method. The result obtained is
displayed in Figure S1 (Supporting Information) and reveals
that the sample consists of four phases with Ca3Mg2Ni13 as the
major phase and Ca2MgNi9, Ni, and CaO as minor phases. The
relative amounts of each phase are 85, 12, 2, and 1 wt %,
respectively. Moreover, as confirmed in Table S1 (Supporting
Information), crystallographic data obtained for Ca3Mg2Ni13
are in good agreement with previous values reported by us.34
2.2. Pressure−Composition Isotherm. Prior to neutron
diffraction experiments, the pressure−composition isotherm of
the Ca3Mg2Ni13−D2 system was measured at 25 °C and
compared further to that of the Ca3Mg2Ni13−H2 system for
consistency. This measurement was taken by the volumetric
method in a typical Sievert-type apparatus which facilitates
accurate determination of the deuterium D content. Figure 2
Figure 1. Structure of a unit cell for a Ca3Mg2Ni13 compound which
illustrates the stacking of three blocks (A, B, C) along the c axis and
sub-blocks of I and II consisting of [CaNi5] and [CaMgNi4] units.
[CaNi5] for the second block. Ca3Mg2Ni13 demonstrates fast
kinetics but unfavorable thermodynamics for hydrogen storage.
The pressure−composition isotherm for the Ca3Mg2Ni13−H2
system at 25 °C clearly shows that the desorption plateau is
very low, and the desorbed hydrogen is less than the absorbed
hydrogen.34 To determine different phases formed during
desorption and to better understand the thermodynamic
properties of this system, two questions must to be clarified:
(i) what interstitial sites are occupied by hydrogen during the
absorption process, and (ii) which sites are still occupied after
the desorption process? To answer these questions, for this
research, we employed neutron diffraction to study the change
in the Ca3Mg2Ni13 hydride crystal structure during the course
of hydrogen desorption. Because hydrogen is not suitable for
neutron diffraction, deuterium is substituted for hydrogen in
this research.
First, the Ca3Mg2Ni13 compound was fully charged by
deuterium gas and was then transferred to the neutron facility
to determine the crystal structure of the full deuteride,
including the sites occupied by deuterium, site occupancy,
and chemical bonding length. Then the fully charged deuteride
was subjected stepwise to partial dedeuteration and examined
again by neutron diffraction to determine which sites are
occupied or no longer occupied by deuterium. Finally, the
mechanism of deuterium desorption from the Ca3Mg2Ni13
deuteride was discussed on the basis of the structural data
obtained.
Figure 2. Pressure−composition isotherm for the Ca3Mg2Ni13−D2
system at 25 °C; deuterium absorption (■) and deuterium desorption
(□).
illustrates the pressure−composition isotherm where the
composition (i.e., the x-axis) is expressed in terms of two
scales: one scale is the number of D atoms per formula unit of
Ca3Mg2Ni13 (D/f.u.), and the other scale is the weight
percentage of hydrogen content (wt % H) for comparison.
Note that among the three minor phases in the prepared
Ca3Mg2Ni13 sample Ni and CaO do not absorb deuterium or
hydrogen, but Ca2MgNi9 is able to absorb deuterium to form a
deuteride-like La2MgNi9D13.35 Therefore, the amounts of
deuterium absorbed by the sample issue from the two
deuterides of Ca3Mg2Ni13 and Ca2MgNi9. Usually, a layered
X−Mg−Ni compound absorbs deuterium up to a concentration of around 1.0 D/M (the ratio of the number of D atoms
to metal atoms);29 thus, 97% of the measured deuterium shown
in Figure 2 is estimated to belong to the Ca3Mg2Ni13 deuteride.
This estimation is further proven by the following analyses of
the neutron diffraction data.
2. EXPERIMENTAL SECTION
2.1. Sample Preparation. As described elsewhere,34
Ca3Mg2Ni13 was prepared by induction melting of appropriate
amounts of pure metals under an argon atmosphere. Losses of
Ca and Mg due to evaporation were determined from
preliminary experiments, and extra 10 wt % of Ca and 16 wt
% of Mg were therefore added to compensate for losses during
induction melting. The sample was remelted three times to
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values obtained from XRD data (see Table S1 in Supporting
Information). Meanwhile, the structural model for the minor
Ca2MgNi9 deuteride was taken from La2MgNi9D13.35 To limit
the number of refined parameters, the isotropic thermal factors
(B values) for all D atoms were constrained at 1.5 Å2. The
outcomes showed that the influence caused by these constraints
on the structural parameters of the major phase is not
significant.
As illustrated in Figure 3, the calculated diffraction pattern of
Sample 1 fits the observed diffraction pattern very well. Crystal
Figure 2 confirms that the deuterium isotherm is consistent
with hydrogen absorption and desorption properties of the
Ca3Mg2Ni13−H2 system.34 To extract and compare structural
features at various stages of the deuterium desorption of the
Ca3Mg2Ni13 deuteride, samples for neutron diffraction were
carefully chosen in terms of the measured pressure−
composition isotherm shown in Figure 2.
2.3. Neutron Diffraction. Three deuteride samples,
Ca3Mg2Ni13D15.6, Ca3Mg2Ni13D5.9, and Ca3Mg2Ni13D0.3, corresponding to before, during, and after deuterium desorption at
25 °C were selected for neutron diffraction. The deuterium
pressures in equilibrium with the three deuterides were 2.9720,
0.0022, and 0.0007 MPa, respectively. For clarity, the status of
the three samples was labeled as Sample 1, Sample 2, and
Sample 3 in Figure 2, respectively.
Neutron diffraction patterns were obtained at the Laboratoire Léon Brilloin in Saclay (France) on a 3T2 instrument in
the range of 4.5° < 2θ < 121° by a step of 0.05° (λ = 1.225 Å).
