Emerging Materials Research
Crystal structures of aluminum-based
hydrides
Vajeeston and Fjellvåg
ice | science
http://dx.doi.org/10.1680/emr.15.00016
Research Article
Received 18/02/2015 Accepted 21/08/2015
Published online 24/08/2015
Keywords: characterization/energy storage/hydrogen storage
ICE Publishing: All rights reserved
Crystal structures of aluminumbased hydrides
1
Ponniah Vajeeston PhD*
2
Researcher, Centre for Materials Science and Nanotechnology,
Department of Chemistry, University of Oslo, Oslo, Norway
1
2
Helmer Fjellvåg Cand. Real.
Professor, Centre for Materials Science and Nanotechnology,
Department of Chemistry, University of Oslo, Oslo, Norway
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
M3AlH6 – Li3AlH6, Na3AlH6, K3AlH6
MAlH4 – LiAlH4, NaAlH4, KAlH4
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. CO2 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
*Corresponding author e-mail address: ponniahv@kjemi.uio.no
1
Emerging Materials Research
Crystal structures of aluminum-based
hydrides
Vajeeston 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.
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 [AlH4]−
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 MgH2 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+(AlH4)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 LiAlH4 (10·6 wt%), NaAlH4 (7·5 wt%), Mg(AlH4)2
(9·3 wt%), Ca(AlH4)2 (7·9 wt%) and LiMg(AlH4)3 (9·7 wt%). The
desorption of hydrogen from alkaline-based tetraalanates takes
place by the following two-step reaction:
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
NaAlH4 by Bogdanović and Schwickardi,6 this finding initiated a
significant effort on hydro aluminates. Magnesium and aluminumbased metal hydrides attracted attention due to their potential use
in reversible hydrogen storage. Such transition metal complexes
(like Mg2NiH47), 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.Alanates
The 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
2
1.
MAlH 4 ⇔ 3M3 AlH 6 + 2 / 3 Al + H 2
2.
M3 AlH 6 ⇔ 3MH + Al + 3 / 2 H 2
where M = Li, Na and K. In the intermediate compounds M3AlH6,
here named hexa-alanates, hydrogen covalently bonded to Al
in [AlH6]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 LiMgAlH6,
that are intermediate phases in the desorption from Ca(AlH4)2
and LiMg(AlH4)3, respectively; (ii) mixed hexa-alanates, like
Na2LiAlH6, K2NaAlH6, K2LiAlH6; and (iii) Ba- and Sr-based
hydrides (examples are BaAlH5, Ba2AlH7 and Sr2AlH7). 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 trihydride
The 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 aluminiumbased hydrides like catalyzed sodium alanate have been studied for
this purpose. Aluminum trihydride (AlH3) 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
Crystal structures of aluminum-based
hydrides
Vajeeston and Fjellvåg
Emerging Materials Research
List
no.
Compound
(space group)
Cell parameters
(in Å)
Coordinates
Reference remarks
1.
α-AlH3 (R-3c)
a = 4·431
Al(6b):0, 0, 0
Ref. 31
c = 11·774
H(18e):0·628(2), 0, 0·25
γ = 120
2.
α′-AlH3 (Cmcm)
a = 6·470(3)
Al(4b):0, 0·5, 0
b = 11·117(5)
Al(8d):0·25, 0·25, 0
c = 6·562
D(8f):0, 0·197(2), 0·451(4)
Ref. 44
D(16h):0·312(2), 0·1000(14) 0·047(3)
D(4c):0, 0·465(3), 0·25
D (8g): 0·298(4), 0·277(2), 0·25
3.
β-AlH3 (Fd-3m)
a = 9·0037
4.
γ-AlH3 (Pnnm)
a = 7·3360(3)
Al(2a): 0, 0, 0
b = 5·3672(2)
Al(4g): 0·4174(5), 0·7127(6), 0
c = 5·7562(1)
D(4g): 0·2044(9), 0·8269(11), 0
Al (16d): 0·5, 0, 0
Ref. 122
D (48f) 0·4301(1), 0·125, 0·125
Ref. 122
D(4g): 0·3668(10), 0·3931(13), 0
D(2d): 0, 0·5, 0·5
D(8h): 0·4174(6), 0·7038(8), 0·3009(6)
5.
LiAlH4 (P21/c)
a = 4·8254
Al(4e): 0·1428(2), 0·2013(1), 0·9311(1)
b = 7·8040
Li(4e): 0·5601(12), 0·4657(6), 0·8236(6)
c = 7·8968
D(4e): 0·1902(10), 0·0933(8), 0·7710(6)
β = 112·268
D(4e): 0·3526(10), 0·3726(7), 0·9769(6)
Ref. 49
D(4e): 0·2384(11), 0·0840(7), 0·1141(7)
D(4e): 0·8024(14), 0·2644(7), 0·8689(8)
6.
NaAlH4 (I41/a)
a = 5·020
Al(4a): 0, 0, 0
c = 11·330
Na(4b): 0, 0, 0·5
Ref. 50
H(16f): 0·228(1), 0·117(2), 0·838(9)
7.
KAlH4 (Pnma)
a = 8·8514
K(4c): 0·1839, 0·250, 0·1522
b = 5·8119
Al(4c): 0·5578, 0·250, 0·8209
c = 7·3457
H(4c): 0·4018, 0·250, 0·9156
Ref. 57
H(4c):0·7055, 0·250, 0·9630
H(8d):0·4209, 0·9741, 0·3098
8.
RbAlH4 (Pnma)
a = 9·5956
Rb(4c): 0·1823, 1/4, 0·1597
b = 5·7662
Al(4c): 0·5615, 1/4, 0·8138
c = 7·7795
H1(4c): 0·4017, 1/4, 0·8990
Ref. 59
H2(4c): 0·6883, 1/4, 0·9610
H3(8d): 0·4198, 0·9762, 0·3121
9.
