Journal of Alloys and Compounds xxx (2006) xxx–xxx
Structure and bonding in BAlH5 (B = Be, Ca, Sr)
from first-principle calculations
A. Klaveness∗ , P. Vajeeston, P. Ravindran, H. Fjellvåg, A. Kjekshus
Department of Chemistry, University of Oslo, Box 1033 Blindern, N-0315 Oslo, Norway
Received 9 May 2006; accepted 7 June 2006
Abstract
Prediction of structures for the hitherto hypothetical compounds BeAlH5 , CaAlH5 , and SrAlH5 have been attained through minimization of the
total energy for 50 different guess structures. In addition to the ground-state phases (here designated ␣ modifications), BeAlH5 and CaAlH5 are also
predicted to form high-pressure modifications (termed ␤ modifications). Bonding analysis shows that Ca and Sr can, to a reasonable approximation,
be regarded as divalent ions, while Be forms iono-covalent (sp3 -hybridized) bonds with H in tetrahedral configuration. The ␤-BeAlH5 , ␣-CaAlH5 ,
␤-CaAlH5 , and SrAlH5 structures exhibit chains of corner-sharing AlH6 octahedra, while ␣-BeAlH5 forms layers of alternating sheets of cornersharing AlH6 octahedra and twin chains of corner-sharing BeH4 tetrahedra. At ambient conditions CaAlH5 and Ca(AlH4 )2 are predicted to be
thermodynamically metastable, while SrAlH5 is anticipated to be stable. However, indications suggest that CaAlH5 can be stabilized under pressure.
© 2006 Elsevier B.V. All rights reserved.
PACS: 71.; 81.05.Je; 71.15.Nc; 71.20.−b; 65.40.−b
Keywords: Hydrogen absorbing materials; Computer simulations; Crystal structure; Enthalpy; Thermodynamic properties
1. Introduction
A possible path toward better understanding of complex
hydrides for potential utilization in hydrogen storage, goes
through accumulation of information for a large number of
compounds which in turn is subjected to comparative analysis.
The BCH5 (B = Group II element, C = Group III element)
series is believed to play a central role in the dehydrogenation
of the present-days popular B(CH4 )2 compounds, but may
also themselves be of considerable interest as potential storage
materials. The present communication focuses on bonding and
trends in the properties of the BAlH5 series. Information on
MgAlH5 and BaAlH5 has recently become available in the
literature [1] whereas BeAlH5 , CaAlH5 , and SrAlH5 constitutes
the main content of this report. The existence of a independent
first-principles study on the structure of CaAlH5 [2] has recently
come to knowledge.
∗
Corresponding author. Tel.: +47 22 85 74 17; fax: +47 22 85 54 41.
E-mail address: arnekla@kjemi.uio.no (A. Klaveness).
URL: http://folk.uio.no/arnekla/.
No information on BeAlH5 is available in the literature, probably owing to the fact that Be is severely toxic. CaAlH5 and
SrAlH5 are reported to form as intermediate products upon gentle heating of Ca(AlH4 )2 [3] and Sr(AlH4 )2 [4], respectively:
∼200 ◦ C
2Ca(AlH4 )2 −−−−−→ 2CaAlH5 + 2Al + 3H2 ,
(1)
260–550 ◦ C
2CaAlH5 −−−−−→ 2CaH2 + 2Al + 3H2 ,
(2)
890–950 ◦ C
CaH2 −−−−−→ Ca + H2 .
(3)
145–165 ◦ C
2Sr(AlH4 )2 −−−−−→ 2SrAlH5 + 2Al + 3H2 ,
(4)
220–320 ◦ C
2SrAlH5 −−−−−→ 2SrH2 + 2Al + 3H2 ,
(5)
355–390 ◦ C
2SrH2 + 4Al −−−−−→ Al4 Sr + SrH2 + H2 ,
(6)
890–950 ◦ C
SrH2 −−−−−→ Sr + H2 .
(7)
SrAlH5 is also reported [4] to form during “mechano-chemical
activation” of Sr(AlH4 )2 .
