Download Magnesium based ternary metal hydrides containing alkali and

Journal of Alloys and Compounds xxx (2006) xxx–xxx
Review
Magnesium based ternary metal hydrides containing
alkali and alkaline-earth elements
Klaus Yvon ∗ , Bernard Bertheville
Laboratoire de Cristallographie, Université de Genève, 24 Quai Ernest Ansermet, CH-1211 Genève, Switzerland
Received 23 December 2005; accepted 17 January 2006
Abstract
A review of structurally characterised ternary metal hydrides containing magnesium and alkali (A) or alkaline-earth (Ae) elements (including divalent Eu and Yb) is given. There exist 28 such compounds that cover 18 different crystal structures. Most were obtained by solid-state
reactions under moderate hydrogen gas pressure while some required hydrostatic pressure in the kbar range. The compounds are saline and
have ordered crystal structures. Their hydrogen contents can be rationalized in terms of mono and divalent metal cations, and hydrogen anions.
Many have fluorine analogues, and those containing calcium and strontium tend to have ytterbium and europium analogues, respectively. Magnesium usually adopts octahedral and rarely tetrahedral or pentagonal bipyramidal hydrogen configurations. The metal configurations around
hydrogen are mostly octahedral and tetrahedral, but square pyramidal, trigonal bipyramidal, trigonal and linear configurations are also found.
Compared to binary hydrides, the metal–hydrogen bonds are generally shortened which is reflected by a volume contraction. The hydrogen storage
efficiencies for A metal compounds reach 6.0 wt.% and 88 g l−1 (NaMgH3 ) and for Ae metal compounds 5.7 wt.% and 99 g l−1 (Ca4 Mg3 H14 ).
The compounds generally desorb hydrogen above 670 K at 1 bar. The europium containing hydrides order magnetically at temperatures below
19–32 K.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Hydrogen storage materials; Chemical synthesis; Crystal structures
Contents
1.
2.
3.
4.
5.
∗
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Synthesis and methods of characterisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Crystal chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1. Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2. Crystal structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1. Weight and volume efficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2. Thermal stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3. Electronic and magnetic properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Corresponding author. Tel.: +41 22 379 6231; fax: +41 22 379 6864.
E-mail address: [email protected] (K. Yvon).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.01.049
JALCOM-13609;
No. of Pages 8
K. Yvon, B. Bertheville / Journal of Alloys and Compounds xxx (2006) xxx–xxx
2
1. Introduction
Due to their light weight and low cost, magnesium based
metal hydrides are of interest as constituents for advanced hydrogen storage systems. Until recently only few ternary hydrides
between magnesium and alkali (A) or alkaline-earth (Ae) elements were known. The first fully characterized example was
Ca4 Mg3 H14 [1] which has been reported in 1992. Since then
quite a few other hydrides of this class have been discovered,
most of them at the University of Geneva. No review on these
compounds has appeared as yet, except for a short chapter in
an article on novel solid-state metal hydrides [2] and a report
on europium based saline metal hydrides [3]. Ternary hydrides
between magnesium and A or Ae elements known up to 1997
are included in the Hydride–Fluoride Crystal Structure Data
Base [4], but are currently not covered by the Hydride Mate-
rials Data Base [5]. The purpose of this article is to review
these hydrides and to discuss their crystal chemistry and some
of their properties. Only ternary compounds (i.e. no solid solutions) are covered whose structures (including hydrogen positions) have been fully characterised. Systems containing divalent
europium and ytterbium are included because of their close crystal chemical relationship with the corresponding alkaline-earth
systems.
