Polyanionic hydrides from polar intermetallics AeE2 (Ae = Ca, Sr, Ba; E = Al, Ga, In)

Article abstract of DOI:10.1021/ja054456y

The hydrogenation behavior of the polar intermetallic systems AeE ₂ (Ae = Ca, Sr, Ba; E = Al, Ga, In) has been investigated systematically and afforded the new hydrides SrGa₂H₂ and BaGa₂H₂. The structure of these hydrides was characterized by X-ray powder diffraction and neutron diffraction of the corresponding deuterides. Both compounds are isostructural to previously discovered SrAI ₂H₂ (space group P3m1, Z = 1, SrGa₂H ₂/D₂: a = 4.4010(4)74.3932(8) A, c = 4.7109(4)/4.699(1) A; BaGa₂H₂/D₂: a = 4.5334(6)/4.5286(5) A, c = 4.9069(9)/4.8991(9) A). The three hydrides SrAI₂H₂, SrGa₂H₂, and BaGa₂H₂ decompose at around 300 °C at atmospheric pressure. First-principles electronic structure calculations reveal that H is unambiguously part of a two-dimensional polyanion [E₂H ₂]^2- in which each E atom is tetrahedrally coordinated by three additional E atoms and H. The compounds AeE₂H₂ are classified as polyanionic hydrides. The peculiar feature of polyanionic hydrides is the incorporation of H in a polymeric anion where it acts as a terminating ligand. Polyanionic hydrides provide unprecedented arrangements with both E-E and E-H bonds. The hydrogenation of AeE₂ to AeE₂H ₂ takes place at low reaction temperatures (around 200 °C), which suggests that the polyanion of the polar intermetallics ([E₂] ^2-) is employed as precursor.

Full text of DOI:10.1021/ja054456y

Published on Web 12/31/2005
Polyanionic Hydrides from Polar Intermetallics AeE2
(Ae ) Ca, Sr, Ba; E ) Al, Ga, In)
Thomas Bjo¨rling,‡ Dag Nore´us,‡ and Ulrich Ha¨ussermann*,†,§
Contribution from the Inorganic Chemistry and Structural Chemistry Departments,
Stockholm UniVersity, 10691 Stockholm, Sweden
Received July 5, 2005; E-mail: Ulrich.Haussermann@asu.edu
Abstract: The hydrogenation behavior of the polar intermetallic systems AeE2 (Ae ) Ca, Sr, Ba; E ) Al,
Ga, In) has been investigated systematically and afforded the new hydrides SrGa2H2 and BaGa2H2. The
structure of these hydrides was characterized by X-ray powder diffraction and neutron diffraction of
the corresponding deuterides. Both compounds are isostructural to previously discovered SrAl2H2 (space
group P3hm1, Z ) 1, SrGa2H2/D2: a ) 4.4010(4)/4.3932(8) Å, c ) 4.7109(4)/4.699(1) Å; BaGa2H2/D2: a )
4.5334(6)/4.5286(5) Å, c ) 4.9069(9)/4.8991(9) Å). The three hydrides SrAl2H2, SrGa2H2, and BaGa2H2
decompose at around 300 °C at atmospheric pressure. First-principles electronic structure calculations
reveal that H is unambiguously part of a two-dimensional polyanion [E2H2]2- in which each E atom is
tetrahedrally coordinated by three additional E atoms and H. The compounds AeE2H2 are classified as
polyanionic hydrides. The peculiar feature of polyanionic hydrides is the incorporation of H in a polymeric
anion where it acts as a terminating ligand. Polyanionic hydrides provide unprecedented arrangements
with both E-E and E-H bonds. The hydrogenation of AeE2 to AeE2H2 takes place at low reaction
temperatures (around 200 °C), which suggests that the polyanion of the polar intermetallics ([E2]2-) is
employed as precursor.
1. Introduction
Ba), were discovered, and the structure of nanocrystalline
MgAl2H8 was finally solved.5
Alanates, which are aluminum hydrides of alkali metals (A)
or alkaline earth metals (Ae), gained recently a tremendous
amount of attention.1-3 This was triggered by the discovery that
some transition metals catalyze the reverse of the two-step
decomposition NaAlH4 f 1/3(Na3AlH6) + 2/3Al + H2 f NaH
+ Al + 3/2H2, which suddenly turned long-known NaAlH4 into
a state-of-the-art media with 5.6 wt % H-storage capacity. In
the past the systems AAlH4 and A3AlH6 have been intensively
investigated with respect to synthesis, dehydrogenation behavior,
structural characterization, and computational modeling of
structural stability and physical properties.4 Further, new alkaline
earth metal alanates, such as BaAlH5 and Ae2AlH7 (Ae ) Sr,
Characteristically, alanates represent fully hydrogenated
systems AmAenAloHm+2n+3o. In 2000, however, Gingl et al.
reported the synthesis and structural characterization of peculiar
and novel SrAl2H2.6 This aluminum hydride compound is not
fully hydrogenated and was obtained by hydrogenating the alloy
SrAl2 at mild conditions (200 °C, 50 bar H2 pressure). 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 a more electronegative p-block metallic
or semimetallic element (the E component). According to the
Zintl concept, Al is formally reduced by the electropositive Sr
and features a three-dimensional four-connected (3D4C) poly-
anionic 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. In
SrAl2H2 the Al network is maintained, although its dimension
is reduced to two. The Al atoms are arranged as puckered
graphitic layers where they have three nearest Al neighbors,
the vacant coordination is taken by hydrogen. Thus, hydrogen
appears to act as a pair of scissors cutting covalent Al-Al bonds
and subsequently terminating them (Figure 1). This retains a
† Inorganic Chemistry Department.