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.06.047
JALCOM-14122;
No. of Pages 8
2
A. Klaveness et al. / Journal of Alloys and Compounds xxx (2006) xxx–xxx
2. Computational details
Structure relaxation, volume optimization, and calculation of
total energies have been performed according to the projectoraugmented-wave method (PAW) [5] as implemented in the Vienna ab initio simulation package (VASP) [6]. The calculations
were performed by utilizing density-functional theory, employing the generalized-gradient-approximation (GGA) functional
of Perdew et al. [7,8]. The structural relaxation process was
continued until all atoms had obtained their equilibrium positions as specified by Hellmann–Feynman forces of less than
10−2 eV Å−1 acting on the atoms. Brillouin-zone integration
are performed with a Gaussian broadening of 0.2 eV during
all relaxations. Forty k points were used for the structure of
SrAlH5 , and corresponding k-point densities were used for the
other structures considered. All calculations were performed
with a 500 eV plane-wave cutoff. Values for the bulk modulus have been obtained using the Birch–Murnaghan formulation
of the universal equation of state to fit the total energy versus
volume relationship. Density-of-states (DOS) plots were generated using the covalent radii of the respective atoms to define the
charge spheres from which the spherical harmonic weights were
derived.
Charge densities and Mulliken populations have been calculated by the Hartree–Foch self-consistent-field (SCF) method
as implemented in the CRYSTAL [9] code. Basis sets with 51-1G [10], HAYWSC-2-1-1G* (unpublished, but available at
the website), HAYWSC-31(3d)G [11], 85-11G [12], and 5-11G
[13] were used for Be, Ca, Sr, Al, and H, respectively.
The same composition-related potential structure types as
used in the study [1] of MgAlH5 and BaAlH5 were considered
as inputs for the present theoretical simulation. From the chosen
structural starting points, full geometry optimization has been
carried out with only symmetry constrains on atomic coordinates
and unit-cell parameters. As structural relaxation may change a
structure drastically, the final structure may not resemble much
of the starting structure. The structure-type specifications found
in Fig. 1 and Table 1 are therefore referring to the symmetry of
the initial guess structure.
Structure descriptions used in calculations of enthalpies
of formation were taken from the inorganic crystal structure
database [14] but were subjected to structural relaxation prior
to use. Effects of zero-point motion were not taken into account in the enthalpy considerations, since estimation of these
requires access to phonon modes which are not addressed
within the scope of the present study. However, experience
[15,16] suggests that the neglect of zero-point-motion effects
does not invalidate the present conclusions on phase stability
at 0 K.
The Born-effective-charge (BEC) concept can be used to
analyze the bonding properties of solids. In a purely ionic
compound, off-diagonal components of the BEC tensor will
be small, and the diagonal components will carry information about how much charge which is transferred from one
site to other sites. Generally the diagonal tensor elements
show values equal to or exceeding the expected valence for
ionic constituents. The King-Smith and Vanderbilt [17] method
was used to calculate the polarization in the perturbed cells,
Fig. 1. Total energy vs. cell volume for (a) BeAlH5 , (b) CaAlH5 , and (c) SrAlH5 in different possible structural arrangements. Only the variants with the lowest
calculated energies are shown. Structure types for the guess structures used as inputs are indicated on the illustrations.
A. Klaveness et al. / Journal of Alloys and Compounds xxx (2006) xxx–xxx
and from these the BEC tensor elements for the constituents
were derived. Locally developed codes were used for pre- and
post-processing.
3
CaAlH5 , respectively. Note that hypothetical phases with very
high transition pressures may have escaped detection.
3.2. Structure descriptions
3. Results and discussion
3.1. Phase relations
Fig. 1 shows total energy versus cell volume for BeAlH5 ,
CaAlH5 , and SrAlH5 , only the structures with the lowest total
energy being included for clarity. High-pressure modifications
have been detected for BeAlH5 and CaAlH5 . According to
the Birch–Murnaghan equation of state with bulk moduli from
Table 1, the pressures at which these phase transformations
takes place at 0 K is around 1.3 and 4.5 GPa for BeAlH5 and
The crystal structure of ␣-BeAlH5 (Fig. 2(a)) exhibits alternating layers of corner-sharing 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. Each BeH4 tetrahedron shares corners with
two AlH6 octahedra and two other BeH4 tetrahedra. As seen
from Table 2, the polyhedra in the ␣-BeAlH5 structure are the
most regular of the entire BAlH5 series. In ␤-BeAlH5 (Fig. 2(b)),
the AlH6 octahedra form corner-connected chains and the BeH4
tetrahedra share corners with only AlH6 octahedra.
