Stepwise phase transition in the formation of lithium amidoborane

Article abstract of DOI:10.1021/ic100308j

A stepwise phase transition in the formation of lithium amidoborane via the solid-state reaction of lithium hydride and ammonia borane has been identified and investigated. Structural analyses reveal that a lithium amidoborane-ammonia borane complex (LiNH₂BH₃?NH₃BH ₃) and two allotropes of lithium amidoborane (denoted as α- and β-LiNH₂BH₃, both of which adopt orthorhombic symmetry) were formed in the process of synthesis. LiNH₂BH ₃?NH₃BH₃ is the intermediate of the synthesis and adopts a monoclinic structure that features layered LiNH ₂BH₃ and NH₃BH₃ molecules and contains both ionic and dihydrogen bonds. Unlike α-LiNH₂BH ₃, the units of the β phase have two distinct Li⁺ and [NH₂BH₃]^- environments. β-LiNH ₂BH₃ can only be observed in energetic ball milling and transforms to α-LiNH₂BH₃ upon extended milling. Both allotropes of LiNH₂BH₃ exhibit similar thermal decomposition behavior, with 10.8 wt % H₂ released when heated to 180 °C; in contrast, LiNH₂BH₃?NH₃BH ₃ releases approximately 14.3 wt % H₂ under the same conditions.

Full text of DOI:10.1021/ic100308j

Inorg. Chem. 2010, 49, 4319–4323 4319
DOI: 10.1021/ic100308j
Stepwise Phase Transition in the Formation of Lithium Amidoborane
Chengzhang Wu,† Guotao Wu,*,† Zhitao Xiong,† William I. F. David,‡,§ Kate R. Ryan,‡,§ Martin O. Jones,‡,§
Peter P. Edwards,§ Hailiang Chu,† and Ping Chen*,†
†Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023,
People’s Republic of China, ‡Rutherford Appleton Laboratory, Harwell Science and Innovation Campus,
Didcot OX11 0QX, U.K., and §Department of Chemistry, ICL, University of Oxford, South Parks Road,
Oxford OX1 2JD, U.K.
Received February 14, 2010
A stepwise phase transition in the formation of lithium amidoborane via the solid-state reaction of lithium hydride and
ammonia borane has been identified and investigated. Structural analyses reveal that a lithium amidoborane-ammonia
borane complex (LiNH2BH3 NH3BH3) and two allotropes of lithium amidoborane (denoted as R- and β-LiNH2BH3, both
3
of which adopt orthorhombic symmetry) were formed in the process of synthesis. LiNH2BH3 NH3BH3 is the intermediate
3
of the synthesis and adopts a monoclinic structure that features layered LiNH2BH3 and NH3BH3 molecules and contains
both ionic and dihydrogen bonds. Unlike R-LiNH2BH3, the units of the β phase have two distinct Liþ and [NH2BH3]-
environments. β-LiNH2BH3 can only be observed in energetic ball milling and transforms to R-LiNH2BH3 upon extended
milling. Both allotropes of LiNH2BH3 exhibit similar thermal decomposition behavior, with 10.8 wt % H2 released when
heated to 180 °C; in contrast, LiNH2BH3 NH3BH3 releases approximately 14.3 wt % H2 under the same conditions.
3
Introduction
diffraction (XRD), and insitu Raman, respectively. NH3BH3
decomposes to hydrogen upon heating,8 and its application
as a hydrogen-storage material was investigated by Wolf
et al.9a and Gutowska et al.9b In the past decade, considerable
attention has been given to NH3BH3 to improve its dehydro-
genation properties.9 More recently, there have been signifi-
cant efforts to chemically modify ammonia borane through
substitution of one of the protic hydrogen atoms with an
alkali or alkaline-earth element.10-13 Xiong et al. reported
that lithium amidoborane, LiNH2BH3, could be synthesized
The development of a viable hydrogen-storage system for
fuel-cell vehicles is of significant importance in the transition
from a carbon-based to a hydrogen-based economy. Recent
research on materials design and synthesis has mainly focu-
sed on chemicals that are composed of light elements and
possess high hydrogen content.1 One significant material that
has been investigated is ammonia borane, NH3BH3, a solid-
state compound with 19.6 wt % hydrogen capacity. Ammo-
nia borane was first synthesized in 1955,2 and its room-
temperature body-centered tetragonal structure was deter-
mined in the subsequent year.3 Hoon and Reynhardt4 obser-
ved a phase transition from a tetragonal to an orthorhombic
structure at low temperatures. Subsequently, Klooster et al.,5
Bowden et al.,6 and Hess et al.7 confirmed this orthorhombic
structure using neutron diffraction, single-crystal X-ray
(8) Hu, M. G.; Geanangel, R. A.; Wendlandt, W. W. Thermochim. Acta
1978, 23, 249–255.
