Hydrogen-induced phase decomposition of Ba7Al13 and the crystal structure of Ba2AlH7

Article abstract of DOI:10.1016/S0925-8388(03)00420-1

The hydrogenation characteristics of Ba₇Al₁₃ have been studied under a hydrogen pressure of 7 MPa at different temperatures for 5 days. Ba₇Al₁₃ reacts with hydrogen to form BaAlH ₅ and Al between 373 and 553 K. When the temperature is in the range from 553 to 603 K, Ba₇Al₁₃ is hydrogenated to Ba ₂AlH₇ and Al. However, further increasing the temperature leads to the hydrogen-induced decomposition of Ba₇Al₁₃ into Ba₂AlH₇ and BaAl₄. The crystal structure of Ba₂AlD₇ was also studied by neutron powder diffraction. Ba₂AlD₇ is isostructural to Sr₂AlD ₇, crystallizing with a monoclinic structure in space group I2/a. The lattice parameters were determined as a=13.197(3) ?, b=10.237(2) ?, c=8.509(2) ?, β=101.290(9)°.

Full text of DOI:10.1016/S0925-8388(03)00420-1

Journal of Alloys and Compounds 361 (2003) 180–186
L
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Hydrogen-induced phase decomposition of Ba7Al13 and the crystal
structure of Ba2AlH7
Q.A. Zhanga, Y. Nakamuraa, K. Oikawab, T. Kamiyamab, E. Akibaa ,
*
aNational Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
bHigh-Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
Received 20 March 2003; accepted 28 March 2003
Abstract
The hydrogenation characteristics of Ba7Al13 have been studied under a hydrogen pressure of 7 MPa at different temperatures for 5
days. Ba7Al13 reacts with hydrogen to form BaAlH5 and Al between 373 and 553 K. When the temperature is in the range from 553 to
603 K, Ba7Al13 is hydrogenated to Ba2AlH7 and Al. However, further increasing the temperature leads to the hydrogen-induced
decomposition of Ba7Al13 into Ba2AlH7 and BaAl4. The crystal structure of Ba2AlD7 was also studied by neutron powder diffraction.
Ba2AlD7 is isostructural to Sr2AlD7, crystallizing with a monoclinic structure in space group I2/a. The lattice parameters were
˚
˚
˚
determined as a513.197(3) A, b510.237(2) A, c58.509(2) A, b5101.290(9)8.
2003 Elsevier B.V. All rights reserved.
Keywords: Hydrogen absorbing materials; Gas–solid reaction; Crystal structure; Neutron diffraction
1. Introduction
that consists of [AlH6] octahedra and [HSr4] tetrahedra
[12]. As the temperature is increased to about 563 K,
Sr2AlH7 decomposes to SrH2, Al and H2.
For the on-board storage of hydrogen in fuel-cell-po-
wered vehicles, it is desirable to develop hydrogen storage
materials which can absorb and desorb a large amount of
hydrogen under moderate conditions of pressure and
temperature. Recently, the alkali metal aluminum hydrides
(alanates) NaAlH4 and LiAlH4 have been studied exten-
sively for this purpose because they have hydrogen con-
tents of 7.4 and 10.5 wt%, respectively [1–6]. It was found
that the kinetic and thermodynamic properties of the
dehydriding process can be improved by the assistance of
surface catalysts [7–10]. Thus alkali metal aluminum
hydrides can be considered as possible candidates for
reversible hydrogen storage.
Alkaline earth metal aluminum hydrides have also
attracted attention recently [11,12]. It was confirmed that
SrAl2 can be hydrogenated to SrAl2H2 at about 463 K.
SrAl2 is a Zintl phase alloy with a three-dimensional Al
network and SrAl2H2 is a Zintl hydride with a two-
dimensional Al–H network [11]. On increasing the tem-
perature to 513 K, SrAl2H2 further absorbs hydrogen to
form Sr2AlH7 and Al. Sr2AlH7 has a new structure type
In the Ba–Al binary system, a BaAl2 intermetallic
compound does not exist, but Ba7Al13 exists in the
equilibrium state [13]. This is a remarkable difference
between the Sr–Al and Ca–Al binary systems. In our latest
work [14], we also found that BaAlH5 can be synthesized
by hydrogenation of Ba7Al13. BaAlH5 is structurally
constructed from [AlH6] octahedra, which share one corner
and form one-dimensional zig-zag chains. Because of this
interesting arrangement of the hydrogen atoms, BaAlH5
has a larger H/M ratio than the most studied alanate
NaAlH4. However, the detailed hydrogenation behavior of
Ba7Al13 has not been clarified. Thus the purpose of the
present study was to obtain more extensive information in
order to understand the hydrogenation characteristics of
Ba7Al13.
2. Experimental
The Ba7Al13 alloy was prepared by arc melting Ba and
Al metals. Before the preparation, the loss of Ba during arc
melting was determined to be about 3 wt%. On the basis of
the stoichiometric amounts of starting materials, an extra 3
*
Corresponding author. Tel./fax: 181-298-61-4541.
E-mail address: e.akiba@aist.go.jp (E. Akiba).
0925-8388/03/$ – see front matter
doi:10.1016/S0925-8388(03)00420-1
2003 Elsevier B.V. All rights reserved.