2. Synthesis and methods of characterisation
The compounds reported so far [1,6–27] are summarized
in Table 1. They were usually synthesised by solid-state reaction between binary hydrides powder mixtures (e.g. Rb2 MgH4 )
or from binary metal alloys (e.g. EuMg2 H6 ) under moderate
hydrogen pressures (up to 160 bar) and temperatures (up to
Table 1
Compositions, structural properties and hydrogen storage efficiencies of magnesium based ternary metal hydrides containing alkali (A) and alkaline-earth (Ae) or
lanthanide elements (Eu, Yb)a
Compound
Space group
Vb
A–Mg
NaMgH3
KMgH3 d
RbMgH3
CsMgH3 (hp)f,g
CsMgH3 (lp)f
Rb4 Mg3 H10
Cs4 Mg3 H10 g
K2 MgH4
Cs2 MgH4 (hp)f,g
Rb2 MgH4
Cs2 MgH4 (lp)f
Rb3 MgH5
Cs3 MgH5
Pnma
Pm-3m
P63 /mmc
R-3m
Pmmn
Cmca
Cmca
I4/mmm
I4/mmm
Pnma
Pnma
I4/mcm
I4/mcm
−5.1
−15.9
−15.3e
−12.3
−8.7
−6.3
−10.0
−9.9
−14.3
+3.9
+3.8
<+0.1
−0.3
Ae–Mg
EuMg2 H6
Sr2 Mg3 H10
Ba2 Mg3 H10
Eu2 Mg3 H10 g
Sr6 Mg7 H26 g
Eu6 Mg7 H26 g
Ba6 Mg7 H26
SrMgH4
EuMgH4
BaMgH4
Ca4 Mg3 H14
Yb4 Mg3 H14
Sr2 MgH6 g
Ba2 MgH6
Eu2 MgH6 g
Ca19 Mg8 H54 g
Yb19 Mg8 H54
P4/mmm
C2/m
C2/m
C2/m
I/2m
I/2m
Immm
Cmc21
Cmc21
Cmcm
P-62m
P-62m
P-3m1
P-3m1
P-3m1
Im-3
Im-3
+8.9
−3.8
−5.1
−3.1
−2.9
−2.6
−5.4
−0.5
−0.2
−4.6
−0.6
+0.7
−7.9
−9.9
−7.3
−4.7
−4.1
a
b
c
d
e
f
g
CNc
wt.% H2
A, Ae
Mg
12
12
12
12
12
10,11
10, 11
9
9
9, 10
9, 11
8, 10
8, 10
6
6
6
6
6
6
6
6
6
4
4
4
4
6.0
4.6
2.7
1.9
1.9
2.4
1.6
3.8
1.2
2.0
1.4
1.8
1.2
12
12
12
12
10, 11
10, 11
12
9
9
13
9
9
12
12
12
8, 10, 12
8, 10, 12
6
6
6
6
6
6
6
6
6
6
7
7
6
6
6
6
6
2.9
3.9
2.8
2.6
3.6
2.4
2.6
3.5
2.2
2.4
5.7
1.8
2.9
2.0
1.8
5.4
1.5
g H2 /liter
88.3
77.1
69.7e
60.0
56.9
56.9
52.7
59.9
48.5
45.5
40.0
42.7
37.1
88.3
95.1
86.5
96.9
92.0
94.5
83.7
88.5
91.2
81.1
98.9
99.6
91.3
78.3
92.9
100.9
97.7
Reference
6
7
8
9
10
11
9
12
9
13
14
13
14
15
16
17
18
19
18
20
21
15
22
1
23
24
25
18
26
27
As of 2005, hydrides structurally characterized by neutron diffraction on deuterides only (except for KMgH3 , see d), listed in decreasing order of Mg content.
Relative volume contraction compared to weighted sums of volumes of corresponding binary hydrides.
Coordination number.
H positions not ascertained but fixed by symmetry.
Values derived from deuteride.
hp, high-pressure; lp, low-pressure modification.
Synthesized under high hydrostatic pressure.
K. Yvon, B. Bertheville / Journal of Alloys and Compounds xxx (2006) xxx–xxx
∼700 K). Some hydrides, however, required the application of
rather high quasi-hydrostatic pressure in a multi-anvil press,
such as Sr2 MgH6 that was obtained at 35 kbar and T = 870 K,
Sr6 Mg7 H26 at 25 kbar and 900 K, CsMgH3 (hp), Cs4 Mg3 H10
and Cs2 MgH4 (hp) at 30 kbar and 820 K, and various Eu–Mg
hydrides at up to 32 kbar and 870 K. Some hydrides were
obtained by hydrogenation of single-phase alloys such as the
solid solution series Eu1−x Srx Mg2 (MgZn2 -type structure) that
yielded Eu1−x Srx Mg2 H6 for x ≤ 0.6 and (Eu1−x Srx )2 Mg3 H10
[28] for x > 0.8, or by hydrogenation of multi-phase alloys
such as mixtures between binary CaMg2 and elemental Ca that
yielded Ca4 Mg3 H14 .
