Comparative studies of the decomposition of alanates followed by in situ XRD and DSC methods

Article abstract of DOI:10.1016/j.jallcom.2005.09.004

The decomposition of various alkali and alkaline earth complex alanates and the formation of intermediate compounds was studied by in situ X-ray high-temperature diffraction and differential scanning calorimetry (DSC) experiments. Differences of the reaction pathways during thermolysis for the alkali and the alkaline earth aluminum hydrides were determined. During the thermolysis of Mg(AlH₄)₂ and Ca(AlH₄)₂, the appearance of metal hydrides in combination with alloys was observed, whereas for the alkali alanates LiAlH₄ and KAlH₄, intermediate aluminum hydrides but no alloys are formed. For the alkali salt-containing KAlH₄ systems a strong influence due to the presence of salts on the decomposition temperatures are observed. In addition, the decomposition temperatures are also significantly influenced by the type of salt present. For the first time, the decomposition of the LiMg(AlH₄)₃ and Na₂LiAlH₆ systems was studied by in situ methods.

Full text of DOI:10.1016/j.jallcom.2005.09.004

Journal of Alloys and Compounds 416 (2006) 303–314
Comparative studies of the decomposition of alanates followed
by in situ XRD and DSC methods
∗
¨
M. Mamatha, C. Weidenthaler , A. Pommerin, M. Felderhoff, F. Schuth
Max-Planck-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mu¨lheim an der Ruhr, Germany
Received 8 August 2005; received in revised form 2 September 2005; accepted 5 September 2005
Available online 27 October 2005
Abstract
The decomposition of various alkali and alkaline earth complex alanates and the formation of intermediate compounds was studied by in
situ X-ray high-temperature diffraction and differential scanning calorimetry (DSC) experiments. Differences of the reaction pathways during
thermolysis for the alkali and the alkaline earth aluminum hydrides were determined. During the thermolysis of Mg(AlH4)2 and Ca(AlH4)2, the
appearance of metal hydrides in combination with alloys was observed, whereas for the alkali alanates LiAlH4 and KAlH4, intermediate aluminum
hydrides but no alloys are formed. For the alkali salt-containing KAlH4 systems a strong influence due to the presence of salts on the decomposition
temperatures are observed. In addition, the decomposition temperatures are also significantly influenced by the type of salt present. For the first
time, the decomposition of the LiMg(AlH4)3 and Na2LiAlH6 systems was studied by in situ methods.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Hydrogen storage materials; Alkali alanates; Alkaline earth alanates
1. Introduction
prominent example being NaAlH4 [2,3]. The LiAlH4 decompo-
sition was studied by in situ X-ray and neutron diffraction inves-
tigations [5,6]. However, we nevertheless decided to include our
results for this system in the present publication. There are two
main reasons for this: firstly, the phase composition of our start-
ing sample was different to the published materials. Secondly,
we were interested in the comparison of different alanates. For
this, the data used for discussion should be measured under the
same experimental conditions which precluded use of the pub-
lished data.
Almost 40 years ago, Dymova et al. proposed a three step
mechanism for the decomposition of KAlH4 established on the
basis of DTA and the measured hydrogen evolution [7,8]. This
mechanism was supported by the study of Morioka et al. [9].
From NMR studies, Tarasov et al. assumed a three step decom-
position [10]. To our knowledge, so far no in situ diffraction
investigations of the decomposition are available.
Detailed studies of the decomposition of NaAlH4 are subject
of several publications [1–4]. The transformation of the alanate
to Na3AlH6, Al, NaH and H2 was investigated not only by ther-
mal analyses but also by in situ X-ray and neutron diffraction
methods. These investigations showed clearly that during the
decomposition phases can appear and disappear which were not
expected from the reaction pathways [3]. Some of these phases
are either metastable and transform during cooling of the sam-
ples, or their temperature window of existence is very small.
The knowledge of all phases occurring during the decomposi-
tion of potential hydrogen storage materials is of great interest
particularly with regard to an optimization of the properties of
those materials. For the analysis of the thermolysis processes of
potential hydrogen storage systems in detail and for the under-
standing why some alanates do not work as a reversible storage
material, in situ X-ray diffraction (XRD) and differential scan-
ning calorimetry (DSC) methods are powerful tools.
Phase formation reactions during the thermolysis of
Ca(AlH4)2 were proposed by Mal’tseva et al. using DTA and
gas evolution measurements [11], but again, diffraction studies
are still lacking.
Up to now, only limited in situ XRD data are available for
alkali and alkaline earth complex aluminum hydrides, the most
The first in situ high-temperature X-ray diffraction studies
of the decomposition of Mg(AlH4)2 were recently published
[12,13]. The comparison of DSC and in situ X-ray data allowed
∗
Corresponding author. Tel.: +49 208 3062181; fax: +49 208 3062989.
E-mail address: weidenthaler@mpi-muelheim.mpg.de (C. Weidenthaler).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2005.09.004