(9) (a) Wolf, G.; Baumann, J.; Baitalow, F.; Hoffmann, F. P. Thermo-
chim. Acta 2000, 343, 19–25. (b) Gutowska, A.; Li, L. Y.; Shin, Y. S.; Wang,
C. M. M.; Li, X. H. S.; Linehan, J. C.; Smith, R. S.; Kay, B. D.; Schmid, B.; Shaw,
W.; Gutowski, M.; Autrey, T. Angew. Chem., Int. Ed. 2005, 44, 3578–3582. (c)
Chandra, M.; Xu, Q. J. Power Sources 2006, 156, 190–194. (d) Stowe, A. C.;
Shaw, W. J.; Linehan, J. C.; Schmid, B.; Autrey, T. Phys. Chem. Chem. Phys.
2007, 9, 1831–1836. (e) Heldebrant, D. J.; Karkamkar, A.; Hess, N. J.; Bowden,
M.; Rassat, S.; Zheng, F.; Rappe, K.; Autrey, T. Chem. Mater. 2008, 20, 5332–
5336. (f ) He, T.; Xiong, Z. T.; Wu, G. T.; Chu, H. L.; Wu, C. Z.; Zhang, T.; Chen,
P. Chem. Mater. 2009, 21, 2315–2318.
(10) Xiong, Z. T.; Yong, C. K.; Wu, G. T.; Chen, P.; Shaw, W.;
Karkamkar, A.; Autrey, T.; Jones, M. O.; Johnson, S. R.; Edwards, P. P.;
David, W. I. F. Nat. Mater. 2008, 7, 138–141.
(11) Diyabalanage, H. V. K.; Shrestha, R. P.; Semelsberger, T. A.; Scott,
B. L.; Bowden, M. E.; Davis, B. L.; Burrell, A. K. Angew. Chem., Int. Ed.
2007, 46, 8995–8997.
*To whom correspondence should be addressed. E-mail: pchen@
dicp.ac.cn (P.C.), wgt@dicp.ac.cn (G.W.).
(1) (a) Orimo, S. I.; Nakamori, Y.; Eliseo, J. R.; Zuttel, A.; Jensen, C. M.
Chem. Rev. 2007, 107, 4111–4132. (b) Chen, P.; Xiong, Z. T.; Luo, J. Z.; Lin,
J. Y.; Tan, K. L. Nature 2002, 420, 302–304.
(2) Shore, S. G.; Parry, R. W. J. Am. Chem. Soc. 1955, 77, 6084–6085.
(3) (a) Hughes, E. W. J. Am. Chem. Soc. 1956, 78, 502–503. (b) Lippert,
E. L.; Lipscomb, W. N. J. Am. Chem. Soc. 1956, 78, 503–504.
(4) Hoon, C. F.; Reynhardt, E. C. J. Phys. C 1983, 16, 6129–6136.
(5) Klooster, W. T.; Koetzle, T. F.; Siegbahn, P. E. M.; Richardson, T. B.;
Crabtree, R. H. J. Am. Chem. Soc. 1999, 121, 6337–6343.
(6) Bowden, M. E.; Gainsford, G. J.; Robinson, W. T. Aust. J. Chem.
2007, 60, 149–153.
(12) Kang, X. D.; Fang, Z. Z.; Kong, L. Y.; Cheng, H. M.; Yao, X. D.;
Lu, G. Q.; Wang, P. Adv. Mater. 2008, 20, 2756.
(7) Hess, N. J.; Bowden, M. E.; Parvanov, V. M.; Mundy, C.; Kathmann,
S. M.; Schenter, G. K.; Autrey, T. J. Chem. Phys. 2008, 128.
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14839.
r
2010 American Chemical Society
Published on Web 03/30/2010
pubs.acs.org/IC

4320 Inorganic Chemistry, Vol. 49, No. 9, 2010
Wu et al.
from the direct reaction of LiH with NH3BH3.
solution will decrease if NH3 is released from the sample and
absorbed by the solution. The synthesis process was stopped at
different stages for sample collection.