The reaction products were usually polycrystalline and
contained secondary phases such as unreacted binary metal
hydrides. Single crystals of sufficient size and quality for
diffraction experiments (e.g. Yb19 Mg8 H54 ) were rare. The
hydrides containing A and/or heavy Ae metals were particularly
sensitive to air and moisture. Given the unfavourable scattering power of hydrogen for both X-ray and neutron powder
diffraction the atomic arrangements had to be determined ab
initio on deuterides. Difficulties during structure determination
arose due to the presence of impurity phases (of which some
were not detected by X-rays), the structural complexity (up
to 16 atomic parameters for Ae6 Mg7 H26 ; Ae = Eu, Sr) and
the limited resolution of conventional powder diffractometers.
In some cases, such as for the strongly neutron absorbing
europium compounds, high-resolution data had to be collected
on advanced synchrotron and high-flux neutron radiation
sources. The data were analysed by the Rietveld method by
using joint refinement procedures, if necessary. For better
convergence the number of refined parameters, in particular
the atomic displacement amplitudes, were reduced by applying
suitable constraints. Due to the absence of single crystals
and/or single-phase powder samples and their sensitivity to
moisture, measurements of physical properties were difficult
and only few have been performed. Magnetic susceptibility
data have been collected only on europium containing hydrides,
and enthalpies of hydride formation have been determined for
very few, relatively unstable hydrides by measuring pressurecomposition isotherms on a thermo-balance (Yb4 Mg3 H14 )
or by calorimetric methods (NaMgH3 [29], KMgH3
[30]).
3
3. Crystal chemistry
3.1. Composition
Magnesium based ternary metal hydrides are known to occur
for all A and Ae elements (including Eu and Yb) except A = Li
and Ae = Be. Their compositions and some of their properties
are listed in Table 1. So far, there exist 28 ternary compounds of
which five occur with Eu, four with Rb, Cs, Sr and Ba each, two
with K, Ca and Yb each, and one with Na. For some hydrides
(CsMgH3 , Cs2 MgH4 ) there exist low-pressure (lp) and highpressure (hp) polymorphs. As expected from valence considerations the hydrogen-to-metal ratios range from H/M = 1.25 (e.g.
Rb3 MgH5 ) to H/M = 2 (e.g. SrMgH4 ). For A metal compounds
the Mg/A ratios cover the range 0.33–1.00 and assume simple
fractions such as Mg/A = 3/4 (Rb4 Mg3 H10 ) and 1/2 (K2 MgH4 ).
For Ae metal compounds the Mg/Ae ratios cover the range
0.42–2.00 and assume complex fractions such as Mg/Ae = 6/7
(Ba6 Mg7 H26 ) and 19/8 (Ca19 Mg8 H54 ). Interestingly, ternary
hydrides between Ae elements other than magnesium, and quaternary hydrides between magnesium, A and Ae elements have
not been reported as yet. This absence is presumably due to
the fact that the binary hydrides CaH2 , SrH2 , BaH2 , EuH2 and
YbH2 are isostructural (PbCl2 -type), thus favouring the formation of solid solutions, whereas MgH2 crystallizes with different
structures (␣-TiO2 - or ␣-PbO2 -type [31]).