CsAlH4 (Pnma)
a = 10·0520
Cs(4c): 0·1868, 1/4, 0·1580
b = 6·0945
Al(4c): 0·5570, 1/4, 0·8078
c = 8·0232
H1(4c): 0·4034, 1/4, 0·8847
Ref. 59
H2(4c): 0·6741, 1/4, 0·9541
H3(8d): 0·4226, 0·9708, 0·3127
Table 1 Crystal structure data (space group, unit cell dimensions and positional parameters) for Al-based hydrides considered in this review.
3
Emerging Materials Research
Crystal structures of aluminum-based
hydrides
Vajeeston and Fjellvåg
List
no.
Compound
(space group)
Cell parameters
(in Å)
Coordinates
Reference remarks
10.
Li3AlH6 (R-3)
a = 8·07117
Li(18f): 0·9576(14), 0·2260(12), 0·2911(9)
Ref. 64
Al(3a): 0, 0, 0
c = 9·5130
Al(3b): 0, 0, 0·5
γ = 120
D(18f): 0·8333(9), 0·8057(7), 0·1007(5)
a = 5·390
Na(2b): 0, 0, 0·5
b = 5·514
Na(4e): −0·006(5), 0·461(4), 0·252(5)
c = 7·725
Al(2a): 0, 0, 0
β = 89·86
D(4e): 0·091(3), 0·041(3), 0·215(3)
D(18f): 0·1582(8), 0·1820(7), 0·3900(5)
11.
Na3AlH6 (P21/n)
Ref. 62
D(4e): 0·234(3), 0·328(3), 0·544(3)
D(4e): 0·165(3), 0·266(3), 0·944(3)
12.
K3AlH6 (P21/n)
a = 6·1771
K(2b): 0, 0, 0·5
b = 5·8881
K(4e): −0·0058, 0·4828, 0·2544
c = 8·6431
Al(2a): 0, 0, 0
β = 89·30
H(4e): 0·0617, 0·0089, 0·2042
Ref. 67
H(4e): 0·2799, 0·3136, 0·5349
H(4e): 0·1786, 0·2281, 0·9652
13.
Na2LiAlH6 (P121/n1)
a = 5·165
Li(2b): 0, 0, 0·5
b = 5·251
Na(4e): 0·99, 0·47, 0·25
c = 7·339
Al(2a): 0, 0, 0
β = 90·03
H(4e): 0·07, 0·02, 0·23
Ref. 66
H(4e): 0·23, 0·30, 0·53
H(4e): 0·20, 0·27, 0·96
14.
Na2LiAlD6 (Fm-3m*)
a = 7·38484
Na(8c): 0·25, 0·25, 0·25
Ref. 70
Li(4b): 0·5, 0·5, 0·5
Al(4a): 0, 0, 0
D(24e): 0·238(4), 0, 0
15.
K2LiAlH6 (Fm-3m)
a = 7·9383
K(8c):1/4, ¼, ¼
Ref. 69
Li(4b): ½, ½, ½
Al(4a): 0, 0, 0
H(24e): 0·201, 0, 0
16.
K2LiAlH6 (R3-mH)
a = 5·62068
Li(6c): 0, 0, 0·4036(8)
c = 27·3986
Al(3a): 0, 0, 0
γ = 120
Al(3b): 0, 0, 0·5
K(6c): 0, 0, 0·1270(1)
K(6c): 0, 0, 0·2853(1)
H(18h): 0·096(7), −0·096(7), 0·466(3)
H(18h): 0·205(5), −0·205(5), 0·638(2)
Table 1 Continued
4
Ref. 71
Emerging Materials Research
Crystal structures of aluminum-based
hydrides
Vajeeston and Fjellvåg
List
no.
Compound
(space group)
Cell parameters
(in Å)
Coordinates
Reference remarks
17.
K2LiAlH6 (P121/n1)
a = 5·528
K(4e): 0, 0·5, 0·25
Ref. 66
b = 5·536
Li(2b): 0, 0, 0·5
c = 7·832
Al(2a): 0, 0, 0
β = 90·03
H(4e): 0, 0, 0·23
H(4e): 0·27, 0·27, 0·5
H(4e): 0·23, 0·23, 0
18.
K2NaAlH6 (Fm-3m)
a = 8·118
K(8c):1/4, ¼, ¼
Ref. 72
Na(4b): ½, ½, ½
Al(4a): 0, 0, 0
H(24e): 0·2167(8), 0, 0
19.
Na5Al3H14 (P4/mnc)
a = 5·733
Na(2b): 0, 0·5, 0·25
b = 5·754
Na(8g): 0·2851, 0·7851, 0·25
c = 8·128
Al(2a): 0, 0, 0
β = 89·97
Al(4c): 0, 0·5, 0
Ref. 73
H(4e): 0, 0, 0·1694
H(8h): 0·7522, 0·0731, 0
H(16i): 0·3207, 0·0420, 0·6175
20.
21.
SrAl2D2 (P-3m1)
SrSiAlH (P3m1)
a = 4·5253
Sr(1a): 0, 0, 0
c = 4·7214
Al(2d): 0·3333, 0·6667, 0·4589(7)
γ = 120
D(2d): 0·3333, 0·6667, 0·0976(4)
a = 4·2113
Sr(1a): 0, 0, 0
c = 4·9518
Al(1c): 0·6667, 0·3333, 0·547(1)
γ = 120
Si(1b): 0·3333, 0·6667, 0·431(2)
Ref. 80
Ref. 123
D(1c): 0·6667, 0·3333, 0·904(1)
22.