Samples were measured at room temperature using a tight
cylindrical stainless steel container. During neutron diffraction
measurements, samples were under constant deuterium
pressure to avoid any desorption effect. Rietveld analysis was
performed using the RIETAN-2000 program for refinement of
neutron diffraction data.36 For the neutron data refinement,
diffraction peaks originating from the stainless steel container
were excluded. It should be noted that there is 2 wt % of Ni in
the prepared sample and three Ni diffraction peaks in the range
of 4.5−70° which overlap those of major phases, thus when we
tried to refine the profile in the range of 4.5−70°, the calculated
Ni abundance was variable with the structural parameters of the
major phases. This suggests that neither the Ni abundance nor
the structural parameters of the main phase could be
determined accurately if the neutron data in the range of
4.5−70° were used for refinement. Fortunately, the diffraction
data of Ni exceeding 70° had good resolution, thus the data
within the range of 4.5−110° were used for the refinement.
Figure 3. Rietveld refinement of the observed power neutron
diffraction pattern for the Ca3Mg2Ni13D15.6 sample. Reflection markers
(from above) are for Ca3Mg2Ni13D15.6 (85 wt %), Ca2MgNi9D13 (12
wt %), Ni (2 wt %), and CaO (1 wt %) phases, respectively.
structural data obtained for the Ca3Mg2Ni13 deuteride are given
in Table 1 with lattice parameters of a = 5.2974(8) Å and c =
39.320(6) Å. Thirteen interstitial sites (denoted as D1, D2, D3,
..., D13) are partially filled by D atoms. The Ca3Mg2Ni13
deuteride crystal structure is illustrated in Figure 4 where the
following features can be seen: (i) D atoms occupy both
[CaNi5] and [CaMgNi4] units. Within the [CaNi5] unit, two
sites (tetrahedral Ni4 [D1 site] and tetragonal pyramidal Ca2Ni3
[D2 site]) are occupied which is in agreement with the
reported La2MgNi9D13 crystal structure.35 (ii) D3 and D4
occupy two Ca(Ca/Mg)Ni2 tetrahedra at the [CaNi5]/
[CaMgNi4] border which is defined by the z position of the
Ni4 atomic position (z = 0.445(1)). If the D3 and D4 atoms are
supposedly equally shared between [CaNi5] and [CaMgNi4]
units, each [CaNi5] unit contains 4.8(2) D (i.e., CaNi5D4.8(2))
which is identical to the deuterium quantity in β-CaNi5D4.8,
although the deuterium distribution is somehow different.40
(iii) Within the [CaMgNi4] unit, D atoms occupy eight sites
(D5 in Ni4, D6 in (Ca/Mg)Ni3, D7 in (Ca/Mg)2Ni2, D8 in
(Ca/Mg)Ni3, D9 in (Ca/Mg)2Ni2, D10 in (Ca/Mg)Ni3, D11 in
(Ca/Mg)2Ni2, and D12 in (Ca/Mg)2Ni2). At the border
between the two [CaMgNi4] units defined by the z atomic
position of the Ni3 atom (z = 0), D13 is located in the (Ca/
Mg)2Ni2 tetrahedron. As a result, each [CaMgNi4] unit has a
deuterium content of 5.4(3) D (i.e., CaMgNi4D5.4(3)). Therefore, based on the sum of the deuterium occupancies of the 13
sites, the deuteride can be described as Ca3Mg2Ni13D15.6(8)
(CaNi5D4.8(2) + 2CaMgNi4D5.4(3)). In this case, the calculated D
content of 15.6(8) D/f.u. agrees with the measured value of
15.8 D/f.u. by the volumetric method (see Figure 2).
Interatomic distances between D atoms and surrounding
metal atoms are listed in Table S3 (Supporting Information).
Ca−D distances are in the range of 2.63−2.70 Å which agrees
fairly well with distances in Ca4Mg3Fe22D22 (2.39−2.76 Å).41
3. RESULTS
3.1. Crystal Structure of Ca3Mg2Ni13D15.6. As revealed by
neutron diffraction, for Mg-free deuterides like La2Ni7D6.5,
Ce2Ni7D4, and CeNi3D2.8, D atoms reside only in [XNi2] units,
not in [XNi5] units.37,38 On the other hand, for Mg-containing
deuterides such as La4MgNi19D21.8, La1.5Mg0.5Ni7D9.1, and
La2MgNi9D13, both [XMgNi4] and [XNi5] units are occupied
by D atoms.29,31,35 Depending on the types of local metal
atoms, various interstitial sites are identified for deuterium
occupation: Ni4, (X/Mg)Ni3, (X/Mg)2Ni2, and (X/Mg)3Ni2 in
[XMgNi4] units and Ni4, XNi3, X2Ni2, and X2Ni4 (or X2Ni3
derived from X2Ni4) in [XNi5] units.29,31,35,39 On the basis of
this structural information and our XRD data in Table S1
(Supporting Information), we have identified 18 sites that
might be available for deuterium occupation in the Ca3Mg2Ni13
structure: five sites (two Ni4, two CaNi3, and one Ca2Ni3 sites)
within [CaNi5] units and ten sites (two Ni4, four (Ca/Mg)Ni3,
and four (Ca/Mg)2Ni2 sites) within [CaMgNi4] units, two
Ca(Ca/Mg)Ni2 sites at the [CaNi5]/[CaMgNi4] border, and
one (Ca/Mg)2Ni2 site at the [CaMgNi4]/[CaMgNi4] border
(see Table S2 in Supporting Information). All these probable
sites have been considered and tested for Rietveld refinement of