3.2. Crystal structures
As can be seen from Table 2 (A metal constituents) and
Table 3 (Ae metal constituents, including Eu and Yb) the
hydrides crystallize with 18 different structures types of which
some are deformation variants of more symmetric ones, such
as NaMgH3 that derives from cubic KMgH3 by an orthorhombic distortion, and Ae6 Mg7 H26 and AeMgH4 (Ae = Sr, Eu) that
derive from orthorhombic Ba6 Mg7 H26 and BaMgH4 , respectively, by small monoclinic distortions. All hydrogen atom
arrangements are ordered, in contrast to metallic metal hydrides
[32] in which they are usually disordered. Many have fluorine
analogues [4,12]. Mixed hydride fluorides exist such as the solid
solution series NaMgH3−x Fx [6], but none have ordered anion
distributions. As expected from ionic size considerations the Yb
Table 2
Ternary metal hydridesa between magnesium and alkali elements
Li
Na
K
Rb
Cs
Structure type (space group)
CsMgH3 (hp)b
CsMgH3 (lp)
Cs4 Mg3 H10 b
Cs2 MgH4 (hp)
Cs2 MgH4 (lp)
Cs3 MgH5
GdFeO3 (Pnma)
CaTiO3 (Pm-3m)
BaTiO3 (P63 /mmc)
BaRuO3 (R-3m)
CsMgH3 (Pmmn)
Cs4 Mg3 F10 (Cmca)
K2 NiF4 (I4/mmm)
␤-K2 SO4 (Pnma)
Cs3 CoCl5 (I4/mcm)
NaMgH3 b
KMgH3 b
RbMgH3 b
Rb4 Mg3 H10 b
K2 MgH4 b
Rb2 MgH4 b
Rb3 MgH5
a
b
Arranged in decreasing order of Mg content.
Fluoride analogue known to exist.
K. Yvon, B. Bertheville / Journal of Alloys and Compounds xxx (2006) xxx–xxx
4
Table 3
Ternary metal hydridesa between magnesium and alkaline-earths or divalent lanthanides
Ca
Sr
Ba
Eu
Yb
Sr2 Mg3 H10
Sr6 Mg7 H26
Ba2 Mg3 H10 b
EuMg2 H6
Eu2 Mg3 H10
Eu6 Mg7 H26
Ba6 Mg7 H2
SrMgH4 b
EuMgH4
BaMgH4 b
Ca4 Mg3 H14
Yb4 Mg3 H14
Sr2 MgH6
Ba2 MgH6 b
Ca19 Mg8 H54
a
b
Eu2 MgH6
Yb19 Mg8 H54
Structure type (space group)
EuMg2 H6 (P4/mmm)
Ba2 Ni3 F10 (C2/m)
Ba6 Zn7 F26 (I2/m)
Ba6 Mg7 H26 (Immm)
BaZnF4 (Cmc21 )
LaNiH4 (Cmcm)
Ca4 Mg3 H14 (P-62m)
K2 GeF6 (P-3m1)
Yb19 Mg8 H54 (Im-3)
Arranged in decreasing order of Mg content (adapted from [3]).
Fluoride analogue known to exist.
compounds are isotypic to the Ca compounds and the Eu compounds are isotypic to the Sr compounds. The only Eu compound
for which no Sr analogue has been reported yet is EuMg2 H6 .
Its solid solution Eu1−x Srx Mg2 H6 has only a limited range
0 ≤ x ≤ 0.6 [28].
The structures show complicated atom arrangements
(Sr6 Mg7 H26 : nine different H sites) and display a rich variety
of coordination numbers (CNs) and geometries. Of major interest are the H atom environments around magnesium. They are
usually octahedral (CN = 6) or distorted octahedral as in binary
MgH2 . Notable exceptions are the tetrahedral H configurations
(CN = 4) in ␤-K2 SO4 -type hydrides (Rb2 MgH4 , lp-Cs2 MgH4 )
and Cs3 CoCl5 -type hydrides (Cs3 MgH5 ), and the pentagonal
bipyramidal H configuration (CN = 7) in Ae4 Mg3 H14 (Ae = Ca,
Yb). As expected, the MgH6 octahedra tend to be isolated at low
Mg contents and connected via corners, edges and faces at higher
Mg contents. However, there is no clear relationship between the
connectivity of the MgH6 octahedra and the Mg content. While
the MgH6 octahedra are isolated in Ae2 MgH6 (Ae = Sr, Ba,
Eu), corner sharing occurs in EuMg2 H6 and AMgH3 (A = Na,
K), and in AeMgH4 (Ae = Sr, Ba, Eu) and A2 MgH4 (A = K,
Cs), leading to three-dimensional slabs and two-dimensional
networks, respectively (see Fig. 1). On the other hand, the octahedra in Ae19 Mg8 H54 (Ae = Ca, Yb) are connected via corners
to groups of 8 that are isolated from each other. Corner and
edge sharing (such as in MgH2 ) leading to three-dimensional
networks occurs in Ae2 Mg3 H10 and Ae6 Mg7 H26 (Ae = Sr, Ba,
Eu). Corner and face sharing occurs only with A metal compounds such as A4 Mg3 H10 (A = Rb, Cs) and AMgH3 (A = K,
Cs). Their Mg octahedra are condensed via common faces to
dimers (RbMgH3 ) or trimers (CsMgH3 ; A4 Mg3 H10 , A = Rb,
Cs) that are linked via corners to two-dimensional slabs or threedimensional networks. Interestingly, the trimers in CsMgH3 are
quasi linear in the high-pressure and cyclic in the low-pressure
modification.