Sr2AlD7 (I12/a1)
a = 12·552
Sr(8 f): 0·3435(3), 0·5798(4), 0·3195(6)
b = 9·7826
Sr(8 f): 0·1109(4), 0·3184(4), 0·0882(6)
c = 7·9816
Al(8 f): 0·921(1), 0·097(1), 0·232(2)
β = 100·286
D(8 f): 0·9994(7), 0·1094(7), 0·077(1)
Ref. 81
D(8 f): 0·8514(7), 0·9606(7), 0·117(1)
D(8 f): 0·0158(6), 0·8978(8), 0·341(1)
D(8 f): 0·8385(6), 0·0798(8), 0·379(1)
D(8 f): 0·9895(6), 0·2419(7), 0·3291(9)
D(8 f): 0·8248(6), 0·2058(7), 0·1157(8)
D(8 f): 0·6875(6), 0·8537(7), 0·3189(9)
23.
BeAlH5 (P21)
a = 4·790
Be(2a): 0·002, 0·230, 0·623
b = 4·324
Al(2a): 0·243, 0·990, 0
c = 6·277
H(2a): 0·247, 0·162, 0·749
β = 89·408
H(2a): 0·001, 0·740, 0·902
Ref. 79
H(2a): 0·501, 0·740, 0·914
H(2a): 0·240, 0·821, 0·251
H(2a): 0·890, 0·965, 0·515
Table 1 Continued
5
Emerging Materials Research
Crystal structures of aluminum-based
hydrides
Vajeeston and Fjellvåg
List
no.
Compound
(space group)
Cell parameters
(in Å)
Coordinates
Reference remarks
24.
MgAlH5 (P21/c)
a = 4·550
Mg(4a): −0·2504, −0·2466, −0·3204
Ref. 79
b = 4·260
Al(4a): 0·2486, 0·2528, −0·4083
c = 13·024
H(4a): −0·4756, −0·0559, 0·4069
H(4a): −0·0300, 0·0912, 0·3051
H(4a): 0·4719, −0·0516, −0·4063
H(4a): 0·0284, 0·0975, −0·3045
H(4a): −0·0024, 0·0916, −0·4994
25.
CaAlD5 (P21/n)
a = 9·8000
Ca(4e): 0·7845(16), 0·2166(19), 0·7382(13)
b = 6·9081
Ca(4e): 0·3275(14), 0·2676(16), 0·1816(11)
c = 12·4503
Al(4e): 0·8017(5), 0·3097(16), 0·4907(12)
β = 137·936
Al(4e): 0·2071(14), 0·2175(14), 0·8706(11)
Ref. 92
D(4e): 0·0058(17), 0·3009(19), 0·5190(14)
D(4e): 0·6406(16), 0·4242(18), 0·3076(12)
D(4e): 0·6070(14), 0·2725(17), 0·4696(13)
D(4e): 0·7010(18), 0·3865(14), 0·8592(15)
D(4e): 0·9589(14), 0·1915(15), 0·6767(10)
D(4e): 0·1259(17), 0·0329(14), 0·9070(13)
D(4e): 0·1154(19), 0·3773(14), 0·9139(15)
D(4e): 0·2848(16), 0·0634(15), 0·8156(14)
D(4e): 0·2612(9), 0·4064(13), 0·8154(13)
D(4e): 0·4470(13), 0·1884(16), 0·0707(12)
26.
27.
SrAlD5 (P212121)
BaAlD5 (Pna21)
a = 4·5253
Sr(1a): 0, 0, 0
c = 4·7214
Al(2d): 0·3333, 0·6667, 0·4589(7)
γ = 120
D(2d): 0·3333, 0·6667, 0·0976(4)
a = 9·194
Ba(4 a): 0·6873(7), 0·156(1), 0·250
b = 7·0403
Al(4 a): 0·049(1), 0·847(2), 0·233(6)
c = 5·1061
D(4 a): 0·006(1), 0·939(1), 0·919(3)
Ref. 79
Ref. 68
D(4 a): 0·576(1), 0·846(1), 0·019(4)
D(4 a): 0·5720(9), 0·805(1), 0·497(4)
D(4 a): 0·3533(8), 0·696(1), 0·240(4)
D(4 a): 0·7112(8), 0·541(1), 0·209(2)
28.
Ba2AlD7 (I2/a)
a = 13·197
Ba(8f): 0·3459, 0·5848, 0·3249
b = 10·237
Ba(8f): 0·1084, 0·3247, 0·0852
c = 8·509
Al(8f): 0·927, 0·096, 0·235
β = 101·290
D(8f): 0·004(1), 0·116(1), 0·077(2)
D(8f): 0·846(1), 0·974(1), 0·135(2)
D(8f): 0·023(1), 0·999(2), 0·325(2)
D(8f): 0·844(1), 0·104(2), 0·387(2)
D(8f): 0·983(1), 0·249(2), 0·324(2)
D(8f): 0·832(1), 0·207(1), 0·115(2)
D(8f): 0·693(1), 0·864(1), 0·322(2)
Table 1 Continued
6
Ref. 93
Emerging Materials Research
Crystal structures of aluminum-based
hydrides
Vajeeston and Fjellvåg
List
no.
Compound
(space group)
Cell parameters
(in Å)
Coordinates
Reference remarks
29.
Mg(AlH4)2 (P-3m1)
a = 5·199
Mg(1a): 0, 0, 0
Ref. 96
c = 5·858
Al(2d): 0·3333, 0·6667,0·7
γ = 120
H(2d): 0·3333, 0·6667, 0·45
a = 13·4491
Ca(8c): 0·8958(1), 0·4662(2), 0·2818(3)
b = 9·5334
Al(8c): 0·4389(3), 0·7757(5), −0·0011(8)
c = 9·0203
Al(8c): 0·8460(3), 0·1060(4), 0·1839(5)
H(6i): 0·16, −0·16, 0·81
30.
