1 ice | science The interest of hydrogen as the future energy is that it is a clean energy carrier, most abundant element in the universe, the lightest fuel, richest in energy per unit mass and unlike electricity it can be easily stored. Hydrogen gas is the most promising fuel for various applications. Hydrogen is already being used as the fuel of choice for space programs around the world. Metal hydrides are a fascinating class of compounds, because the small mass and size of hydrogen and its medium electronegativity cause a large flexibility in terms of metal–ligand interactions. These manifest itself in a vast variety of possible compositions, chemical bonding, crystal structures and physical properties. In this review, we presented the structural details of all up-to-date known Al-based hydrides. Notations M 3 AlH 6 – Li 3 AlH 6, Na 3 AlH 6, K 3 AlH 6 MAlH 4 – LiAlH 4 , NaAlH 4 , KAlH 4 MH – LiH, NaH, KH 1. Introduction The world is facing energy shortage and has become increasingly dependent on new methods to store and convert energy for new, environmentally friendly methods of transportation and electrical energy generation as well as for portable electronics. Mobility – the transport of people and goods – is a socioeconomic reality that will surely increase in the coming years. Non-renewable fossil fuels are projected to decline sharply after 25–30 years. CO 2 emission from burning such fuels is the main cause of global warming. Currently, the whole world is seeking international commitment to cut emissions of greenhouse gasses by 60% by 2050. There is a constant search for alternate fuel to solve the energy shortage that can provide us energy without pollution. Hence, most frequently discussed source is the hydrogen which when burnt in air produces a clean form of energy. The interest of hydrogen as the future energy is that it is a clean energy carrier, most abundant element in the universe, the lightest fuel, richest in energy per unit mass and unlike electricity it can be easily stored. Hydrogen gas is the most promising fuel for various applications, such as to generate electricity, useful in cooking food, fuel for automobiles, hydrogen-powered industries, jet planes, hydrogen village and for all our domestic energy requirements. Hydrogen that can produce with little or no harmful emissions has projected as a long-term solution for a secure energy in future. Increasing application of hydrogen energy is the only way forward to meet the objectives of Department of Energy (DOE), USA – that is, reducing greenhouse gases, increasing energy security and strengthening the developing countries economy. Any transition from a carbon-based/fossil-fuel energy system to a hydrogen-based economy involves overcoming significant scientific, technological and socioeconomic barriers to ultimate implementation of hydrogen as the clean energy source of the future. Hydrogen as a fuel has already found applications in experimental cars and all the major car companies are in competition to build a commercial car and most probably they may market hydrogen fuel automobiles in the near future. Hydrogen is already being used as the fuel of choice for space programs around the world. It will be used to power aerospace transport to build the international space station, as well as to provide electricity and portable water for its inhabitants. Hydrogen is the simplest and lightest element of our universe with only one proton and one electron. 1 Hydrogen is not available as an element but in the form of compounds such as water Crystal structures of aluminum-based hydrides Vajeeston and Fjellvåg ICE Publishing: All rights reserved Keywords: characterization/energy storage/hydrogen storage 1 2 *Corresponding author e-mail address: [email protected]1 Ponniah Vajeeston PhD* Researcher, Centre for Materials Science and Nanotechnology, Department of Chemistry, University of Oslo, Oslo, Norway 2 Helmer Fjellvåg Cand. Real. Professor, Centre for Materials Science and Nanotechnology, Department of Chemistry, University of Oslo, Oslo, Norway Crystal structures of aluminum- based hydrides Emerging Materials Research http://dx.doi.org/10.1680/emr.15.00016 Research Article Received 18/02/2015 Accepted 21/08/2015 Published online 24/08/2015
Transcript
1
ice | science
The interest of hydrogen as the future energy is that it is a clean energy carrier, most abundant element in the universe,
the lightest fuel, richest in energy per unit mass and unlike electricity it can be easily stored. Hydrogen gas is the most
promising fuel for various applications. Hydrogen is already being used as the fuel of choice for space programs around
the world. Metal hydrides are a fascinating class of compounds, because the small mass and size of hydrogen and
its medium electronegativity cause a large flexibility in terms of metal–ligand interactions. These manifest itself in a
vast variety of possible compositions, chemical bonding, crystal structures and physical properties. In this review, we
presented the structural details of all up-to-date known Al-based hydrides.
NotationsM
3AlH
6 – Li
3AlH
6, Na
3AlH
6, K
3AlH
6
MAlH4 – LiAlH
4, NaAlH
4, KAlH
4
MH – LiH, NaH, KH
1. IntroductionThe world is facing energy shortage and has become increasingly dependent on new methods to store and convert energy for new, environmentally friendly methods of transportation and electrical energy generation as well as for portable electronics. Mobility – the transport of people and goods – is a socioeconomic reality that will surely increase in the coming years. Non-renewable fossil fuels are projected to decline sharply after 25–30 years. CO
2 emission
from burning such fuels is the main cause of global warming. Currently, the whole world is seeking international commitment to cut emissions of greenhouse gasses by 60% by 2050. There is a constant search for alternate fuel to solve the energy shortage that can provide us energy without pollution. Hence, most frequently discussed source is the hydrogen which when burnt in air produces a clean form of energy. The interest of hydrogen as the future energy is that it is a clean energy carrier, most abundant element in the universe, the lightest fuel, richest in energy per unit mass and unlike electricity it can be easily stored. Hydrogen
gas is the most promising fuel for various applications, such as to generate electricity, useful in cooking food, fuel for automobiles, hydrogen-powered industries, jet planes, hydrogen village and for all our domestic energy requirements. Hydrogen that can produce with little or no harmful emissions has projected as a long-term solution for a secure energy in future. Increasing application of hydrogen energy is the only way forward to meet the objectives of Department of Energy (DOE), USA – that is, reducing greenhouse gases, increasing energy security and strengthening the developing countries economy. Any transition from a carbon-based/fossil-fuel energy system to a hydrogen-based economy involves overcoming significant scientific, technological and socioeconomic barriers to ultimate implementation of hydrogen as the clean energy source of the future.
Hydrogen as a fuel has already found applications in experimental cars and all the major car companies are in competition to build a commercial car and most probably they may market hydrogen fuel automobiles in the near future. Hydrogen is already being used as the fuel of choice for space programs around the world. It will be used to power aerospace transport to build the international space station, as well as to provide electricity and portable water for its inhabitants. Hydrogen is the simplest and lightest element of our universe with only one proton and one electron.1 Hydrogen is not available as an element but in the form of compounds such as water
Crystal structures of aluminum-based hydridesVajeeston and Fjellvåg
Crystal structures of aluminum-based hydridesVajeeston and Fjellvåg
needed for survival of human beings and hydrocarbons being used as a fuel today. Hydrogen has potential to solve fuel needs having three times higher energy efficient compared to petroleum. Lots of research are going to find the commercially viable solutions for hydrogen production, storage and utilization, but hydrogen storage is very challenging, as application part of hydrogen energy totally depends on this. The interest in hydrogen as an energy alternative initiated in the late 1960s and has grown more and more in the 1990s.2 There have been tremendous efforts to produce it on a large scale.3 However, there are still many problems to implement hydrogen economy in daily life, out of which hydrogen storage is a major bottleneck. High-pressure storage4 and cryo-storage5 are not suitable ways for practical vehicular application due to their low energy density and also due to the safety reasons associated with them.