Ca3Mg2Ni13 deuteride neutron data. The starting model for the
structure was based on the atomic structure of a Ca3Mg2Ni13
compound derived from XRD analysis. During the refinement,
Ca/Mg occupancy ratios for the two 6c sites were fixed to the
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Table 1. Atomic Coordinates, Isotropic Thermal Parameters (B Values), and Occupation Numbers (g Values) for
Ca3Mg2Ni13D15.6 Refined from Neutron Diffraction Data of Sample 1a
atom
site
g
x
y
z
B (Å2)
Ca1
Ca2/Mg2
Ca3/Mg3
Ni1
Ni2
Ni3
Ni4
D1
D2
D3
D4
D5
D6
D7
D8
D9
D10
D11
D12
D13
3b
6c
6c
6c
6c
9e
18h
6c
18h
18h
18h
6c
6c
18h
18h
18h
18h
36i
36i
18h
1
0.45/0.55b
0.53/0.47
1
1
1
1
0.37(1)
0.56(2)
0.12(1)
0.11(1)
0.08(1)
0.49(2)
0.08(1)
0.14(1)
0.25(1)
0.19(1)
0.20(1)
0.18(1)
0.09(1)
0
0
0
0
0
1/2
0.499(1)
0
0.174(1)
0.851(1)
0.149(1)
0
0
0.783(1)
0.217(1)
0.213(1)
0.787(1)
0.625(1)
0.946(1)
0.851(1)
0
0
0
0
0
0
−x
0
−x
−x
−x
0
0
−x
−x
−x
−x
0.035(1)
0.697(3)
−x
1/2
0.045(1)
0.404(1)
0.163(1)
0.279(1)
0
0.445(1)
0.205(2)
0.847(2)
0.443(2)
0.443(2)
0.680(3)
0.098(2)
0.978(1)
0.978(1)
0.578(2)
0.580(2)
0.038(2)
0.262(2)
0
2.8(3)
1.1(2)
2.9(3)
1.3(1)
2.4(2)
0.3(1)
2.3(2)
1.5c
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
a
Space group R3̅m (No. 166); cell parameters: a = 5.2974(8) Å and c = 39.320(6) Å; Z = 3. Rwp = 2.42%, Rp = 1.93%, RI = 1.02%, S = 1.21. bg values
for Ca2/Mg2 and Ca3/Mg3 were determined by the XRD pattern of Ca3Mg2Ni13 (see Table S1 in Supporting Information). cB values for all D
atoms were constrained as 1.5.
Supporting Information) and can be only partially filled by D
atoms.
3.2. Occupancies of D Sites in Ca3Mg2Ni13D5.9. To
understand the deuterium unloading mechanism from interstitial sites of Ca3Mg2Ni13D15.6, Sample 1 was subjected to
partial desorption. Figure 5 shows the refined diffraction
Figure 4. Crystal structure of Ca3Mg2Ni13D15.6, showing the stacking
of [CaNi5D4.8] and [CaMgNi4D5.4] units. Thirteen interstitial sites are
filled by D atoms; two sites are located within the [CaNi5] unit; two
sites are at the border of [CaNi5] and [CaMgNi4] units; eight sites are
within the [CaMgNi4] unit; and one site is at the border of
[CaMgNi4] and [CaMgNi4] units.
Figure 5. Rietveld refinement of the observed power neutron
diffraction pattern for the Ca3Mg2Ni13D5.9 sample. Reflection markers
(from above) are for phases Ca 3 Mg 2 Ni 13 D 5.9 (65 wt %),
Ca3Mg2Ni13D0.4 (20 wt %), Ca2MgNi9D7.2 (9 wt %), Ca2MgNi9D0.8
(3 wt %), Ni (2 wt %), and CaO (1 wt %), respectively.
pattern of the partially dedeuterated sample (Sample 2 in
Figure 2) which reveals the coexistence of a Ca3Mg2Ni13
deuteride and a deuterium-poor Ca3Mg2Ni13 solid solution.
To limit the number of refined parameters, atomic coordinates
and isotropic thermal parameters for the deuterium-poor solid
solution were fixed to Ca3Mg2Ni13D15.6 parameters (see Table
S5 in Supporting Information). For a minor Ca2MgNi9
deuteride and deuterium-poor Ca2MgNi9 solid solution, atomic
coordinates were fixed to those of La2MgNi9D13,35 and
Ca/Mg−D distances are in the range of 2.03−2.24 Å. Shorter
distances should correspond to Mg−D bonds because they are
close to the Mg−D distance (1.96 Å) observed for MgD2.42
Larger distances are attributed to Ca−D distances since they
are close to the Ca−D distances (2.24−2.63 Å) in CaD2.43 Ni−
D distances (1.61−1.68 Å) are in good agreement with
LaNi5D7 distances (1.53−1.71 Å).44 Note that some neighboring D sites are less than 2.0 Å in distance (see Table S4 in
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Table 2. Atomic Coordinates, Isotropic Thermal Parameters (B Values), and Occupation Numbers (g Values) for
Ca3Mg2Ni13D5.9 Refined from Neutron Diffraction Data of Sample 2a
atom
site
g
x
y
z
B (Å2)
Ca1
Ca2/Mg2
Ca3/Mg3
Ni1
Ni2
Ni3
Ni4
D1
D2
D3
D4
D5
D6
D7
D8
D9
D10
D11
D12
D13
3b
6c
6c
6c
6c
9e
18h
6c
18h
18h
18h
6c
6c
18h
18h
18h
18h
36i
36i
18h
1
0.45/0.55b
0.53/0.47
1
1
1
1
0.13(1)
0.16(1)
0
0
0
0.39(2)
0.05(1)
0.12(1)
0.10(1)
0.08(1)
0.15(1)
0
0
0
0
0
0
0
1/2
0.498(1)
0
0.173(1)



0
0.783(1)
0.217(1)
0.213(1)
0.785(2)
0.625(1)


0
0
0
0
0
0
−x
0
−x



0
−x
−x
−x
−x
0.035(1)


1/2
0.045(1)
0.404(1)
0.165(1)
0.277(1)
0
0.445(1)
0.206(2)
0.847(2)



0.098(2)
0.976(2)
0.977(1)
0.578(2)
0.580(2)
0.038(2)


2.2(2)
3.8(2)
1.0(3)
2.1(3)
2.7(2)
1.7(1)
2.2(2)
1.5c
1.5



1.5
1.5
1.5
1.5
1.5
1.5


a
Space group R3̅m (No. 166); cell parameters: a = 5.1369(5) Å and c = 37.732(4) Å; Z = 3. Rwp = 2.25%, Rp = 1.74%, RI = 3.35%, S = 1.11. bg values
for Ca2/Mg2 and Ca3/Mg3 were determined by the XRD pattern of Ca3Mg2Ni13 (see Table S1 in Supporting Information). cB values for all D
atoms were constrained as 1.5.
