Synthesis, crystal structure and thermal decomposition of LiCa(AlH 4)3

Article abstract of DOI:10.1016/j.jallcom.2012.01.011

Lithium-calcium alanate, LiCa(AlH₄)₃, was synthesized by a mechano-chemically activated reaction of LiAlH₄ with CaCl ₂ in a molar ratio of 3:1. The results of X-ray diffraction (XRD) and Fourier transform infra-red (FTIR) spectroscopy indicated that the reactants converted entirely into LiCa(AlH₄)₃ accompanied with a by-product LiCl after 30 h ball-milling. LiCa(AlH₄)₃ crystallized in a hexagonal structure with space group P6₃/m (No. 176), and with cell parameters a = b = 8.9197(12) and c = 5.8887(7) . Thermal decomposition of LiCa(AlH₄)₃ initiated around 120 °C and proceeded mainly in three steps. The first two steps could be assigned to the formation of CaH₂, LiH and Al, with LiCaAlH₆ as a possible intermediate, and the third step was the further formation of some CaAl intermetallic phases.

Full text of DOI:10.1016/j.jallcom.2012.01.011

Journal of Alloys and Compounds 520 (2012) 202–206
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Synthesis, crystal structure and thermal decomposition of LiCa(AlH4)3
∗
D.M. Liu, Z.X. Qian, T.Z. Si, Q.A. Zhang
School of Materials Science and Engineering, Anhui University of Technology, Maanshan, Anhui 243002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 November 2011
Received in revised form
Lithium–calcium alanate, LiCa(AlH ) , was synthesized by a mechano-chemically activated reaction of
LiAlH4 with CaCl2 in a molar ratio of 3:1. The results of X-ray diffraction (XRD) and Fourier transform infra-
red (FTIR) spectroscopy indicated that the reactants converted entirely into LiCa(AlH4)3 accompanied
4
3
3
0 December 2011
with a by-product LiCl after 30 h ball-milling. LiCa(AlH4)3 crystallized in a hexagonal structure with
Accepted 3 January 2012
Available online 10 January 2012
space group P63/m (No. 176), and with cell parameters a = b = 8.9197(12) and c = 5.8887(7) A˚ . Thermal
◦
decomposition of LiCa(AlH4)3 initiated around 120 C and proceeded mainly in three steps. The first two
steps could be assigned to the formation of CaH2, LiH and Al, with LiCaAlH6 as a possible intermediate,
and the third step was the further formation of some Ca Al intermetallic phases.
Keywords:
Lithium–calcium alanate
Synthesis
©
2012 Elsevier B.V. All rights reserved.
Crystal structure
Dehydrogenation property
1
. Introduction
also investigated [12,34–37]. It was reported that LiMg(AlH ) and
4 3
LiMgAlH6 crystallized in space groups P2 /c and P321, respec-
1
Since NaAlH4 was discovered to reversibly store hydrogen
tively, and LiMgAlH was the intermediate decomposition product
6
under moderate conditions by doping with Ti catalysts [1], alkali
metal alanates have been extensively investigated for hydrogen
storage applications [2–10]. Recently, considerable efforts have
further been carried out to develop alkaline-earth metal alanates
as potentially promising hydrogen storage materials [11–21]. For
example, Mg(AlH ) (∼9.3 wt.% H) and Ca(AlH ) (∼7.9 wt.% H) can
of LiMg(AlH ) [34,35].
4 3
In order to explore the mixed calcium alanates and under-
stand their crystal structures and hydrogen storage properties, our
recent researches focused on the preparation and characterization
of MCa(AlH4)2+n (M: alkali or alkaline-earth metal) alanates. In the
present paper, we report on the mechano-chemical synthesis of
lithium–calcium alanate, LiCa(AlH ) , with a total hydrogen con-
4
2
4 2
be synthesized not only by wet-chemical or mechano-chemical
metathesis reaction of alkali metal alanates with alkaline-earth
4
3
tent of 8.6 wt.%. Furthermore, its crystal structure determined by
XRD and thermal decomposition behavior are introduced.
metal chlorides [11–15] but also by direct ball-milling of AlH with
3
MgH2 or CaH2 hydrides [16]. It was found that Mg(AlH4)2 and
Ca(AlH ) might decompose and release hydrogen in two and four
2
. Experimental details
4
2
steps, respectively [12,14–16], and their dehydrogenation temper-
ature could be reduced by adding catalysts [15,19–21].