‡ Structural Chemistry Department.
§ Present address: Department of Chemistry and Biochemistry, Arizona
State University, Tempe, AZ 85287-1604.
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9
10.1021/ja054456y CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 817-824
817

A R T I C L E S
Bjo¨rling et al.
article we report the results of our hydrogenation experiments
on AeE2 (Ae ) Ca, Sr, Ba; E ) Ga, In), along with thermal
behavior and electronic structure characterizations of the
obtained compounds. The latter investigations include already
known SrAl2H2. On the basis of this fundamental study we draw
general conclusions on bonding and structural and thermal
stability of polyanionic hydrides involving metallic group 13
elements as the E component.
Figure 1. Crystal structures of SrAl2 (left) and SrAl2H2 (right) viewed
along [110]. Red, blue, and green circles denote Sr, Al, and H atoms,
respectively.
2. Experimental Section
Synthesis. All steps of synthesis and sample preparation for
diffraction and thermal analysis experiments were performed in an Ar-
filled glovebox (O2 concentration <1 ppm). The binary phases SrAl2,
CaGa2, CaGa2+x (x ) 0.1-0.2), SrGa2, BaGa2, SrIn2, and BaIn2 were
synthesized from the pure elements which where mixed in stoichio-
metric ratios (total sample amounts between 0.5 and 1.0 g). For
preparing SrAl2, elemental mixtures were pressed into pellets, which
subsequently were arc melted. This compound melts incongruently,
and quantitative yields can only be obtained by the arc-melting
procedure. For preparing the gallides and indides, reactant mixtures
were transferred into Ta and stainless steel ampules, respectively, which
were sealed and placed in a fused quartz Schlenk tube under reduced
pressure. The metal mixtures were heated at 200 K/h to 800 °C, held
at 800 °C for 24 h, and cooled at 100 K/h to room temperature. The
binary intermetallics were obtained as ingots that were ground, and
the powdered samples were used for subsequent characterization and
hydrogenation. For their hydrogenation the intermetallic compounds
were loaded in corundum crucibles, which were placed in stainless steel
autoclaves. Reactions were carried out at hydrogen pressures around
50 bar and at temperatures varying between 150 and 400 °C. All
products obtained (intermetallics and hydrides) were characterized by
Guinier powder diagrams (Cu KR; Si standard) and their Ae:E ratios
were confirmed with the EDX (energy-disperse X-ray) method in a
JEOL 820 scanning electron microscope.
Structural Characterization. Lattice parameters of the prepared
intermetallics and their hydrides were obtained from least-squares
refinement of measured and indexed lines of the corresponding Guinier
powder diffractograms.12 The considered phases AeE2 and CaGa2+x (x
) 0.1-0.2) are all described in the literature.13 In some cases we
undertook a structural reinvestigation from single-crystal X-ray dif-
fraction data. Atomic positions of SrGa2H2 and BaGa2H2 were deter-
mined from Rietveld profile refinements of neutron powder diffraction
data14 from deuterized samples measured at Studsvik Neutron Research
Laboratory, Sweden (room temperature, λ ) 1.4700 Å for SrGa2D2
and 1.5514 Å for BaGa2D2, data resolution ∆d/d ) 2 × 10-3). These
samples were obtained by reacting SrGa2 and BaGa2 with deuterium
at a pressure of 50 bar and at a temperature of 200 °C for 4 days. The
SrGa2D2 sample contained some SrGa2, which was due to insufficient
supply of D2 during the reaction. The BaGa2D2 sample contained a
small amount of BaGa4, which occurred as a byproduct during the
synthesis of BaGa2. Due to overlapping reflections, a multiphase
refinement was performed. Initial values for the atomic coordinates
were taken from SrAl2H2. The results are presented in Tables 1 and 3.
Thermal Investigations. The thermal behavior of powdered samples
of SrAl2H2, SrGa2H2, and BaGa2H2 was investigated by differential
thermal analysis (DTA-TG, Setaram Labsys 1600). Samples were
placed in a steel container, which were sealed with gold foil to prevent
(two-dimensional) polyanionic structure [Al2H2]2- in the hydride
with both Al-Al and Al-H bonds and is remarkable against
the alanate structures which consist of tetrahedral [AlH4] or
octahedral [AlH6] entities.
Considering the large number and structural variety of Zintl
phases, the immediate question arises: Did the discovery of
SrAl2H2 open up a new class of hydride compounds with
unprecedented polyanions consisting of E-E and E-H bonds?