Ca(AlD4)2 (Pbca)
Ref. 108
D(8c): 0·3710(9), 0·6842(11), 0·1087(12)
D(8c): 0·5280(8), 0·8546(12), 0·0825(14)
D(8c): 0·4877(9), 0·6706(12), −0·1183(13)
D(8c): 0·3647(8), 0·8817(11), −0·0835(13)
D(8c): 0·8264(10), 0·0829(11), 0·0086(8)
D(8c): 0·8094(8), 0·2610(8), 0·2337(14)
D(8c): 0·9590(5), 0·0702(12), 0·2407(16)
D(8c): 0·7762(9), −0·0075(10), 0·2636(16)
31.
LiMgAlH6(P321)
a = 7·985550
Mg(3e): 0,0·3570(13), 0
c = 4·378942
Li(3f): 0, 0·686(6), 0·5
γ = 120
Al(1a): 0, 0, 0
Ref. 103
Al(2d): 0·3333, 0·6667, 0·492(10)
D(6g): 0·540(3), 0·763(2), 0·278(3)
D(6g): 0·119(3), 0·576(2), 0·734(3)
D(6g): 0·904(2), 0·117(2), 0·228(3)
32.
LiMg(AlH4)3 (P21/c)
a = 8·37113
Mg(4e): 0·6305(6), 0·5292(4), 0·8833(3)
b = 8·73910
Li(4e): 0·127(3), 0·4720(19), 0·3822(14)
c = 14·3012
Al(4e): 0·7615(5), 0·6282(4), 0·1512(3)
β = 124·8308
Al(4e): 0·4745(5), 0·8809(4), 0·8581(3)
Ref. 104
Al(4e): 0·9593(5), 0·2510(4), 0·4986(3)
D(4e): 0·6057(14), 0·5722(12), 0·1782(9)
D(4e): 0·6523(1), 0·5907(11), 0·0190(6)
D(4e): 0·7843(17), 0·8088(9), 0·1721(10)
D(4e): 0·9475(12), 0·5201(10), 0·2158(9)
D(4e): 0·4888(15), 0·7127(10), 0·8153(9)
D(4e): 0·6918(11), 0·9294(11), 0·9554(8)
D(4e): 0·3783(15), 0·9895(12), 0·7474(8)
D(4e): 0·3312(15), 0·8752(13), 0·8981(10)
D(4e): 0·9500(15), 0·3124(13), 0·3908(8)
D(4e): 0·7599(14), 0·1597(12), 0·4549(10)
D(4e): 0·1293(13), 0·1222(10), 0·5635(8)
D(4e): 0·9941(14), 0·3727(11), 0·5902(7)
Table 1 Continued
7
Emerging Materials Research
Crystal structures of aluminum-based
hydrides
Vajeeston and Fjellvåg
List
no.
Compound
(space group)
Cell parameters
(in Å)
Coordinates
Reference remarks
33.
LiAlMg10H24 (P121)
a = 8·9885,
Mg(2e): 0·251, 0, 0·003
Ref. 106
b = 8·9848,
Mg(2e): 0·243, 0·337, 0
c = 4·4846,
Mg(2e): 0·246, 0·666, 0·006
β: 89·655
Mg(1d): 0·5, 0·168, 0·5
Mg(1d): 0·5, 0·501, 0·5
Mg(1d): 0·5, 0·834, 0·5
Mg(1b): 0, 0·172, 0·5
Li(1b): 0, 0·497, 0·5
Al(1b): 0, 0·832, 0·5
H(2e): 0·402, 0·999, 0·307
H(2e): 0·401, 0·333, 0·302
H(2e): 0·401, 0·671, 0·305
H(2e): 0·909, 0·980, 0·316
H(2e): 0·901, 0·329, 0·303
H(2e): 0·908, 0·697, 0·312
H(2e): 0·654, 0·165, 0·195
H(2e): 0·653, 0·504, 0·198
H(2e): 0·656, 0·834, 0·186
H(2e): 0·149, 0·159, 0·195
H(2e): 0·149, 0·506, 0·195
H(2e): 0·141, 0·838, 0·223
34.
LiCa(AlH4)3 (P63/m)
a = 8·9197
(9·093*)
Li(2a): 0, 0, ¼
Ref. 107
c = 5·8887
(5·996*)
Ca(2d): 2/3, 1/3, ¼
*Ref. 109
Al(6h): 0·281,0·903, ¼ (0·3, 0·9, ¼*)
*DFT results from
theory.
H(6h): (0·544, 0·501, ¼*)
H(6h): (0·807, 0·815, ¼*)
H(12i): (0·535, 0·754 0·029*)
35.
36.
37.
38.
La3AlH6 (R3m)
Ce3AlH6(R3m)
Pr3AlH6(R3m)
Nd3AlH6(R3m)
a = 6·4732
La(3b): 0, 0, 0·5
c = 6·2765
Al(3a): 0, 0, 0
γ = 120
H(18h): 0·2149, 0·7851, 0·4904
a = 6·4637
Ce(3b): 0, 0, 0·5
c = 6·2609
Al(3a): 0, 0, 0
γ = 120
H(18h): 0·2147, 0·7853, 0·4910
a = 6·4217
Pr(3b): 0, 0, 0·5
c = 6·2028
Al(3a): 0, 0, 0
γ = 120
H(18h): 0·2139, 0·7861, 0·4894
a = 6·3846
Nd(3b): 0, 0, 0·5
c = 6·741
Al(3a): 0, 0, 0
H(18h): 0·2132, 0·7868, 0·4883
γ = 120
Table 1 Continued
8
Ref. 110
Ref. 110
Ref. 110
Ref. 110
Emerging Materials Research
Crystal structures of aluminum-based
hydrides
Vajeeston and Fjellvåg
List
no.
Compound
(space group)
Cell parameters
(in Å)
Coordinates
Reference remarks
39.
Th2AlH4 (I4/mcm)
a = 7·629
Al(4a): 0, 0, 0·25
Ref. 111
c = 6·517
Th(8h): 0·162, 0·662, 0
D(16l): 0·368, 0·868, 0·137
40.