Metal hydrides are a fascinating class of compounds because the small mass and size of hydrogen and its medium electronegativity cause a large flexibility in terms of metal–ligand interactions. These manifest itself in a vast variety of possible compositions, chemical bonding, crystal structures and physical properties. The reversibility and improved kinetics have been found in Ti-enhanced NaAlH
4 by Bogdanović and Schwickardi,6 this finding initiated a
significant effort on hydro aluminates. Magnesium and aluminum-based metal hydrides attracted attention due to their potential use in reversible hydrogen storage. Such transition metal complexes (like Mg
2NiH
47), however, do not fulfill modern day’s requirements
for weight efficiency. Due to these reasons tremendous efforts have been made to search reliable materials that can hold hydrogen reversibly. As a target given by DOE, USA, a solid hydrogen storage material should have few commandments such as: (a) storage capacity to be at least 6·5 wt%, (b) desorption temperature to be 60–120°C, (c) low cost and (d) low toxicity. Metal hydrides,8,9 carbon-based materials,10–12 activated charcoal13,14 have been tested to fulfill above requirements, but unfortunately none of them could show satisfactory performance for commercial vehicular application.15–17 Recently, complex hydrides offered a possibility to design a potential hydrogen storage system due to their light weight and number of atoms per metal atom. Complex hydride termed as a group of materials that are a combination of hydrogen and group 1, 2, 3 light metals, for example, Li, Na, B and Al.18 Typical complex hydrides include alanates, borohydrides, amides, imides and so on. In the present review, the fundamental understanding of the physical, chemical and structural properties alanates has been presented.
2. AlanatesThe term ‘alanate’ also known as ‘aluminum hydrides’ refers to a family of compounds consisting of hydrogen and aluminum. The alanates are usually referred to as complex hydrides because of the presence of anionic metal complexes. These compounds have mixed ionic–covalent bonding features.19–21 The anionic complex
hydrogen covalently bonded to aluminum.19–21 The cation ionically bonded to the anionic complex. The Al–H bond is relatively weak, and in alanates the bonding strength is influenced by the cation matrix. The H atoms covalently bonded to Al in [AlH
4]−
tetrahedra.19–21 NaAlH4 is the most popular material of this family.
The search of reversibility of NaAlH4 was done by Bogdanovic
et al.6 Many methods of preparation have been developed to prepare these aluminohydrides and to study their structural and thermodynamical properties.22–26 The attractive feature of alanates is related to their easy accessibility. While sodium and lithium alanates are commercially available, magnesium alanate can be readily synthesized from sodium alanate and MgH
2 by way of a metathesis
reaction.27 Potassium alanate can also be formed from potassium hydride and aluminum under high pressure and temperature.28 The tetra-alanates have the general formula Mx+(AlH
4)
x (tetra-alanates)
and Mx+Ny+(AlH4)
x+y in so-called mixed tetra-alanates. M and N are
typically alkalies (x; y =1) or earth-alkali metals (x; y = 2), but elements from group III and IV in the Periodic Table also form alanates. Examples of promising tetra-alanates with high hydrogen contents are LiAlH
4 (10·6 wt%), NaAlH
4 (7·5 wt%), Mg(AlH
4)
2
(9·3 wt%), Ca(AlH4)
2 (7·9 wt%) and LiMg(AlH
4)
3 (9·7 wt%). The
desorption of hydrogen from alkaline-based tetraalanates takes place by the following two-step reaction:
1.
2.
where M = Li, Na and K. In the intermediate compounds M3AlH
6,
here named hexa-alanates, hydrogen covalently bonded to Al in [AlH
6]3− octahedra. There exist several other alanates with
[AlH6]3− octahedra based on alkali and earth-alkali elements.
Examples of such compounds are: (i) CaAlH5 and LiMgAlH
6,
that are intermediate phases in the desorption from Ca(AlH4)
2
and LiMg(AlH4)
3, respectively; (ii) mixed hexa-alanates, like
Na2LiAlH
6, K
2NaAlH
6, K
2LiAlH
6; and (iii) Ba- and Sr-based
hydrides (examples are BaAlH5, Ba
2AlH
7 and Sr
2AlH
7). To optimize
a material for hydrogen storage, one should have knowledge of their structural, thermodynamical and kinetics of hydrogenation properties. In this review, we present the crystal structure of most of the aluminohydride’s one by one (Table 1).
3. Crystal structure of alanates
3.1 Aluminum trihydrideThe current interest in the development of novel metal hydrides stems from their potential use as reversible hydrogen storage devices at low and medium temperatures. Various aluminium-based hydrides like catalyzed sodium alanate have been studied for this purpose. Aluminum trihydride (AlH
3) is an imperative material
because it is one of the by-products in most of the dehydriding reactions in Al-based hydrides. In addition, it has application as an energetic component in rocket propellants and a reducing agent
M MAlH 3 AlH 2 3 Al H4 3 6 2⇔ + +/
M M3 6 2AlH 3 H Al 3 2 H⇔ + + /
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Crystal structures of aluminum-based hydridesVajeeston and Fjellvåg
Crystal structures of aluminum-based hydridesVajeeston and Fjellvåg
Figure 1. Crystal structures of AlH3 in (a) α-AlH3, (b) α′-AlH3, (c) β-AlH3
and (d) γ-AlH3
H
(a) (b)
(c) (d)
HH
Al
AlAl
Al
H
c
a bc
b
c
c
baa
in alkali batteries and polymerization catalysts. Further, AlH3 is a
unique binary hydride having at least six crystalline phases with different physical properties and at the same time store up to 10·1 wt % of hydrogen.29 Its gravimetric hydrogen density is two times higher than liquid hydrogen and much greater than that of most of the known metal hydrides. In addition, elemental Al is a commonly available and recyclable material that could be an acceptable component of the future sustainable society. Thus, it is considered as a possible hydrogen storage material.30
The crystal structure of α-AlH3 has been well studied31 in
the literature, and less attention has focused on the other polymorphs. Recent theoretical study by Ke et al.32 found two new phases of AlH
3 which are energetically more favorable
than the stable α-modification. Followed by this study, Brinks et al.33,34 and Yartys et al.35 experimentally solved the structure of α′, β and γ-AlH
3 phases. Experimental results have shown
that α modification is the most stable at ambient conditions. The structural aspects of irradiated AlH
3 in comparison with the
various phases are also investigated in refs. 36–39. Similarly, the electronic structure32,40 and thermodynamic stability41 of α-AlH
3
are also well studied.
3.1.1 α-AlH3
α-AlH3 is the kinetically stable form and can be stored for several
years.42 This stabilization is probably caused by (hydr-)oxide layers on the particles, and the stability has also been reported to be dependent on the particle size of α-AlH
3. As a consequence,
AlH3 may be used as a chemical hydride. α-AlH
3 releases
hydrogen at ≥60°C.29,43 The structure of the α- modification was solved already in 1968 based on powder X-ray diffraction (PXD) and powder neutron diffraction (PND) data for both the hydride and the deuteride.31 Recently, α-AlH
3 was reexamined by
Brinks et al, and in this study the data were refined with Rietveld method.44 The α-AlH
3 structure may be described as a ReO
3-type
structure with rotated corner-sharing AlH6 octahedra (Figure
1(a)). In this phase, Al–H bond distance is 1·712 Å and H–Al–H angle is almost 90°.
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Crystal structures of aluminum-based hydridesVajeeston and Fjellvåg
3.1.2 α′-AlH3
PND and synchrotron powder X-ray diffraction (SRPXD) determined the structure of α′-AlH
3.44 The sample contained both the α′ and the
α modification, see above. α′-AlH3 takes a β-AlF
3 related structure
with space group Cmcm.44 The structure consists of AlH6 octahedra
where all H atoms shared between two octahedra (Figure 1(b)). The calculated average Al–H distance is 1·736 Å and the θ
H−Al−H varies
from 87·5 to 92·5°. The connectivity of the octahedra is significantly different from the α-AlH
3. For α′-AlH
3, four of the six octahedra
in the first coordination sphere are interconnected in pairs, whereas in α-AlH
3 none of the octahedra in the first coordination sphere are
connected with each other. The corner-sharing network is more open in α′-AlH
3 giving hexagonal-shaped pores with a diameter of 3·9 Å.