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M. Mamatha et al. / Journal of Alloys and Compounds 416 (2006) 303–314
a clear assignment of the different decomposition steps. The first
step of decomposition of Mg(AlH4)2 between 110 and 130 ◦C
resulting in the formation of MgH2 and Al is in accordance with
the reaction mechanism proposed by several authors [14–16].
From thermogravimetric measurements, Claudy et al. [14] and
Fichtner et al. [15] proposed the subsequent decomposition of
MgH2 to Mg and H2 to take place at about 300 ◦C. Dymova
et al. [16] observed different temperatures during the decom-
position of Mg(AlH4)2 and deduced the formation of an alloy.
In a previous paper, we already discussed the thermolysis of
Ca(AlH4)2 and LiMg(AlH4)3 from DSC and X-ray diffraction
data and proposed a reaction pathway [13]. In this work, we
present recent in situ high-temperature X-ray diffraction data to
support the previously proposed mechanism. For LiMg(AlH4)3
and Na2LiAlH6, no data were available and a comparative study
with the single alanate systems can be of great interest.
2.3.1. Magnesium alanate Mg(AlH4)2
For the preparation of Mg(AlH4)2 + 2NaCl mixtures, 0.567 g (10.5 mmol)
of NaAlH4 and 0.500 g (5.25 mmol) of MgCl2 were weighed in the vial of the
milling apparatus and milled for 3 h [13].
2.3.2. Calcium alanate Ca(AlH4)2
The Ca(AlH4)2 + 2NaCl mixtures were prepared by milling 0.486 g
(9.0 mmol) of NaAlH4 and 0.500 g (4.5 mmol) of CaCl2 [13].
2.3.3. Lithium alanate LiAlH4
The lithium aluminum hydride (LiAlH4, 95%) was provided by Aldrich and
was subjected to further purification in dry diethyl ether. Two grams of LiAlH4
was suspended in 150 ml diethyl ether and the suspension was kept for stirring
overnight with a high-speed stirrer (∼2500 rpm). The LiAlH4, which is soluble
in diethyl ether, was obtained in the filtrate. During continuous removal of the
solvent by heating in an oil bath, LiAlH4 began to crystallize from the solution
at 60 ◦C. The suspension was kept for 10–15 min at the same temperature and
100 ml of pentane were added. The suspension was stirred for 50 min and the
LiAlH4 crystals were separated from the mother liquor by filtration, washed
with pentane and dried in vacuum (10−2 mbar) at room temperature, yielding
1.6 g of LiAlH4.
2. Experimental
2.1. Differential scanning calorimetry
2.3.4. Lithium magnesium alanate LiMg(AlH4)3
The LiMg(AlH4)3 + 2 LiCl mixture was prepared starting from 0.611 g
(16.0 mmol) of LiAlH4 and 0.500 g (5.25 mmol) of MgCl2 [13].
The DSC measurements were performed on a Mettler-Toledo DSC 27HP
instrument, using a fully automated program for the evaluation of the DSC
data. The peak areas of the DSC curves are expressed in mJ units. The peak
areas were calibrated on the basis of averaged heats of fusion of In, Sn and Bi
as standards. The measurements were carried out under argon using typically
5–6 mg of a sample heated in an aluminum crucible with a heating rate of 5 and
20 ◦C min−1. To prevent oxidation during transportation of the samples from
the glove box to the DSC unit, an aluminum stopper was used to cover the
crucible.
2.3.5. Potassium alanate KAlH4
2.3.5.1. Salt-containing KAlH4. The KAlH4 + Na(Li)Cl mixtures were pre-
pared from 0.362 g (6.7 mmol) of NaAlH4, 0.254 g (6.7 mmol) of LiAlH4 and
0.500 g (6.7 mmol) of KCl.
2.3.5.2. Salt-free KAlH4. KAlH4 was synthesized in an one-step direct hydro-
genation process under ball milling in a special mill, which has been described
in detail in a previous paper from our group [18]. The so-called “direct process”
for the synthesis of potassium alanate was carried out by ball-milling of KH/Al
mixtures in 1:1 molar ratio under hydrogen pressure according to the reaction
shown below:
2.2. X-ray diffraction
The X-ray powder patterns were recorded on a Stoe STADI P transmis-
˚
sion diffractometer in Debye–Scherrer geometry (Cu K␣1: 1.54060 A) with a
primary monochromator (curved germanium (1 1 1)) and a curved position sen-
sitive detector. For the in situ high-temperature experiments, a capillary furnace
was attached to the diffractometer. Data were collected in the range between 20◦
and 90◦ 2θ with a step width of 0.01◦ 2θ. For the measurements, the samples
were filled into Ø 0.5 mm quartz glass capillaries in a glove box. The capillaries
were sealed to prevent contact with air during the transport to the diffractometer.
During the in situ experiment, the samples were kept under argon. Heating rates
of 5 and 20 ◦C min−1 were applied. The measured patterns were evaluated qual-
itatively by comparison with entries from the PDF-2 powder pattern database or
with calculated patterns using literature structure data.
10 h b.m.
KH + Al + 100 bar H2 −→ KAlH4 (b.m. : ball milling)
For the one-step direct synthesis of the potassium alanate, a mixture of 1.0 g
(24.9 mmol) of KH and 0.67 g (24.9 mmol) of Al powder was milled in the auto-
clave of Fritsch P7 mill under hydrogen (99.999%, 100 bar initial pressure, 10 h)
without external heating. For the direct synthesis of the Ti-catalyzed potassium
alanate, a mixture of 1.0 g (24.9 mmol) of KH, 0.67 g (24.9 mmol) of Al powder
and 2 mol% TiCl3 was milled in the autoclave of the Fritsch mill under hydrogen
(99.999%, 100 bar initial pressure, 10 h) without external heating.
2.3.6. Sodium lithium alanate Na2LiAlH6
Na2LiAlH6 was prepared from 0.500 g (9.25 mmol) of NaAlH4, 0.222 g
(9.25 mmol) of NaH and 0.073 g (9.25 mmol) of LiH.
2.3. Synthesis
All operations in the described experiments were performed under argon as
a protecting atmosphere, using a glove box or Schlenk techniques.
3. Results and discussion
Starting materials: NaAlH4 (Chemetall) was re-crystallized by addition of
pentane to a NaAlH4 tetrahydrofurane (THF) solution [17]. LiAlH4 for ball
milling experiments was purified by dissolving it in diethyl ether, filtering the
solution and evaporating the diethyl ether (purity 99% according to thermovol-
umetric measurement). Anhydrous MgCl2, CaCl2 and TiCl3 (Aldrich 99.9%)
were used without further purification; Al (Strem Chemicals 99.7%, 325 mesh),
KH in mineral oil (Aldrich, 30 wt.%) was filtered and washed several times with
hexane to separate KH from the oil, followed by evaporation of the solvent.
Pentane and diethyl ether were dried over K–Na alloy, and THF was puri-
fied over magnesium anthracene. The different alanates were synthesized by
mechanochemical preparation using a Retsch ball mill 200 MM (30 Hz) in 25 ml
milling vials with 2 steel milling balls, 6.3 g each. The starting mixtures were
ball milled for 3 h. For the preparation of KAlH4, a modified Fritsch Pulverisette
7 planetary mill was used.
3.1. Mg(AlH4)2
Heating the starting material, consisting of a mixture of
Mg(AlH4)2 and NaCl (Fig. 1), to higher temperature results
in the decomposition of Mg(AlH4)2 in the temperature range
between 110 and 130 ◦C and the formation of MgH2 and Al
(Fig. 2a) [13]. These results are in accordance with the ther-
mal analyses published by Claudy et al. [14] and Dymova et al.
[16]. The formation of an intermediate phase MgAlH5 as sug-
gested by Dymova et al. cannot be confirmed. Fossdal et al. [12]