Figure 2). The refined diffraction pattern for Sample 3 is shown
in Figure 6, and the deuterium-poor solid solution was
isotropic thermal parameters for metal and D atoms were
constrained as 1 and 1.5 Å2, respectively (see Tables S6 and S7
in Supporting Information). Rietveld refinement confirmed that
Sample 2 consists of Ca 3 Mg 2 Ni 13 D 5.9 (65 wt %),
Ca 3Mg2Ni13D0.4 (20 wt %), Ca 2MgNi9D7.2 (9 wt %),
Ca2MgNi9D0.8 (3 wt %), Ni (2 wt %), and CaO (1 wt %).
Hence, the deuterium content in the sample can be calculated
as 0.54 wt % H (here the D weight content is converted into
the H weight content for the purpose of comparison) which is
close to the measured value of 0.68 wt % H by the volumetric
method outlined in Figure 2.
Ca3Mg2Ni13D5.9 crystallographic parameters are listed in
Table 2. Compared to the initial Ca3Mg2Ni13D15.6, the
following changes in D occupancies are observable: (i) D3
and D4 sites in the tetrahedron Ca(Ca/Mg)Ni2 at the [CaNi5]/
[CaMgNi4] border and D13 atoms in the tetrahedron (Ca/
Mg)2Ni2 at the [CaMgNi4]/[CaMgNi4] border are empty; i.e.,
D atoms are completely released from these sites; (ii) D
occupancies at sites within [CaNi5] and [CaMgNi4] units are
generally reduced which indicates the partial release of D atoms
from these sites; (iii) the refined D content can be expressed as
Ca3Mg2Ni13D5.9(5) (CaNi5D1.22(8) + 2CaMgNi4D2.34(20)) which
indicates that nearly 40% of D atoms remain in interstices of
[CaNi5] and [CaMgNi4] units relative to Ca3Mg2Ni13D15.6.
Interatomic distances between D atoms and surrounding
metal atoms in Ca3Mg2Ni13D5.9 are listed in Table S8
(Supporting Information). Ca−D, Ca/Mg−D, and Ni−D
distances are 2.62, 1.96−2.12, and 1.54−1.63 Å, respectively.
These values are very close to the individual values of CaD2 or
MgD2 deuterides, which suggests that these sites are energetically favorable for the D atoms.
3.3. Occupancies of D Sites in Ca3Mg2Ni13D0.3. To
determine which sites are still occupied in the structure of the
deuterium-poor Ca3Mg2Ni13 solid solution, Sample 2 was
subjected to additional desorption to obtain Sample 3 (see
Figure 6. Rietveld refinement of the observed power neutron
diffraction pattern for the Ca3Mg2Ni13D0.3 sample. Reflection markers
(from above) are for Ca3Mg2Ni13D0.3 (85 wt %), Ca2MgNi9D0.7 (12 wt
%), Ni (2 wt %), and CaO (1 wt %) phases, respectively.
determined to be Ca3Mg2Ni13D0.3(1). Sample 3 is composed
of Ca3Mg2Ni13D0.3 (85 wt %), Ca2MgNi9D0.7 (12 wt %), Ni (2
wt %), and CaO (1 wt %); thus, the total amount of deuterium
in the sample can be estimated at around 0.04 wt % H which
complies with the measured value by the volumetric method in
Figure 2.
Table 3 contains structural data for Ca3Mg2Ni13D0.3 which
indicate that a few D atoms remain in the tetrahedral Ni4 and
tetragonal pyramidal Ca2Ni3 sites within [CaNi5] units. Similar
deuterium occupation is also observed in the minor phase
Ca2MgNi9D0.7 (see Table S9 in Supporting Information). In
Ca3Mg2Ni13D0.3, the Ca−D (2.55 Å) and Ni−D (1.50−1.57 Å)
distances (see Table S10 in Supporting Information) further
suggest that the Ni4 and Ca2Ni3 interstices within the [CaNi5]
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Article
Table 3. Atomic Coordinates, Isotropic Thermal Parameters (B Values), and Occupation Numbers (g Values) for
Ca3Mg2Ni13D0.3 Refined from Neutron Diffraction Data of Sample 3a
atom
site
g
x
y
z
B (Å2)
Ca1
Ca2/Mg2
Ca3/Mg3
Ni1
Ni2
Ni3
Ni4
D1
D2
3b
6c
6c
6c
6c
9e
18h
6c
18h
1
0.45/0.55b
0.53/0.47
1
1
1
1
0.05(1)
0.03(1)
0
0
0
0
0
1/2
0.499(1)
0
0.174(1)
0
0
0
0
0
0
−x
0
−x
1/2
0.044(1)
0.404(1)
0.164(1)
0.278(1)
0
0.443(1)
0.206(1)
0.849(2)
2.0(3)
0.4(1)
1.1(1)
0.5(1)
1.0(1)
1.0(1)
0.9(1)
1.5c
1.5
a
Space group R3̅m (No. 166); cell parameters: a = 4.9762(4) Å and c = 36.358(6) Å; Z = 3. Rwp = 3.52%, Rp = 2.68%, RI = 3.29%, S = 1.64. bg values
for Ca2/Mg2 and Ca3/Mg3 were determined by the XRD pattern of Ca3Mg2Ni13 (see Table S1 in Supporting Information). cB values for all D
atoms were constrained as 1.5.