α-Al(BH4)3 (C12/c1)
a = 22·834
Al(8f): 0·3797, 0·5943, 0·8366
b = 6·176
B(8f): 0·3205, 0·3121, 0·8239
c = 22·423
H(8f): 0·3809, 0·3078, 0·8439
β = 111·67
H(8f): 0·3002, 0·5071, 0·8128
Ref. 115
H(8f): 0·3067, 0·2479, 0·8677
H(8f): 0·3019, 0·2214, 0·7725
B(8f): 0·3899, 0·7542, 0·7555
H(8f): 0·4165, 0·5751, 0·7794
H(8f): 0·3516, 0·8141, 0·7820
H(8f): 0·3575, 0·7139, 0·7005
H(8f): 0·4309, 0·8894, 0·7685
B(8f): 0·4298, 0·7324, 0·9297
H(8f): 0·3703, 0·7520, 0·8985
H(8f): 0·4541, 0·6038, 0·9002
H(8f): 0·4349, 0·6401, 0·9787
41.
β-Al(BH4)3 (PNA21)
a = 18·649
H(8f): 0·4531, 0·9080, 0·9304
Al(4a): 0·8703, 0·1558, 0·2098
b = 6·488
B(4a): 0·7800, 0·0057, 0·0633
c = 6·389
H(4a): 0·8456, −0·0341, 0·0213
Ref. 115
H(4a): 0·7751, 0·1384, 0·2112
H(4a): 0·7552, −0·1515, 0·1331
H(4a): 0·7551, 0·0858, −0·0906
B(4a): 0·9168, 0·0183, 0·4855
H(4a): 0·9353, −0·0248, 0·2979
H(4a): 0·8712, 0·1661, 0·4885
H(4a): 0·9700, 0·0865, 0·5674
H(4a): 0·8870, −0·1319, 0·5547
B(4a): 0·9115, 0·4349, 0·0722
H(4a): 0·8623, 0·4281, 0·2121
H(4a): 0·9337, 0·2529, 0·0277
H(4a): 0·9619, 0·5214, 0·1520
42.
Ti0.75Al0.25H0.17 (Im3m)
a = 3·280
H(4a): 0·8837, 0·5009, −0·0844
Ti(2a): 0, 0, 0 (occupancy 0·75)
Ref. 119
Al(2a): 0, 0, 0 (occupancy 0·25)
H (6b): 0·5, 0, 0 (occupancy 0·17)
43.
Ti0.75Al0.25H1.25 (Fm3m)
a = 4·350
Ti(2a): 0, 0, 0 (occupancy 0·75)
Ref. 119
Al(2a): 0, 0, 0 (occupancy 0·25)
H (6b): 0·5, 0, 0 (occupancy 0·63)
9
Emerging Materials Research
Crystal structures of aluminum-based
hydrides
Vajeeston and Fjellvåg
H
H
Al
Al
c
a
b
b
c
(a)
H
(b)
H
Al
Al
c
b
a
a
c
(c)
(d)
Figure 1. Crystal structures of AlH3 in (a) α-AlH3, (b) α′-AlH3, (c) β-AlH3
and (d) γ-AlH3
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 AlH3 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-AlH3 phases. Experimental results have shown
that α modification is the most stable at ambient conditions.
The structural aspects of irradiated AlH3 in comparison with the
10
various phases are also investigated in refs. 36–39. Similarly, the
electronic structure32,40 and thermodynamic stability41 of α-AlH3
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-AlH3. As a consequence,
AlH3 may be used as a chemical hydride. α-AlH3 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-AlH3 was reexamined by
Brinks et al, and in this study the data were refined with Rietveld
method.44 The α-AlH3 structure may be described as a ReO3-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°.
Emerging Materials Research
Crystal structures of aluminum-based
hydrides
Vajeeston and Fjellvåg
3.1.2 α′-AlH3
PND and synchrotron powder X-ray diffraction (SRPXD) determined
the structure of α′-AlH3.44 The sample contained both the α′ and the
α modification, see above. α′-AlH3 takes a β-AlF3 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-AlH3. For α′-AlH3, four of the six octahedra
in the first coordination sphere are interconnected in pairs, whereas
in α-AlH3 none of the octahedra in the first coordination sphere are
connected with each other. The corner-sharing network is more open
in α′-AlH3 giving hexagonal-shaped pores with a diameter of 3·9 Å.
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′-AlH3 can be regarded as an intermediate
between α-AlH3 and β-AlH3. 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-AlH3
(45·6 Å3) than α-AlH3 (33·5 Å3) and α′-AlH3 (39·3 Å3).
3.1.3 β-AlH3
The combined PND and SRPXD diffraction methods have
determined that the β-AlH3 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
Li
B
H
Al
c
A
o
(a)
4.
A
H
Al
H
Na
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 cornersharing AlH6 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 edgesharing, 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- AlH3 modifications.
Al
o
c
B
(c)
(b)
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
Complex hydrides
The 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 [AlH4]−, [NH2]−, [BH4]−, that is, alanates, amides, and
borohydrides46 and TM complex hydrides that have anionic
(TMHx)− complexes such as [FeH6]4− attached to a cationic light
metal, for example, Mg2+, in Mg2FeH6.45 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
Mg2CoH5, Mg2NiH4 and Mg3MnH7 (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
11
Emerging Materials Research
Crystal structures of aluminum-based
hydrides
Vajeeston and Fjellvåg
H
Al
H
Al
Na
Li
(a)
(b)
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
H
Na
Na
Li
Li
(a)
(b)
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
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.
carried out a more detailed atomic structure determination
of LiAlH4 based on the combined powder neutron and X-ray
diffraction studies. The compound crystallized in the space group
P21/c. The atomic structure found to consist of isolated [AlH4]−
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 LiAlH4 were found to vary by less than
1·5° from the angles of a perfect tetrahedron. The Li−H distances
5.