3.1.3 β-AlH3
The combined PND and SRPXD diffraction methods have determined that the β-AlH
3 takes the detailed pyrochlore
structure.34 The atomic arrangement is shown in Figure 1(c). Similar to the α and the α′ modification, the structure consists of
corner-sharing AlH6 octahedra. The octahedra are close to regular
with θ H−Al−H
= 87·1−92·9° and Al–H distances of 1·724 Å. All octahedra connected to two of the neighbouring octahedra and the surrounding octahedral to form two groups of three octahedra that are interconnected. α′-AlH
3 can be regarded as an intermediate
between α-AlH3 and β-AlH
3. The connectivity of the octahedra
leads to an open framework and channels of about 3·9 Å formed in several directions. The volume per formula unit is larger for β-AlH
3
(45·6 Å3) than α-AlH3 (33·5 Å3) and α′-AlH
3 (39·3 Å3).
3.1.4 γ-AlH3
The structure of the γ modification has recently been studied both by SRPXD of the hydride35 and combined PND and SRPXD of the deuteride.34 The space group is Pnnm. In addition to corner-sharing AlH
6 octahedra (as found in the other alane modifications),
γ-AlH3 also contains edge-sharing octahedra. The structure has
two types of octahedra. First one is involving only corner-sharing (average Al–H distance of 1·719 Å). The second type is connected to one octahedra by way of edge-sharing and four by way of edge-sharing, the average Al–H distance is 1·706 Å in this octahedra. The structure is shown in Figure 1(d). Two-third of the octahedra has edge-sharing. The crystal structure may describe as chains formed by pairs of edge-sharing octahedra connected by way of corner-sharing in the chain and by way of corner-sharing octahedra only between the different chains. The shortest Al–Al distances of 2·585 Å are found between the edge-sharing octahedra. The shortest Al–Al distance between corner-sharing octahedra is 3·155 Å, and this is similar to the shortest Al–Al distances in the other alane modifications. The average Al–H distances are very similar for the α-, α′-, β- and γ- AlH
3 modifications.
4. Complex hydridesThe term ‘complex’ hydride rather liberally applied to a rather large group of hydrides by various authors. In the broadest sense, these are hydrides composed of an anionic metal–hydrogen complex or non-metal–hydrogen complex bonded to a cationic alkali or transition metal (TM).45 Hence, the entire large group can be roughly subdivided into two categories. Group I and II – salts of [AlH
4]−, [NH
2]−, [BH
4]−, that is, alanates, amides, and
borohydrides46 and TM complex hydrides that have anionic (TMHx)− complexes such as [FeH
6]4− attached to a cationic light
metal, for example, Mg2+, in Mg2FeH
6.45 Their bonding is usually
an ionic–covalent mix. Similar transition metal ternary complex hydrides exist in the Mg–Co, Mg–Ni and Mg–Mn systems forming Mg
2CoH
5, Mg
2NiH
4 and Mg
3MnH
7 (the latter was synthesized
under 20 kbar H2 at ~ 800°C.47
A number of complex solid hydrides have very high theoretical gravimetric and volumetric hydrogen capacities combined with relatively low desorption temperature range due to quite favorable thermodynamics (enthalpies). It has made them a very attractive topic of research in the past 15 years. Unfortunately, a number of them are
Figure 2. Crystal structures of (a) LiAlH4, (b) NaAlH4 and (c) KAlH4.
Both RbAlH4 and CsAlH4 also have the same structure type (space
group Pnma) as KAlH4
(a)
Al
Al
Al
Na
H
H
H
LiB
B
o
o
c
A
A
c
(b)
(c)
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Crystal structures of aluminum-based hydridesVajeeston and Fjellvåg
still plagued by fundamental problems like high kinetic barriers to dehydrogenation and irreversibility. Nevertheless, in the all honesty, it must be said that if a very restrictive target of reversibility established by DOE could be moderated then at least a few complex hydrides would be quite close for vehicular applications in the near future.
5. Alkali-based tetra alanates
5.1 LiAlH4
Sklar and Post initially determined the crystal structure of LiAlH
448 through an X-ray diffraction study. Hauback et al.49
carried out a more detailed atomic structure determination of LiAlH
4 based on the combined powder neutron and X-ray
diffraction studies. The compound crystallized in the space group P2
1/c. The atomic structure found to consist of isolated [AlH
4]−
tetrahedra surrounded by lithium atoms (Figure 1(a)). The minimum Al−Al distance between tetrahedra was 3·754(0·01) Å at 295 K. The Al−H distances averaged 1·619(0·07) Å at 295 K, which are longer than the distances ranging from 1·516 to 1·578 Å that were deduced from the X-ray structure determination.48 The H−Al−H angles of LiAlH
4 were found to vary by less than
1·5° from the angles of a perfect tetrahedron. The Li−H distances
Figure 3. Crystal structures of (a) Li3AlH6 and (b) Na3AlH6. The
complex [AlH6]3− anions depicted as octahedra and the Li and Na
cations as spheres
H
(a) (b)
Al
Li
Na
H
Al
Figure 4. (a) Theoretically predicted (space group P21/c) and (b)
experimentally observed (space group Fm-3m) crystal structure of
Na2LiAlH6. The complex [AlH6]3− anions are depicted as octahedra and
the Li and Na cations as spheres
(a) (b)
HH
Na
Na
Li Li
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Crystal structures of aluminum-based hydridesVajeeston and Fjellvåg
ranged from 1·831(0·06) to 1·978(0·08) Å at 295 K and from 1·841(0·09) to 1·978(0·12) Å at 8 K.
5.2 NaAlH4
Lauher et al50 determined the atomic structure of NaAlH4 through a
single-crystal X-ray diffraction study in 1979. Refinement of their data in space group I4
1/a showed the compound to consist of isolated
[AlH4]− tetrahedra in which the Na atoms are surrounded by eight
[AlH4]− tetrahedra in a distorted square antiprismatic geometry.
Their results gave an Al−H bond length of 1·532(0·07) Å. These findings were significantly shorter than the Al−H bond distances that were previously determined from a single-crystal X-ray study of LiAlH
448 (average value of 1·548(0·17) Å). According to Bel’skii
et al.,51 it was inconsistent with the implications of the infrared (IR) spectra of the compounds. The Al−H stretching frequency of NaAlH
4 observed at a lower frequency than that of LiAlH
4 (1680 and
1710 cm−1, respectively). A second single crystal study generated data that converged to give an Al−H distance of 1·61(0·04) Å51 that was in agreement with the IR data. X-ray diffraction data tend to give erroneously short metal−hydrogen distances and colossal uncertainties in the determination of hydrogen coordinates.
Powder neutron diffraction data have determined the structure of NaAlD
4 at 8 and 295 K.52 The atomic structure was found to be made
up of isolated [AlD4]− tetrahedra surrounded by sodium atoms (Figure
1(b)). The shortest Al−Al separations were 3·737(0·01) and 3·779(0·01) Å at 8 and 295 K, respectively. The two unique Na−D bond distances were nearly equal, such as 2·403(0·02) and 2·405(0·02) Å at 8 K and 2·431(0·02) and 2·439(0·02) Å at 295 K. The Al−D distances were found to be 1·626(0·02) and 1·627(0·02) Å at 8 and 295 K, respectively. Previously, X-ray data by Bel’skii et al.51 reported a shorter and much more uncertain Al−D distance of 1·61(0·04) Å. On cooling from 295 to 8 K, the Al−D distances showed no significant change. The two unique D−Al−D bond angles in the [AlD
4]− tetrahedron were reported to be
107·32° and 113·86° at 295 K.