2
.1. Sample preparation
Element substitution has been becoming an effective approach
to adjust the de-/hydrogenation thermodynamics of conventional
metal hydrides and novel complex hydrides [22–25]. Several mixed
alanates with two alkali metal cations, such as Na LiAlH , K LiAlH
Commercial LiAlH4 (97%, Alfa Aesar) and anhydrous CaCl2 (99.9%, Alfa Aesar)
powders were used as-received. To synthesize LiCa(AlH4)3, the powders of LiAlH4
and CaCl2 in a molar ratio of 3:1 were mixed manually, and then ball-milled for
1
planetary mill at a rotation speed of 400 rpm, with stainless steel vials (250 ml in
volume) and balls (10 mm in diameter). The ball to powder weight ratio was 30:1.
For comparison, Ca(AlH4)2(+LiCl) was prepared from the mixture of 2LiAlH4 + CaCl2
by the same method described above. For keeping the sample from air-exposure,
all the sample handling was carried out in an Ar-filled glove box equipped with a
purification system, in which the typical O2/H2O levels were below 1 ppm.
0–50 h under argon atmosphere. The ball-milling was performed using a QM-1SP2
2
6
2
6
and K NaAlH , have been synthesized and characterized [26–33].
2
6
Among them, Na LiAlH and K NaAlH take the same cubic ordered
2
6
2
6
¯
perovskite-type structure (space group Fm3m) [27], while K LiAlH
has a hexagonal–rhombohedral structure (space group R3m) [28].
2
6
¯
As an extension research of mixed alanates, LiMg(AlH4)3 and
LiMgAlH with mixed alkali and alkaline-earth metal cations were
2.2. Dehydrogenation property determination
6
The temperature dependence of dehydrogenation was determined by using
a
carefully calibrated Sieverts-type apparatus with volumetric method. After
◦
evacuation, the sample was heated from ambient temperature to 550 C at a
heating rate of 2 C min . The temperature and the pressure changes in the testing
system were digitally recorded; as a result the hydrogen amount desorbed as a
∗ Corresponding author. Tel.: +86 555 2311891; fax: +86 555 2311570.
E-mail address: zhang03jp@yahoo.com.cn (Q.A. Zhang).
◦
−1
0
925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2012.01.011

D.M. Liu et al. / Journal of Alloys and Compounds 520 (2012) 202–206
203
function of temperature can be derived from the gas equation of state modified
by a compressibility factor. The weight of the by-product LiCl was not taken
into account in calculating the dehydrogenation amounts of the LiCa(AlH4)3 and
Ca(AlH4)2 samples. The differential scanning calorimetry (DSC) measurement for
the LiCa(AlH4)3 sample was carried out on a Netzsch STA 409 PC/PG unit under
results of structure analysis (see Section 3.2), the new phase can
be determined to be LiCa(AlH ) . In order to increase the yield
4
3
of LiCa(AlH ) , the ball-milling time was prolonged to 20 h. As
4
3
shown in Fig. 1c, the diffraction peaks from LiCa(AlH4)3 and LiCl
are dominant, while those of LiAlH4 and CaCl2 are very weak in
the XRD pattern. After ball-milling for 30 h, the diffraction peaks
of LiAlH and CaCl disappear completely, and the product is com-
−1
◦
−1
.
argon flow (25 ml min ) at a heating rate of 10 C min
2.3. Structure characterization
4
2
posed of LiCa(AlH ) and the by-product LiCl (see Fig. 1d). Further
4
3
To evaluate the phase structures of the samples, XRD measurements were car-
ried out using a Rigaku D/Max 2500VL/PC diffractometer with Cu K␣ radiation at
0 kV and 200 mA. The XRD samples were loaded and sealed in a special holder
that can keep the samples under argon atmosphere in the course of measurement.