M. Mamatha et al. / Journal of Alloys and Compounds 416 (2006) 303–314
305
different compounds. The magnesium hydride formed remains
stable in a temperature window of almost 100 ◦C before it also
starts to decompose at about 210–220 ◦C (Fig. 2b). The pow-
der patterns collected at temperatures above 250 ◦C show the
appearance of reflections in the range between 34.5 and 38.9◦
2θ, which are assigned to an alloy with a composition very sim-
ilar to Al3Mg2 (Fig. 2c). The second reaction step occurring at
higher temperatures was interpreted by Claudy et al. [14] and
Fichtner et al. [15] as the decomposition of MgH2 to Mg and
H2. However, the appearance of metallic Mg cannot be veri-
fied by our in situ X-ray investigations. This contradiction could
be due to the fact, that both groups based their interpretation
mainly on DTA/TG data without checking the true phase com-
position by X-ray diffraction experiments. Fichtner et al. assign
the formation of an intermetallic compound from Mg and Al
at much higher temperatures of about 400 ◦C. Our investigation
clearly shows that the formation of Al3Mg2 can be correlated
with the second thermal effect of the DSC data, which starts
at about 250 ◦C (Fig. 3). The in situ XRD studies reveal the
existence of the aluminum rich alloy up to 450 ◦C prior to its
melting (Fig. 2d). The formation and decomposition steps can
be summarized as follows:
Fig. 1. Starting material (a) and calculated pattern for Mg(AlH4)2 (b) using the
crystal structure data given by Fichtner et al. [19].
report the onset of the dissociation of Mg(AlH4)2 to be at 150 ◦C
resulting in the formation of MgH2 and a so-called ␣-phase. This
phase represents a solid solution of Mg in Al. As a proof for the
formation of the ␣-phase, the authors discuss the increase of
the unit cell parameter of Al. In contrast to these observations,
our studies show a rapid decomposition of Mg(AlH4)2 already
between 110 and 120 ◦C. The powder patterns measured up to
these temperatures do not show significant changes, which could
be attributed to effects other than the thermal expansion of the
b.m.
MgCl2 + 2NaAlH4−→ Mg(AlH4)2 + 2NaCl
(1)
110−130 ◦C
Mg(AlH4)2 + 2NaCl −→ MgH2 + 2NaCl + 2Al + 3H2 ↑
(2)
Fig. 2. Selection of in situ powder patterns obtained during the decomposition of Mg(AlH4)2 between (a) room temperature and 140 ◦C, (b) 150 and 210 ◦C, (c) 250
and 310 ◦C and (d) section of the 2θ range between 26◦ and 48◦ collected at 420 ◦C.