5.3 KAlH4
PXD determined the structure of KAlH4 at room temperature.
Available data on the crystal structure of potassium tetrahydro aluminate are scant and contradictory. In particular, the structure of KAlH
4 is solved in the monoclinic system with a = 5·897 Å, b =
7·360 Å and c = 8·815 Å in ref. 53 and in the tetragonal system with a = 7·47 Å and c = 9·31 Å in ref. 54 In the later study, it concluded that KAlH
4 crystallize in the orthorhombic system with the unit cell
parameters: a = 8·814, b = 5·819 and c = 7·551.53 In the above works, neither the hydrogen, aluminum and potassium atoms are located nor the Al–H and K–H distances are determined. The structure of KAlH
4
with the location of all atoms in the lattice is recently calculated from first principles in the framework of density functional theory (DFT).55 According to these calculations, the KAlH
4 lattice in the ground state
is orthorhombic with a = 9·009 Å, b = 5·757 Å and c = 7·393 Å, which is consistent with experimental data.56 The ground state of the KAlH
4
lattice is composed of slightly distorted tetrahedra with r(Al–H) = 1·654 Å separated by K+ cations. Each potassium atom is surrounded by 12 hydrogen atoms at distances 2·717–3·204 Å.
The powder diffraction pattern simulated on the basis of the calculated structure of KAlH
4 almost coincides with the
experimental X-ray powder diffraction pattern in ref. 56 The calculated reflections pertaining determined the Slight differences in position and intensity between the calculated and measured reflections in the diffraction patterns to a perfect defect-free lattice at 0 K, whereas the experimental diffraction pattern is obtained at room temperature on a sample containing impurities and defects.
Recently, the KAlD4 structure at 8 and 295 K was determined
by neutron diffraction.57 KAlD4 has a BaSO
4-type structure with
space group Pnma. The structure (Figure 1(c)) consists of isolated [AlD
4]− tetrahedra in which potassium atoms were surrounded
by seven of the tetrahedra (ten D atoms total). The average Al−D distance was 1·631 Å at 8 K and 1·618 Å at 295 K. The minimum Al−Al distance between the tetrahedra was 4·052 Å at 295 K. Also, D−Al−D bond angles were close to ideal and ranged from 106·4 to 113·3° at 8 K and 106·2−114·6° at 295 K. In addition, the minimum K−D distance was 2·596 Å at 295 K (larger than the Na−D distance of NaAlD
4 and the Li−D distance of LiAlD
4).
Figure 5. Crystal structures of Na5Al3H14. The complex [AlH6]3− anions
depicted as octahedra and the Na cations are displayed as sphere
HNa
Al
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Crystal structures of aluminum-based hydridesVajeeston and Fjellvåg
5.4 RbAlH4 and CsAlH4
Bastide et al.58 found RbAlH4 and CsAlH
4 to have the same
structure type (space group Pnma) as KAlH4. This is in agreement
with the DFT predictions by Vajeeston et al.59 However, no detailed structural studies have so far reported for these compounds. It is noteworthy that variations in the crystal structures of MAlH
4
compounds (M = Li, Na, K) can be due to the differences in the size of the alkali cations of Li+, Na+, and K+, which result in coordination numbers of 5, 8 and 10, respectively.59
6. Alkali metal hexahydro aluminatesThe crystal lattice of alkali-metal hexahydro aluminates built of isolated octahedral anions [AlH
6]3+ and alkali-metal cations
M+. The first X-ray powder diffraction studies of M3AlH
6 made
it possible to determine the type of crystal system and unit-cell parameters for the lithium,60 sodium61 and potassium53 salts. The complete crystal structures of Li
3AlD
661 and Na
3AlD
662 have been
solved, including the location of the deuterium atoms, on the basis of combined synchrotron X-ray and neutron diffraction data.
6.1 Li3AlH6
The lithium hexa-alanate is the intermediate phase during decomposition of LiAlH
4.63 The combined PND and SR-PXD
methods determined the structure of Li3AlH
6 data at room
temperature giving the space group R-3.64 The structure consists of
isolated and close to regular [AlH6]3− octahedra, which connected
by way of six-coordinated Li (Figure 2(a)). The structure described as a distorted bcc of [AlH
6]3− units with half the tetrahedral sites
filled with Li. The average Al–H and Li–H distances (within the first coordination sphere) are 1·744 Å and 2·001 Å, respectively. The structure can also be described as a distorted bcc structure of [AlH
6]3− units with all tetrahedral sites filled by Li. The
experimental structural model based on PND data measured at 9 K has the same space group as at room temperature and reveals magnificent correspondence with the DFT-calculated structure.21
It is important to note that, in general, all aluminohydrides with known structures, only lithium hexahydro aluminate Li
3AlH
6
and its isotopomer Li3AlD
6 crystallize in different space groups.
According to X-ray powder diffraction study at 298 K,60 Li3AlH
6
crystallizes in the monoclinic space group P21/c with a = 5·667 Å, b = 8·107 Å, c = 7·917 Å, β = 92·17° and z = 4. At the same time, lithium hexadeuteroaluminate Li
3AlD
6 at 295 K crystallized in the
orthorhombic space group R-3 with a =8·07117 Å, c = 9·5130 Å and z = 6.64
6.2 Na3AlH6
Na3AlH
6 takes a monoclinic P2
1/n structure.62 The structure consists
of isolated [AlD6]3− octahedra with the Al atoms arranged in a
pseudo fcc sublattice (Figure 2(b)). The average Al–D distances within the [AlD
6]3− octahedra are similar for Na
3AlH
6 (1·758 Å)
Figure 6. Crystal structures of (a) SrAl2H2, (b) SrSiAlH and (c) Sr2AlH7
H
H
H
Al
Al
Al
Sr(a)
(b)
(c)
Si
Sr
Sr
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Crystal structures of aluminum-based hydridesVajeeston and Fjellvåg
and Li3AlH
6 (1·744 Å). The Na coordination number is both 8 and
6 in Na3AlH
6 compared to 6 in Li
3AlH
6 because of different sizes
of the alkali ions. It will give a shorter average Li–D distance than Na–D range within the first coordination sphere. The distance between the Al tetrahedra is also somewhat larger in the Na hexa-alanate (shortest Al–Al distance: 5·390 Å) than in the Li hexa-alanate (shortest Al–Al distance: 4·757 Å).
6.3 K3AlH6
The only X-ray powder diffraction study of potassium hexahydroaluminate at 298 K determined only unit cell parameters.53 According to these data, K
3AlH
6 is tetragonal with
a = 8·445 Å and c = 8·584 Å. On the other hand, according to the DFT calculations, K
3AlH
6 has been stabilized in the same structure
type as Na3AlH
6 (space group P21/n).65,66 From the total energy
calculations at 0 K and ambient pressure with unit-cell dimensions: a = 6·1771, b = 5·8881, c = 8·6431 Å and β = 89·30°.66 To the best of our knowledge, no structural data on hexahydroaluminates of heavy alkali metals are available.
7. Alkali metal mixed hexahydroaluminatesThe alkali alanates discussed above have a low kinetics and low enthalpies and, thus, require very high pressures for the rehydrogenation of the material. These shortcomings led the researchers to find some other materials that could retain the high capacity, but at ambient condition. It will generate the idea of mixed alanates containing more than one alkali or alkaline earth atom. Using DFT calculations, Løvvik and Swang65 investigated existing and hypothetical compounds of the form A
3-xB
xAlH
6, where
A and B are Li, Na or K while x = 0, 1, 2 or 3. The following four bi-alkali alanates were predicted to be thermodynamically stable: Na
2LiAlH
6, K
2LiAlH
6, K
2NaAlH
6 and KNa
2AlH
6, and from these
only KNa2AlH
6 does not find experimentally.