The software program, TREOR90 [38], was firstly used to index the XRD pattern for
LiCa(AlH4)3; the crystal structure of LiCa(AlH4)3 was then determined with the EXPO
program [39,40]. Based on the structural model, the XRD profile was finally refined
by the Rietveld program RIETAN-2000 [41]. FTIR spectra of the LiCa(AlH4)3 and
Ca(AlH4)2 samples (KBr dispersion) were recorded at ambient condition by using
a Nicolet 6700 FTIR spectrometer.
increasing the milling time to 50 h, the XRD pattern (Fig. 1e) is
hardly changed as compared with Fig. 1d. The results above indi-
cate that LiCa(AlH ) can be readily synthesized by ball-milling the
5
4
3
3
LiAlH + CaCl2 mixture, and reaction (1) can proceed completely
4
after 30 h ball-milling.
To further confirm the formation and give more structural
information of LiCa(AlH4)3, FTIR spectrum of the 3LiAlH4 + CaCl2
mixture after 30 h ball-milling was collected and presented in
Fig. 2. For comparison, the FTIR spectrum of the Ca(AlH4)2 sam-
ple prepared by ball-milling 2LiAlH4 + CaCl2 mixture for 30 h is
also given in Fig. 2, which shows a Al H stretching vibration fre-
3
. Results and discussion
−
1
3.1. Synthesis of LiCa(AlH4)3
quency (1800 cm ) agreeing well with that reported elsewhere
(
−
1
1798 cm [19]). It can be seen from Fig. 2 that there is a great
LiCa(AlH4)3 was synthesized by a mechano-chemically acti-
difference between the IR spectra of LiCa(AlH4)3 and Ca(AlH4)2,
due to the distinct chemical environment of Al H bond in these
two compounds. On the one hand, the as-synthesized LiCa(AlH4)3
sample exhibits two active infra-red stretching vibrations of the
vated reaction of LiAlH4 with CaCl , which can be superficially
2
expressed as follows:
3
LiAlH + CaCl → LiCa(AlH ) + 2LiCl
(1)
−1
4
2
4 3
Al H bond (1886 and 1819 cm ). On the other hand, the Al H
stretching vibration frequency for LiCa(AlH4)3 is larger than that
In order to examine the formation process of LiCa(AlH ) , the
4
3
−
1
for Ca(AlH ) . No bands from LiAlH4 (1757 and 1615 cm [42])
3
LiAlH + CaCl mixtures ball-milled for different times were sub-
4 2
4
2
can be detected in the IR spectrum of LiCa(AlH ) sample, implying
jected to XRD measurements, and the results are presented in
Fig. 1. It can be seen that some additional peaks (indicated by solid
square) and the peaks from LiCl emerge in Fig. 1b, indicating the
formation of a new phase together with LiCl when ball-milling the
4 3
that the starting materials, 3LiAlH + CaCl , have reacted entirely
4
2
to form LiCa(AlH ) and LiCl after 30 h ball-milling. Such a result is
4 3
consistent with that obtained from the XRD study.
3
LiAlH + CaCl mixture for 10 h. According to reaction (1) and the
4
2
3.2. Crystal structure of LiCa(AlH4)3
LiCa(AlH4)3 crystallized in a hexagonal structure with space
LiAlH4
LiCl
CaCl2
group P6 /m (No. 176), which is different from those of lithium
3
LiCa(AlH4)3
alanate (LiAlH , monoclinic structure, space group P2 /c [43]), cal-
4
1
cium alanate (Ca(AlH ) , orthorhombic structure, space group Pbca
4
2
[
44]) and lithium–magnesium alanate (LiMg(AlH ) , monoclinic
4 3
(e) BM 50 h
-
ν(AlH )
4
1640
1886
(d) BM 30 h
1819
(a) LiCa(AlH4)3
(c) BM 20 h
(b) Ca(AlH4)2
1800
(b) BM 10 h
(
a) BM 0 h
2
200
2000
1800
11600
1400
-
Wave numbers (cm )
15
25
35
45
55
65
75
Fig. 2. FTIR spectra of (a) LiCa(AlH4)3 sample prepared by ball milling the
Two Theta (degree)
3LiAlH4 + CaCl2 mixture for 30 h and (b) Ca(AlH4)2 sample prepared by ball milling
−
1
the 2LiAlH4 + CaCl2 mixture for 30 h. The band at 1640 cm corresponds to water
bending vibration [20].