Hydrogen displays an electronegativity similar to that of the E
component of Zintl phases (i.e. groups 13-15 metallic and
semimetallic elements), and thus, the E-H bond should be of
covalent nature. This would place the structure and bonding
properties of polyanionic hydrides between those of the ionic,
salt-like, active metal hydrides and the molecular or polymeric
covalent hydrides EnHm. Alternatively, polyanionic hydrides can
be viewed as intermediates between saline metal hydrides and
Zintl phases. The hydrogen content of polyanionic hydrides is
comparably low, and thus they may not be appropriate as
hydrogen storage materials. However, if physical properties
inherent to saline metal hydrides and Zintl phases can be
combined in polyanionic hydrides, unexpected prospects are
opened by extending hydride materials research beyond the
traditional quest for hydrogen storage materials.7
We have started to examine systematically the issue of
polyanionic hydrides. Specifically, we are interested in exploring
which E elements or combinations of E elements form polymeric
anions with hydrogen and how hydride formation is influenced
by the choice of the countercation. It is important to note that
the expression “polyanionic hydride” implies a clear definition
applying exclusively to compounds in which hydrogen atoms
are part of a polymeric anion. This is in line with the original
description of SrAl2H2 by Gingl et al.6 and excludes compounds
with covalently bonded but separated entities [EHn]x-, such as
the alanates and systems such as KSiH3, AGeH3, APH2 (A )
K, Rb, Cs), and A2PH (A ) Rb, Cs), ASH, ASeH.8 Importantly,
this also excludes Zintl phases containing interstitial hydrogen.
Such phases were especially described by Corbett et al. and
comprise Ba5Ga6H2,9 Ca3SnH2 ,10 and Ae5Tt3Hx (Ae ) Ca, Sr,
Ba; Tt ) Si, Ge, Sn)11 where H is not attached to the semimetal
but surrounded exclusively by the cationic component. In this
(7) Bjo¨rling, T.; Nore´us, D.; Jansson, K.; Andersson, M.; Leonova, E.; Ede´n,
M.; Hålenius, U.; Ha¨ussermann, U. Angew. Chem., Int. Ed. 2005, 44, 7269.
(8) (a) Mundt, O.; Becker, G.; Hartmann, H. M.; Schwarz, W. Z. Anorg. Allg.
Chem. 1989, 572, 75. (b) Thirase, G.; Weiss, E.; Hennig, H. J.; Lechert,
H. Z. Anorg. Allg. Chem. 1975, 417, 221. (c) Bergerhoff, G.; Schultze-
Rhonhof, E. Acta Crystallogr. 1962, 15, 420. (d) Jacobs, H.; Hassiepen,
K. M. Z. Anorg. Allg. Chem. 1985, 531, 108. (e) Somer, M.; Carrillo-
Cabrera, W.; Peters, E.-M.; Peters, K.; von Schnering, H. G. Z. Kristallogr.s
New Crystal Struct. 1997, 212, 299. (f) von Schnering, H. G.; Somer, M.;
Peters, K.; Carrillo-Cabrera, W.; Grin, Y. Z. Kristallogr.sNew Crystal
Struct. 2001, 216, 42.
(11) Alejandro Leon-Escamilla, E.; Corbett, J. D. J. Solid State Chem. 2001,
159, 149.
(12) Werner, P.-E. Ark. Kemi 1969, 31, 513.
(13) Villars, P.; Calvert, L. D. Pearsons Handbook of Crystallographic Data
for Intermetallic Compounds, 2nd ed.; ASM International: Materials Park,
OH, 1991; Desk Edition, 1997.
(14) Rodriguez-Carvajal, J. FULLPROF: A Program for Rietveld Refinement
and Pattern Matching Analysis. Abstracts of the Satellite Meeting on Powder
Diffraction of the XV Congress of the IUCr; Toulouse, France, 1990; p
127.
(9) Henning, R. W.; Alejandro Leon-Escamilla, E.; Zhao, J.-T.; Corbett, J. D.
Inorg. Chem. 1997, 36, 1282.
(10) Huang, B.; Corbett J. D. Inorg. Chem. 1997, 36, 3730.
9
818 J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006

Polyanionic Hydrides from Polar Intermetallics
A R T I C L E S
Table 1. Refinement Resultsa for AeGa2H2/D2
) Ca, Sr, Ba; E ) Ga, In). These six isoelectronic compounds
occur in three different structures (Figure 2). SrGa2 and BaGa2
crystallize with the hexagonal AlB2-type (P63/mmm), CaGa2,
CaIn2, and SrIn2 with the hexagonal CaIn2-type (P63/mmc), and
BaIn2 with the orthorhombic CeCu2-type (Imma).19,20 Addition-
ally, there exists slightly nonstoichiometric CaGa2+x with the
AlB2 structure.21 The three structure types are closely related.19
In the simple AlB2 structure E atom hexagon layers are stacked
on top of each other, and Ae atoms are sandwiched between
two hexagon rings of neighboring layers. The CaIn2 and CeCu2
structures have 3D4C E atom networks and are easily derived
from the AlB2-type. In the CaIn2-type E atom hexagon layers
are corrugated as in gray As and connected as in hexagonal
diamonds. In the CeCu2-type E atom hexagon layers are
corrugated as in black P (which yields a smoother corrugation
compared to CaIn2) and connected to give a ladder of four-
membered rings.
compound
SrGa2H2/D2
BaGa2H2/D2
space group, Z P3hm1, 1
P3hm1, 1
a, Å
4.4010(4)/4.3932(8)
4.7109(4)/4.699(1)
79.02/78.54
295
4.5334(6)/4.5286(5)
4.9069(9)/4.8991(9)
87.33/87.01
295
c, Å
V, Å3
T, K
RBragg
Rp
4.73
11.9
4.51
10.4
Rwp
12.0
11.5
Ae 1a/Beq
E 2d/Beq
D 2d/Beq
0, 0, 0/0.45(10)
1/3, 2/3, 0.4656(7)/0.17(6) 1/3, 2/3, 0.4680(4)/0.78(4)
1/3, 2/3, 0.1067(8)/2.03(8) 1/3, 2/3, 0.1232(4)/1.75(5)
0, 0, 0/0.61(7)
aCell parameters obtained from X-ray data.