7.1 Na2LiAlH6
Na2LiAlH
6 has known for several years, and a cubic unit cell of
7·405 Å has been proposed from PXD data.67 According to the DFT study by Løvvik and Swang, Na
SrAlH5 and (e) BaAlH5. AlH6 octahedra are marked in colour
H HH
H
Mgc
c
c
b
bba
a
a
c ba
cb
a
CaBe
Ba
Al
Al
Al
AlAl
Sr
H
(a)
(b)
(c)
(d) (e)
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Crystal structures of aluminum-based hydridesVajeeston and Fjellvåg
group P21/c at 0K.65 Recent SR-PXD data showed that the structure takes the space group Fm-3m with a = 7·40641 Å.68 The detailed structure has been determined by PND and SR-PXD experiments of the deuterium analogue Na
2LiAlH
6 giving a = 7·384845 Å.69 It is
interesting to note that this compound has an ordered perovskite-type structure with Li and Al in octahedral positions (see Figure 3(a)). The structure consists of a three-dimensional network of corner-sharing [AlH
6]3− and [LiH
6]3− octahedra where each octahedra is
surrounded by six octahedra all of them of the different type. Na is 12-coordinated with 12 H atoms from four different octahedra. The Na––H distance (2·612 Å) is larger than in Na
3AlH
6, probably
because of the increased coordination number from 6/8 to 12. The structure of Na
2LiAlH
6 may also be described as: (i) An ordered
perovskite A2BB′X
6 with A = Na in a 12-coordinated site and both
Li = B and Al = B′ in octahedral positions. The structure is very similar to Na
3AlH
6 where Na is substituted by Li in one of the Na
(A) positions. (ii) A lattice of cubic closest packing geometry of AlH
6 entities with Na filling the tetrahedral positions and Li the
octahedral position.
7.2 K2LiAlH6
From DFT work65 and SR-PXD data68, the structure of K2LiAlH
6
was reported to be isostructural with the Na2LiAlH
6 (space group
Fm-3m; see Figure 3(b)), but the PXD data were not of sufficient quality for a conclusion on the structure. Rietveld refinements of PXD data70 found that a correct description of the structure is with the space group R-3m. Since the refinements are based on PXD data, it is likely that the reported refined atomic coordinates are only approximate. The refined Al–H distances of 1·31–1·47 Å are short as compared to other alanates. This may be due to the low scattering factor of hydrogen in X-ray diffraction, leading to unreliable hydrogen atom positions, which are usually determined by collecting neutron diffraction spectra of a deuterated sample. Unfortunately, deuterated samples of K
2LiAlD
6 were not prepared
due to the difficulty of preparing pure KD. However, it was concluded in literature70 that the structure of K
2LiAlH
6 is likely
isostructural with the hexagonal–rhombohedral form of K2LiAlF
6.
Accurate PND data are needed for an accurate description of the structure. According to DFT calculation, both Cs
2NaAlF
6 type
structure with a symmetry C2/m and high-temperature fluorite structure (K
2LiAlF
6) with symmetry R-3 are having similar total
energy. The calculated Al–H bond distance in these structures varies between 1·77 Å and 1·79 Å.
7.3 K2NaAlH6
K2NaAlH
6 takes the same structure as Na
2LiAlH
6.68,69 The detailed
structure of the hydride was determined by PND.71 The Al–H/D distances are nearly equal in K
2NaAlH
6 and Na
2LiAlH
6. The
average Al–H distance is 1·761 Å and 1·756 Å for K2NaAlH
6 and
Na2LiAlH
6, respectively. In spite of the significant differences
in cation sizes, these compounds are isostructural. However, all intermetallic distances are about 10% longer in K
2NaAlH
6 than in
Na2LiAlH
6 due to expanded unit cell and metal atoms in particular
positions in the structure.
The decomposition temperature and enthalpy depend on the size of the alkali metals A and B. In general, the decomposition temperature and enthalpy increase with the size of the alkali metal. This trend found in both mono tetra- and hexa-alanates. This tendency is also applied to partial substitution of the metal atoms.68 For example, for A = K and B = Li, Na and K the stability of these materials follows this sequence: K
3AlH
6 > K
2NaAlH
6 >
K2LiAlH
6. However, one exception from the general rule is the
substitution of Li for Na where Na2LiAlH
6 is more stable than
Na3AlH
6.
7.4 Na5Al3H14
Ojwang et al.72 investigated the structure of Na5Al
3H
14 by using DFT
study. Na5Al
3H
14 is crystallized in the space group P4/mnc with two
formula units per unit cell and it is one of the possible intermediates of the thermal decomposition of NaAlH
4. The structure thought of
as a slightly distorted perovskite, and it has layers of AlH6 octahedra
(Figure 4). There are two types of AlH6 octahedra whose symmetries
are different, which form shifted independent [Al3H
14]
n5n- layers
perpendicular to the c axis. Within the unit cell, a third of the octahedra share four corners and the remaining share only two. The sharing of cis two vertices of octahedra can lead to either a zigzag chain or cyclic molecules. The doubly bridged and tetra-bridged octahedra form a linear chain due to sharing of trans vertices and at the same time are involved in a cyclic network of eight octahedra due to sharing of cis vertices. In reality, this Na
5Al
3H
14 phase was
not observed in experiments because, if it forms during the thermal decomposition of NaAlH
4 then, they are quasi-stationary states. In
particular, the inclusion of Na5Al
3H
14 in the decomposition pathway
of NaAlH4 nicely explains how the lattice structure of NaAlH
4 is
disrupted and the mobile alane species are formed.
Figure 8. Crystal structures of (a) Mg(AlH4)2 and Ca(AlH4)2
H
H
(a) (b)
Al
Ca
Mg
Al
Emerging Materials Research
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Crystal structures of aluminum-based hydridesVajeeston and Fjellvåg
8. Alkaline earth-based aluminohydridesData on the preparation and crystal structures of alkaline earth aluminohydride’s are limited compared to the alkaline-based alumina hydrides. For example, in the AAlH
5 (A = any one of the
alkaline earth elements) series only MgAlH5 and BaAlH
5 have been
identified experimentally,73 whereas the crystal structure of BeAlH5,
CaAlH5 and SrAlH
5 phases are not yet solved experimentally. No
information on BeAlH5 is available in the literature, probably
because Be is severely toxic. CaAlH5 and SrAlH
5 are reported to
form as intermediate products on gentle heating of Ca(AlH4)
274 and
Sr(AlH4)
2,75 respectively. SrAlH
5 is also reported76 to form during
‘mechanochemical activation’ of Sr(AlH4)
2. DFT calculations have
predicted the crystal structure of BeAlH5, CaAlH
5 and SrAlH
5
phases and experimental verification is needed.77,78
8.1 SrAl2H2
SrAl2 is usually considered as a member of the large family of Zintl
phases that form between active metals (alkali, alkaline earth or rare earth metals) and more electronegative p-block metallic or semimetallic elements (Al, Ga In). According to the Zintl concept, Al is formally reduced by the electropositive Sr and features a three-dimensional four-connected (3D4C) polyanionic network in which each Al atom is surrounded by four neighbors in a distorted tetrahedral fashion. This arrangement fits the electron count of Al-, which is isoelectronic to Si. SrAl
2H
2 synthesized by the reaction
of SrAl2 with H
2 at 50 bar and temperatures below 200°C and
their structures studied by X-ray and neutron diffraction.79 As the temperature increased to 513 K, SrAl
2H
2 absorbs an extra portion
of hydrogen to form Sr2AlH
7.80 SrAl
2H
2 is the first known alkaline
earth-based aluminum hydride and it crystallizes with a new structure type in the trigonal space group P-3m1 (164).79 Three crystallographic sites occupied; apart from Sr in 1a and Al in 2d there is one D atom located on another 2d site. The Al atoms are located in slightly puckered hexagonal nets perpendicular to the trigonal c axis. One hydrogen atom is bonded to each Al atom,
alternating below and above the net (Figure 5(a)). The Sr atoms are located in the space between the nets and are surrounded by 6H neighbors. The average distance between the Al nets is 4·72 Å, the distances between Sr and its 12 Al neighbors are 3·39–3·65 Å. The Al–D bond (1·71 Å) is longer than the Al–H bonds in LiAlH
4
(1·54–1·58 Å) or NaAlH4 (1·61 Å).