Alkali-based tetra alanates
5.1LiAlH4
Sklar and Post initially determined the crystal structure of
LiAlH448 through an X-ray diffraction study. Hauback et al.49
12
Emerging Materials Research
Crystal structures of aluminum-based
hydrides
Vajeeston 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.
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 [AlD4]− tetrahedron were reported to be
107·32° and 113·86° at 295 K.
5.2NaAlH4
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 I41/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 LiAlH448 (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
NaAlH4 observed at a lower frequency than that of LiAlH4 (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
NaAlD4 at 8 and 295 K.52 The atomic structure was found to be made
Al
Na
H
Figure 5. Crystal structures of Na5Al3H14. The complex [AlH6]3− anions
depicted as octahedra and the Na cations are displayed as sphere
5.3KAlH4
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 KAlH4 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 KAlH4 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 KAlH4
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 KAlH4 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 KAlH4
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 KAlH4 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 BaSO4-type structure with
space group Pnma. The structure (Figure 1(c)) consists of isolated
[AlD4]− 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 NaAlD4 and the Li−D distance of LiAlD4).
13
Emerging Materials Research
Crystal structures of aluminum-based
hydrides
Vajeeston and Fjellvåg
H
H
Al
Sr
(a)
Al
H
Si
Sr
Al
Sr
(c)
(b)
Figure 6. Crystal structures of (a) SrAl2H2, (b) SrSiAlH and (c) Sr2AlH7
5.4 RbAlH4 and CsAlH4
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 [AlH6]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 [AlH6]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
6.
The crystal lattice of alkali-metal hexahydro aluminates built
of isolated octahedral anions [AlH6]3+ and alkali-metal cations
M+. The first X-ray powder diffraction studies of M3AlH6 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 Li3AlD661 and Na3AlD662 have been
solved, including the location of the deuterium atoms, on the basis
of combined synchrotron X-ray and neutron diffraction data.
It is important to note that, in general, all aluminohydrides with
known structures, only lithium hexahydro aluminate Li3AlH6
and its isotopomer Li3AlD6 crystallize in different space groups.
According to X-ray powder diffraction study at 298 K,60 Li3AlH6
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 Li3AlD6 at 295 K crystallized in the
orthorhombic space group R-3 with a =8·07117 Å, c = 9·5130 Å
and z = 6.64
6.1Li3AlH6
6.2Na3AlH6
Bastide et al.58 found RbAlH4 and CsAlH4 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 MAlH4
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
Alkali metal hexahydro aluminates
The lithium hexa-alanate is the intermediate phase during
decomposition of LiAlH4.63 The combined PND and SR-PXD
methods determined the structure of Li3AlH6 data at room
temperature giving the space group R-3.64 The structure consists of
14
Na3AlH6 takes a monoclinic P21/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 [AlD6]3− octahedra are similar for Na3AlH6 (1·758 Å)
Emerging Materials Research
Crystal structures of aluminum-based
hydrides
Vajeeston and Fjellvåg
Be
H
H
H
Ca
Al
Al
Mg
c b
a
a
a
Al
b
b
c
c
(a)
(c)
(b)
H
H
Sr
Al
Al
Ba
b
c
a
a
c b
(d)
(e)
Figure 7. Crystal structures of (a) BeAlH5, (b) MgAlH5, (c) CaAlH5, (d)
SrAlH5 and (e) BaAlH5. AlH6 octahedra are marked in colour
and Li3AlH6 (1·744 Å). The Na coordination number is both 8 and
6 in Na3AlH6 compared to 6 in Li3AlH6 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 hexaalanate (shortest Al–Al distance: 5·390 Å) than in the Li hexaalanate (shortest Al–Al distance: 4·757 Å).
6.3K3AlH6
The only X-ray powder diffraction study of potassium
hexahydroaluminate at 298 K determined only unit cell
parameters.53 According to these data, K3AlH6 is tetragonal with
a = 8·445 Å and c = 8·584 Å. On the other hand, according to the
DFT calculations, K3AlH6 has been stabilized in the same structure
type as Na3AlH6 (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 hexahydroaluminates
The 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 A3-xBxAlH6, 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:
Na2LiAlH6, K2LiAlH6, K2NaAlH6 and KNa2AlH6, and from these
only KNa2AlH6 does not find experimentally.
7.1Na2LiAlH6
Na2LiAlH6 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, Na2LiAlH6 crystallize with the space
15
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Crystal structures of aluminum-based
hydrides
Vajeeston 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 Na2LiAlH6 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 cornersharing [AlH6]3− and [LiH6]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 Na3AlH6, probably
because of the increased coordination number from 6/8 to 12. The
structure of Na2LiAlH6 may also be described as: (i) An ordered
perovskite A2BB′X6 with A = Na in a 12-coordinated site and both
Li = B and Al = B′ in octahedral positions. The structure is very
similar to Na3AlH6 where Na is substituted by Li in one of the Na
(A) positions. (ii) A lattice of cubic closest packing geometry of
AlH6 entities with Na filling the tetrahedral positions and Li the
octahedral position.
Na2LiAlH6 due to expanded unit cell and metal atoms in particular
positions in the structure.
7.2K2LiAlH6
From DFT work65 and SR-PXD data68, the structure of K2LiAlH6
was reported to be isostructural with the Na2LiAlH6 (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 K2LiAlD6 were not prepared
due to the difficulty of preparing pure KD. However, it was
concluded in literature70 that the structure of K2LiAlH6 is likely
isostructural with the hexagonal–rhombohedral form of K2LiAlF6.
Accurate PND data are needed for an accurate description of the
structure. According to DFT calculation, both Cs2NaAlF6 type
structure with a symmetry C2/m and high-temperature fluorite
structure (K2LiAlF6) 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.
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: K3AlH6 > K2NaAlH6 >
K2LiAlH6. However, one exception from the general rule is the
substitution of Li for Na where Na2LiAlH6 is more stable than
Na3AlH6.