Table 2. Computationally Relaxed Structural Parameters for
AeE2H2
a
SrAl2H2
SrGa2H2
BaGa2H2
a, Å
c, Å
E (1/3, 2/3, z)
H (1/3, 2/3, z)
4.5289 (4.5283)
4.7248 (4.7215)
0.4614 (0.4589)
0.0988 (0.0976)
4.4104
4.7291
0.4658
0.1061
4.5604
4.9700
0.4703
0.1287
In systems AeE2 the group 13 E atoms are formally singly
charged and thus isoelectronic to a group 14 element. The
formation of 3D4C polyanionic networks for compounds with
(distorted) tetrahedrally coordinated E atoms is reasonable. This
holds also for AlB2-type SrGa2 and BaGa2 when considering
the hexagon layers electronically as graphitic layers. Thus, these
compounds are apparently charge-balanced and conform to the
Zintl concept although they adopt different structures. AeAl2
displays a somewhat different structural chemistry. Only SrAl2
crystallizes with a structure that agrees with the Zintl concept
(CeCu2-type).22 CaAl2 forms the cubic Laves phase structure
MgCu2 and BaAl2 does not exist but decomposes into complex
Ba21Al40 and BaAl4.23
aExperimental values for SrAl2H2 according to ref 6 are given in
parentheses.
Table 3. Selected Interatomic Distances (Å) and Angles (deg) in
AeE2D2
a
SrAl2D2
SrGa2D2
BaGa2D2
Ae-D
Ae-E
Ae-E
E-D
6x
6x
6x
1x
3x
1x
3x
3x
2.653(1)
3.394(2)
3.654(2)
1.706(4)
2.641(1)
1.706(4)
2.653(1)
2.770(1)
117.88(2)
98.5(2)
2.586(1)
3.350(2)
3.569(2)
1.687(5)
2.557(1)
1.687(5)
2.586(1)
2.727(3)
118.42(6)
97.3(1)
2.683(1)
3.477(1)
3.692(2)
1.689(3)
2.6334)
1.689(3)
2.683(1)
2.880(2)
118.60(4)
96.8(1)
E-E
D-E
D-Ae
D-D
E-E-E
E-E-D
Hydrogenation reactions. The prepared phases CaGa2,
CaGa2+x, SrGa2, BaGa2, CaIn2, SrIn2, and BaIn2 were exposed
to a hydrogen atmoshphere of 50 bar at various temperatures.
Additionally we included SrAl2 in our investigation. The result
of these reaction series is compiled in Scheme 1. For SrAl2,
SrGa2, and BaGa2 we observe hydride formation at temperatures
between 170 and 200 °C. These hydrides corresponds to
AeE2H2. At hydrogenation temperatures between 250 and 300
°C AeGa2H2 decomposes upon formation of AeH2 and AeGa4.
This is in contrast to SrAl2H2, which is further hydrogenated to
Sr2AlH7.5a,5b Sr2AlH7 represents a fully hydrogenated alanate
system and apparently there is no equivalent for gallium
compounds. CaIn2-type CaGa2 and AlB2-type CaGa2+x do not
yield any hydride but decompose immediately into roentgen-
amorphous CaH2 and monoclinic CaGa4. CaIn2, and SrIn2
remain unaffected upon hydrogenation up to temperatures of
400 °C. Most likely CaIn2 and SrIn2 decompose into AeH2 and
E at higher temperatures. This decomposition is already observed
for BaIn2 at very low temperatures (around 200 °C). The low
melting temperature of elemental Ga prompted us to attempt
a Values for SrAl2D2 are taken from ref 6.
exposure to air and moisture, and the temperature was raised from 40
to 400 °C. The experiments were performed under a flow of dry Ar
and with a temperature increase rate of 5 K/min. It should be pointed
out that these investigations do not monitor hydride decomposition
strictly at thermodynamic equilibrium conditions.
Electronic Structure Calculations. Total energy calculations for
AeE2 and AeE2H2 (Ae ) Ca, Sr, Ba; E ) Al, Ga, In) were performed
in the framework of the frozen core all-electron Projected Augmented
Wave (PAW) method,15 as implemented in the program VASP.16 The
energy cutoff was set to 500 eV. Exchange and correlation effects were
treated by the generalized gradient approximation (GGA), usually
referred to as PW91.17 The integration over the Brillouin zone was
done on special k points determined according to the Monkhorst-Pack
scheme.18 Total energies were converged to at least 1 meV/atom.
