Band structures and native defects of ammonia borane
D. West,
1,
* Sukit Limpijumnong,
2
and S. B. Zhang
1
1
Physics Department, Rensselaer Polytechnic Institute, Troy, New York 12180, USA
2
School of Physics, Suranaree University of Technology and Synchrotron Light Research Institute, Nakhon Ratchasima 30000, Thailand
Received 17 December 2008; published 24 August 2009
Band structures and native defects in molecular solid ammonia borane AB are investigated by using
first-principles calculations based on the density-functional theory DFT. The native defects include NH
3
and
BH
3
antisites, interstitials, and vacancies in the various charge states. Despite that the AB crystal is an insulator
with a DFT band gap larger than 6 eV, some of the charge-neutral native defects can be abundant, especially
at the NH
3
-rich growth conditions, due to their exceptionally low formation energies. This is in sharp contrast
to defects in semiconductors for which the formation energies of charge-neutral defects are usually high and
correspondingly the concentrations of those defects are low.
DOI: 10.1103/PhysRevB.80.064109 PACS numbers: 61.72.y, 71.15.Mb, 71.20.b
I. INTRODUCTION
Ammonia borane AB, also known as borazane, is a mo-
lecular solid at room temperature with a unit cell of NH
3
BH
3
molecules. It has recently received considerable attention
due to its high hydrogen content 19.6 wt %, which offers a
good deal of promise as a candidate for onboard hydrogen
storage applications.
1
Furthermore, its volumetric energy
density 4.94 kWh/L is superior to that of liquid hydrogen
2.36 kWh/L. The AB solid is stable at room temperature,
with a melting point of 383–388 K.
2,3
The formation of solid
is originated from the difference in the electronegativity be-
tween B and N. The electronegative N takes electrons from
its three neighboring H atoms, leaving a partial positive
charge. The electropositive boron, on the other hand, gives
electrons away to its three neighboring H atoms, leaving a
partial negative charge. The solid is held together due to the
attraction between the opposite charges on the hydrogen at-
oms, or dihydrogen bonds. On average, each AB molecule
has six dihydrogen bonds. At sufficiently low temperatures,
the AB solid forms ordered orthorhombic phase. In this
phase, the NH
3
BH
3
molecules are locked into position and
are unable to rotate. At 225 K, however, the ammonia borane
undergoes an order-disorder phase transition
4,5
to form a new
tetragonal phase.
There have been several thermal decomposition studies of
the ammonia borane.
6–9
These studies consistently showed
that the thermal decomposition of NH
3
BH
3
takes place in
two exothermic steps, each of which releases a formula unit
of H
2
. The first step is the creation of the H
2
and polyami-
noborane PAB, i.e., NH
3
BH
3
x
→ BH
2
NH
2
x
+ xH
2
. This
reaction has an associated H =-0.22 eV per H
2
. Although
the reaction is exothermic, a barrier exists so thermal energy
is required for the reaction to move forward. It is found that
the second step of the decomposition starts at temperatures
near 150 ° C. The second step is the continued decomposi-
tion of PAB to H
2
and polyiminoborane PIB, following the
reaction: NH
2
BH
2
x
→ NHBH
x
+ xH
2
. Further decomposi-
tion requires a much higher temperature than that suitable for
hydrogen storage. While the first step in this decomposition
occurs very rapidly
7
at the boiling point of AB 114 °C, it is
relatively slow in the temperature regime suitable for on-
board H storage, which is 30–85 °C.
10
Increasing reaction kinetics at low temperature is a very
active area of research. Ball milling and doping the AB ma-
terials have shown some promise.
11
AB in solutions has been
studied in the form of acid-catalyzed
12
and
transition-metal-catalyzed
13
AB or solvated AB in ionic
liquids.
14
With transition-metal catalysis, it is possible to get
up to nearly 2.5 H
2
off NH
3
BH
3
.
13
The AB material can also
be embedded in porous matrices, both in nanoscaffolds of
mesoporous silica
9
and in carbon cryogels,
15,16
which has
been shown to markedly improve the kinetics and thermody-
namics of the H desorption. Moreover, the replacement of
hydrogen by lithium and sodium in the form of NH
2
LiBH
3
and NH
2
NaBH
3
has been suggested
17
to accelerate the reac-
tion kinetics.
First-principles studies ranging from density-functional
theory DFTRefs. 18 and 19 to quantum chemistry
20–23
calculations have been performed in the past to determine the
reaction paths and energetics of H desorption. While this is a
very important problem, none of the previous works consid-
ered the potential role of defects. As a matter of fact, for all
hydride materials there are only a few studies of defects in
hydrogen release systems. For hydride materials that are in-
sulators with considerable band gaps, native defects might
not play an important role. It is known that for wide-band-
gap materials, neutral native defects are high in energy and
only charged defects can have reasonably low formation en-
ergies at certain Fermi levels. Under p-type conditions where
the Fermi level is near the valence-band edge, some posi-
tively charge native defects donors may have low energy.
On the other hand, under n-type conditions, where the Fermi
level is near the conduction-band edge, some negatively
charged native defects acceptors may have low energy. In
an insulator, however, the Fermi level is located in the gap,
far away from both valence-band and conduction-band
edges; resulting in no native defects with low energy. There-
fore, it may be reasonable to ignore the effects of native
defects on those materials.
The purpose of this work is to contribute to the fundamen-
tal understanding of the material properties of AB solid. We
consider the AB solid here because as a molecular solid, its
physical behavior can be qualitatively different from many
other hydrides. Indeed, our calculation shows that unlike a
typical wide-gap semiconductor, the formation energy of a
PHYSICAL REVIEW B 80, 064109 2009
1098-0121/2009/806/06410910 ©2009 The American Physical Society 064109-1