The H atom environments around A atoms and Ae atoms
other than magnesium show a great variety of CNs and configurations that differ substantially from those in the corresponding
binary hydrides. As expected from their larger size, heavy A and
Ae atoms have higher CNs than Mg atoms. They range from
CN = 8 (A3 MgH5 ) to CN = 13 (BaMgH4 ), in contrast to the 12fold configuration in binary NaCl-type hydrides AH (A = K, Rb,
Cs) and the 9-fold configuration in binary PbCl2 -type hydrides
AeH2 (Ae = Ca, Sr, Ba, Eu, Yb). The influence of cation size
on CNs and H atom configurations is particularly apparent in
barium and strontium based analogues. As shown in Fig. 2
the bigger Ba atoms in orthorhombic Ba6 Mg7 H26 occupy two
sites with CN = 12, while the smaller Sr atoms in monoclinic
Sr6 Mg7 H26 occupy three sites with CN = 10 and 11 that have
quite different H atom configurations. Similar differences in CNs
and H atom configurations also occur between centrosymmetric BaMgH4 (CN = 13) and its non-centrosymmetric strontium
congener SrMgH4 (CN = 9). Another example for the influence
of cation size is the perovskite-related series AMgH3 . As the
atomic size of A decreases the structures change from rhombohedral BaRuO3 -type (A = Cs) to hexagonal BaTiO3 -type (A = Rb),
cubic CaTiO3 -type (A = K) and orthorhombic GdFeO3 -type
(A = Na). A similar size effect is also observed in the A2 MgH4
series (A = K, Rb, Cs) that changes from the orthorhombic ␤K2 SO4 -type (tetrahedral Mg coordination) for the relatively big
Rb and Cs atoms (CN = 9, 10) to the tetragonal K2 NiF4 -type
(octahedral Mg coordination) for the smaller K atom (CN = 9),
a transition that can also be induced by external pressure as
shown for A = Cs. Clearly, pressure stabilizes the octahedral
MgH6 groups in CsMgH3 , Cs4 Mg3 H10 and Cs2 MgH4 , and these
have smaller volume requirements and thus better volume efficiencies for hydrogen storage than the tetrahedral MgH4 groups
formed under lower hydrogen gas pressure as in Cs2 MgH4 and
Cs3 MgH5 .
The metal configurations around the H atom sites display
a large inventory of coordination numbers and site symmetries. Typical examples are shown in Fig. 3. Many are specific
to ternary hydrides as they do not occur in the corresponding
binary hydrides. They range from linear (CN = 2) to trigonal
planar or nearly planar as in ␣- and ␥-MgH2 , respectively
(CN = 3), distorted tetrahedral (CN = 4), square pyramidal or
trigonal-bipyramidal (CN = 5), and octahedral (CN = 6). The linear Mg–H–Mg co-ordination in EuMg2 H6 and its strontium containing solid solution Eu1−x Srx Mg2 H6 is remarkable because
such a geometry is atypical for saline compounds. The H site
symmetries range from relatively high symmetric 4/mmm in
EuMg2 H6 to low symmetric 1 in Ba6 Mg7 H26 .
The metal–deuterium distances are consistent with those in
the corresponding binary deuterides, but tend to be shorter.
K. Yvon, B. Bertheville / Journal of Alloys and Compounds xxx (2006) xxx–xxx
5
Fig. 1. Connectivity of MgH6 octahedra in magnesium based ternary metal hydrides containing alkali (A) and alkaline-earth (Ae) or lanthanide (Eu, Yb) elements;
lp = low-pressure modification, hp = high-pressure modification.