Structural parameters were relaxed until forces had converged to less
than 0.01 eV/Å. The computationally obtained parameters for SrAl2H2,
SrGa2H2, and BaGa2H2 are listed in Table 2.
(18) Monkhorst; H. J.; Pack, J. D. Phys. ReV. B 1972, 13, 5188.
(19) Nuspl, G.; Polborn, K.; Evers, J.; Landrum, G. A.; Hoffmann, R. Inorg.
Chem. 1996, 35, 6922.
3. Results
(20) (a) Bruzzone, G.; Bonino, G. B. Atti Accad. Nat. Lincei 1970, 48, 235. (b)
Iandelli, A. Z. Anorg. Allg. Chem. 1964, 330, 221. (c) Bruzzone, G.
Bollettino Scientifico della Facolta di Chimica Industriale di Bologna 1966,
24, 113. (d) Iandelli, A. J. Less-Common Met. 1987, 135, 195.
(21) Bruzzone, G.; Fornasini, M. L.; Merlo, F. J. Less-Common Met. 1989, 154,
67.
(22) (a) Nagorsen, G.; Posch, H.; Scha¨fer, H.; Weiss, A. Z. Naturforsch. 1969,
24b, 1191. (b) Cordier, G.; Czech, E.; Scha¨fer, H. Z. Naturforsch. 1982,
37b, 1442.
(23) Amerioun, S.; Yokosawa, T.; Lidin, S.; Ha¨ussermann, U. Inorg. Chem.
2004, 43, 4751.
Zintl Phases AeE2. The discovery of the prototype poly-
anionic hydride SrAl2H2 from SrAl2 prompted the investigation
of the hydrogenation behavior of the related phases AeE2 (Ae
(15) (a) Blo¨chl, P. E. Phys. ReV. B 1994, 50, 17953. (b) Kresse, G.; Joubert, J.
Phys. ReV. B 1999, 59, 1758.
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A R T I C L E S
Bjo¨rling et al.
Figure 2. Crystal structures of AlB2-type SrGa2, BaGa2 (a) CaIn2-type CaGa2, CaIn2, SrIn2 (b) and CeCu2-type SrAl2, BaIn2 (c). Red and blue circles denote
Ae and E atoms, respectively.
Scheme 1
Figure 3. DTA heating curves for SrAl2H2, SrGa2H2, and BaGa2H2.
It is interesting to mention that the formation of SrGa2H2 was
difficult to recognize from the X-ray powder pattern because
the synthesis of AeGa2H2 from stoichiometric mixtures of AeH2
SrGa2 and SrGa2H2 have hexagonal cells with very similar axes
and Ga under a hydrogen pressure of 50 bar. However, already
(a ) 4.36 and 4.40 Å, c ) 4.70 and 4.71 Å, respectively).
However, there are small but significant intensity differences
at reactions temperatures as low as 100 °C BaAl4-type AeGa4
was obtained, which behaves completely inert to hydrogen.24
for a few reflections (e.g. 102, 212, and 104), which are a
In general, the polyanionic hydrides AeE2H2 are highly moisture
consequence of the different arrangement of Ga atoms in the
sensitive and decompose rapidly when exposed to air. For the
two structures (planar hexagon layers vs puckered ones). The
sake of completeness we note that the Laves phase CaAl2 has
formation of SrAl2H2 from SrAl2 is accompanied by a volume
been the subject of earlier hydrogenation experiments.25 This
increase of 8.9 Å3/Z, whereas the volume changes connected
compound decomposes into CaH2 and Al at high temperatures.
with the hydrogenation of SrGa2 and BaGa2 are very small
Ba21Al40 (previously denoted as Ba7Al13) has a structure closely
(increase by about 1 Å3/Z). This different behavior can be
related to the Laves phases23 and affords the alanates BaAlH5
explained by fact that the hexagon nets formed by Ga atoms in
and Ba2AlH7 when exposed to a hydrogen atmosphere under
AlB2-type AeGa2 are widely separated, whereas in CeCu2-type
SrAl2 the corresponding layers of Al are connected by bonds
and thus much closer together (cf. Figure 2). Upon hydride
elevated temperature.5c,5d
Hydride structures. The structures of SrGa2H2 and BaGa2H2
were established from the Rietveld refinement of neutron
formation these bonds are broken, and the layers move farther
diffraction powder data. Both compounds are isostructural to
apart when terminated by hydrogen.
Thermal Stability. Figure 3 displays DTA heating curves
for AeE2H2 at atmospheric pressure (inert gas). We observe a
sharp endothermic rise around 280 °C for the gallium com-
pounds. For SrAl2H2 a small thermal effect occurs in this
temperature range, which may indicate the onset of hydrogen
desorption. The main desorption, however, should coincide with
the steep rise at about 320 °C. The three hydrides display a
very similar thermal stability; their decomposition takes place
at around 300 °C. For SrGa2H2 a second endothermic effect is
seen at around 350 °C, for BaGa2H2 at around 450 °C (not
included in Figure 3). We attribute this effect to further reactions
already reported SrAl2H2 (cf. Figure 1).6 In the trigonal structure
type of SrAl2H2 (space group P3hm1) E atoms form slightly
puckered graphitic layers. Additionally each E atom is coordi-
nated to one hydrogen atom. This yields a two-dimensional
polyanion [E2H2]2- which is formally electron precise. The
occurrence of polyanions with both, E-E and E-H bonds has
not been observed earlier and is the unique feature of poly-
anionic hydrides. Al-H and Ga-H distances are very similar,
around 1.70 Å, Al-Al and Ga-Ga distances are in a range
typically observed for Zintl phases containing polyanions with
E-E single bonds (Table 3).