Table 4. Lattice Expansions of Ca3Mg2Ni13D15.6, Ca3Mg2Ni13D5.9, and Ca3Mg2Ni13D0.3
a (Å)
c (Å)
V (Å3)
Δa/a (%)
Δc/c (%)
ΔV/V (%)
c[CaNi5] (Å)
c[CaMgNi4] (Å)
Δc/c[CaNi5] (%)
Δc/c[CaMgNi4] (%)
V[CaNi5] (Å3)
V[CaMgNi4] (Å3)
ΔV/V[CaNi5] (%)
ΔV/V[CaMgNi4] (%)
Ca3Mg2Ni13
Ca3Mg2Ni13D15.6
Ca3Mg2Ni13D5.9
Ca3Mg2Ni13D0.3
4.9615(5)
36.066(3)
768.9(1)
5.2974(8)
39.320(6)
955.6(2)
6.8
9.0
24.3
4.33 (3)
4.38(2)
7.4
9.6
105.2(5)
106.5(5)
22.4
25.0
5.1369(5)
37.732(4)
862.3(2)
3.5
4.6
12.1
4.15(3)
4.21(2)
2.9
5.4
94.8(5)
96.2(5)
10.3
13.0
4.9762(4)
36.358(6)
779.7(2)
0.3
0.8
1.4
4.14(3)
3.99(2)
2.6
−0.1
88.8(5)
85.6(5)
3.3
0.5
4.032(7)
3.995(4)
85.96(1)
85.17(1)
[CaMgNi4]. However, this deuterium distribution is somewhat
different from the case of individual (Ca0.67Mg0.33)Ni2D2
deuteride where only (Ca/Mg)Ni3 and (Ca/Mg)2Ni2 tetrahedra are occupied.46 The difference may be caused by the
constraint on the [CaMgNi4] unit coming from the adjacent
[CaNi5] unit in Ca3Mg2Ni13D15.6. Such constraint is necessary
to adjust the Ni4 tetrahedra size suitable for D occupation.
D-occupied sites in the [CaNi5] unit of Ca3Mg2Ni13D15.6 are
identical to the [LaNi5] unit of La2MgNi9D1335 but are
somewhat different from the [LaNi5] unit in La4MgNi19D21.8
and La1.5Mg0.5Ni7D9.129,31 which may be related to the number
of [XNi5] units. For Ca3Mg2Ni13D15.6 and La2MgNi9D13, there
is one [XNi5] unit per block, whereas La4MgNi19D21.8 has three
adjacent [LaNi5] units per block and La1.5Mg0.5Ni7D9.1 two
adjacent [LaNi5] blocks. Likewise, the difference in D
distribution in their [CaMgNi4] units may also be linked to
the number of [XMgNi4] units. Ca3Mg2Ni13D15.6 has two
adjacent [CaMgNi4] units per block, but La4MgNi19D21.8,
La1.5Mg0.5Ni7D9.1, and La2MgNi9D13 have only one [XMgNi4]
unit per block.29,31,35
4.2. Mechanism of Deuterium Desorption. The analysis
of the D-site occupancies in Ca3Mg2Ni13D5.9 clearly shows that
D3 and D4 sites at the [CaNi5]/[CaMgNi4] border and the
D13 site at the [CaMgNi4]/[CaMgNi4] border are fully
released (see Table 2) which suggests that D atoms at unit
borders are released first during deuterium desorption. This
result may be related to the strain caused by the mismatch
between two adjacent units.29 Moreover, compared to
units are suitable for D atom accommodation. Note that the D
atom distribution in the Ca3Mg2Ni13D0.3 is different from
La1.64Mg0.36Ni7D0.56 where D atoms occupy only the LaNi3
tetrahedron within [LaMgNi4] units.44 This difference in D
distribution may be related to the interstice size that is strongly
dependent on Mg/X ratios in [XMgNi4] units (with 1:1 for
Ca3Mg2Ni13D0.3 and 9:16 for La1.64Mg0.36Ni7D0.56) and atomic
radii of X.45
4. DISCUSSION
4.1. Deuterium Distribution in Subunits of
Ca3Mg2Ni13D15.6. Although Mg atoms reside only in the
[CaMgNi4] unit in Ca3Mg2Ni13,34 cell parameters of both
[CaMgNi4] and the [CaNi5] units decreased with the
introduction of Mg atoms compared to CaNi5 and (Ca,Mg)Ni2
compounds. Hence, site occupancies of D atoms in [CaNi5]
and [CaMgNi4] units of Ca3Mg2Ni13D15.6 are distinct from
units of CaNi5 and (Ca,Mg)Ni2 compounds, respectively. For
Ca3Mg2Ni13D15.6, D atoms occupy Ni4 and Ca2Ni3 sites within
the [CaNi5] unit and two Ca(Ca/Mg)Ni2 tetrahedra at the
[CaNi5]/[CaMgNi4] border and thus reach a deuterium
content of 4.8 D/[CaNi5]. In a binary CaNi5 compound, D
atoms initially occupy octahedral Ca2Ni4 and tetrahedral
Ca2Ni2 sites to form a solid solution CaNi5D0.3. Further
deuterium absorption leads to structural transitions from α′, β,
to γ phase up to a deuterium content of 6.1 D/f.u.40 Similarly,
D atoms occupy the Ni4, (Ca/Mg)Ni3, and (Ca/Mg)2Ni2
tetrahedra in the [CaMgNi4] unit up to a content of 5.4 D/
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The Journal of Physical Chemistry C
■
Ca3Mg2Ni13D15.6, all D site occupancies within [CaNi5] and
[CaMgNi4] units are decreased in Ca3Mg2Ni13D5.9 which
implies a simultaneous deuterium desorption from both
[CaNi5] and [CaMgNi4] units and corresponds to the
accommodation of mismatch between two units with a same
a-axis length. The deuterium content is significantly reduced by
9.7 D/f.u. as Ca3Mg2Ni13D15.6 transforms into Ca3Mg2Ni13D5.9,
which means that a large solid solution domain exists between
the two deuterides Ca3Mg2Ni13D5.9 and Ca3Mg2Ni13D15.6 and
results in an increased branch in the pressure−composition
isotherm.