Table 1
Optimized equilibrium structural parameters, bulk modulus (B0 ) and the derivative of bulk modulus (B0 ) for ␣- and ␤-BeAlH5 , ␣- and ␤-CaAlH5 , and SrAlH5
B0 (GPa)
B0
0.623
1.000
0.749
0.902
0.914
0.251
0.515
31.3
3.1
0.333,
0.000,
0.904,
0.777,
0.044,
0.250
0.000
0.250
0.881
0.913
30.6
2.9
0.260,
0.809,
0.005,
0.128,
0.177,
0.971,
0.085,
0.317,
0.929,
0.987,
0.143,
0.302,
0.097,
0.187,
0.231,
0.270,
0.190,
0.221,
0.696,
0.793,
0.357,
0.903,
0.213,
0.195,
0.385,
0.557,
0.031,
0.063,
0.044
0.151
0.810
0.343
0.224
0.362
0.197
0.045
0.370
0.983
0.836
0.038
0.211
0.837
42.6
3.9
Ca(4a)
Al(4a)
H1(4a)
H2(4a)
H3(4a)
H4(4a)
H5(4a)
0.089,
0.165,
0.242,
0.074,
0.416,
0.076,
0.257,
0.610,
0.126,
0.374,
0.365,
0.140,
0.119,
0.138,
0.456
0.060
0.234
0.915
0.427
0.385
0.744
33.5
5.4
Sr(4a)
Al(4a)
H1(4a)
H2(4a)
H3(4a)
H4(4a)
H5(4a)
0.908,
0.165,
0.763,
0.078,
0.093,
0.079,
0.254,
0.104,
0.117,
0.859,
0.337,
0.860,
0.114,
0.116,
0.036
0.071
0.278
0.918
0.945
0.374
0.768
34.8
5.1
Compound (structure type; space group)
Unit cell (Å or ◦ )
Site
Positional parameters
␣-BeAlH5 (Te2 O5 ; P21 )
a = 4.790
b = 4.324
c = 6.277
β = 89.408
Be(2a)
Al(2a)
H1(2a)
H2(2a)
H3(2a)
H4(2a)
H5(2a)
0.002,
0.243,
0.247,
0.001,
0.501,
0.240,
0.890,
0.230,
0.990,
0.162,
0.740,
0.740,
0.821,
0.965,
␤-BeAlH5 (at ∼1.3 GPa)
(CaAlF5 ; C2/c)
a = 5.959
b = 7.008
c = 6.241
β = 116.205
Be(4e)
Al(4a)
H1(4e)
H2(8f)
H3(8f)
0.00,
0.000,
0.000,
0.902,
0.688,
␣-CaAlH5 (BaFeF5 ; P21 /n)
a = 8.340
b = 6.948
c = 9.714
β = 93.848
Ca1(4e)
Ca2(4e)
Al1(4e)
Al2(4e)
H1(4e)
H2(4e)
H3(4e)
H4(4e)
H5(4e)
H6(4e)
H7(4e)
H8(4e)
H9(4e)
H10(4e)
␤-CaAlH5 (at ∼4.5 GPa)
(BaAlF5 ; P21 21 21 )
a = 11.911
b = 4.832
c = 4.196
SrAlH5 (␣-BaAlF5 ; P21 21 21 )
a = 12.679
b = 5.200
c = 4.508
Structure type refers to the input for the structural relaxation computation while pressure values specify conditions on the quoted structure data.
4
A. Klaveness et al. / Journal of Alloys and Compounds xxx (2006) xxx–xxx
Fig. 2. Crystal structure of (a) ␣-BeAlH5 , (b) ␤-BeAlH5 , (c) ␣-CaAlH5 , (d) ␤-CaAlH5 , and (e) SrAlH5 . AlH6 octahedra are shown in gray tone and BeH4 tetrahedra
are represented in pink color. Ca and Sr ions are shown as orange and green spheres, respectively. (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of the article.)
The crystal structure of ␣-CaAlH5 (Fig. 2(c)) resembles
that of SrAlH5 and BaAlH5 , and consists of non-linear chains
of AlH6 octahedra and isolated Ca ions. Essentially the same
ground-state structure for CaAlH5 is reported by the independent computational-based investigation of Weidenthaler et al.
[2]. The chains in the ␣-CaAlH5 structure resemble spirals, while
those in SrAlH5 and BaAlH5 take distinct zig-zag forms. The
␣-CaAlH5 structure provides a good example of a relaxed structure that has lost most of the traits of character of the initial
BaFeF5 -type starting structure (which comprises both isolated
FeF6 octahedra and FeF4 tetrahedra). ␤-CaAlH5 (Fig. 2(d)) takes
an isostructural atomic arrangement to SrAlH5 , confirming the
close relationship of the two phases. The structure of SrAlH5
(Fig. 2(e)) contains as mentioned zig-zag chains of AlH6 octahedra and more isolated Sr ions.