Q.A. Zhang et al. / Journal of Alloys and Compounds 361 (2003) 180–186
181
wt% of Ba was added to compensate for the loss of Ba
during arc melting. The ingot was remelted four times in
the arc melt furnace to ensure homogeneity. The alloy was
then ground to a powder of particle size smaller than 45
mm. Powder samples of 1–2 g were loaded into stainless
steel containers and placed in stainless steel autoclaves.
Hydrogenation experiments were carried out under a
hydrogen pressure of 7 MPa at different temperatures for 5
days. After hydrogenation, XRD measurements of the
hydrogenated samples were performed using a Rigaku
RINT-2500V diffractometer with Cu Ka radiation at 50 kV
and 200 mA. The XRD profiles were analyzed with the
present case, the lattice parameters were calculated as
a56.1006(2) A and c517.2697(5) A by the Rietveld
method.
˚
˚
Fig. 2a presents the XRD pattern for the sample
hydrogenated at 333 K for 5 days. The diffraction peaks
are only from Ba7Al13 and Ba3Al5, indicating that no other
phase was formed during hydrogenation. However, addi-
tional weak peaks can be seen in the XRD pattern for the
sample hydrogenated at 373 K (Fig. 2b). When the
temperature was increased to 413 K, the diffraction peaks
from the reaction product can clearly be observed and were
indexed to be BaAlH5 and Al, as shown in Fig. 2c. From
these results, we found that Ba7Al13 can react with
hydrogen to form BaAlH5 and Al starting at about 373 K.
The structure of BaAlH5 was studied in our latest work
[14]. It crystallizes with a new orthorhombic structure in
space group Pna21. Using this structure model, we refined
the XRD pattern for the sample hydrogenated at 513 K. As
shown in Fig. 3, the calculated pattern is in good agree-
ment with the measured pattern. The lattice parameters of
Rietveld refinement program RIETAN-97 [15].
A
deuteride sample was prepared under a deuterium pressure
of 7 MPa at 613 K for 10 days. Neutron powder diffraction
data of the deuteride sample were collected at room
temperature using the backward bank of the time-of-flight
(TOF) diffractometer Vega [16] at the Neutron Science
Laboratory of the High Energy Accelerator Research
Organization of Japan. The neutron diffraction profiles
were analyzed by the program RIETAN-2001T [17].
˚
BaAlH5 were calculated to be a59.187(1) A, b57.047(1)
˚
˚
A and c55.1238(4) A, which compare well with those of
BaAlD5 [14]. This sample also contained 3 wt% of
˚
unreacted Ba7Al13 with lattice parameters a56.099(3) A
3. Results
˚
and c517.272(5) A. Compared with those of the as-cast
Fig. 1 shows the calculated and observed XRD patterns
for the Ba7Al13 alloy sample. It can be seen that the
sample contains the major phase Ba7Al13 and an impurity
phase Ba3Al5. We attempted to prepare a single-phase
Ba7Al13 sample, but all efforts were unsuccessful under
the present experimental conditions. From the Rietveld
analysis, we found that the amounts of Ba7Al13 and
Ba3Al5 were about 80 and 20 wt%, respectively. Ba7Al13
sample, the lattice parameters of the unreacted Ba7Al13 do
not change. This indicates that the solubility of hydrogen
in Ba7Al13 is very small and a small amount of hydrogen
absorption readily leads to the decomposition and hydro-
genation of Ba7Al13.
Fig. 4 compares the XRD patterns of the samples
hydrogenated at 553, 603 and 653 K for 5 days. We found
that a second hydride was formed besides BaAlH5, Al and
unreacted Ba7Al13 when the hydrogenation temperature
¯
has trigonal symmetry in space group P3m1 [18]. In the
Fig. 1. Rietveld refinement (———) of the observed XRD pattern (1) for the Ba7Al13 alloy sample. Reflection markers are for Ba7Al13 and Ba3Al5
(from above), respectively. Rwp 514.68%, Rp 511.20%, S53.37.