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M. Mamatha et al. / Journal of Alloys and Compounds 416 (2006) 303–314
tified phase appear. We assume that the unidentified reflections
belong to CaAlH5 as an intermediate phase with a crystal struc-
ture similar to BaAlH5 [20]. The decomposition of Ca(AlH4)2
is completed at about 100 ◦C (Fig. 4a).
In disagreement with our investigation, Mal’tseva et al. [11]
describe the decomposition of Ca(AlH4)2 to occur at about
200 ◦C. A possible reason for this discrepancy could be the
preparation of Ca(AlH4)2 using different starting materials.
Mal’tseva et al. started from a mixture of CaH2 and AlCl3
whereas we started from a mixture of NaAlH4 and CaCl2. The
authors describe the second step of decomposition, involving
the decomposition of CaAlH5 to CaH2, Al and H2, to appear
between 260 and 550 ◦C. The final step, the decomposition of
CaH2 and Al to Al4Ca (the authors wrote Ca4Al but we think
this is a typo) is discussed to take place between 600 and 700 ◦C.
Our in situ XRD and DSC investigations clearly show, that
all decomposition steps happen at significantly lower temper-
atures than discussed in literature (Figs. 4 and 5). The phase
mixture of Al, NaCl and CaAlH5, formed after the first step of
decomposition, is stable up to about 180 ◦C before, in a second
step, CaAlH5 starts to decompose and CaH2 is formed (Fig. 4b).
The decomposition is finished at about 220 ◦C. Between 350 and
360 ◦C, the intensities of the CaH2 and Al reflections decrease
and an alloy of the composition Al4Ca is formed (Fig. 4c). At
about 400 ◦C, the remaining CaH2 decomposes completely and
the composition of the alloy changes again. The alloy transforms
from an Al-rich Al4Ca composition to a Ca-richer Al2Ca-alloy
Fig. 3. DSC curve of the Mg(AlH4)2–2NaCl mixture displayed for the temper-
ature range between 100 and 500 ◦C. Heating rate 20 ◦C min−1
.
MgH2 + 2NaCl + 2Al
250 ◦C
−→ 0.5Al3Mg2 + 2NaCl + 0.5Al + H2 ↑
(3)
3.2. Ca(AlH4)2
At room temperature, the sample consists of a mixture of
Ca(AlH4)2 andNaCl. Between80and90 ◦C, Ca(AlH4)2 startsto
decompose and small reflections belonging to Al and an uniden-
Fig. 4. Selection of in situ powder patterns obtained during the decomposition of Ca(AlH4)2. Displayed temperature ranges between (a) room temperature and
120 ◦C, (b) 190 and 250 ◦C, (c) 330 and 390 ◦C and (d) 390 and 450 ◦C.

M. Mamatha et al. / Journal of Alloys and Compounds 416 (2006) 303–314
307
ferent data collection times of 60 and 300 s. The patterns of the
sample measured 300 s per pattern (Fig. 7a) do not show an
amorphous state of the sample indicating the formation of melt.
At 160 ◦C, the sample consists of LiAlH4, whereas at 170 ◦C,
the sample has decomposed to Li3AlH6 and Al. The patterns
collected for 60 s each allow the detection of a melt in the tem-
perature range between 180 and 190 ◦C (Fig. 7b). Between 230
and 240 ◦C, Li3AlH6 decomposes and LiH and Al are formed
(Fig. 7c).
The results of the in situ X-ray investigations summarized
in the reaction steps given below are confirmed by the DSC
experiment (Fig. 8).
◦
LiAlH4␣ꢀ70−−1→00 CLiAlH4
(8)
(9)
Fig. 5. DSC curve of Ca(AlH4)2–2NaCl mixture, heating rate 20 ◦C min−1
.
◦
3LiAlH4170−−→180 CLi3AlH6 + 2Al + 3H2 ↑
(Fig. 4d). This alloy is stable up to 450 ◦C, which represents the
highest temperatures of the analysis presented in this work. The
results of the in situ XRD studies starting with the synthesis of
Ca(AlH4)2 are summarized in the following reactions:
◦
Li3AlH6 + 2Al230−−→240 C3LiH + 3Al + 1.5H2 ↑
(10)
Applying different heating rates of 5 and 20 ◦C min−1 does
not result in a change of reaction temperatures. However, Brinks
et al. [6a] report slightly lower temperatures for the first step of
decomposition of 148–155 ◦C. The second step of decomposi-
tion to LiH, Al and H2 is reported to occur between 165 and
220 ◦C. Our in situ X-ray investigations restrict the temperature
range for the second step to 230–240 ◦C.
b.m.
CaCl2 + 2NaAlH4−→ Ca(AlH4)2 + 2NaCl
(4)
(5)
(6)
(7)
Ca(AlH4)2 + 2NaCl
◦
80−−1→00 CCaAlH5 + 2NaCl + Al + 1.5H2 ↑
3.4. KAlH4
CaAlH5 + 2NaCl + Al
180 ◦C
During the in situ investigations of salt-containing KAlH4
samples, a significant influence of the presence of salt and the
type of salt on the decomposition temperatures and the reaction
pathways became obvious. For this reason, a salt-free KAlH4
sample prepared via the direct synthesis route from KH and Al
was analyzed in addition to salt (NaCl/LiCl)-containing sam-
ples. Additionally, all samples were doped with Ti-catalysts and
compared with the undoped samples. The DSC curves for pure
−→ CaH2 + 2NaCl + 2Al + 1.5H2 ↑
CaH2 + 2NaCl + 2Al
◦
340−−→450 CAl–Ca-alloy + 2NaCl + H2 ↑
3.3. LiAlH4
The starting material used for the in situ investigation con-
sists of a mixture of LiAlH4 and a second phase (Fig. 6). The
main reflections, which do not overlap with LiAlH4 reflec-
˚
tions are at d-values of 3.52, 2.64, 2.49, 2.09 and 1.99 A. The
comparison with the ICCD database showed a match with a
LiAlH4 phase named LiAlH4-␣ꢀ by Bastide et al. [21]. This
phase with a larger unit cell than known for the LiAlH4 phase
is metastable and transforms to the ␣-form at about 70 ◦C. To
our knowledge, the appearance of this phase was not described
in later publications. Our in situ investigation confirms the dis-
appearance of the LiAlH4-␣ꢀ during the temperature increase to
100 ◦C and an increase of the intensities of the LiAlH4 reflec-
tions [22] indicating a transformation of the metastable form
(Fig. 6).
At higher temperatures between 170 and 180 ◦C, the decom-
position of LiAlH4 to Li3AlH6 and Al starts. Prior to the forma-
tion of Li3AlH6, the LiAlH4 phase melts. This is only visible
when the system is not only heated applying very fast heating
rates but also very quick data acquisition times. Two samples
were heated with the same heating rate of 20 ◦C min−1 but dif-
Fig. 6. Comparison of the powder patterns collected at room temperature and
collected in situ at 140 ◦C. The peak positions for both LiAlH4 phases are shown
by the vertical bars.