leading to the formation of BaAl4-type AeGa4. As a matter of
(24) We also performed a hydrogenation study of BaAl4-type polar intermetallics
AeE4 (Ae ) Sr, Ba, E ) Al, Ga). These compounds do not react with
hydrogen under the autoclave conditions employed (maximum hydrogen
pressure 70 bar, maximum temperature 700 °C).
(25) (a) Veleckis, E. J. Less-Common Met. 1981, 80, 241. (b) Zhang, Q. A.;
Enoki, H.; Akiba, E. J. Alloys Compd. 2001, 322, 257.
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Polyanionic Hydrides from Polar Intermetallics
A R T I C L E S
Figure 4. Electronic density of states (DOS) for CeCu2-type SrAl2 (a) and AlB2-type SrGa2 (b).
fact, from powder X-ray analysis we identify a considerable
amount of AeE4 in all three systems after atmospheric pressure
decomposition.
In AeGa2H2 they are widened to 2.56 and 2.63 Å for Ae ) Sr
and Ba, respectively. This is in accord with the idea that the
stronger bonded Ga-Ga bonded π-system is destroyed in the
precursor polyanion upon hydrogenation and the Ga atoms
become four-coordinated in the hydride. In the case of the pair
SrAl2/SrAl2H2, puckered Al hexagon layers are present in both
structures (cf. Figure 1). In CeCu2-type SrAl2 these layers are
connected. Al-Al distances are 2.776 and 2.783 Å within the
layers and are somewhat longer, 2.882 Å, between layers.27
Upon hydride formation the long, layer-connecting bonds
become dissected and subsequently terminated by hydrogen. The
Al atoms in the corrugated hexagon layer and the Sr atoms
rearrange slightly. Although the metal atom arrangements in
the CeCu2-type precursor and the trigonal hydride are very
similar, their space group symmetries (Imma and P3hm1,
respectively) can only be related via hexagonal P6/mmm
(AlB2).28 The Al-Al distance in SrAl2H2 is considerably
reduced (2.64 Å) compared to the interlayer distances in SrAl2
and corresponds much closer to what is considered an Al-Al
single-bonded distance (cf. Table 3).
4. Discussion
The discovery of SrGa2H2 and BaGa2H2 shows that SrAl2H2
is not a singularity and that polyanionic hydrides may indeed
represent a wider class of compounds. Importantly, polyanionic
hydrides AeE2H2 were obtained from the hydrogenation of
phases AeE2 only when applying low reaction temperatures
(180-230 °C). Higher temperatures eventually result in a
mixture of AeH2 and BaAl4-type AeE4 or AeH2 and E metal.
(In the case of SrAl2H2 an intermediate alanate is obtained).
The synthesis reaction AeH2 + 2Ga, which in principle allows
low reaction temperatures because of the low melting point of
Ga, does not afford polyanionic hydrides at the conditions
applied by us. Also, it is not possible to obtain polyanionic
hydrides from the compounds CaAl2 and Ba21Al40 with Laves
phase structures. These observations point strongly to the fact
that the polyanion [E2]2-, which is present in Zintl phases AeE2,
is utilized as precursor in the hydrogenation reaction. In this
case the E-E bonded part of final [E2H2]2- is completely
preformed, and the hydrogenation reactionsleading to covalent
E-H bond formationscan take place at mild, low-temperature
conditions. The prerequisite for a precursor reaction is the
possibility to preserve the countercation arrangement of the Zintl
phase in the hydride. As a matter of fact, the SrAl2H2 structure
is closely related to that of the parent Zintl phases. The insertion
of hydrogen into AeE2 induces only a minor rearrangement of
the metal atoms and is similar to a topotactic reaction.
In the next step we compare the electronic structures of
formally electron-precise AeE2 and AeE2H2.29 For this we
computed the electronic density of states (DOS) at the theoreti-
(26) The cell parameters for AlB2-type AeGa2 obtained by us are a ) 4.3614-
(7) Å, c ) 4.696(1) Å for Ae ) Sr and a ) 4.4323(3) Å, c ) 5.0823(8)
Å for Ae ) Ba. The Ga-Ga distance is (x3a)/3.
(27) These distances are based on our reinvestigation of the crystal structure of
SrAl2 from single-crystal X-ray data: space group Imma; a ) 4.7954(6)
1
Å, b ) 7.8956(9) Å, c ) 7.953(1) Å; Sr 4e (0, /4, 0.05047(4): Al 8h (0,
0.9325(1), 0.3388(1)). The parameters differ slightly from the data presented
in the original reports (ref 20). The reinvestigation of the crystal structure
of SrAl2 also confirmed the stoichiometric composition of this compound
although it was prepared by high-temperature arc-melting.