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Q.A. Zhang et al. / Journal of Alloys and Compounds 361 (2003) 180–186
Fig. 4. XRD patterns for the samples hydrogenated at (a) 553, (b) 603
and (c) 653 K for 5 days.
Fig. 2. XRD patterns for the sample hydrogenated at (a) 333, (b) 373 and
(c) 413 K for 5 days.
Fig. 3. Rietveld refinement (———) of the observed XRD pattern (1) for the sample hydrogenated at 513 K for 5 days. Reflection markers are for
BaAlH5, Al and Ba7Al13 (from above), respectively. Rwp 59.81%, Rp 57.81%, S52.50.

Q.A. Zhang et al. / Journal of Alloys and Compounds 361 (2003) 180–186
183
was 553 K. As the temperature was raised to 603 K,
BaAlH5 disappeared entirely and the second hydride was
observed together with Al and BaAl4. The diffraction
peaks from the second hydride were indexed to be a
monoclinic unit cell isostructural to Sr2AlH7 [12]. Thus we
can conclude that Ba7Al13 is hydrogenated to Ba2AlH7
and Al when the temperature exceeds 553 K. It should be
noted that the increase of temperature resulted in the
precipitation of BaAl4 as well as Al accompanying the
formation of Ba2AlH7 (Fig. 4b). On further increasing the
temperature to 653 K, as shown in Fig. 4c, only BaAl4
precipitated instead of Al, while Ba2AlH7 was formed.
In order to confirm the structure of Ba2AlH7, a deuteride
sample was prepared under a deuterium pressure of 7 MPa
at 613 K for 10 days. The structure refinement was first
carried out from the XRD data of the deuteride sample. For
the refinement, the metal atom coordinates of Sr2AlD7 [12]
were used as a starting model for Ba2AlD7. Fig. 5 shows
the calculated and measured XRD patterns for the
deuteride sample. It can be seen that the calculated pattern
fits the measured pattern very well. The refined Ba and Al
atom coordinates were then fixed during refinement of the
deuterium positions in the neutron diffraction data. The
neutron diffraction data were analyzed by the Rietveld
refinement program RIETAN-2001T [17]. The deuterium
positions were initially located by using those of Sr2AlD7.
The occupation factors of the deuterium positions were
refined to be close to 1 (the differences were smaller than
3s), thus they were fixed at unity during the final
refinement. The observed and calculated neutron powder
diffraction patterns are shown in Fig. 6. Tables 1 and 2 list
the final refinement results and selected inter-atomic
distances of Ba2AlD7, respectively.
Isostructural to Sr2AlD7 [12], Ba2AlD7 crystallizes with
a monoclinic structure in the I2/a (No. 15) space group
(Z58). The lattice parameters were determined to be
˚
˚
˚
a513.197(3) A, b510.237(2) A, c58.509(2) A, b5
101.290(9)8. Similarly, the crystal structure of Ba2AlD7
can be built up from isolated [AlD6] units and infinite
one-dimensional chains of edge-sharing [DBa4] tetrahedra.
The Al–D bond lengths in the [AlD6] octahedra are from
˚
1.67 to 1.85 A, and thus the octahedra are rather distorted
˚
compared with those in Sr2AlD7 (1.71–1.76 A [12]).
However, similar distorted [AlD6] octahedra were also
˚
found in BaAlD5 (1.69–1.85 A [14]). Therefore, we
suggest that the distortion of the [AlD6] octahedra is
related to the larger size of Ba. The Ba–D bond lengths
˚
(2.59–2.86 A) in the [DBa4] tetrahedra of Ba2AlD7 are in
˚
good agreement with those in binary BaD2 (2.57–2.98 A
[19]). As far as we are aware, [DBa4] tetrahedra also exist
in Ba5Ga6D2 [20], but the twisted chain of edge-sharing
[DBa4] tetrahedra is an interesting feature of the present
structure.
4. Discussion
4.1. Decomposition of Ba7 Al13 under hydrogen
atmosphere
As reported above, Ba7Al13 is transformed into BaAlH5
and Al during hydrogenation at 373–553 K. Prior to
decomposition, the lattice parameters of Ba7Al13 do not
change, indicating that only a small amount of hydrogen
can dissolve in it. When the temperature exceeds 553 K,
Fig. 5. Calculated (———) and measured (1) XRD patterns of Ba2AlD7 prepared under a deuterium pressure of 7 MPa at 613 K for 10 days. Reflection
markers are for Ba2AlD7 and BaAl4 (from above), respectively. Rwp 56.47%, Rp 54.50%, S51.72.