LiH þ NH3BH3 f LiNH2BH3 þ H2
One of the driving forces suggested for the formation of
LiNH2BH3 is the high chemical potential for the combina-
tion of the protic Hδþ in NH3 with the hydridic Hδ- in alkali-
metal hydrides to form H2. LiNH2BH3 crystallizes in the
ð1Þ
Dehydrogenation. A homemade temperature-programmed
desorption-mass spectrometry (Hiden HPR-20) combined sys-
tem was employed to detect the gaseous products evolved in the
thermal decomposition processes. Volumetric release for quan-
titative measurements of hydrogen desorption from samples
was performed on a HyEnergy PCT apparatus. The heat flow in
the thermal dehydrogenation was also monitored by a Netzch
449C thermogravimetric/differential scanning calorimetry (TG/
DSC) unit. The ammonia concentration in the gaseous phase
was measured from solution thermoconductivity.
˚
orthorhombic space group Pbca, with a = 7.11274(6) A,
3
˚
˚
˚
b=13.94877(14) A, c=5.15018(6) A, and V=510.970(15) A ,
which is consistent with Wu et al.’s results.13 LiNH2BH3
releases 10.9 wt % of hydrogen without borazine formation
at 91 °C.10 The systematic first-principles study of the struc-
tural and energetic properties of LiNH2BH3 by Ramzan et al.
is in agreement with the experimental data.14 The mechanism
of dehydrogenation was modeled by Kim et al.15 More
recently, a new allotrope of LiNH2BH3 was synthesized that
also crystallizes in space group Pbca but with different lattice
Characterization. Structural identification was undertaken
on a PANalytical X’pert diffractometer (Cu KR, 40 kV,
40 mA) and also on the high-resolution diffractometers ID31
at the European Synchrotron Radiation Facility, Grenoble,
˚
France, at a wavelength of 0.79825(1) A and BL14B1 at the
Shanghai Synchrotron Radiation Facility, Shanghai, China, at
˚
a wavelength of 1.2398 A.
˚
˚
˚
constants: a=15.146(6) A, b=7.721(3) A, c=9.268(4) A,
3
˚
and V = 1083.7(8) A . This new allotrope is denoted as
Results and Discussion
β-LiNH2BH3 to distinguish it from the originally discovered
form of LiNH2BH3 (henceforth denoted as R-LiNH2BH3)
reported by Xiong et al.10 The derived space group implies an
Phase Transition in the Formation of Lithium Amido-
borane. To explore the formation mechanism of lithium
amidoborane, a 1:1 molar ratio of LiH and NH3BH3 was
mechanically milled, and the extent of reaction was moni-
tored by recording the increase in the pressure in the milling
vial as a function of the time. The gaseous products were
analyzed by mass spectrometry and an ammonia detection
device. The ammonia concentration was found to be less
than 300 ppm upon completion of the reaction, and thus the
majority of the gaseous product is H2 (eq 1). The extent of
reaction is correlated with the amount of H2 generated.
Solid residues collected at different stages of the reaction
were subject to structural characterization. As shown in
3
˚
asymmetric unit with a volume of 135.5(1) A that is essen-
3
˚
tially twice the volume of R-LiNH2BH3 (63.8 A ), implying
that there are two crystallographically distinct Li and
NH2BH3 ions.
The formation of LiNH2BH3 via a solution-based route,
i.e., by using tetrahydrofuran (THF) as a solvent, was
monitored recently by in situ NMR.16 A gradual shift in
the resonance of 11B was observed, which indicated that the
formation of LiNH2BH3 was a stepwise process. Therefore, it
is of significant importance to characterize the composition-
ally induced structural changes obtained when using a con-
ventional solid-state synthesis. In this study, we systemati-
cally investigated the formation of LiNH2BH3 via the mecha-
nical milling of molar equivalents of LiH and NH3BH3. Our
experimental results show that the amidoborane-ammonia
borane complex is formed at the initial stage of the reaction
and is subsequently converted to LiNH2BH3, with two
phases of LiNH2BH3, namely, R-LiNH2BH3 and β-LiNH2-
BH3, identified in the process. Interestingly, both phases of
lithium amidoborane exhibit essentially identical features in
dehydrogenation.