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M. Mamatha et al. / Journal of Alloys and Compounds 416 (2006) 303–314
Fig. 7. Powder patterns collected in situ during the decomposition of LiAlH4 in the temperature range between 160 and 210 ◦C and a data collection time of 300 s
per scan (a) and a data collection time of 60 s per scan (b). The temperature range between 220 and 250 ◦C and a data collection time of 300 s per scan is shown in
(c).
KAlH4, KAlH4 doped with Ti, KAlH4/LiCl and KAlH4/NaCl
are shown in Fig. 9.
3.4.1. KAlH4 pure system
ThesampleprepareddirectlyfromKHandAlconsistsofpure
KAlH4 as the comparison with the simulated powder pattern
shows (Fig. 10a). The compound is stable up to 300 ◦C before it
decomposes and Al and a second phase appear. The analysis of
the pattern collected at 340 ◦C does not allow the assignment of
Fig. 9. DSC curves for (a) pure KAlH4, (b) KAlH4 doped with Ti, (c)
KAlH4/NaCl and (d) KAlH4/LiCl in the temperature range between 100 and
Fig. 8. DSC curve of LiAlH4, heating rate 20 ◦C min−1
.
500 ◦C. Heating rate 20 ◦C min−1
.

M. Mamatha et al. / Journal of Alloys and Compounds 416 (2006) 303–314
309
Fig. 10. (a) Powder pattern of the parent material measured at room temperature (I) and simulated powder pattern (II) using data from Chung and Morioka [23]; (b)
selected powder patterns collected in situ at different temperatures.
the peaks to any known phase. The reflections at 25.6◦, 27.2◦,
29.7◦, 40.9◦ and 42.5◦ 2θ do neither belong to K3AlH6 or AlH3
nor to KH. Between 350 and 360 ◦C, this unidentified phase
decomposes and KH is formed, which represents together with
Al the only crystalline phases. The formation and decomposition
of the pure system can be summarized by the two following
steps:
of K3AlH6 as proposed in a later study [7] and by other authors
[9,10] cannot be confirmed by our in situ X-ray diffraction
studies.
3.4.2. KAlH4–NaCl
After preparation, the sample consists of KAlH4 and NaCl
(Fig. 11a), which remain stable up to temperatures of 180 ◦C.
In the temperature range between 180 and 190 ◦C, a very weak
reflection appears at 28.3◦ 2θ. At this stage, it is not possible
to assign the reflection unambiguously but during further tem-
perature increase it becomes obvious that the reflection belongs
to KCl. The amount of KCl increases rapidly and at 300 ◦C
KCl represents the main crystalline phase. The pattern shows
also the appearance of another weak reflection present up to
temperatures of 240 ◦C. Even though, due to the low intensity
of this reflection, an assignment is difficult, it represents the
main reflection of K3AlH6 as proposed by Bastide et al. [21].
ThoughK3AlH6 hasformed, theamountsseemtobemuchlower
than expected. During further temperature increase, KAlH4 and
KH + Al + H2(100 bar)10−h→b.m.KAlH4
(11)
(12)
(13)
300 ◦C
KAlH4 −→ unidentified phase + Al
◦
unidentified phase + Al350−−→360 CKH + Al
The temperatures of decomposition of pure KAlH4 are in
accordance with the work of Dymova et al. [7,8]. In their early
work, the authors assign the first decomposition product to
have the composition KAlH2. Since no specifications about the
structure or a measured powder pattern were given, we cannot
compare our data to this early work. Nevertheless, the formation
Fig. 11. Selection of powder patterns collected in situ during the decomposition of the mixture KAlH4–NaCl between (a) room temperature and 320 ◦C and (b) 330
and 430 ◦C.