A striking example is the Mg–D bond of 1.77 Å in the linear Mg–D–Mg co-ordination of EuMg2 D6 which is significantly shorter than the Mg–D bonds in MgD2 (1.95 Å). It is
the shortest known among magnesium containing solid-state
metal deuterides and reflects the partially covalent character of magnesium–deuterium (hydrogen) bonds. A shortening
of metal–deuterium bonds also occurs for other constituents,
in particular A metals such as Na (2.27 Å in NaMgD3 versus 2.44 Å in NaD), K (2.78 Å in K2 MgD4 versus 2.85 Å in
KD), Rb (2.77 Å in Rb2 MgD4 versus 3.01 Å in RbD) and Cs
(2.94 Å in Cs4 Mg3 D10 versus 3.19 Å in CsD). The shortest
metal–deuterium distances for Ae metals other than Mg and
for divalent lanthanides do generally not much differ from
those in the corresponding binary deuterides, such as for Ca
(2.29 Å in Ca4 Mg3 D14 versus 2.24 Å in CaD2 ), Sr (2.43 Å in
SrMgD4 versus 2.43 Å in SrD2 ), Ba (2.55 Å in Ba2 Mg3 D10 versus 2.57 Å in BaD2 ), Eu (2.37 Å in EuMgD4 versus 2.38 Å in
EuD2 ) and Yb (2.27 Å in Yb4 Mg3 D14 versus 2.24 Å in YbD2 ).
However, a significant shortening occurs in those deuterides that
were obtained by application of high quasi-hydrostatic pressure, such as Ca19 Mg8 D54 (Ca–D = 2.20 Å) and Eu6 Mg7 D26
(Eu–D = 2.20 Å). As expected, the metal–deuterium distances
also depend on coordination numbers and matrix effects. The
average Eu–D distances, for example, generally increase with
the CN of europium and range from 2.53 Å in EuMgD4 (CN = 9)
to 2.56–2.57 Å in Eu6 Mg7 D26 (CN = 10), 2.64 Å in Eu6 Mg7 D26
(CN = 11) and 2.65–2.71 Å in Eu2 Mg3 D10 (CN = 12) [3]. The
anomalously small value in EuMg2 D6 (2.58 Å for CN = 13)
is presumably a result of its unusual crystal structure. The
importance of matrix effects on metal–deuterium bond lengths
6
K. Yvon, B. Bertheville / Journal of Alloys and Compounds xxx (2006) xxx–xxx
Fig. 2. Deuterium coordination polyhedra around the Ba sites (top) in Ba6 Mg7 D26 (left and centre) and BaMgD4 (right), and around the corresponding Sr sites
(bottom) in the less symmetric Sr analogues Sr6 Mg7 D26 and SrMgD4 ; same shading (colour) of H atoms indicate symmetry equivalent sites.
and bond angles can be seen in K2 NiF4 -type deuterides such
as A2 + MgD4 (A+ = K, Cs) in which the bigger Cs+ lengthens the Mg–D bonds and flattens the MgD6 octahedron (axial
Mg–D = 2.00 (K), 2.01 Å (Cs), equatorial Mg–D = 2.02 (K),
2.16 Å (Cs)). As in the corresponding binary deuterides, the
Eu–D distances in the ternary deuterides are similar to, but
slightly shorter than, the corresponding Sr–D distances in the
strontium analogues, while the Yb–D distances in the ternary
deuterides are similar to, but slightly longer than, the correspond-
ing Ca–D distances in the calcium analogues, thus underlining
the closely related hydride chemistry of Ae metals and divalent
lanthanides. The observed differences (∼0.04 Å, on the average,
between the Sr and Eu analogues, and ∼0.02 Å, on the average,
between the Ca and Yb analogues) are somewhat larger than
expected from tabulated effective ionic metal radii for oxides and
fluorides (Sr2+ : 1.36 Å versus Eu2+ : 1.35 Å for CN = 10; Ca2+ :
1.14 Å versus Yb2+ : 1.12 Å for CN = 8 [33]) which suggests
that different sets of metal radii should be used for rationalizing
Fig. 3. Examples for deuterium coordinations in magnesium based ternary metal hydrides containing alkali and alkaline-earths or divalent lanthanides; small spheres:
D atoms, medium spheres: Mg atoms, large spheres: A or Ae atoms (perspective views).