8.2 SrSiAlHThe unexpected discovery of superconductivity at 39 K in MgB
281
has attracted much attention to the layer-structured compounds with the AlB
2-type structure because of their potential in the search for
Si and (Ga and Al) atoms arranged in the hexagonal honeycomb layers, and alkaline-earth metals intercalated between them and become superconductors with transition temperatures Tc of 3·5, 7·8 and 5·1 K, respectively.84–87 Similar isotypic ternary compounds M(Ga
0.5Si
0.5)
2 (M = Ca, Sr and Ba) have also been reported to
become superconductors at Tc = 4·1–5·2 K.85,86 Furthermore, some compositions, MAlSi, react with hydrogen and form the monohydride MAlSiH.88 Hydrogen may be incorporated in the polymeric anion where it acts as a covalently bonded terminating ligand to a p-block metal atom. The H incorporation imposes only small changes to the structure. However, electronic structures can be dramatically influenced by the inclusion of H.88
The structure of SrAlSiH is very similar to that of SrAl2H
2:
Half of the [Al–H] entities in the polyanionic layer of SrAl2H
2
is replaced by isoelectronic Si (Figure 5(b)). This replacement occurs in a strictly ordered way; that is, each Si atom is surrounded by three [Al–H] entities and vice versa. In SrAlSiH, the center of inversion present in SrAl
2H
2 is lost, and the space group
symmetry reduced to P3m1. The local coordination of H is the same; however, the isoelectronic replacement of Al–H by Si has drastic consequences on the electronic structure and properties:
Figure 9. The crystal structure of the (a) LiMg(AlH4)3, (b) LiCa(AlH4)3-
and (c) LiAlMg10H24 compounds.
H
H
Al
Al
Mg
CaAl
H
Li
Mg
(c) (c)(a)
Li
LiB
C
C
BA
A
O
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Crystal structures of aluminum-based hydridesVajeeston and Fjellvåg
SrAl2H
2 is a metallic conductor, whereas for SrAlSiH a gap is
opened in the density of states at the Fermi level (e.g. = 0·63 eV), which makes the compound a unique example of a narrow-gap semiconductor hydride. Additionally, thermal stability is raised dramatically. The hydrogen desorption temperature of SrAlSiH exceeds 650°C (1 atm) and is the highest known for aluminum hydride compounds. The most conspicuous difference between the two structures is that in SrAlSiH, Sr is only coordinated by three H atoms. The Sr–H distance in SrAlSiH is considerably shorter than that in SrAl
2H
2 (2·48 vs. 2·65 Å) and the Al–H
distance is significantly larger (1·77 vs. 1·71 Å). Additionally, in SrAlSiH, [Al–H] entities are well separated while, in SrAl
2H
2,
H atoms approach each other at a distance of 2·77 Å. A series of compounds in this MAlM′H (M = Ca, Sr and Ba; M′ = Si and Ge) family was also like SrAlGeH, BaAlGeH, CaAlGeH, CaAlSiH, SrAlSiH and BaAlSiH; and are also stabilized in space-group symmetry P3m1.88–90
8.3 Sr2AlH7
Sr2AlH
7 crystallize in a new monoclinic structure in space group I2
(No. 5). Sr2AlH
7 is the first example that consists of isolated [AlH
6]
units and infinite one-dimensional twisted chains of edge-sharing [HSr
4] tetrahedra along the crystallographic c axis. The crystal
structure of Sr2AlD
7/H
7, (Figure 5(c)) is built up from isolated
[AlH6] units and infinite one-dimensional chains of edge-sharing
[HSr4] tetrahedra.80 In the Al-centered [AlH
6] octahedron, the Al–H
bond lengths ranging from 1·71 to 1·76 Å are in good agreement with those in Na
3AlH
6 10 (1·75–1·77 Å). The H-centered [HSr
4]
tetrahedra are inverted alternately and share one edge to form infinite one-dimensional twisted chains along the c axis. The Sr–H bond lengths (2·46–2·55 Å) compare well with those in binary SrH
2
(2·36–2·80 Å).
8.4 BeAlH5
The crystal structure of BeAlH5 (Figure 6(a)) has been predicted
by Klaveness et al.78 and it exhibits alternating layers of corner-sharing AlH
6 octahedra which are connected by twin chains of
BeH4 tetrahedra. Each AlH
6 octahedron shares corners with four
other AlH6 octahedra and two BeH
4 tetrahedra (see Figure 6(a)).
Each BeH4 tetrahedron shares corners with two AlH
6 octahedra and
two other BeH4 tetrahedra. The polyhedra in the BeAlH
5 structure
are the most regular of the entire AAlH5 series.
8.5 MgAlH5
DFT predicted that MgAlH5 stabilize in a monoclinic P21/c
structure (CaFeF5-type).77 The MgAlH
5 structure has comprising
AlH6 octahedra and capped MgH
7 octahedra. The AlH
6 octahedra
share corners and edges with capped MgH7 octahedra. The Al–H
and Mg–H bond distances in MgAlH5 fall in the ranges 1·68–1·78
and 1·86–2·31 Å, respectively. The H–Al–H bond angles (81·72°–97·96°) demonstrate that also the AlH
6 octahedra of MgAlH
5 are
highly distorted. The H–Mg–H angles in the MgH7 polyhedra
take values between 62·02° and 99·56°. One interesting structural feature is the MgH
7 configuration which distinguishes MgAlH
5
from the AAlH4 (A = alkali metal) series. In the latter series, A
cannot be ascribed meaningful coordinations. This suggests that MgAlH
5 displays a somewhat different bonding situation for the
hydrogen atoms than in the AAlH4 series.
8.6 CaAlH5
From DFT calculations, it has been predicted that CaAlH5 stabilize
in BaFeF5-type monoclinic P21/n structure.78 CaAlH
5 consisted
of non-linear chains of AlH6 octahedra and isolated Ca ions (see
Figure 6(c)). Essentially, the same ground-state structure for CaAlH
5 is reported by the independent computational-based
Figure 10. The crystal structure of the (a) REAlH6 (with RE = La, Ce, Pr
and Nd) and (b) Th2AlH4 compounds
(a)
La H
HAl
Th
AO
B
C
(b)
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Crystal structures of aluminum-based hydridesVajeeston and Fjellvåg
investigation of Weidenthaler et al.91 The chains in the CaAlH5
structure resemble spirals. According to the theory, at 4·5 GPa, monoclinic CaAlH
5 transform into high-pressure orthorhombic
(SrAlH5; P2
12
12
1) modification. Both of these monoclinic and
orthorhombic modifications are not yet identified experimentally.