With further desorption, Ca3Mg2Ni13D5.9 directly transforms
into a hydrogen-poor solid solution phase in the two-phase
coexistence state. However, the two-phase region of the
pressure−composition isotherm is very narrow (about 5.6 D/
f.u.) in the Ca3Mg2Ni13−D2 system. The relatively narrow twophase region together with a relatively wide single-deuteride
domain is identical to the scenario as previously reported for
the NdNi4Mg−H2 system.47 As Ca3Mg2Ni13D5.9 transforms
totally into Ca3Mg2Ni13D0.3, D atoms occupy Ni4 and Ca2Ni3
sites in the [CaNi5] units only (see Table 3).
4.3. Changes in Lattice Expansion with Deuterium
Desorption. Since D atoms are distributed in both [CaMgNi4]
and [CaNi5] units, Ca3Mg2Ni13D15.6 shows a rather isotropic
lattice expansion (Δa/a = 6.8%, Δc/c = 9.0%, and ΔV/V =
24.3%) (see Table 4). In addition, the [CaNi5] unit lattice
expansion is smaller than the [CaMgNi4] unit lattice expansion.
Considering that the a-axis expansion is constrained to be the
same for the [CaNi5] and [CaMgNi4] units, slight differences in
expansion and D content may be related to the accommodation
of the mismatch between the two units.
When Ca3Mg2Ni13D15.6 is dedeuterated into Ca3Mg2Ni13D5.9
and the deuterium-poor solid solution, expansions of [CaNi5]
and [CaMgNi4] units simultaneously are smaller but are still
isotropic. When the two phases transform into the solid
solution phase, Ca3Mg2Ni13D0.3, the volume expansion occurs
mainly in the [CaNi5] units (ΔV/V[CaNi5] = 3.3%). The
expansion along the c-axis (Δc/c[CaNi5] = 2.6%) is larger than
the expansion along the a-axis (Δa/a[CaNi5] = 0.3%) which may
be related to the constraint of the adjacent [CaMgNi4] units.
However, this small expansion along the a-axis still causes a
slight volume expansion (ΔV/V[CaMgNi4] = 0.5%) of the
[CaMgNi4] units, even though there is no deuterium
occupation in this unit and the c-axis (3.99(2) Å) maintains
nearly the same value (3.995(4) Å) as the initial compound,
Ca3Mg2Ni13, before deuterium absorption.
Article
ASSOCIATED CONTENT
S Supporting Information
*
Crystallographic parameters of Ca3Mg2Ni13 in as-prepared
alloys, Ca3Mg2Ni13D0.4, Ca2MgNi9D7.2, and Ca2MgNi9D0.8 in
Sample 2, and Ca2MgNi9D0.7 in Sample 3; and interatomic
distances of the occupied interstices in Ca3Mg2Ni13D15.6,
Ca3Mg2Ni13D5.9, and Ca3Mg2Ni13D0.3 structures. This material
is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail: dlsun@fudan.edu.cn (D.S.).
*E-mail: memzhu@scut.edu.cn (M.Z.).
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work was financially supported by the National Natural
Science Foundation of China (Nos. 50925102 and 51271002)
and the Ministry of Science and Technology of China (No.
2010CB631302). M.L. and J.Z. wish to thank Mrs F. Porcher
for her assistance in neutron data acquisition at LLB (Saclay,
France).
■
REFERENCES
(1) Li, Y.; Zhou, G.; Fang, F.; Yu, X.; Zhang, Q.; Ouyang, L.; Zhu,
M.; Sun, D. De-/re-hydrogenation Features of NaAlH4 Confined
Exclusively in Nanopores. Acta Mater. 2011, 59, 1829−1838.
(2) Liu, D.; Liu, Q.; Si, T.; Zhang, Q.; Fang, F.; Sun, D.; Ouyang, L.;
Zhu, M. Superior Hydrogen Storage Properties of LiBH4 Catalyzed by
Mg(AlH4)2. Chem. Commun. 2011, 47, 5741−5743.
(3) Wolf, G.; Baumann, J.; Baitalow, F.; Hoffmann, F. P. Calorimetric
Process Monitoring of Thermal Decomposition of B−N−H
Compounds. Thermochim. Acta 2000, 343, 19−25.
(4) Xiong, Z.; Yong, C. K.; Wu, G.; Chen, P.; Shaw, W.; Karkamkar,
A.; Autrey, T.; Jones, M. O.; Johnson, S. R.; Edwards, P.; et al. HighCapacity Hydrogen Storage in Lithium and Sodium Amidoboranes.
Nat. Mater. 2007, 7, 138−141.
(5) Wu, H.; Zhou, W.; Yildirim, T. Alkali and Alkaline-Earth Metal
Amidoboranes: Structure, Crystal Chemistry, and Hydrogen Storage
Properties. J. Am. Chem. Soc. 2008, 130, 14834−14839.
(6) Zhang, Q.; Tang, C.; Fang, C.; Fang, F.; Sun, D.; Ouyang, L.;
Zhu, M. Synthesis, Crystal Structure, and Thermal Decomposition of
Strontium Amidoborane. J. Phys. Chem. C 2010, 114, 1709−1714.
(7) Schlapbach, L.; Zuttel, A. Hydrogen-Storage Materials for Mobile
Applications. Nature 2001, 414, 353−358.
(8) Kadir, K.; Sakai, T.; Uehara, I. Synthesis and Structure
Determination of a New Series of Hydrogen Storage Alloys;
RMg2Ni9 (R=La, Ce, Pr, Nd, Sm and Gd) Built from MgNi2 LavesType Layers Alternating with AB5 Layers. J. Alloys Compd. 1997, 257,
115−121.
(9) Kadir, K.; Kuriyama, N.; Sakai, T.; Uehara, I.; Eriksson, L.
Structural Investigation and Hydrogen Capacity of CaMg2Ni9: A New
Phase in the AB2C9 System Isostructural with LaMg2Ni9. J. Alloys
Compd. 1999, 284, 145−154.