K. Yvon, B. Bertheville / Journal of Alloys and Compounds xxx (2006) xxx–xxx
metal hydride structures. In this context it is useful to recall
that metal–hydrogen distances in such compounds cannot be
described in terms of a fixed hydrogen radius. This is due to the
strong polarizability of the hydride anion which leads to a wide
spread of molar hydrogen volumes [34]. The D–D contact distances in all deuterides studied so far all exceed 2.0 Å, which can
be taken as evidence for repulsive D− –D− interactions. Repulsive interactions also occur between the metal cations as shown,
for example, by the displacements of the Mg2+ cations from
the centres of the face-sharing H octahedra in the [3MgH3 ]3−
trimers of the CsMgH3 polymorphs (Mg–D = 1.94–1.96 Å at the
periphery, 2.00–2.08 Å at the centre of the trimers). Finally,
the metal–hydrogen distances tend to be slightly longer than
the corresponding distances in the deuterides, thus leading to
hydride volumes that are typically 0.5% bigger than those of the
corresponding deuterides which is consistent with the usually
observed isotope effect.
4. Properties
4.1. Weight and volume efficiencies
As shown in Table 1 the gravimetric hydrogen storage efficiencies of Mg based ternary metal hydrides reach 6.0 wt.%
for A metal constituents (NaMgH3 ) and 5.7 wt.% for Ae
metal constituents (Ca4 Mg3 H14 ). The volume efficiencies reach
101 g H2 l−1 (Ca19 Mg8 H54 ) and thus come close to that of binary
MgH2 (109 g H2 l−1 ) while exceeding by far that of liquid hydrogen (71 g H2 l−1 ). This is partly due to the fact that saline ternary
metal hydrides tend to be denser than the weighted mixtures of
the corresponding binary hydrides (see V values in Table 1).
This contraction presumably originates from packing effects.
It is reflected by a general shortening of the metal–hydrogen
bonds [12] and can be discussed in terms of atomic volume increments [34]. A particularly large volume contraction occurs in the
high-pressure phase of Cs2 MgH4 (V = −14%), while volume
expansions occur in the (low-pressure) ␤-K2 SO4 -type phases
A2 MgH4 (A = Rb: V = +3.8, A = Cs: V = +3.9%) which are
presumably due to the relatively large space requirement of
the tetrahedral [MgH4 ]2− units. The exceptionally big volume
expansion of EuMg2 H6 (V = +9%) is presumably a consequence of its relatively open structure displaying linear coordinated H atoms.
4.2. Thermal stability
The hydrogen dissociation temperatures at atmospheric pressure usually exceed 670 K, i.e. Mg based ternary hydrides
are thermally more stable than binary MgH2 (∼550 K). The
least stable ternary hydrides known so far are Rb2 MgH4 and
Rb3 MgH5 that decompose at 630–640 K. Note that the dehydrogenation reactions usually proceed in several steps. Sr6 Mg7 H26 ,
for example, decomposes at 700 K and 30 bars hydrogen pressure into SrMgH4 and SrH2 , while Yb4 Mg3 H14 decomposes
according to the reaction
Yb4 Mg3 H14 → 4YbH2 + 3Mg + 3H2
7
Fig. 4. Pressure-composition isotherms during desorption of Yb4 Mg3 H14 (from
[23]).
as can be seen from the pressure-composition isotherms shown
in Fig. 4. The desorption enthalpy associated with the latter
reaction (H = −103 kJ/mol H2 ) is intermediate between those
of binary MgH2 (−74 kJ/mol H2 ) and YbH2 (−182 kJ/mol H2 )
[27]. These finding are in line with similar results obtained
for complex transition metal hydrides such as CaMgNiH4 ,
Yb4 Mg4 Fe3 H22 and Ca4 Mg4 Fe3 H22 [35]. The desorption
enthalpies of these quaternary hydrides are significantly higher
than those of the corresponding ternary hydrides Mg2 NiH4
and Mg2 FeH6 , respectively, thus reflecting a greater strength
of Ca–H versus Mg–H bonds. On the other hand, the formation enthalpies of NaMgH3 (H = −154 kJ/mol H2 [29]) and
KMgH3 (H = −185 kJ/mol H2 [30]) are considerably larger
than those of the corresponding binary A metal hydrides NaH
(H = −114 kJ/mol H2 ) and KH (H = −116 kJ/mol H2 ).