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M. Mamatha et al. / Journal of Alloys and Compounds 416 (2006) 303–314
K3AlH6 decompose and above 270 ◦C, KCl and Al are the main
crystalline phases. At 290 ◦C, small NaH reflections become vis-
ible gaining intensity with increasing temperatures. At 310 ◦C,
NaCl has almost completely transformed to KCl and NaH. This
phase composition is stable up to 410 ◦C before NaH starts to
melt and KCl and Al remain as crystalline phases (Fig. 11b).
On first sight surprisingly, during thermolysis, NaCl which had
been generated during the preparation procedure is consumed
and KCl is formed. However, if one compares the thermody-
namic data, the metathesis reaction NaCl + KH ꢀ KCl + NaH is
understandable. The standard free enthalpy for this reaction is
−32 kJ mol−1, i.e. it should proceed readily towards the product
side.
The formation and decomposition steps can be summarized
as follows:
Fig. 12. Selection of powder patterns collected in situ during the decomposition
of the mixture KAlH4–LiCl.
• ball milling of KCl and NaAlH4 results in a mixture of KAlH4
and NaCl;
• upon heating to 180–190 ◦C → partial decomposition of
KAlH4 starts, resulting in a mixture of reformed KCl, Al,
and remaining NaCl and KAlH4;
(ꢀGRØ (298) = −76 kJ mol−1) compared to −32 kJ mol−1 in
case of NaCl. The driving force with LiCl as admixture is thus
higher and the reaction can occur at lower temperature. The fol-
lowing steps are involved during synthesis and decomposition
of LiCl-containing KAlH4:
• below 240 ◦C → partial dehydrogenation of KAlH4 →
K3AlH6, KCl, NaCl, KAlH4, Al;
• above 240 ◦C → decomposition of KAlH4 and K3AlH6:
substitution of Na+ in NaCl by K+ → KCl is formed;
reaction of Na+ with H− → NaH is formed;
• at 420 ◦C → decomposition of NaH, remaining crystalline
phases are KCl and Al.
• ball milling of KCl and LiAlH4 results in a mixture of KAlH4
and LiCl;
• upon heating to 160–170 ◦C → partial decomposition of
KAlH4 starts, resulting in a mixture of reformed KCl, Al,
and remaining LiCl and KAlH4;
3.4.3. KAlH4–LiCl
• at 200 ◦C → decomposition of KAlH4:
substitution of Li+ in LiCl by K+ → KCl is formed;
reaction of Li+ with H− → LiH is formed.
Similar to the KAlH4–NaCl sample, first compositional
changes for the KAlH4–LiCl system become visible between
160 and 170 ◦C when the main KCl reflections appear. Com-
pared to the NaCl system, the intensity of the KCl reflection is
already very high at 190 and 200 ◦C, KAlH4 decomposes rapidly
to KCl, LiH and Al (Fig. 12). Again, no KH is formed, instead of
this, LiH forms after the decomposition of KAlH4. This trans-
formation occurs at lower temperatures than in case of the NaCl
system, which can tentatively be attributed to the higher free
enthalpy of the metathesis reaction for LiCl + KH ꢀ KCl + LiH
For both systems, KAlH4 and NaCl/LiCl, we do not observe
the formation of KH and also the formation of the intermediate
phase K3AlH6 is difficult to prove. Dymova et al. [16] deter-
mined much higher decomposition temperatures for the first step
to an intermediate phase (KAlH2) and Al at about 300 ◦C. Also
for the second and third steps, the given temperatures of 340 ◦C
Fig. 13. Powder patterns collected in situ during the decomposition of the Ti-doped but salt-free KAlH4 sample in the temperature range between (a) room temperature
and 294 ◦C and (b) 294 and 440 ◦C.

M. Mamatha et al. / Journal of Alloys and Compounds 416 (2006) 303–314
311
Fig. 14. Sample at 420 ◦C (a) without catalyst and (b) with catalyst.
(KAlH2 to KH + Al + H2) and 430 ◦C (KH to K and H2) are
much higher than observed in our studies. A possible reason
for this discrepancy is the presence of the salts in our samples,
which significantly lower the decomposition temperature, due to
the additional driving force provided by the coupled metathesis
reaction.
Fig. 15. Selection of powder patterns collected in situ during the decomposition
of the Ti-doped LiCl-containing KAlH4.
presence of a catalyst has also some influence on the structure of
the KH formed during thermolysis. The patterns of doped and
undoped samples collected at 420 ◦C shows that the positions
of the KH reflections in the undoped sample (Fig. 14a) appear
at higher angles than in the doped sample (Fig. 14b) indicat-
ing smaller unit cell parameters. The shift has some significance
since the positions of the Al reflections are not influenced.
3.4.4. KAlH4 without salt but with catalyst
After doping of the pure KAlH4 sample with the titanium,
a small amount of KCl is observed indicating a partial decom-
position of the material right after doping (Fig. 13). As already
observed for the undoped sample, reflections belonging to the
new phase I appear after the decomposition of KAlH4 between
280 and 290 ◦C. The decomposition temperature of the alanate
is about 20 ◦C lower than for the undoped sample. In addition,
a reflection at about 31◦ 2θ is observed which is assigned to a
new phase II and which is not observed for the undoped sample.
Between 330 and 340 ◦C, the powder pattern changes again and
both the new phases and KCl have disappeared and KH and Al
are the remaining stable phases. This also happened at 10–20 ◦C
lower temperatures than observed for the undoped sample. The
3.4.5. KAlH4 with LiCl and catalyst
Doping the salt-containing sample with titanium results in a
partial decomposition of the alanate already at room temperature
as the mixture of KAlH4, LiCl, KCl and Al shows (Fig. 15). At
130 ◦C, KAlH4 and LiCl decompose and KCl and Al are formed.
The temperature of the decomposition is, therefore, significantly
lower than for the salt- and/or catalyst-free systems. Although
LiH can be expected to be formed, we cannot detect LiH to be
present by our XRD data. Due to the very fast data acquisition
Fig. 16. Powder patterns collected in situ during the decomposition of the mixture LiMg(AlH4)3 in the temperature range between (a) room temperature and 160 ◦C
and (b) 170 and 230 ◦C.