(10) Liang, G.; Schulz, R. Phase Structures and Hydrogen Storage
Properties of Ca−Mg−Ni Alloys Prepared by Mechanical Alloying. J.
Alloys Compd. 2003, 356−357, 612−616.
(11) Kadir, K.; Sakai, T.; Uehara, I. Structural Investigation and
Hydrogen Capacity of YMg2Ni9 and (Y0.5Ca0.5)(MgCa)Ni9: New
Phases in the AB2C9 System Isostructural with LaMg2Ni9. J. Alloys
Compd. 1999, 287, 264−270.
(12) Kohno, T.; Yoshida, H.; Kawashima, F.; Inaba, T.; Sakai, I.;
Yamamoto, M.; Kanda, M. Hydrogen Storage Properties of New
Ternary System Alloys: La2MgNi9, La5Mg2Ni23, La3MgNi14. J. Alloys
Compd. 2000, 311, L5−L7.
5. CONCLUSIONS
Neutron diffraction studies of Ca3Mg2Ni13D15.6 confirmed that
D atoms occupy the interstitial sites of [CaNi5] and [CaMgNi4]
units as well as sites at unit borders. After deuterium
desorption, D atoms located at the unit border are first
released. D atoms in [CaNi5] and [CaMgNi4] units are then
simultaneously released which reveals the coexistence of
Ca3Mg2Ni13D5.9 and a deuterium-poor solid solution phase.
With further desorption, the Ca3Mg2Ni13D5.9 transforms into
Ca3Mg2Ni13D0.3 where D atoms are located only in the [CaNi5]
units. These findings provide not only important information
on the mechanism of hydrogen desorption but also some useful
hints for tailoring intermetallic hydride properties with layered
features in crystal structures.
4632
dx.doi.org/10.1021/jp412378r | J. Phys. Chem. C 2014, 118, 4626−4633
The Journal of Physical Chemistry C
Article
(33) Lemort, L.; Latroche, M.; Knosp, B.; Bernard, P. Elaboration
and Characterization of New Pseudo-Binary Hydride-Forming Phases
Pr1.5Mg0.5Ni7 and Pr3.75Mg1.25Ni19: A Comparison to the Binary Pr2Ni7
and Pr5Ni19 Ones. J. Phys. Chem. C 2011, 115, 19437−19444.
(34) Zhang, Q. A.; Pang, G.; Si, T. Z.; Liu, D. M. Crystal Structure
and Hydrogen Absorption-Desorption Properties of Ca3Mg2Ni13. Acta
Mater. 2009, 57, 2002−2009.
(35) Denys, R. V.; Yartys, V. A.; Webb, C. J. Hydrogen in
La2MgNi9D13: The Role of Magnesium. Inorg. Chem. 2012, 51, 4231−
4238.
(36) Izumi, F.; Ikeda, T. A Rietveld-Analysis Programm RIETAN−98
and its Applications to Zeolites. Mater. Sci. Forum 2000, 321−324,
198−205.
(37) Yartys, V. A.; Riabov, A. B.; Denys, R. V.; Sato, M.; Delaplane,
R. G. Novel Intermetallic Hydrides. J. Alloys Compd. 2006, 408−412,
273−279.
(38) Filinchuk, Y. E.; Yvon, K.; Emerich, H. Tetrahedral D Atom
Coordination of Nickel and Evidence for Anti-isostructural Phase
Transition in Orthorhombic Ce2Ni7D∼4. Inorg. Chem 2007, 46, 2914−
2920.
(39) Latroche, M.; Paul-Boncour, V.; Percheron-Guégan, A.
Structural Properties of Two Deuterides LaY2Ni9D12.8 and CeY2Ni9D77
Determined by Neutron Powder Diffraction and X-ray Absorption
Spectroscopy. J. Solid State Chem. 2004, 177, 2542−2549.
(40) Yoshikawa, A.; Uyenishi, Y.; Iizumi, H.; Matsumoto, T.; Takano,
N.; Terasaki, F. Determination of the Position and Occupancy of
Deuterium in CaNi5 Deuterides by Neutron Diffraction. J. Alloys
Compd. 1998, 280, 204−208.
(41) Huang, B.; Yvon, K.; Fischer, P. Ca4Mg4Fe3H22, a New
Quaternary Transition Metal Hydride Containing Octahedral
[FeH6]4− Complex Anions. J. Alloys Compd. 1992, 190, 65−68.
(42) Zachariasen, W. H.; Holley, C. E.; Stamper, J. F. Neutron
Diffraction Study of Magnesium Deuteride. Acta Crystallogr. 1963, 16,
352−353.
(43) Andresen, A. F.; Maeland, A. J.; Slotfeldt-Ellingsen, D. Calcium
Hydride and Deuteride Studied by Neutron Diffraction and NMR. J.
Solid State Chem. 1977, 20, 93−101.
(44) Guzik, M. N.; Hauback, B. C.; Yvon, K. Hydrogen Atom
Distribution and Hydrogen Induced Site Depopulation for the
La2‑xMgxNi7−H System. J. Solid State Chem. 2012, 186, 9−16.
(45) Denys, R. V.; Yartys, V. A. Effect of Magnesium on the Crystal
Structure and Thermodynamics of the La3‑xMgxNi9 Hydrides. J. Alloys
Compd. 2011, 509S, S540−S548.
(46) Nakamura, Y.; Kim, H.; Yamazaki, S.; Sakaki, K.; Proffen, T.;
Akiba, E. Structural Studies of Hydrogen Storage Alloys Using X-ray/
Neutron Diffraction and Total Scattering. In Proceedings of 2011 MRS
Spring Meeting; San Francisco, CA, April 25−29, 2011; Materials
Research Society: Warrendale, PA, 2011.
(47) Guenee, L.; Favre-Nicolin, V.; Yvon, K. Synthesis, Crystal
Structure and Hydrogenation Properties of the Ternary Compounds
LaNi4Mg and NdNi4Mg. J. Alloys Compd. 2003, 348, 129−137.