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Q.A. Zhang et al. / Journal of Alloys and Compounds 361 (2003) 180–186
Fig. 6. Calculated (———) and measured (1) neutron powder diffraction patterns of Ba2AlD7 prepared under a deuterium pressure of 7 MPa at 613 K
for 10 days. Reflection markers are for Ba2AlD7 and BaAl4 (from above), respectively. Rwp 52.88%, Rp 52.27%, S52.23.
Ba7Al13 is hydrogenated to Ba2AlH7 and Al. Although our
hydrogenation experiments were carried out at constant
temperature, we suggest that the procedure for the hydro-
gen-induced phase decomposition of Ba7Al13 proceeds in
two steps. In the first step starting at about 333 K, Ba7Al13
reacts with hydrogen to form BaAlH5 and Al. As the
temperature is raised to 553 K, BaAlH5 becomes unstable
thermodynamically. Thus the decomposition of BaAlH5
into Ba2AlH7, Al and H2 takes place as the second step.
It is interesting to note that BaAl4 appears together with
Al when Ba2AlH7 is formed at 603 K, as shown in Fig. 4b.
When the temperature increases to 653 K, however, BaAl4
entirely replaces Al during decomposition (Figs. 4c and 5).
These results are probably related to the diffusibility of
metal atoms. Phase decomposition is a process of atom
diffusion and, therefore, temperature is a key factor in
controlling it. The diffusion behavior of metal atoms is
responsible for the decomposition products at different
temperatures.
4.2. Structural comparison of Ba2 AlH7 with fluorides
The structural analogies between hydrides and fluorides
have been studied in a systematic way [21]. First, Messer
[22] found structural analogies between the binary hy-
drides and fluorides of the alkali metals, the alkaline earth
metals and aluminum. He also suggested that ternary
hydrides might be analogous to the corresponding ternary
fluorides, at least in those cases where both binary
hydride–fluoride pairs have similar compositions and
structures. For example, ternary alkaline earth hydrides
Ba2Mg3H10 [23] and Ba6Mg7H26 [24], and alkali–alkaline
earth hydrides KMgH3 and ALiH3 (A5Sr, Ba) [25] are
related to the corresponding fluorides. Recently, an alkali
metal aluminum hydride Na3AlH6 [26] was also found to
be isostructural to Na3AlF6 [27]. So far, however, we have
not found any fluoride analogous to Ba2AlH7. In the
Ba–Al–F system [28], five ternary fluorides, namely
trimorphic BaAlF5, Ba3Al2F12, Ba5Al3F19, polymorphic
Ba3AlF9 and Ba5AlF13, were found, but nobody reported
the compound ‘Ba2AlF7’. Even though ‘Sr2AlF7’ was
reported by Ravez [29], it was recently suggested to be
Sr5Al2F16 with an orthorhombic structure [30]. Moreover,
the existence of Ca2AlF7 (Pnma) [31] and Sr2InF7 (P21 /c)
[32] was confirmed, but their structures are quite different
from that of Ba2AlH7. Therefore, further work is necessary
to clarify whether the structural analogy concept is suitable
for alkaline earth metal aluminum hydrides and fluorides.
Table 1
Atomic coordinates and isotropic thermal parameters for Ba2AlD7 refined
from neutron powder diffraction dataa
2
˚
Atom
Site
g
x
y
z
B (A )
Ba1
Ba2
Al1
D1
D2
D3
D4
D5
D6
D7
a
8f
8f
8f
8f
8f
8f
8f
8f
8f
8f
1
1
1
1
1
1
1
1
1
1
0.3459
0.1084
0.927
0.004(1)
0.846(1)
0.023(1)
0.844(1)
0.983(1)
0.832(1)
0.693(1)
0.5848
0.3247
0.096
0.116(1)
0.974(1)
0.999(2)
0.104(2)
0.249(2)
0.207(1)
0.864(1)
0.3249
0.0852
0.235
0.077(2)
0.135(2)
0.325(2)
0.387(2)
0.324(2)
0.115(2)
0.322(2)
3.1(6)
0.2(4)
2.6(7)
1.6(4)
1.6(5)
2.2(5)
2.9(6)
2.4(5)
0.5(4)
1.5(4)
˚
Space group I2/a (No. 15); cell parameters: a513.197(3) A, b5
3
˚
˚
˚
10.237(2) A, c58.509(2) A, b5101.290(9)8, V51127.2(4) A , Z58;
wp 52.88%, Rp 52.27%, RI 56.28%, S52.23. The Ba and Al atom
coordinates were fixed as those refined from the X-ray diffraction data.
R