7.4Na5Al3H14
Ojwang et al.72 investigated the structure of Na5Al3H14 by using DFT
study. Na5Al3H14 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 NaAlH4. 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 [Al3H14]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 Na5Al3H14 phase was
not observed in experiments because, if it forms during the thermal
decomposition of NaAlH4 then, they are quasi-stationary states. In
particular, the inclusion of Na5Al3H14 in the decomposition pathway
of NaAlH4 nicely explains how the lattice structure of NaAlH4 is
disrupted and the mobile alane species are formed.
H
H
16
Ca
Al
7.3K2NaAlH6
K2NaAlH6 takes the same structure as Na2LiAlH6.68,69 The detailed
structure of the hydride was determined by PND.71 The Al–H/D
distances are nearly equal in K2NaAlH6 and Na2LiAlH6. The
average Al–H distance is 1·761 Å and 1·756 Å for K2NaAlH6 and
Na2LiAlH6, respectively. In spite of the significant differences
in cation sizes, these compounds are isostructural. However, all
intermetallic distances are about 10% longer in K2NaAlH6 than in
Al
Mg
(a)
(b)
Figure 8. Crystal structures of (a) Mg(AlH4)2 and Ca(AlH4)2
Emerging Materials Research
Crystal structures of aluminum-based
hydrides
Vajeeston and Fjellvåg
8.
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 LiAlH4
(1·54–1·58 Å) or NaAlH4 (1·61 Å).
Alkaline earth-based aluminohydrides
Data on the preparation and crystal structures of alkaline earth
aluminohydride’s are limited compared to the alkaline-based
alumina hydrides. For example, in the AAlH5 (A = any one of the
alkaline earth elements) series only MgAlH5 and BaAlH5 have been
identified experimentally,73 whereas the crystal structure of BeAlH5,
CaAlH5 and SrAlH5 phases are not yet solved experimentally. No
information on BeAlH5 is available in the literature, probably
because Be is severely toxic. CaAlH5 and SrAlH5 are reported to
form as intermediate products on gentle heating of Ca(AlH4)274 and
Sr(AlH4)2,75 respectively. SrAlH5 is also reported76 to form during
‘mechanochemical activation’ of Sr(AlH4)2. DFT calculations have
predicted the crystal structure of BeAlH5, CaAlH5 and SrAlH5
phases and experimental verification is needed.77,78
8.1SrAl2H2
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. SrAl2H2 synthesized by the reaction
of SrAl2 with H2 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, SrAl2H2 absorbs an extra portion
of hydrogen to form Sr2AlH7.80 SrAl2H2 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,
8.2SrSiAlH
The unexpected discovery of superconductivity at 39 K in MgB281
has attracted much attention to the layer-structured compounds with
the AlB2-type structure because of their potential in the search for
non-cuprate superconductors.82,83 Ternary silicides Sr(Ga0.37Si0.63)2,
Ca(Al0.5Si0.5)2 and Sr(Al0.5Si0.5)2 adopt the AlB2 structure, in which
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(Ga0.5Si0.5)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 SrAl2H2:
Half of the [Al–H] entities in the polyanionic layer of SrAl2H2
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 SrAl2H2 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:
C
Li
O
Mg
Ca
B
Al
C
Li
H
Mg
H
Al
H
Al
A
A
(a)
B
(c)
Li
(c)
Figure 9. The crystal structure of the (a) LiMg(AlH4)3, (b) LiCa(AlH4)3and (c) LiAlMg10H24 compounds.
17
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Crystal structures of aluminum-based
hydrides
Vajeeston and Fjellvåg
SrAl2H2 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 SrAl2H2 (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 SrAl2H2,
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
8.4BeAlH5
8.3Sr2AlH7
Sr2AlH7 crystallize in a new monoclinic structure in space group I2
(No. 5). Sr2AlH7 is the first example that consists of isolated [AlH6]
units and infinite one-dimensional twisted chains of edge-sharing
[HSr4] tetrahedra along the crystallographic c axis. The crystal
structure of Sr2AlD7/H7, (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 [AlH6] octahedron, the Al–H
bond lengths ranging from 1·71 to 1·76 Å are in good agreement
with those in Na3AlH6 10 (1·75–1·77 Å). The H-centered [HSr4]
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 SrH2
(2·36–2·80 Å).
The crystal structure of BeAlH5 (Figure 6(a)) has been predicted
by Klaveness et al.78 and it exhibits alternating layers of cornersharing AlH6 octahedra which are connected by twin chains of
BeH4 tetrahedra. Each AlH6 octahedron shares corners with four
other AlH6 octahedra and two BeH4 tetrahedra (see Figure 6(a)).
Each BeH4 tetrahedron shares corners with two AlH6 octahedra and
two other BeH4 tetrahedra. The polyhedra in the BeAlH5 structure
are the most regular of the entire AAlH5 series.
8.5MgAlH5
DFT predicted that MgAlH5 stabilize in a monoclinic P21/c
structure (CaFeF5-type).77 The MgAlH5 structure has comprising
AlH6 octahedra and capped MgH7 octahedra. The AlH6 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 AlH6 octahedra of MgAlH5 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 MgH7 configuration which distinguishes MgAlH5
from the AAlH4 (A = alkali metal) series. In the latter series, A
cannot be ascribed meaningful coordinations. This suggests that
MgAlH5 displays a somewhat different bonding situation for the
hydrogen atoms than in the AAlH4 series.
8.6CaAlH5
From DFT calculations, it has been predicted that CaAlH5 stabilize
in BaFeF5-type monoclinic P21/n structure.78 CaAlH5 consisted
of non-linear chains of AlH6 octahedra and isolated Ca ions (see
Figure 6(c)). Essentially, the same ground-state structure for
CaAlH5 is reported by the independent computational-based
C
H
Al
La
H
Th
B
A
O
(a)
Figure 10. The crystal structure of the (a) REAlH6 (with RE = La, Ce, Pr
and Nd) and (b) Th2AlH4 compounds
18
(b)
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Crystal structures of aluminum-based
hydrides
Vajeeston 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 CaAlH5 transform into high-pressure orthorhombic
(SrAlH5; P212121) modification. Both of these monoclinic and
orthorhombic modifications are not yet identified experimentally.