3.3. Thermodynamic stability of CaAlH5 and SrAlH5
The standard enthalpy of formation of CaAlH5 and SrAlH5
can be attained via the constructed reaction BH2 + AlH3 →
BAlH5 . This is done by adding the calculated reaction enthalpies
at 0 K to the corresponding experimental-based standard enthalpies of formation for the reactants, in this case CaH2 or SrH2
(−181.5 or −180.3 kJ mol−1 ) [19] and AlH3 (−11.7 kJ mol−1 )
[20]. The reactants are chosen as the set of compounds which,
together, best resembles the bonding situation in the corresponding products. The idea is that the phonon spectra of the reactants,
in sum, shall approximately imitate that of the product in question. The use of this method requires that the standard enthalpies
of formation for the reactants are available.
From the theoretically calculated total energies for the three
constituents of the above reference reaction, the reaction energies at 0 K is estimated as −30 and −56 kJ mol−1 for B = Ca and
Sr, respectively. The estimated values for the standard enthalpy
of formation for CaAlH5 and SrAlH5 are accordingly −224 and
−248 kJ mol−1 , respectively. Similarly the standard enthalpy of
formation of Ca(AlH4 )2 has been estimated as −214 kJ mol−1
via the reaction 2AlH3 + CaH2 → Ca(AlH2 )2 , using the structure established by Løvvik [21] to calculate the theoretical value
for the total energy of the product.
A Gibbs-free-energy-based criterion [22] for stability of
hydrides toward dehydrogenation at ambient temperature and
A. Klaveness et al. / Journal of Alloys and Compounds xxx (2006) xxx–xxx
5
Table 2
Interatomic distances and bond valence [18] data for ␣- and ␤-BeAlH5 , ␣- and ␤-MgAlH5 , ␣- and ␤-CaAlH5 , SrAlH5 , and BaAlH5
B/C H
␣-BeAlH5
Be H
Al H
␤-BeAlH5
Be H
Al H
␣-MgAlH5
Mg H
Al H
␤-MgAlH5
Mg H
Al H
␣-CaAlH5
Ca1 H
Ca2 H
Al1 H
Al2 H
␤-CaAlH5
Ca H
Al H
SrAlH5
Sr H
Al H
BaAlH5
Ba H
Al H
CN
Distance (Å)
Bond valence B/C
Minimum
Maximum
Mean
Variance
Minimum
Maximum
Sum
4
6
1.433
1.703
1.450
1.739
1.442
1.723
0.008
0.015
0.399
0.457
0.418
0.505
1.631
2.872
4
6
1.430
1.708
1.433
1.725
1.432
1.719
0.002
0.008
0.417
0.476
0.421
0.498
1.676
2.899
7
6
1.857
1.682
2.314
1.775
2.044
1.731
0.152
0.034
0.120
0.415
0.413
0.534
1.890
2.822
7
6
1.849
1.686
2.191
1.752
2.021
1.725
0.137
0.028
0.167
0.442
0.423
0.528
1.985
2.865
10
10
6
6
2.290
2.316
1.698
1.696
2.590
2.578
1.784
1.842
2.367
2.390
1.739
1.745
0.086
0.077
0.033
0.056
0.128
0.132
0.406
0.347
0.288
0.269
0.512
0.515
2.401
2.246
2.759
2.733
12
6
2.280
1.690
2.615
1.792
2.422
1.731
0.095
0.034
0.120
0.397
0.297
0.523
2.499
2.817
12
6
2.274
1.694
2.868
1.818
2.480
1.747
0.159
0.041
0.061
0.369
0.302
0.517
2.244
2.702
12
6
2.650
1.704
2.990
1.780
2.794
1.732
0.107
0.028
0.125
0.410
0.313
0.503
2.647
2.340
pressure states that a given hydride will be stable, unless the
enthalpy for any possible dehydrogenation reaction is less than
39 kJ mol−1 H2 . The enthalpy difference for dehydrogenation of
CaAlH5 according to reaction (2) is estimated to be 28 kJ mol−1
H2 at ambient temperature and pressure, indicating that this
compound is metastable under these conditions. However, the
compound might be stable at pressures around 100 bar, but this
value is subject to considerable uncertainty. The corresponding
reaction for SrAlH5 exhibits a larger estimated enthalpy difference of 45 kJ mol−1 H2 indicating that this compound should be
stable. The dehydrogenation reaction of Ca(AlH4 )2 according
to reaction (1) is estimated at −7 kJ mol−1 H2 , a result which
would attribute metastability to Ca(AlH4 )2 . Similar considerations on the stability of BeAlH5 could not be made since
experimental thermodynamic data for appropriate reactants are
not available for this compound. In fact, no hydrogen-containing
compound of Be is hitherto recorded in the literature.