1
Figure 1, when approximately /3 mol equiv of H2 had
evolved, the solid residue was composed of unreacted LiH
and LiNH2BH3 NH3BH3, a new phase, recently identified
3
by the authors, that can be obtained by the mechanical
milling of a 1:2 molar ratio of LiH and NH3BH3 or a 1:1
molar ratio of LiNH2BH3 and NH3BH3.17 It should be
noted that the absence of unreacted NH3BH3 in the XRD
pattern is likely due to the deformation of NH3BH3 into an
amorphous phase under energetic ball milling. After
2
ca. /3 mol equiv of H2 had evolved, R-LiNH2BH3 and
LiNH2BH3 NH3BH3 were found to coexist in the solid
residue. When ca. /4 mol equiv of H2 had evolved, both
3
3
Experimental Section
R-LiNH2BH3 and β-LiNH2BH3 phases were present in the
residue. Upon the release of ca. 1.0 mol equiv of H2, the only
phase detected was β-LiNH2BH3. On the basis of the above
observations and analyses, the following reaction steps are
proposed for the formation of LiNH2BH3 from LiH and
NH3BH3 (wherein AB, LiAB, and LiABAB are the abbre-
Synthesis. Lithium hydride (LiH, 99%, Aldrich) and ammo-
nia borane (NH3BH3, 98%, Aldrich) in a molar ratio of 1:1 were
mechanically milled together under 1 bar of argon at 200 rpm on
a planetary mill (Retsch, PM400). The extent of reaction was
monitored by recording the pressure change in the milling vial.
The content of the gaseous product(s) was analyzed by mass
spectrometry (Hiden HPR-20 Gas Analysis System) and a
thermoconductivity meter (accuracy, 0.1 μs/cm), where the out-
let gas was introduced to a dilute H2SO4 solution whose ion
conductivity was monitored with the progression of dehydro-
genation. It should be noted that the ionic conductivity of the
viations for NH3BH3, LiNH2BH3, and LiNH2BH3
NH3BH3, respectively):
3
2LiH þ 2AB
f LiH þ LiABAB þ H2
(14) Ramzan, M.; Silvearv, F.; Blomqvist, A.; Scheicher, R. H.; Lebegue,
S.; Ahuja, R. Phys. Rev. B 2009, 79.
(15) Kim, D. Y.; Singh, N. J.; Lee, H. M.; Kim, K. S. Chem.;Eur. J.
2009, 15, 5598–5604.
f 2LiAB þ 2H2
ð2Þ
(16) Xiong, Z. T.; Chua, Y. S.; Wu, G. T.; Xu, W. L.; Chen, P.; Shaw, W.;
Karkamkar, A.; Linehan, J.; Smurthwaite, T.; Autrey, T. Chem. Commun.
2008, 5595–5597.
(17) Wu, C. Z.; Wu, G. T.; Han, X. W.; Xiong, Z. T.; Chu, H. L.; He, T.;
Chen, P. Chem. Mater. 2010, 22, 3.

Article
Inorganic Chemistry, Vol. 49, No. 9, 2010 4321
Figure 2. XRD patterns of LiH-NH3BH3 ball-milled for 5, 8, and 16 h,
respectively. β-LiNH2BH3 tends to transform to R-LiNH2BH3 through
extended milling (3, R-LiNH2BH3; 1, β-LiNH2BH3; f, postdecomposed
product of LiNH2BH3 NH3BH3).
3
Figure 2, β-LiNH2BH3 gradually converted to R-LiNH2BH3
upon mechanical milling at 200 rpm for 16 h. It should be
noted that no pressure change was detected in such a
transformation, which suggests that the chemical compo-
sition of the solid remains unchanged.
Figure 1. XRD patterns of LiH-NH3BH3 samples ball-milled for
various times. The XRD pattern of LiNH2BH3 NH3BH3 is also pre-
sented for comparison (3, R-LiNH2BH3; 1, β-LiNH2BH3; f, post-
3
Structural information for LiNH2BH3 NH3BH3 and
3
decomposed product of LiNH2BH3 NH3BH3; g, LiH). The diffraction
3
R- and β-LiNH2BH3 is presented in Table 1. It can be seen
peak at 2θ = 22.7° is the product of the decomposition of LiNH2BH3
3
NH3BH3.17
.