For the formation of AeGa2H2 from AlB2-type AeGa2
hydrogen is added to a three-bonded, sp2-hybridized E- entity,
which changes into an sp3-hybridized one upon binding to H.
The arrangement of Ae atoms is not affected at all, and the
space group of the hydride (P3hm1) is a translationengleiche (t2)
subgroup of the precursor (P6/mmm). The Ga-Ga distances in
AeGa2 are 2.52 and 2.57 Å for Ae ) Sr and Ba, respectively.26
(28) Hoffmann, R.-D.; Po¨ttgen, R. Z. Kristallogr. 2001, 216, 127.
(29) A comparison of the electronic structures of SrAl2 and SrAl2H2 has been
recently performed by Orgaz and Aburto (Orgaz, E.; Aburto, A. Int. J.
Quantum Chem. 2005, 101, 783) on the basis of LAPW calculations and
the experimental crystal structures. While the DOS for SrAl2H2 resembles
strongly our result, the one for CeCu2-type SrAl2 looks completely different
and does not correspond to what is expected for a polar intermetallic
compound.
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J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006 821

A R T I C L E S
Bjo¨rling et al.
Figure 5. DOS for SrAl2H2, SrGa2H2, and BaGa2H2. The distribution of the Ae, E, and H site-projected DOS is very similar for the three compounds and
only shown for SrAl2H2.
cally obtained equilibrium structures. The results are shown in
Figures 4 and 5. The DOS of SrAl2 (Figure 4a) displays a
pseudogap, which separates occupied from nonoccupied states.
Al states dominate the DOS below the Fermi level, and the
pseudogap is a consequence of strong covalent Al-Al interac-
tions within the polyanionic network. To the contrary, in the
DOS for the AlB2-type gallides this pseudogap is absent, and
the value of the density of states at the Fermi level is rather
high (Figure 4b). The metallic behavior of CeCu2-type SrAl2
and AlB2-type AeGa2 is due to a large portion of E-p bands
crossing the Fermi level. This is especially the case for the latter
compounds. As a matter of fact, the electronic structure of a
graphitic layer is not recognized in the occupied bands, as
perhaps anticipated for charge-balanced AlB2-type AeGa2. This
is simply because pπ- and pσ-type bands are not energetically
separated. Thus, although formally charge-balanced, the elec-
tronic structure of intermetallic systems AeE2 does not com-
pletely conform to what is expected for Zintl phases, and
therefore, they should be classified more appropriately as polar
intermetallics. The presence of substantial polarity is recognized
in the low contribution of Sr atoms to the occupied states, which
sharply rises above the Fermi level.
polyanionic character. Second, in the hydrides the E atom s band
is detached from the p band. The contribution of H is mainly
into the intermediate energy regions of the DOS (between -6.5
and -2 eV), which are the parts where E atom s and p bands
strongly mix. The hybridization of H s and E s,p states strongly
indicates that H is covalently bonded to the Al framework and
thus part of the 2D polyanion [E2H2]2-. This is corroborated
by the analysis of the charge density F. In Figure 6 F and the
difference density ∆F are compared for the pair SrGa2 and
SrGa2H2 in the (110) plane. The charge density distribution
shows bond critical points associated with high values of F along
the Ga-Ga distance in both systems, and additionally along
the Ga-H distance in the hydride. This is characteristic for
covalent bonding. The polar character of both compounds is
revealed in the ∆F map, which clearly shows charge depletion
around Sr and accumulation between the polyanion-forming
atoms. Concerning the Ga-H bond ∆F suggests substantial
charge accumulation around the position of the proton. Un-
doubtedly, H possesses a higher electronegativity than Ga, and
the Ga-H bond is polarized. However, this situation appears
exaggerated in the ∆F map because of the absence of core
electrons in H.
The electronic structures of SrAl2H2 and AeGa2H2 are
virtually identical, but rather different from those of the under-
lying intermetallics. First we observe a very pronounced pseu-
dogap at the Fermi level. Compared to the precursor materials
the hydrides are poorer metals. Thus, the incorporation of
hydrogen in the polyanionic E atom network enhances its
Finally, modern first-principles methods allow in principle
the quantification of structural stability and even reaction
energies. We computed energies of formation (i.e., formation
enthalpies at zero temperature) for all combinations AeE2H2 and
considered the two formation reactions AeE2 + H2 f AeE2H2
(1) and AeH2 + 2E f AeE2H2 (2). We point out that the applied
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Polyanionic Hydrides from Polar Intermetallics
A R T I C L E S
Figure 6. Charge density F and difference density ∆F (crystalline - superposed atomic density) distribution in the (110) plane for SrGa2 and SrGa2H2.
reaction involves a gaseous species. This leads to a decisive
entropy contribution when, more appropriately, the Gibbs free
energy is considered.30 In a recent paper Ke and Tanake
investigated the decomposition reaction for NaAlH4 and
Na3AlH6 and found that the entropy contribution T∆S to the
reaction is approximately equal to the entropy contribution of
the H2 gas molecule produced in that reaction.31 This is an
important result since this contribution can be calculated from
thermochemical data. The temperature-dependent Gibbs free
energy for a H2 gas molecule at 1 atm with respect to 0 K is
-0.32 eV at 300 K and -0.76 eV at 600 K.31 Assuming that
sodium alanates and AeE2H2 systems behave similarly, the first
value would imply that only SrAl2H2, SrGa2H2, and BaGa2H2
Figure 7. Computed zero-temperature formation energies for systems
are stable with respect to AeE2 + H2 at atmospheric pressure
AeE2H2 (Ae ) Ca, Sr, Ba; E ) Al, Ga, In). The reactions AeE2 + H2 (1,
around room temperature. The second value would indicate a
solid symbols) and AeH2 + 2E (2, open symbols) were considered. A
decomposition of these three hydrides at temperatures around
600 K, which is in good agreement with the results of our
thermal analysis experiments.