3.4. Bonding
The calculated total and site-projected density-of-states for ␣BeAlH5 , ␣-CaAlH5 , and SrAlH5 are shown in Fig. 3. All phases
are proper insulators at 0 K with estimated (GGA) band gaps of
3.12, 2.72, and 3.51 eV, respectivly, which are of similar size to
those of the A3 AlH6 series (A = Group I element). For atom
sites with approximately equal partial DOS, the first labeled
site refers to the illustration whereas sites with resembling DOS
profiles are indicated in parenthesis.
The site-projected DOSs for ␣-BeAlH5 (Fig. 3(a)) reveal (as
a probably acceptable approximation) sp3 -hybridization within
the BeH4 tetrahedra and d2 sp3 -hybridization within the AlH6
octahedra. This is in good agreement with the charge-density
map in Fig. 4(a), which confirms distinct covalent bonding between Be and H as well as Al and H. The Born-effective charges
of ␣-BeAlH5 in Table 3 are also in agreement with this inference, showing somewhat varying diagonal tensor elements with
values often well below the expected ionic valences. The overall conclusion is that the Al H and Be H bonds have largely
covalent character but still carry a significant woof of ionisity.
The ␣-CaAlH5 phase shows similar site-projected DOSs to
those of SrAlH5 and BaAlH5 , features that, above all, originate
from the pronounced structural similarity. The classic d2 sp3 hybridization picture, which largely is appropriate for the B
constituents of ␣-BeAlH5 and ␣-MgAlH5 , is invalid for those
with B = Ca, Sr, and Ba. This is brought about by the asymmetric way the octahedra are connected. A distinctly modified
hybridization scheme in which two of the hybridized orbitals
become separated in energy from the others, can be constructed
to explain this situation, but the details are of little interest in the
current context.
The charge-density map for ␣-CaAlH5 (Fig. 4(b)) shows similar characteristics to that of ␣-MgAlH5 , SrAlH5 , and BaAlH5
with regard to appreciable covalent character for the Al H bond,
and distinctly ionic character for Ca. The Born-effective charges
further emphasizes the ionic character for Ca (revealing too high
values for the diagonal tensor elements in Table 3) while Al
6
A. Klaveness et al. / Journal of Alloys and Compounds xxx (2006) xxx–xxx
Fig. 3. Total and partial density of states (DOS) for (a) ␣-BeAlH5 , (b) ␣-CaAlH5 , and (c) ␣-SrAlH5 . Shading marks s states, dashed line denotes p states, and solid
line in unshaded fields signifies d states. The Fermi level is indicated by a vertical broken line.
Fig. 4. Charge-density maps of (a) ␣-BeAlH5 , (b) ␣-CaAlH5 , and (c) ␣-SrAlH5 . The two first numbers below each plot specifies minimum and maximum value
within the plot, and the third number the separation between two successive contours. White is set at zero in the color representation. Stars indicate three atoms which
define the mapping plane.