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M. Mamatha et al. / Journal of Alloys and Compounds 416 (2006) 303–314
necessary to analyze the catalyzed samples, the quality of the
data is not good enough to allow an unambiguous assignment of
LiH. The hydride is a very weakly scattering compound, which
is difficult to identify at all.
3.5. LiMg(AlH4)3
The parent material consists of a mixture of LiMg(AlH4)3
and LiCl. At about 120 ◦C, the reflections of the LiMg(AlH4)3
become very weak and new reflections appear, which cannot be
assigned to any known phase (Fig. 16a). The first assumption
that the reflections belong to Li3AlH6 turned out to be wrong.
We assume the formation of a new intermediate component of
the composition LiMgAlH6 [13], but as the structure of this com-
pound is not known, we cannot verify this phase unambiguously.
In addition to the new phase, aluminum has crystallized and the
phase mixture consisting of LiCl, Al and the unknown phase is
stable up to 170 ◦C. A further increase of temperature leads to
the decomposition of the unidentified phase and the formation of
MgH2, whichisstableupto220 ◦C(Fig. 16b). Itcanbesupposed
that during the decomposition of the alanate and/or the interme-
diate phase MgH2 and LiH are formed. Compared to the LiAlH4
system, the LiH reflections are not visible. This could be due to
the very weak scattering properties of the LiH in combination
with the lower amounts of LiH formed in the LiMg(AlH4)3 sys-
tem. The powder pattern collected at 230 ◦C shows Al and LiCl
as crystalline phases. The decomposition behavior of the mixed
alanate is different to the pure LiAlH4 and Mg(AlH4)2 com-
Fig. 17. DSC curve of LiMg(AlH4)3, heating rate 20 ◦C min−1
.
pounds. The thermal stability of the complex alanate is similar to
the stability of Mg(AlH4)2 (110–130 ◦C) but lower than the sta-
bility of LiAlH4 (170–180 ◦C). In a first step of decomposition,
a new phase is formed which later on decomposes to MgH2. At
a temperature of about 230 ◦C, which is very similar to the pure
systems, MgH2 decomposes. There is no subsequent formation
of an intermetallic compound as observed for the Mg(AlH4)2
system. The decomposition steps, which can be also followed
by DSC (Fig. 17) proceed as:
• ball milling of MgCl2 and LiAlH4 results in a mixture of
LiMg(AlH4)3 and LiCl;
Fig. 18. Selection of in situ powder patterns obtained at different temperatures during the decomposition of Na2LiAlH6. (a) Temperature range between room
temperature and 400 ◦C, (b) section of the powder patterns collected in the temperature range between 160 and 200 ◦C, (c) decomposition of the alanate in the
temperature range between 250 and 320 ◦C and (d) enlargement of the 2θ range showing the presence of LiH.