precursor BaAl2 does not exist; thus, the reaction BaAl2 + H2 was not taken
into account.
computational method very well reproduces the experimental
structural parameters of AeE2H2. This is shown in Table 2. The
obtained energies of formation are compiled in Figure 7. Four
important results can be extracted from this figure: (i) At least
one of the zero-temperature formation energies, (1) or (2), is
negative for all systems, except for CaIn2H2 and SrIn2H2.
Reaction 2 yields typically a more positive value (except for
BaIn2H2). (ii) SrAl2H2 is hardly stable against disproportionation
into SrH2 + 2Al (reverse formation reaction 2). Hypothetical
CaAl2H2 would be stable against decomposition into CaAl2 +
H2, but not against CaH2 + 2Al. (iii) Ba compounds, where
the 2D [E2H2]2- layers are separated most, are more stable with
respect to disproportionation into (AeE2 + H2) or (AeH2 + 2E)
than are SrE2H2 and CaE2H2 systems. Also, In compounds
display a lower stability with respect to these disproportionations
than Al and Ga compounds. (iv) The experimentally obtained
compounds SrAl2H2, SrGa2H2, and BaGa2H2 have the lowest
zero-temperature formation energies (1), around -0.75 eV/Z.
The zero-temperature formation energy (1) represents only a
crude estimate of stability for AeE2H2 because the hydrogenation
5. Conclusions
The recent discovery of the peculiar, polyanionic hydride
SrAl2H2 by Gingl et al.6 stimulated the search for further
examples of such hydrides to prove their wider significance. A
general study of the hydrogenation behavior of polar interme-
tallic compounds AeE2 (Ae ) Ca, Sr, Ba; E ) Ga, In) afforded
SrGa2H2 and BaGa2H2, which are isostructural to SrAl2H2. A
further example of a polyanionic hydride is SrAlSiH.7 The close
structural relationship between AeE2 and AeE2H2 together with
the low reaction temperatures employed during hydrogena-
tion suggest that the polyanion [E2]2- present in the starting
material acts as a precursor. Then there are two mechanisms
for AeE2H2 formation: Either E-E contacts in a 3D4C frame-
work of the precursor material are partly dissected and sub-
sequently terminated by H, or H is added to a formally sp2
(30) Additionally for hydrides the zero-point energy can give a substantial
contribution to formation energies.
(31) Ke, X.; Tanaka, I. Phys. ReV. B 2005, 71, 024117.
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J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006 823

A R T I C L E S
Scheme 2
Bjo¨rling et al.
at the moment it is not clear whether polyanionic hydrides with
entities E(n > 13)-H exist and how hydride formation depends
on the choice and especially the concentration of the counter-
cation. The hydrogenation of AeE(13)2 yields only a small
number of Zintl-phase hydrides. Most likely, their formation
requires that the polyanion is preformed in Ae(E13)2. Still, this
need not to be a general condition for the formation of
polyanionic hydrides. The stability of hydrides AeE(13)2H2 with
respect to the precursor materials and H2 can be assessed
correctly from first-principles total energy calculations, and we
believe that such calculations will be a helpful tool for the further
exploration of polyanionic hydrides.
hybridized E atom entity which changes into an sp3 hybridized
one upon binding to H.
The peculiar feature of polyanionic hydrides is the incor-
poration of H as terminating ligand in a polymeric anion
composed of p-block metal or semimetal atoms E (e. g., Al,
Ga, In, Si, Ge, Sn, P, As, Sb). This provides unprecedented
arrangements with both E-E and E-H bonds. Entities E(n)-H
are isoelectronic to entities E(n+1): (see Scheme 2). Therefore
the metal/semimetal arrangement in polyanionic hydrides
MxE(n)yHz may often correspond to crystal structures of Zintl
phases MxE(n)y-zE(n+1)z (M ) countercation, E(n) ) element
n). Indeed, the metal arrangement in AeE(13)2H2 corresponds
to the CeCd2 structure type which is adopted by several alkaline
earth metal silicides and germanides. This important relationship
was already pointed out by Gingl et al.6 Polyanionic hydrides
may be considered as a novel class of hydrides. However, their
compositional potential is virtually unexplored. For instance,
Acknowledgment. This work has been supported by the
Swedish Research Council (VR). Additionally the European
Union is acknowledged for financial support through the
European Project HYSTORY with the contract number ENK6-
CT-2002-00600.
Supporting Information Available: Rietveld fits to the
neutron powder diffraction pattern of SrGa2D2 and BaGa2D2.
This material is available free of charge via the Internet at
http://pubs.acs.org.
JA054456Y
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824 J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006