8.8SrAlH5
At ambient conditions, SrAlH5 crystalize in orthorhombic P212121
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.
8.7BaAlH5
The reaction of the Ba7Al13 alloy with H2/D2 at ~333 K and 7 MPa
yields barium alumina hydride BaAlH5; at 603 K, the reaction
results in Ba2AlH7.73,92 BaAlH5 crystallize in the orthorhombic
space group Pna21.92 The crystal structure of BaAlH5 (Figure
6(c)) contains corner-sharing AlH6 octahedra, which form onedimensional 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 BaAlH5. The calculated positional
parameters for BaAlH5 make the AlH6 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 Li3AlH6 [1·73 Å
(ref. 64) 1·75 Å (ref. 21)] and Na3AlH6 [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 BaAlH5 is 2·25 Å and complies accordingly
with the 2 Å rule.93,94
8.9Ba2AlH7
Ba7Al13 reacts with hydrogen to form BaAlH5 and Al between 373
and 553 K. When the temperature is in the range from 553 to 603
K; Ba7Al13 is hydrogenated to Ba2AlH7 and Al. Ba2AlH7 is isostructural to Sr2AlH7, crystallizing with a monoclinic structure in
space group I2/a.92 Ba2AlH7 consists of isolated [AlH6] 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 [AlH4]− tetrahedra surrounded by six Mg atoms in a distorted
H
B
Al
O
B
C
B
O
A
(a)
A
(b)
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
19
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Crystal structures of aluminum-based
hydrides
Vajeeston 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.
AlH6 − octahedron and six Mg/LiH6 octahedra. All corners are
connected to Mg, Li and Al octahedra.
8.11Ca(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(AlH4)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(AlH4)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 [AlH4] units and four oxygen
atoms from four THF molecules. Attempts were made to predict
the crystal structure of solvent-free Ca(AlH4)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(AlH4)2 from DFT calculations (Figure
7(b)). The most stable structure is Pbca, which is derived from the
CaB2F8 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 AlH4 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
9.1LiMgAlH6
The crystal structure of LiMgAlH6 has been investigated using
synchrotron radiation XRD, PND and DFT calculations by
Grove et al.102 LiMgAlH6 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 Mg3Al and Li3Al2
layers as shown in Figure 9(a). In the Li3Al2 and Mg3Al layers,
AlH6 octahedra are sharing edges with three LiH6 and three
MgH6 octahedron, respectively. Each LiH6 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 Mg3Al
units. These layers are interconnected by corner-sharing of the
20
9.2LiMg(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 AlH4
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 AlH4 tetrahedra so that
the structure consists of a corner-sharing network of alternating
AlH4 tetrahedra and LiH6 or MgH6 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 AlH4 tetrahedra, with Li and Mg occupying
2/3 of the interstitial octahedral sites. At 130°C, LiMg(AlH4)3
decomposes to LiMgAlH6. Thus, LiMg(AlH4)3 is not useful for
reversible hydrogen storage while LiMgAlH6 is quite stable by
thermodynamic consideration.104
9.3LiAlMg10H24
DFT predicted that LiAlMg10H24 is crystallized in monoclinic P121
structure (Figure 9(b)).105 This phase was formed when Li and Al are
coupled-substituted into the MgH2 structure to assume the composition
of LiAlMg10H24, 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 LiAlMg10H24 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
MgH2 and LiAlMg10H24 structures.105 Therefore, there could be a nontrivial likelihood that the LiAl-substituted MgH2 structure might have
already been produced in a lab but undetected.
9.4LiCa(AlH4)3
Recently, Liu et al . synthesized LiCa(AlH4)3 by a mechano­
chemically activated reaction of LiAlH4 with CaCl2 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
LiAlH4, (monoclinic structure, space group P21/c26,49), Ca(AlH4)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
Crystal structures of aluminum-based
hydrides
Vajeeston 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 LiAlH449 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.
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 Th2AlD4 and also the structure is
completely ordered. The volume expansion during hydrogenation
of Th2Al 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
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(AlH4)3
agree well with the experimental results. This structure is similar
to the CdTh(MoO4)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 REAlH6 (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 REAlH6 are built up of isolated [AlH6]3−
octahedra which are not directly connected. The presentation of the
LaAlH6 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 [AlH6]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. Rareearth hydrides (REHx) are formed as primary dehydrogenation
products; the final products are RE-aluminum alloys.
10.2Th2AlH4
Th2AlH4 crystallize in space group I4/mcm with the lattice parameters
a = 7·629, c = 6·517 Å.110,111 Th2AlH4 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 Th2AlH4 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
11. Mixed aluminium borohydride
Aluminum borohydride Al(BH4)3 is a liquid at ambient temperatures
with the melting point of 209 K. The structures of the solid phase have
been investigated by XRD measurements.113 Cooling liquid Al(BH4)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 Pna21; 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 hydrides
Titanium 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 Ti2Al 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 Ti2Al/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 TiH2 on heating above 473 K. The FCC
phase has been observed but the existence of the BCC phase has
been questioned. Ti2AlH was determined by neutron diffraction
21
Emerging Materials Research
Crystal structures of aluminum-based
hydrides
Vajeeston 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. Ti2AlH8-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
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20. Singh, D. J. Comment on “structural stability and electronic
structure for Li3AlH6”. Physical Review B 2005, 71, 216101.
21. Vajeeston, P.; Ravindran, P.; Kjekshus, A.; Fjellvåg, H.
Structural stability and electronic structure for Li3AlH6.
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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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