Q.A. Zhang et al. / Journal of Alloys and Compounds 361 (2003) 180–186
185
Table 2
Selected inter-atomic distances (A) and angles (deg) in Ba2AlD7
BaAl4. Ba2AlH7 is isostructural to Sr2AlH7, crystallizing
with a monoclinic structure in space group I2/a. The
lattice parameters of its deuteride Ba2AlD7 are a5
13.197(3) A, b510.237(2) A, c58.509(2) A, b5
101.290(9)8.
˚
Ba1–D5
D3
2.48(2)
2.49(1)
2.59(1)
2.70(1)
2.76(1)
2.79(1)
2.79(1)
2.81(1)
2.86(2)
2.86(1)
2.87(1)
3.23(2)
3.73(1)
4.06(2)
D1–Al1
Ba2
1.85(1)
2.54(1)
2.70(1)
2.86(1)
˚
˚
˚
D7
D1
D6
D6
D4
D6
D2
D1
D2
D4
D4
D2
Ba1
Ba1
D2–Al1
Ba2
1.75(1)
2.79(1)
2.86(2)
2.87(1)
3.70(1)
4.06(2)
Ba1
Ba1
Ba2
Ba1
Acknowledgements
This work was carried out as part of the ‘WE-NET’
project. Q.A. Zhang would like to thank the New Energy
and Industrial Technology Department Organization
(NEDO) of Japan for providing the NEDO fellowship.
D3–Al1
Ba1
1.67(2)
2.49(1)
2.70(2)
2.91(2)
Ba2
Ba2
Ba2–D1
D5
2.54(1)
2.61(2)
2.62(1)
2.70(2)
2.72(1)
2.79(1)
2.86(1)
2.91(2)
2.93(2)
2.96(2)
3.64(1)
3.70(1)
D7
D3
D7
D2
D7
D3
D4
D5
D4–Al1
Ba1
1.85(1)
2.79(1)
2.93(2)
3.23(2)
3.64(1)
3.73(1)
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Ba1
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D5
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D7–Ba1
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2.59(1)
2.62(1)
2.72(1)
2.86(1)
D1–Al1–D2
D1–Al1–D3
D1–Al1–D4
D1–Al1–D5
D1–Al1–D6
D2–Al1–D3
D2–Al1–D4
D2–Al1–D5
D2–Al1–D6
D3–Al1–D4
D3–Al1–D5
D3–Al1–D6
D4–Al1–D5
D4–Al1–D6
D5–Al1–D6
95.5(7)
85.6(7)
170.7(6)
88.4(7)
86.6(6)
97.7(7)
89.0(8)
165.7(8)
83.5(6)
101.8(7)
96.3(8)
172.2(7)
85.4(7)
85.9(7)
83.0(7)
Ba2
Ba2
Ba1–D7–Ba2
Ba1–D7–Ba2
Ba1–D7–Ba2
Ba2–D7–Ba2
Ba2–D7–Ba2
Ba2–D7–Ba2
111.9(3)
116.9(3)
103.7(3)
106.1(3)
108.3(3)
109.8(3)
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5. Conclusions
The hydrogen-induced phase decomposition of Ba7Al13
has been studied at different temperatures. Ba7Al13 can
react with hydrogen to form BaAlH5 and Al in the
temperature range between 373 and 553 K. As the
temperature increases to about 553–603 K, Ba7Al13 de-
composes into Ba2AlH7 and Al under a hydrogen atmos-
phere. Further increasing the hydrogenation temperature
leads to the decomposition of Ba7Al13 into Ba2AlH7 and
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