Fig. 1. XRD patterns of 3LiAlH4 + CaCl2 mixtures ball-milled for different times.

2
04
D.M. Liu et al. / Journal of Alloys and Compounds 520 (2012) 202–206
Table 1
8
Atomic sites, occupancy, coordinates and isotropic thermal parameters for
LiCa(AlH4)3 refined from X-ray powder diffraction data.
(
a)
(b)
7
6
5
4
3
2
1
0
III
II
(c)
B ( A˚ 2
Atom
Site
g
x
y
z
)
Li
Ca
Al
2a
2d
6h
1
1
1
0
2/3
0.2805(3)
0
1/3
0.9027(4)
1/4
1/4
1/4
1.1(2)
2.6(3)
0.4(4)
Note: Space group P63/m (No. 176); cell parameters a = b = 8.9197(12) and
c = 5.8887(7) A˚ ; Rwp = 7.28%; Rp = 5.48%; S = 1.69. The positions of H atoms were not
determined.
(
a)
(b)
(c)
LiCa(AlH4)3
4
3
2
1
000
000
000
000
0
LiAlH4
Ca(AlH4)2
I
5
0 100 150 200 250 300 350 400 450 500 550
Temperature (°C)
Fig. 5. Hydrogen desorption curves for (a) LiCa(AlH ) ; (b) LiAlH and (c) Ca(AlH4)2
4
3
4
samples (LiCl is eliminated for calculating the dehydrogenation amounts of
LiCa(AlH4)3 and Ca(AlH4)2 samples).
characteristics of Al H, Ca H and Li H bonds in LiCa(AlH ) were
4
3
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75
Two Theta (degree)
unknown yet. Even so, it was still found that the Li Al distance of
A˚ is slightly shorter than 3.214–3.415 A˚ in LiAlH4 [43] and
3.035
.255 A˚ in LiMg(AlH4)3 [34], and the Ca Al distance of 3.774 A˚ is
3
Fig. 3. Rietveld refinement of the XRD pattern for LiCa(AlH4)3 sample prepared
by ball milling the 3LiAlH4 + CaCl2 mixture for 30 h. The vertical bars (from above)
indicate the positions of Bragg diffraction for LiCa(AlH4)3 and LiCl, respectively.
˚
slightly longer than 3.578 A in Ca(AlH ) [44]. Further studies on
4
2
the structure of LiCa(AlD4)3 by means of neutron diffraction are
currently underway.
structure, space group P2 /c [34]). The cell parameters were refined
1
3.3. Dehydrogenation performance of LiCa(AlH4)3
to be a = b = 8.9197(12), c = 5.8887(7) A˚ and the coordinates of non-
hydrogen atoms are listed in Table 1. Fig. 3 shows the observed
and calculated XRD patterns of the LiCa(AlH4)3 sample prepared
by ball-milling the 3LiAlH + CaCl mixture for 30 h. It can be seen
that the refined pattern fits the observed data points very well.
The crystal structure of LiCa(AlH4)3 is illustrated in Fig. 4. Due
to the low sensitive of X-ray to hydrogen, the atomic coordinates
for H atoms were not determined in the present work and the
Fig. 5 gives the temperature-programmed desorption (TPD)
curve for the LiCa(AlH4)3 sample prepared by ball-milling the
LiAlH + CaCl mixture for 30 h. For comparison, the TPD curves for
4
2
3
4
2
LiAlH and Ca(AlH ) are also presented in Fig. 5. It can be seen from
4
4 2
◦
Fig. 5 that LiCa(AlH4)3 starts to release hydrogen around 120 C,
which is similar to that of Ca(AlH ) and is little lower than that
of LiAlH4 (140 C). The thermal dehydrogenation of LiCa(AlH ) is
4
2
◦
4 3
stepwise and can be divided superficially into three steps in the
◦
◦
temperature range of 50–550 C. Step I occurs at about 120–180 C,
with 3.6 wt.% of hydrogen released. The XRD pattern of the product
◦
isothermally dehydrogenated at 150 C is given in Fig. 6b, indicat-
ing that a new phase and Al emerge with the disappearance of
the starting LiCa(AlH ) . Step II starts following Step I and ends
4
3
◦
at about 300 C, releasing 2.4 wt.% of hydrogen. The XRD pattern
see Fig. 6c) of the product isothermally dehydrogenated at 300 C
◦
(
indicates that the characteristic peaks from CaH2 Clx, LiH and Al
−x
are present, but those from the new phase formed in Step I have
disappeared completely. Here, the formation of CaH2 Clx may be
−x
the result of that H in CaH2 was partially substituted by Cl [45].