that the lattice parameters of LiNH2BH3 NH3BH3 are
close to those of R-LiNH2BH3 but approximately half of
those of β-LiNH2BH3. The coordination environments of
3
We have examined the interaction of LiNH2BH3
3
NH3BH3 synthesized separately17 with 1.0 mol equiv of
LiH to confirm the second step in the process described
above, i.e., that the formation of LiNH2BH3 is the result
Liþ in R-LiNH2BH3, β-LiNH2BH3, and LiNH2BH3
3
NH3BH3 are presented in Figure 3. The strongest inter-
action in all of the structures is the ionic bond formed
between the Liþ cation and nitrogen atom in the amido-
borane anion and Hδ- in the BH3 group. As shown in
Figure 3a, the þLi-N bond length in R-LiNH2BH3 is
of a further substitution of H by Li in LiNH2BH3
NH3BH3. Our results show that β-LiNH2BH3 is formed,
after the addition of ca. 1.0 mol equiv of H2, from the
3
reaction of LiNH2BH3 NH3BH3 with LiH. Interestingly,
increasing the LiH content for the reaction of
LiNH2BH3 NH3BH3 with LiH did not lead to a further
˚
2.02 A; each Li is surrounded by three BH3 groups of
3
[NH2BH3]- ions with Li-B distances in the range of
˚
2.64- 2.68 A. The crystal structure of β-LiNH2BH3 is an
intergrowth of two different LiNH2BH3 layers oriented
3
substitution of H by Li; i.e., there is no evidence for the
formation of speculated species “Li2NHBH3”.18
It should be noted that a tetragonal phase with a unit
perpendicular to the a axis, and here the bond lengths for
˚
Li1-N and Li2-N are 1.93 and 2.04 A, respectively.
˚
˚
Moreover, the tetrahedral coordination of Liþ ions in
cell of a=4.0320(4) A, c=17.023(4) A, and V=276.73(8)
3
A was present in most of the samples. This phase likely
˚
β-LiNH2BH3 is distorted: the Li1-B distances are 2.56,
originated from the dehydrogenation/decomposition of
˚
2.57, and 2.78 A, respectively. However, the Li2-B dis-
tances are 2.50, 2.51, and 2.92 A (Figure 3b), respectively.
LiNH2BH3 NH3BH3.17
˚
3
For the intermediate LiNH2BH3 NH3BH3, each Liþ
It is interesting to note that LiNH2BH3 initially formed
is of the R phase, which then transforms to the β phase
as the reaction progresses. Moreover, we noted that
β-LiNH2BH3 subsequently transformed back to
R-LiNH2BH3 upon extended ball milling. As shown in
3
bonds with one [NH2BH3]- ion (Li-N distance 2.05 A)
˚
and is also coordinated with one BH3 group of NH3BH3
and two BH3 groups of [NH2BH3]- ions with Li-B
˚
distances of 2.48-2.58 A (Figure 3c).
The symmetry of β-LiNH2BH3 is lower than that of
R-LiNH2BH3 and could suggest that it is a metastable
phase that arises as a result of the energetic mechanical
(18) Armstrong, D. R.; Perkins, P. G.; Walker, G. T. THEOCHEM 1985,
23, 189–203.

4322 Inorganic Chemistry, Vol. 49, No. 9, 2010
Wu et al.
Table 1. Lattice Parameters of LiNH2BH3 NH3BH3, R-LiNH2BH3, and β-LiNH2BH3
3
lattice parameters
3
V (A )
˚
a (A)
˚
b (A)
˚
c (A)
˚
crystal structure
space group
β (deg)
17
LiNH2BH3 NH3BH3
R-LiNH2BH3
monoclinic
orthorhombic
orthorhombic
P21/c
Pbca
Pbca
7.05
7.11
15.15
14.81
13.95
7.72
5.13
5.15
9.27
97.5
90.0
90.0
531.6
511.0
1083.7
3
10
β-LiNH2BH3
Figure 3. Tetrahedral coordination of Liþ ions around amidoborane anions for (a) R-LiNH2BH3, (b) β-LiNH2BH3, and (c) LiNH2BH3 NH3BH3.
3
milling synthesis process. There are numerous cases
where nonequilibrium phase transitions in solid materials
occur under mechanical milling conditions,19 for exam-
ple, the metastable γ-MgH2 phase (high-pressure phase)
is formed upon the milling of stable β-MgH2.20 To date,
β-LiNH2BH3 has only been obtained via mechanical
milling synthesis.16 Dissolving β-LiNH2BH3 in THF
and then recrystallizing results in the formation of
R-LiNH2BH3 (as characterized by XRD), which suggests
that β-LiNH2BH3 may be kinetically stabilized and that
R-LiNH2BH3 is the more thermodynamically stable
species. This is supported by the fact that β-LiNH2BH3
transforms to R-LiNH2BH3 upon extended ball milling.
Although the detailed mechanism for this phase transi-
tion is the subject of further study, the extended ball
milling could lead to the regain of the equilibrium
(thermodynamically stable) state.