8.7 BaAlH5
The reaction of the Ba7Al
13 alloy with H
2/D
2 at ~333 K and 7 MPa
yields barium alumina hydride BaAlH5; at 603 K, the reaction
results in Ba2AlH
7.73,92 BaAlH
5 crystallize in the orthorhombic
space group Pna21.92 The crystal structure of BaAlH5 (Figure
6(c)) contains corner-sharing AlH6 octahedra, which form one-
dimensional zigzag chains along the crystallographic c axis. These chains are surrounded by Ba atoms which form a distorted hexagonal network. This feature is an important structural element of the stability of BaAlH
5. The calculated positional
parameters for BaAlH5 make the AlH
6 octahedra highly distorted.
The H–Al–H bond angles range between 81·72° and 97·96° and the Al–H bond lengths between 1·69 and 1·85 Å. The average Al–H bond length (1·77 Å) is close to that in Li
3AlH
6 [1·73 Å
(ref. 64) 1·75 Å (ref. 21)] and Na3AlH
6 [1·76 Å (ref. 62)]. The
closest shell of H atoms around Ba resides at distances ranging from 2·65 to 3·07 Å and consists of 14 H atoms. The shortest H–H separation in BaAlH
5 is 2·25 Å and complies accordingly
with the 2 Å rule.93,94
8.8 SrAlH5
At ambient conditions, SrAlH5 crystalize in orthorhombic P2
12
12
1
structure which is predicted by DFT.78 The structure of SrAlH5
(Figure 6(e)) contains zigzag chains of AlH6 octahedra and more
isolated Sr ions. Crystal structure of this phase is not yet identified experimentally.
8.9 Ba2AlH7
Ba7Al
13 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; Ba
7Al
13 is hydrogenated to Ba
2AlH
7 and Al. Ba
2AlH
7 is iso-
structural to Sr2AlH
7, crystallizing with a monoclinic structure in
space group I2/a.92 Ba2AlH
7 consists of isolated [AlH
6] units and
infinite one-dimensional twisted chains of edge-sharing [HBa4]
tetrahedra along the crystallographic c axis (see Figure 5(c)).
8.10 Mg(AlH4)2
The structure of crystalline magnesium alanate [Mg(AlH4)
2] was
determined by Fichtner et al.95 using powder XRD on the basis of DFT calculations (Figure 7(a)). A more detailed study of the crystal structure was performed by Fossdal et al. using XRD, PND and synchrotron radiation.96 The space group was confirmed to be P3m1. The structure consists of a sheet-like arrangement composed of [AlH
4]− tetrahedra surrounded by six Mg atoms in a distorted
Figure 11. Crystal structures of (a) α- and (b) β-modification of
Al(BH4)3. The magnified molecular unit of Al(BH4)3 is shown in the
circle
HB
(a) (b)
Al
C
B
B
O
O
AA
Emerging Materials Research
20
Crystal structures of aluminum-based hydridesVajeeston and Fjellvåg
MgH6 octahedral geometry (Figure 7(a)). The Al−H distances
ranged from 1·606(0·10) to 1·634(0·04) Å at 8 K and from 156·1(0·12) to 167·2(0·04) pm at 295 K. It should be noted that these distances are in the same range as those found for lithium, sodium and potassium alanate.
8.11 Ca(AlH4)2
Calcium alanate Ca(AlH4)
2 was first synthesized by Schwab
and Wintersberger in 1950.97 Since little has yet been reported on the crystal structure of Ca(AlH
4)
2, Fichtner et al.74 said the
structure of Ca(AlH4)
2·4THF using powder XRD and found that
Ca(AlH4)
2·4THF crystallized in the monoclinic space group
P21/n with two formula units per unit cell. They found a similar molecular structure of Ca(AlH
4)
2·4THF compared to that of Mg(
AlH4)
2·4THF.98,99 It consists of a central calcium ion occupying a
crystallographic inversion center which is octahedrally coordinated by two hydrogen atoms of two [AlH
4] units and four oxygen
atoms from four THF molecules. Attempts were made to predict the crystal structure of solvent-free Ca(AlH
4)
2, but it proved not to
be possible, probably due to rapidly rotating the AlH4 tetrahedra.
Recently, Løvvik100; Wolverton and Ozolins101 proposed a crystal structure of solvent-free Ca(AlH
4)
2 from DFT calculations (Figure
7(b)). The most stable structure is Pbca, which is derived from the CaB
2F
8 structure. In this structure, the hydrogen is found to be
coordinated around Al in slightly distorted tetrahedra with Al–H bond length 1·61–1·63 Å and H–Al–H angle between 106·8 and 113·2°. Ca is eight-coordinated to H in distorted square antiprisms, with each corner shared by an AlH
4 tetrahedron. The structure
is found to be relatively loose with large voids. Due to this, the barrier of rotation for the tetrahedral is small, which is the reason suggested by Lovvik100 for the difficulty to confirm the structure experimentally.
9. Alkali and alkaline earth-based mixed aluminohydrides
9.1 LiMgAlH6
The crystal structure of LiMgAlH6 has been investigated using
synchrotron radiation XRD, PND and DFT calculations by Grove et al.102 LiMgAlH
6 was found to have crystallizing in
trigonal space group P321, consisting of isolated AlH6 octahedra
connected through octahedrally coordinated Mg and Li atoms. The structure could be described as alternating Mg
3Al and Li
3Al
2
layers as shown in Figure 9(a). In the Li3Al
2 and Mg
3Al layers,
AlH6 octahedra are sharing edges with three LiH
6 and three
MgH6 octahedron, respectively. Each LiH
6 octahedron is sharing
edges with 2 Al-octahedra and forming a two dimensional networks during the Mg – octahedron shares an edge with only one Al octahedron resulting in the formation of isolated Mg
3Al
units. These layers are interconnected by corner-sharing of the
AlH6 − octahedron and six Mg/LiH
6 octahedra. All corners are
connected to Mg, Li and Al octahedra.
9.2 LiMg(AlH4)3
The structure of LiMg(AlH4)
3 was investigated by Grove et al using
synchrotron radiation powder X-ray diffraction, PND and DFT atomic simulations.103 The P21/c structure consists of isolated AlH
4
tetrahedra, connected separately through the four corner H atoms to two Li and two Mg atoms. Each Li and Mg atom are octahedrally coordinated to the corner H atoms of six AlH
4 tetrahedra so that
the structure consists of a corner-sharing network of alternating AlH
4 tetrahedra and LiH
6 or MgH
6 octahedra. In this structure
Al–H distances are 1·621 Å, the Li–H distances vary from 1·873 to 2·093 Å and the Mg–H distances vary from 1·86114 to 1·91911 Å. The shortest H–H distance of 2·52519 Å was found within the Al-tetrahedra. The shortest interpolyhedral H–H distance is 2·78016 Å. The structure can be described as a distorted hexagonal closed packed geometry of AlH
4 tetrahedra, with Li and Mg occupying
2/3 of the interstitial octahedral sites. At 130°C, LiMg(AlH4)
3
decomposes to LiMgAlH6. Thus, LiMg(AlH
4)
3 is not useful for
reversible hydrogen storage while LiMgAlH6 is quite stable by
thermodynamic consideration.104
9.3 LiAlMg10H24
DFT predicted that LiAlMg10
H24
is crystallized in monoclinic P121 structure (Figure 9(b)).105 This phase was formed when Li and Al are coupled-substituted into the MgH
2 structure to assume the composition
of LiAlMg10
H24
, the resulted structure was found to retain that of the parent material but implies substantial differences in bonding and associated properties. The crystal lattice of the LiAlMg
10H
24 structure
is actually pseudo-tetragonal, with a and b differ by less than 0·004 Å and β close to 90° (by a difference of <0·4°), and bears strong resemblance to the parent structure. The resemblance is almost similar to the simulated powder X-ray diffraction patterns of the MgH
2 and LiAlMg
10H
24 structures.105 Therefore, there could be a non-
trivial likelihood that the LiAl-substituted MgH2 structure might have
already been produced in a lab but undetected.