A. Klaveness et al. / Journal of Alloys and Compounds xxx (2006) xxx–xxx
7
Table 3
Calculated Born-effective-charge-tensor elements (Z∗ ) for the constituents of ␣-BeAlH5 , ␣-CaAlH5 , and SrAlH5
xx
yy
zz
xy
yz
zx
xz
zy
yx
␣-BeAlH5
Be
Al
H1
H2
H3
H4
H5
0.856
2.803
−0.713
−0.934
−1.142
−0.583
−0.291
1.325
2.742
−0.598
−0.966
−0.993
−0.621
−0.884
1.516
2.042
−0.845
−0.516
−0.468
−0.977
−0.754
−0.056
0.012
0.152
−0.349
0.444
−0.110
−0.059
0.076
−0.352
0.178
0.051
−0.052
0.245
−0.412
0.097
−0.094
−0.285
0.052
0.083
0.198
−0.064
0.081
−0.092
−0.275
0.053
0.088
0.204
−0.065
−0.049
0.139
0.193
0.042
−0.053
0.239
−0.329
0.076
−0.352
0.178
0.051
−0.052
0.245
−0.412
␣-CaAlH5
Ca1
Ca2
Al1
Al2
H1
H2
H3
H4
H5
H7
H8
H9
H10
2.104
2.421
2.557
2.509
−1.366
−1.115
−0.946
−0.694
−1.117
−0.918
−0.664
−0.821
−0.955
2.024
2.031
2.405
2.112
−0.604
−0.604
−0.752
−1.058
−0.552
−1.172
−1.013
−1.249
−0.962
2.501
2.331
1.905
2.375
−0.783
−0.945
−0.979
−0.883
−0.916
−0.831
−0.925
−0.855
−0.850
0.259
−0.251
0.215
0.243
0.289
−0.006
0.021
0.122
0.073
−0.235
−0.096
−0.305
0.129
0.027
0.006
0.114
0.120
0.007
0.068
0.028
0.094
−0.018
−0.003
−0.048
−0.212
0.009
0.058
−0.044
0.190
0.003
0.038
0.024
−0.036
−0.142
0.128
−0.012
−0.141
−0.101
−0.022
0.162
−0.086
0.169
−0.030
0.023
−0.025
0.008
−0.116
0.147
−0.011
−0.160
−0.073
−0.025
−0.006
−0.022
0.031
0.208
0.025
0.044
0.110
0.166
−0.017
−0.057
−0.118
−0.264
0.022
0.027
0.006
0.114
0.120
0.007
0.068
0.028
0.094
−0.018
−0.003
−0.048
−0.212
0.009
SrAlH5
Sr
Al
H1
H2
H3
H4
H5
2.489
2.338
−0.968
−0.920
−0.934
−0.940
−1.032
2.468
2.046
−0.924
−0.944
−0.914
−0.977
−0.760
2.233
2.176
−0.731
−0.903
−0.910
−0.868
−1.010
−0.094
−0.188
0.053
0.130
−0.098
0.011
0.099
−0.079
−0.317
0.130
0.127
−0.081
0.022
−0.250
0.013
−0.151
0.150
−0.033
−0.033
0.124
0.365
−0.042
−0.041
0.076
−0.029
−0.071
0.052
0.416
−0.050
0.008
0.167
0.158
−0.114
−0.008
−0.241
−0.079
−0.317
0.130
0.127
−0.081
0.022
−0.250
shows iono-covalent bonding with diagonal components below
the values of the expected ionic valence.
As already mentioned, the bonding situation for SrAlH5 resembles that for ␣-CaAlH5 and BaAlH5 . The site-projected
DOSs (Fig. 3(c)), Born-effective-charge values (Table 3), and
charge-density (Fig. 4(c)) support this conclusion.
The outlined distinctions in the bonding characteristics between the BeAlH5 and MgAlH5 phases and the rest of the series
is also nicely reflected by similarities in the bond valence sums
for the B and C constituents (Table 2).
SrAlH5 and BaAlH5 . The shape of the chains appears to be solely
determined by the size of the alkaline-earth ion, since the Al H
bond length seems to be rather invariant throughout the series.
As the chains are key elements in the structural arrangement they
also have great influence on the shapes and gaps in the valence
band. The dominant covalence of the Al H bonds seems rather
unaffected by the geometry of the chain arrangements.
While MgAlH5 and CaAlH5 appear to be metastable, SrAlH5
and BaAlH5 are likely to be stable at ambient temperature and
pressure. The stability of the BAlH5 seems to increase with the
size of the alkaline-earth ion.
4. Conclusion on trends in the BAlH5 series
All structures for this series of compounds, except those for
␣- and ␤-BeAlH5 , comprise divalent alkaline-earth metal ions
and corner-sharing AlH6 octahedra arranged in chains. BeAlH5
adopts a layered structure which exhibits AlH6 sheets. This distinction is caused by the covalent character of the Be H bond
which allows one hydrogen atom to be common to two BeH4
tetrahedra. The end of this is that each AlH6 unit can form two
more linkages to other AlH6 octahedra, and presto, the result is
a layered structure.
The other ground-state phases of the BAlH5 series exhibit
chains of AlH6 octahedra; rather linear chains in ␣-MgAlH5 ,
spiral shaped in ␣-CaAlH5 , and zig-zag formed arrengements in
Acknowledgement
The authors gratefully acknowledge the Research Council
of Norway for financial support and for computer time at the
Norwegian supercomputer facilities.
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