By analogy with the thermolysis of LiMg(AlH4)3 and alkali metal
alanates [12,33,34], reactions (2) and (3) can be presumed to have
happened in Steps I and II, respectively:
LiCa(AlH ) → LiCaAlH + 2Al + 3H
(2)
(3)
4
3
6
2
LiCaAlH → CaH + LiH + Al + 3/2H
6
2
2
According to reactions (2) and (3), the ratio of hydrogen amount
desorbed in Step II to Step I should be 2, which is somewhat higher
than the measured one (1.5). The reason for the difference in the
theoretical and experimental values can be attributed to that reac-
tion (2) is not completely finished in Step I.
Fig. 4. Crystal structure of LiCa(AlH4)3 compound. The Li, Ca and Al atoms are,
respectively, represented as green, red and yellow spheres. The positions of H atoms
were not determined. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of the article.)

D.M. Liu et al. / Journal of Alloys and Compounds 520 (2012) 202–206
205
(
the second DSC peak) and Step III (the third and fourth DSC peaks)
LiCa(AlH4)
3
LiCl
Al
LiH
are endothermic. The results imply that reaction (2) is not likely
to occur reversibly, but that reactions (3) and (4) should be ther-
modynamically reversible. Meanwhile, the post-TPD sample was
New phase
CaH Cl
Al Ca
Al Ca
2
-x
x
2
4
◦
subjected to hydrogenation at 300 C under 3 MPa. Only 1.3 wt.% of
(d) 500 °C
hydrogen could be reabsorbed, indicating the poor reversibility of
LiCa(AlH ) .
4
3
4. Conclusions
(c) 300 °C
In this paper, the synthesis process, crystal structure and ther-
mal decomposition of LiCa(AlH ) were studied by means of XRD, IR
4
3
as well as TPD analysis. The results indicated that LiCa(AlH ) could
4
3
be successfully synthesized by ball-milling the 3LiAlH + CaCl mix-
4
2
ture, and that 30 h of milling duration was sufficient to make
the reactants convert entirely into LiCa(AlH4)3 and LiCl. The
structure investigation showed that LiCa(AlH4)3 had a hexagonal
(b) 150 °C
structure in space group P6 /m (No. 176), with cell parameters
3
a = b = 8.9197(12) and c = 5.8887(7) A˚ . Hydrogen started to release
◦
(a) RT
from LiCa(AlH4)3 at about 120 C. The first two dehydrogenation
steps of LiCa(AlH4)3 could be assigned to the formation of CaH2,
LiH and Al, releasing 6.0 wt.% of hydrogen with LiCaAlH as a pos-
6
sible intermediate, and the third step was the further formation of
some Ca Al intermetallic phases.
15
25
35
45
55
65
75
Two Theta (degree)
Fig. 6. XRD patterns of LiCa(AlH4)3 + 2LiCl mixtures isothermally dehydrogenated
for 2 h at different temperatures.
Acknowledgements
This work is financially supported by the National Natural
Science Foundation of China (No. 50901001) and the Scientific
Research Foundation of Education Department of Anhui Province
of China (No. KJ2010A044).
◦
Step III proceeds in the temperature range of 300–510 C, and
releases 1.8 wt.% of hydrogen. As shown in Fig. 6d, the product
isothermally dehydrogenated at 500 C is composed of Al Ca, Al Ca,
◦
4
2
LiH and the by-product LiCl. Thus, the third-step dehydrogenation
for LiCa(AlH ) can be reasonably expressed as follows:
4
3
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CaH + 3Al → 1/2Al Ca + 1/2Al Ca + H
(4)
2
4
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