9.4 LiCa(AlH4)3
Recently, Liu et al . synthesized LiCa(AlH4)
3 by a mechano-
chemically activated reaction of LiAlH4 with CaCl
2 in a molar
ratio of 3:1.106 LiCa(AlH4)
3 crystallized in a hexagonal structure
with space group P63/m (No. 176)), and with cell parameters a = b = 8·9197 Å and c = 5·8887 Å, which is different from those of LiAlH
4, (monoclinic structure, space group P21/c26,49), Ca(AlH
4)
2
(orthorhombic structure, space group Pbca107) and LiMg(AlH4)
3,
monoclinic structure, space group P21/c26,103). Due to the low sensitive of X-ray to hydrogen, the atomic coordinates for H atoms were not determined in this work and the characteristics
Emerging Materials Research
21
Crystal structures of aluminum-based hydridesVajeeston and Fjellvåg
of Al–H, Ca–H and Li–H bonds in LiCa(AlH4)
3 were unknown
yet. Even so, it was still found that the Li–Al distance of 3·035 Å is slightly shorter than 3·214–3·415 Å in LiAlH
449 and 3·255 Å
in LiMg(AlH4)
3,103 and the Ca–Al distance of 3·774 Å is slightly
longer than 3·578 Å in Ca(AlH4)
2. Further studies on the structure
of LiCa(AlH4)
3 by means of neutron diffraction are necessary.
The H position in LiCa(AlH4)
3 is investigated by way of DFT
calculations.108 Based on the experimentally determined hexagonal symmetry (P63/m, no. 176), hydrogen atoms positions are identified and the optimized crystal structure parameters of LiCa(AlH
4)
3
agree well with the experimental results. This structure is similar to the CdTh(MoO
4)
3 structure (see Figure 8(b)). In this structure,
Al–H distances vary from 1·624 to 1·633 Å, the Li–H distances are 1·727 Å, and Ca–H distances vary from 2·234 to 2·305 Å.
10. Rare-earth aluminum hydrides
10.1 REAlH6 (RE = La, Ce, Pr and Nd)A series of rare-earth aluminum hydride has been prepared by Weidenthaler et al.109 using mechanochemical preparation. The crystal structure of the REAlH
6 (with RE = La, Ce, Pr and Nd)
compounds was calculated by DFT methods and confirmed by structure refinements.109 All these phases crystallized in trigonal crystal structure with R-3m space group. The crystal structures of the new compounds REAlH
6 are built up of isolated [AlH
6]3−
octahedra which are not directly connected. The presentation of the LaAlH
6 structure, which was chosen as a typical example, along the
crystallographic c axis shows [AlH6]3− octahedra alternating with
the RE cations forming a chain-like arrangement (Figure 9(c)). The view along the crystallographic b axis shows the chain-like arrangement of [AlH
6]3− octahedra between which the RE cations
are located. The coordination of the La cations is 12 with 6 coordinating hydrogen atoms in the ab plane and 6 hydrogen atoms coordinating the rare-earth cation along the c axis with a slightly larger distance. The investigation of the rare-earth aluminum hydrides during the thermolysis shows a decrease of thermal stability with increasing atomic number of the RE element. Rare-earth hydrides (REHx) are formed as primary dehydrogenation products; the final products are RE-aluminum alloys.
10.2 Th2AlH4
Th2AlH
4 crystallize in space group I4/mcm with the lattice parameters
a = 7·629, c = 6·517 Å.110,111 Th2AlH
4 is the only compound in
the Al-based hydrides where one can tune the H content from 2 to 4 and this compound does not belong to complex hydrides. The crystal structure of Th
2AlH
4 is illustrated in Figure 9(d). The
crystal structure of the Th2Al contains four crystallographically
different interstitial sites, which are the suitable sites for hydrogen accommodation, 16l and 4b each coordinate to four Th, 32m coordinates to three Th and one Al, and 16k coordinates to two Th, and two Al. Each 16l-based intersite tetrahedron shares a common
face with another 16l-based tetrahedron, whereas the 4b-based tetrahedron shares each of its four faces with 16l-based tetrahedra. Some of the tetrahedral intersites are firmly separated owing to the face sharing of the coordination polyhedral (for more details, see ref. 110). According to the experimental findings (ref. 110), the 16l sites are fully occupied in Th
2AlD
4 and also the structure is
completely ordered. The volume expansion during hydrogenation of Th
2Al is 12·47%, ΔV/H atom is 10·32 Å3. This volume expansion
is strongly anisotropic and proceeds predominantly perpendicular to the basal plane of the tetragonal unit cell; Δa/a = 50·026%, Δc/c = 512·41%.112
with the melting point of 209 K. The structures of the solid phase have been investigated by XRD measurements.113 Cooling liquid Al(BH
4)
3,
the orthorhombic β-phase (monoclinic, space group C2/c; Figure 10(b)) was initially grown and then the transition to the monoclinic α-phase (orthorhombic, space group Pna2
1; Figure 10(a)) occurred
at temperatures in the range 180–195 K. These structures have been solved using XRD along with the parameters deduced for the gaseous molecule113 by electron diffraction and those calculated by ab initio methods.114 The crystal structures of both phases of aluminium tris (tetra-borohydride) are made up of discrete molecular Al(BH4)3 units. The geometry of the Al(BH4)3 molecule itself varies little between the two phases (see Figure 10), the most significant differences affecting the B(Ht)2 angles, which appear to be less uniform in the α-phase. The principal difference between the α- and β-phases relates to the packing of the molecular units. In the β phase, there are two ‘nearest neighbour’ molecules positioned above and below the triangular faces of the trigonal-prismatic Al(µ-H)6 unit such that the shortest Al–H distances are 3·6 Å. By contrast, in the α-phase there is only one such ‘nearest neighbour’, with the asymmetric units spiraling around a 21 axis (see ref. 113; Figure 11).
12. Ti-based aluminium hydridesTitanium is an important industrial metal primarily due to its large strength-to-weight ratio. Many of its alloys are used as structural components in the aerospace industry, in marine applications and in the field of medical implants. The strength and other mechanical properties of Ti may be improved by alloying with Al to form titanium aluminides such as Ti
2Al and TiAl. However,
the penalty for such improvements is a large loss in ductility. In the Ti–Al phases, several hydrogenated compounds also known in the literature.115,116 Based on hydrogen absorption data and X-ray studies, two ternary hydride phases in the Ti
2Al/H system below
473K: a BCC phase117 for 0·4<x<0·5 and a FCC phase for x>1 where x refers to the hydrogen to metal ratio (H/M) are reported by Rudman et al.118. The FCC phase was said to be metastable and disproportionated to give TiH
2 on heating above 473 K. The FCC
phase has been observed but the existence of the BCC phase has been questioned. Ti
2AlH was determined by neutron diffraction
Emerging Materials Research
22
Crystal structures of aluminum-based hydridesVajeeston and Fjellvåg
measurements to be of the cubic (perovskite like) E21 type.119 The Al atoms in this structure occupy the corners of the cube, while the Ti atoms are in the face-centered positions and hydrogen is located at the center of the cube surrounded by a perfect octahedron of Ti atoms. Ti
2AlH
8-z phase also known in the literature and this hydride
is metastable and disproportionates in to TiH2 and amorphous TiAl
or elemental Al at relatively low temperatures.115
Remark: The crystal structures of listed compounds in the review are downloadable from the following link (http://folk.uio.no/ponniahv/Database/al-str).
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