Figure 4. Time dependence of hydrogen release from LiNH2BH3
Dehydrogenation Properties. Thermal decomposition
3
NH3BH3, R-LiNH2BH3, and β-LiNH2BH3 at two temperature stages,
91 and 180 °C, respectively. (Please note that the time-temperature
profile for each sample is different.) For lithium amidoboranes, about
8.8 wt % H2 released at 91 °C (close to 20 h, ca. 1.6 mol equiv of H2) and
a total of 2.0 mol equiv of H2 released (10.8 wt %) during heating up to
properties of LiNH2BH3 NH3BH3, β-LiNH2BH3, and
3
R-LiNH2BH3 were investigated on a PCT apparatus at 91
and 180 °C, respectively. As shown in Figure 4, β-LiNH2BH3
and R-LiNH2BH exhibit essentially identical dehydrogena-
tion features; i.e., both release 8.8 wt % H2 at 91 °C (ca. 1.6
mol equiv of H2. Note that H2 desorption is less than what
we reported previously, which is due to the addition of
carbon to the LiNH2BH3 sample10). Upon further heating
to 180 °C, ca. 2.0 mol equiv of H2 was released [see reaction
(3)]. To further verify the similarity of the thermal decom-
position of these two LiNH2BH3 phases, DSC measure-
ments were also performed. Our results show that both
R- and β-LiNH2BH3 exhibit an endothermic feature
(starting from 82 °C) prior to the exothermic dehydrogena-
tion, which is likely due to the melting of the material. The
structural difference will, therefore, vanish under such a
180 °C. For LiNH2BH3 NH3BH3, first 6 wt % H2 released at 91 °C
3
and 8.3 wt % H2 released at 180 °C. (The ammonia concentrations
after thermal decomposition are ∼300 ppm for R-LiNH2BH3 and
β-LiNH2BH3 and less than 3000 ppm for LiNH2BH3 NH3BH3.)
3
condition. The dehydrogenation should mainly reflect the
chemical nature of the “molten” LiAB species.
LiNH2BH3 NH3BH3 prepared by directly milling a 1:2
3
molar ratio of LiH/NH3BH3 was observedtorelease 6.0 wt
% (ca. ∼2 mol equiv) H2 at 91 °C and 8.3 wt % (ca. ∼3 mol
equiv) H2 at 180 °C, a total of 14.3 wt % H2, which is close
to its theoretical capacity of 14.8 wt % [see reaction (4)].17
nLiNH2BH3ðsÞ f ðLiNBHÞnðsÞ þ 2nH2ðgÞ
ð3Þ
nLiNH2BH3 NH3BH3ðsÞ f ðLiN2B2HÞnðsÞ þ 5nH2ðgÞ
(19) Suryanarayana, C. Prog. Mater. Sci. 2001, 46, 1–184.
(20) Varin, R. A.; Czujko, T.; Wronski, Z. Nanotechnology 2006, 17,
3856–3865.
3
ð4Þ

Article
Inorganic Chemistry, Vol. 49, No. 9, 2010 4323
Conclusion
LiNH2BH3 NH3BH3 releases approximately 14.3 wt % H2
under the same conditions.
3
In summary, LiNH2BH3 NH3BH3 and R and β phases of
LiNH2BH3 were observed as products of the solid-state
reaction of LiH and NH3BH3. The LiNH2BH3 NH3BH3
3
Acknowledgment. The authors acknowledge financial
support from the Hundred Talents Project and Knowledge
Innovation Program of CAS (Grants KGCX2-YW-806
and KJCX2-YW-H21), National Program on Key Basic
Research Project (973 Program, No. 2010CB631304), and
NationalHigh-techR&DProgramofChina(863Program,
No. 2009AA05Z108). We further acknowledge support of
the EPSRC funding bodies and the United Kingdom
Sustainable Hydrogen Energy Consortium (UK-SHEC),
an EPSRC SUPERGEN.
3
complex, possessing a monoclinic structure and composed of
alternative LiNH2BH3 and NH3BH3 layers, is the intermedi-
ate phase in the formation of LiNH2BH3. β-LiNH2BH3, a
metastable phase formed under mechanical milling condi-
tions, converts back to R-LiNH2BH3 upon either extended
ball milling or recrystallization from THF. Both polymorphs
of lithium amidoborane (R- and β-LiNH2BH3) release app-
roximately 10.8 wt % H2 upon heating to 180 °C, whereas