4.3. Electronic and magnetic properties
Most compounds are colourless or white. Those containing
europium are red (EuMg2 H6 ), red-orange (Eu1−x Srx Mg2 H6 ;
x ≤ 0.6), yellow-grey (Eu1−x Srx )2 Mg3 H10 ; x > 0.8 or brown
(EuMgH4 ), which are typical colours for divalent Eu2+ . Ytterbium containing samples are often grey due to the presence of
some unreacted YbH2+x . All hydrides are presumably insulating or semi-conducting and can be considered as saline compounds, in agreement with quantum mechanical calculations
[36]. Magnetic properties are known for Eu containing compounds such as Eu2 MgH6 , EuMgH4 and EuMg2 H6 . They show
Curie–Weiss paramagnetism and ferromagnetic ordering similar to EuH2 which is a ferromagnetic semi-conductor [37].
Interestingly, as the minimum Eu–Eu distance in these hydrides
increases (from 3.72 to 3.93 Å) the Curie temperature decreases
(from 31.6 to 18.7 K). The extrapolated magnetic moments
(7.5–8.1 ␮B ) are consistent with divalent europium (Eu2+ free
ion value: 7.95 ␮B ). The solid solution series Eu1−x Srx Mg2 H6
and (Eu1−x Srx )2 Mg3 H10 show paramagnetic behaviour with
magnetic moments close to the free ion value of Eu2+ , and they
order ferromagnetically below TC = 21 and 15 K for x = 0.2 and
0.4, respectively [28].
8
K. Yvon, B. Bertheville / Journal of Alloys and Compounds xxx (2006) xxx–xxx
5. Conclusions and outlook
Ternary metal hydrides based on magnesium and A or Ae
elements crystallize with a great variety of crystal structures in
which the valence and size of the constituents play dominant
roles. The compounds have saline character and display numerous structural analogies with ternary fluorides. The calcium and
strontium members tend to have ytterbium and europium analogues, respectively. As expected, the hydrogen configurations
depend on the size and nature of the metal cations. Relatively
small cations such as Mg2+ show low CNs and usually adopt
octahedral (CN = 6) and rarely tetrahedral (CN = 4) or pentagonal bipyramidal (CN = 7) H configurations, while large cations
such as K+ , Rb+ and Cs+ , or Ca2+ , Sr2+ and Ba2+ (CN = 8–13)
show high CNs and usually adopt irregular H configurations.
Magnesium clearly occupies a special position within the Ae
series. The frequent occurrence and relative stiffness of the
MgH6 octahedra is consistent with covalent (directional) contributions to the metal–hydrogen interactions as in binary MgH2 ,
in contrast to the relatively flexible H configurations around
the other metal cations that suggest predominantly ionic (nondirectional) metal–hydrogen interactions. Compared to binary
hydrides, the metal–hydrogen bonds in ternary hydrides are
generally shortened which is reflected by a volume contraction. Due to the strong polarizability of the hydride anion,
these bonds cannot be rationalized in terms of a fixed hydrogen
radius. The hydrogen storage efficiencies for A metal compounds reach 6.0 wt.% and 88 g l−1 (NaMgH3 ) and for Ae metal
compounds 5.7 wt.% and 99 g l−1 (Ca4 Mg3 H14 ). They generally
desorb hydrogen only above 670 K and thus are not suitable for
reversible hydrogen storage applications at room temperature.
However, some of them may be suitable as chemical hydrides
for non-reversible hydrogen storage applications. This possibility needs to be explored. Furthermore, a search for quaternary
metal hydrides based on magnesium, alkali and other alkalineearth elements is worthwhile. Such compounds are likely to exist
but have not yet been reported.
Acknowledgment
This work was supported by the Swiss National Science
Foundation and the Swiss Federal Office of Energy.
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