M. Mamatha et al. / Journal of Alloys and Compounds 416 (2006) 303–314
313
Forthermodynamiccalculations, itismandatorytodetermine
the reaction pathway during thermolysis correctly. A wrong
assignment of DSC data can result in an incorrect mechanism
later on used for thermodynamic studies.
The alkaline earth alanates decompose at much lower temper-
atures than the alkali alanates. For the mixed complex hydrides
LiMg(AlH4)3 and Na2LiAlH6, the first steps of decomposition
are shifted to much lower temperatures as compared to LiAlH4
or NaAlH4.
The formation of alloys are observed for both the Mg(AlH4)2
and the Ca(AlH4)2 system.
TheKAlH4 systemsshowsaverysignificantinfluenceofboth
the presence of alkali chloride salts formed during the synthesis
procedure and the type of salt on the dissociation temperatures
of the alanates, which can be explained by the thermodynamics
of the different systems.
Fig. 19. DSC curve of Na2LiAlH6, heating rate 20 ◦C min−1
.
• upon heating at 120 ◦C → decomposition of LiMg(AlH4)3
starts; a new phase of the assumed composition LiMgAlH6 is
formed together with Al;
Acknowledgments
• upon further heating at 170 ◦C → decomposition of
LiMgAlH6, MgH2 and Al are formed; LiCl remains stable;
• at 220 ◦C → MgH2 decomposes: Al and LiCl remain as the
only crystalline phases.
The authors thank the hydrogen storage team at the MPI. This
work was partially supported by GM Fuel Cell Activities and by
the basic funding of the Max-Planck-Gesellschaft. C.W. thanks
Silvia Palm (MPI) for XRD analyses.
3.6. Na2LiAlH6
References
The starting material consists mainly of Na2LiAlH6 and a
small amount of Na3AlH6. The main steps of the decomposition
are shown in Fig. 18a. At 160 ◦C, weak reflections belonging
to NaH appear (Fig. 18b), which start to gain intensity above
240 ◦C. At the same time, Na3AlH6 already starts to decompose.
The data collected at higher temperatures show the coexis-
tence of Al and NaH as main crystalline phases (Fig. 18a and
c). A closer look on the data collected in the temperature range
between 250 and 320 ◦C shows the presence of very weak reflec-
tions, which nevertheless, can be assigned to LiH (Fig. 18d). The
thermal reaction at 350 ◦C (Fig. 19) represents the decomposi-
tion of NaH, which also becomes apparent from the decreasing
intensities in the X-ray powder data. From the in situ data fol-
lows that the formation and decomposition reactions proceeds
according to:
[1] P. Claudy, B. Bonnetot, G. Chahine, J.M. Letoffe, Thermochim. Acta
38 (1980) 75.
[2] K.J. Gross, G. Sandrock, G.J. Thomas, J. Alloy Compd. 330–322 (2002)
691.
[3] K.J. Gross, S. Guthrie, S. Takara, G. Thomas, J. Alloy Compd. 297
(2000) 270.
[4] M.P. Balogh, G.G. Tibbetts, F.E. Pinkerton, G.P. Meisner, C.H. Olk, J.
Alloy Compd. 350 (2003) 136.
[5] V.P. Balema, V.K. Pecharsky, K.W. Dennis, J. Alloy Compd. 313 (2000)
69.
[6] (a) H.W. Brinks, B.C. Hauback, D. Blanchard, C.M. Jensen, M. Ficht-
ner, H. Fjellvag, in: D., Chandra, R.G., Bautista, L., Schlapbach (Eds.),
Advanced Materials For Energy Conversion II, Minerals, Metals &
Materials Society, Warrendale, 2004, p. 195;
(b) H.W. Brinks, B.C. Hauback, P. Norby, H. Fjellvag, J. Alloy Compd.
351 (2003) 222;
(c) D. Blanchard, H.W. Brinks, B.C. Hauback, P. Norby, J. Muller, J.
Alloy Compd., web release 19 July 2005.
[7] T.N. Dymova, S.I. Bakum, U. Mirsaidov, Doklady Akademii Nauk SSSR
216 (1974) 87.
[8] T.N. Dymova, M.S. Selivokhina, N.G. Eliseeva, Doklady Akademii Nauk
SSSR 153 (1963) 1330.
[9] H. Morioka, K. Kakizaki, S.C. Chun, A. Yamada, J. Alloy Compd. 353
(2003) 310.
[10] V.P. Tarasov, S.I. Bakum, A.V. Novikov, Zh. Neorg. Khim. 46 (2001)
474.
[11] N.N. Mal’tseva, A.I. Golovanova, T.N. Dymova, D.P. Aleksandrov, Russ.
J. Inorg. Chem. 46 (2001) 1793.
[12] A. Fossdal, H.W. Brinks, M. Fichtner, B.C. Hauback, J. Alloy Compd.,
web release 14 July 2005.
• ball milling of NaAlH4, LiH and NaH results in a mixture of
Na2LiAlH6 and a small amount of Na3AlH6;
• upon heating above 160 ◦C → decomposition of Na2LiAlH6
starts; LiH, NaH and Al appear, Na3AlH6 remains;
• uponfurtherheatingat250 ◦C → decompositionofNa3AlH6;
Al, LiH and NaH remain;
• at 350 ◦C → LiH and NaH decompose, Al remains as crys-
talline phase.
4. Conclusion
[13] M. Mamatha, B. Bogdanovic´, M. Felderhoff, A. Pommerin, W. Schmidt,
F. Schu¨th, C. Weidenthaler, J. Alloy Compd., web release 22 August
2005.
[14] P. Claudy, B. Bonnetot, J.M. Letoffe, J. Thermal Anal. 15 (1979) 119.
[15] M. Fichtner, O. Fuhr, O. Kircher, J. Alloy Compd. 356–357 (2003) 418.
[16] T.N. Dymova, V.N. Konoplev, A.S. Sizareva, D.P. Aleksandrov, Russ. J.
Coord. Chem. 25 (1999) 313.
The in situ high-temperature X-ray diffraction studies of
different alkali and alkaline earth aluminum hydrides show sig-
nificant differences in the decomposition behavior dependent on
the light metal element used.

314
M. Mamatha et al. / Journal of Alloys and Compounds 416 (2006) 303–314
[17] B. Bogdanovic´, R.A. Brand, A. Marjanovic´, M. Schwickardi, J. To¨lle,
J. Alloy Compd. 302 (2000) 36.
[20] Q.A. Zhang, Y. Nakamura, K. Oikawa, T. Kamiyama, E. Akiba, Inorg.
Chem. 41 (2002) 6941.
[18] J.M. Bellosta von Colbe, M. Felderhoff, B. Bogdanovic´, F.
Schu¨th, C. Weidenthaler, Chem. Commun., web release 26 August
2005.
[19] M. Fichtner, J. Engels, O. Fuhr, A. Glo¨ss, O. Rubner, R. Ahlrichs, Inorg.
Chem. 42 (2003) 7060.
[21] J.P. Bastide, B. Bonnetot, J.M. Letoffe, P. Claudy, Mater. Res. Bull. 20
(1985) 999.
[22] B.C. Hauback, H.W. Brinks, H. Fjellva˚g, J. Alloy Compd. 346 (2002)
184.
[23] S.-C. Chung, H. Morioka, J. Alloy Compd. 372 (2004) 92.