(13) Zhang, J.; Zhou, G.; Chen, G.; Latroche, M.; Percheron-Guégan,
A.; Sun, D. Relevance of Hydrogen Storage Properties of ANi3
Intermetallics (A = La, Ce, Y) to the ANi2 Subunits in Their Crystal
Structures. Acta Mater. 2008, 56, 5388−5394.
(14) Zhang, J.; Fang, F.; Zheng, S.; Zhu, J.; Chen, G.; Sun, D.;
Latroche, M.; Percheron-Guégan, A. Hydrogen-Induced Phase
Transitions in RNi3 and RY2Ni9 (R = La, Ce) Compounds. J. Power
Sources 2007, 172, 446−450.
(15) Si, T. Z.; Pang, G.; Zhang, Q. A.; Liu, D. M.; Liu, N. Solid
Solubility of Mg in Ca2Ni7 and Hydrogen Storage Properties of
(Ca2‑xMgx)Ni7 Alloys. Int. J. Hydrogen Energy 2009, 34, 4833−4837.
(16) Si, T. Z.; Zhang, Q. A.; Pang, G.; Liu, D. M.; Liu, N. Structural
Characteristics and Hydrogen Storage Properties of Ca3.0‑xMgxNi9 (x =
0.5, 1.0, 1.5 and 2.0) Alloys. Int. J. Hydrogen Energy 2009, 34, 1483−
1488.
(17) Zhang, Q.; Zhao, B.; Fang, M.; Liu, C.; Hu, Q.; Fang, F.; Sun,
D.; Ouyang, L.; Zhu, M. (Nd1.5Mg0.5)Ni7-Based Compounds:
Structural and Hydrogen Storage Properties. Inorg. Chem. 2012, 51,
2976−2983.
(18) Iwase, K.; Sakaki, K.; Nakamura, Y.; Akiba, E. Phase
Transformation and Crystal Structure of La2Ni7Hx Studied by In
Situ X-ray Diffraction. Inorg. Chem. 2010, 49, 8763−8768.
(19) Iwase, K.; Sakaki, K.; Matsuda, J.; Nakamura, Y.; Ishigaki, T.;
Akiba, E. Synthesis and Crystal Structure of a Pr5Ni19 Superlattice
Alloy and Its Hydrogen Absorption-Desorption Property. Inorg. Chem.
2011, 50, 4548−4552.
(20) Iwase, K.; Mori, K.; Hoshikawa, A.; Ishigaki, T. Synthesis of
New Compound Gd5Ni19 with a Superlattice Structure and Hydrogen
Absorption Properties. Inorg. Chem. 2011, 50, 11631−11635.
(21) Buschow, K. H. J.; van Der Goot, A. S. The Crystal Structure of
Rare-Earth Nickel Compounds of the Type R2Ni7. J. Less-Common
Met. 1970, 22, 419−428.
(22) Khan, Y. The Crystal Structure of R5Co19. Acta Crystallogr.
1974, B30, 1533−1537.
(23) Hayakawa, H.; Akiba, E.; Gotoh, M.; Kohno, T. Crystal
Structures of La−Mg−Nix(x = 3−4) System Hydrogen Storage Alloys.
Mater. Trans. 2005, 46, 1393−1401.
(24) Chai, Y. J.; Sakaki, K.; Asano, K.; Enoki, H.; Akiba, E.; Kohno, T.
Crystal Structure and Hydrogen Storage Properties of La−Mg−Ni−
Co Alloy with Superstructure. Scr. Mater. 2007, 57, 545−548.
(25) Escobar, D.; Srinivasan, S.; Goswami, Y.; Stefanakos, E.
Hydrogen Storage Behavior of ZrNi 70/30 and ZrNi 30/70
Composites. J. Alloys Compd. 2008, 458, 223−230.
(26) Miraglia, S.; Girard, G.; Fruchart, D.; Liang, G.; Huot, J.; Schulz,
R. Structural Characterization and Some Hydrogen Absorption
Properties of (MgxCa1‑x)Ni2.6: A New Phase in the Mg−Ca−Ni
System. J. Alloys Compd. 2009, 478, L33−L36.
(27) Chai, Y.; Asano, K.; Sakaki, K.; Enoki, H.; Akiba, E. Phase
Transformation of the La0.7Mg0.3Ni2.8Co0.5-H2 System Studied by In
Situ X-ray Diffraction. J. Alloys Compd. 2009, 485, 174−180.
(28) Ferey, A.; Cuevas, F.; Latroche, M.; Knosp, B.; Bernard, P.
Elaboration and Characterization of Magnesium-Substituted La5Ni19
Hydride Forming Alloys as Active Materials for Negative Electrode in
Ni-MH Battery. Electrochim. Acta 2009, 54, 1710−1714.
(29) Nakamura, J.; Iwase, K.; Hayakawa, H.; Nakamura, Y.; Akiba, E.
Structural Study of La4MgNi19 Hydride by In Situ X-ray and Neutron
Powder Diffraction. J. Phys. Chem. C 2009, 113, 5853−5859.
(30) Zhang, Q.; Fang, M.; Si, T.; Fang, F.; Sun, D.; Ouyang, L.; Zhu,
M. Phase Stability, Structural Transition, and Hydrogen AbsorptionDesorption Features of the Polymorphic La4MgNi19 Compound. J.
Phys. Chem. C 2010, 114, 11686−11692.
(31) Denys, R. V.; Riabov, A. B.; Yartys, V. A.; Sato, M.; Delaplane,
R. G. Mg Substitution Effect on the Hydrogenation Behaviour,
Thermodynamic and Structural Properties of the La2Ni7-H(D)2
System. J. Solid State Chem. 2008, 181, 812−821.
(32) Crivello, J. C.; Zhang, J.; Latroche, M. Structural Stability of ABy
Phases in the (La, Mg)−Ni System Obtained by Density Functional
Theory Calculations. J. Phys. Chem. C 2011, 115, 25470−25478.
4633
dx.doi.org/10.1021/jp412378r | J. Phys. Chem. C 2014, 118, 4626−4633