First-principles molecular dynamics study of the structure and dynamic behavior of liquid
Li
4
BN
3
H
10
David E. Farrell,* Dongwon Shin, and C. Wolverton
†
Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA
Received 28 September 2009; published 10 December 2009
We have applied density-functional theory based ab initio molecular dynamics to examine Li
4
BN
3
H
10
at
temperatures both above and below the experimental melting point. We examine the structure of the liquid,
diffusivity, vibrational spectra and compare to both experimental data and analogous properties from solid-state
calculations. We find the following: 1 the liquid state, like the solid state, is primarily a mixture of Li
+
, BH
4
-
,
and NH
2
-
with ionic interactions between the BH
4
-
and NH
2
-
anions and the Li
+
cations. 2 We observe the
reaction of two amide anions exchanging hydrogen to form ammonia and an imide anion: 2NH
2
-
→NH
3
+NH
2-
. 3 The liquid demonstrates wide bond-angle distributions in the BH
4
-
and NH
2
-
units and thus these
anionic units are not simply rigid complexes. 4 The Li
+
sublattice disorders before the anionic sublattices and
the liquid exhibits very fast Li
+
diffusion. We calculate the activation energy and pre-exponential factor for Li
+
diffusivity in the liquid to be 20 kJ / mol and 15 10
-4
cm
2
/ s, respectively. 5 Finally, we find that the
liquid contains the same generic types of vibrational modes as the solid, however the lower-frequency anionic
vibration and rotation modes become more prominent with increasing temperature.
DOI: 10.1103/PhysRevB.80.224201 PACS numbers: 61.20.Ja
I. INTRODUCTION
In recent years, numerous research groups have put forth
much effort toward the realization of efficient, high-capacity
hydrogen storage for hydrogen-based vehicles.
1
One promis-
ing class of materials for this application is complex
hydrides.
2,3
These materials have the possibility of very high
gravimetric and volumetric hydrogen storage densities, mak-
ing them attractive materials if they can be shown to have
high rates of hydrogen desorption and absorption at near-
ambient conditions.
One particular complex hydride that has received signifi-
cant attention due to its high hydrogen storage capacity is the
quaternary compound Li
4
BN
3
H
10
, found to release more
than 10 wt % H
2
. The hydrogen storage properties and crys-
tal structure of this material was studied experimentally by
Pinkerton et al.
4
and subsequently by Filinchuk et al.
5
and
Chater et al.
6
These experimental studies prompted several
theoretical studies that examined the crystal structure, vibra-
tional spectra and reaction energetics of solid Li
4
BN
3
H
10
.
7–9
The experimental and theoretical analysis reveal solid
Li
4
BN
3
H
10
to be an ionic compound consisting of four Li
+
cations, one BH
4
-
, and three NH
2
-
anionic units per formula
unit,
6
whose solid structure has bcc symmetry space group
I2
1
3 and a lattice parameter of roughly 10.6 Å.
8
However,
the initial experiments indicated that hydrogen and ammonia
release occur at approximately 520 K, above the melting
temperature of 460 K,
4,5
though a more recent experimen-
tal study of NiCl
2
catalyzed Li
4
BN
3
H
10
demonstrated hydro-
gen desorption at approximately 400 K, with no change in
the ammonia release temperature.
10
The fact that hydrogen
and ammonia release occur near or above the melting point
indicates that structural characterization of the liquid state is
important in understanding the mechanism of hydrogen re-
lease from this material. Here, we use first-principles calcu-
lations based on density-functional theory DFT to elucidate
the detailed atomic structure, diffusivity, and vibrational
properties of liquid Li
4
BN
3
H
10
.
DFT-based calculations have been used to predict and ex-
plore the ground-state crystal structures, vibrational proper-
ties and thermodynamics of a number of hydrides,
11–16
in-
cluding complex hydrides such as LiBH
4
,
17
NaAlH
4
,
18
LiNH
2
,
19
Li
2
NH,
20,21
CaAlH
4
2
,
22
Li
2
MgNH
2
,
23
and
CaBH
4
2
.
24
Additionally, groups have applied nonzero tem-
perature ab initio molecular dynamics AIMD to materials
such as NaAlH
4
Ref. 25 to explore diffusion in the solid
hydride as well as structural and vibrational properties of
hydride clusters
26
and to predict solid-state transformations
in LiBH
4
.
27
Molecular dynamics has also been used very
recently to investigate the structure and diffusion character-
istics in Li
2
NH.
28
One advantage of AIMD is the ability to
explicitly follow the trajectory of a given atom through time,
allowing one to see any diffusion events or chemical reac-
tions as they occur. However, AIMD is generally limited to a
small time scale, often shorter than the typical time scale for
solid-state diffusion. This time-scale limitation may be par-
tially overcome by studying materials where the phenom-
enon of interest occurs in the liquid state, such as
Li
4
BN
3
H
10
, because the kinetics are generally much faster
than in the solid.
We have used AIMD to examine the structural, vibra-
tional, and diffusion characteristics of liquid Li
4
BN
3
H
10
for
temperatures from 300 to 2000 K. We find that the liquid
state, like the solid state, is a mixture of Li
+
, BH
4
-
, and NH
2
-
with ionic interactions between the BH
4
-
and NH
2
-
units and
the Li
+
ions. However, we show that the BH
4
-
and NH
2
-
units
are not rigid complexes but rather undergo wide bond-angle
fluctuations about their ideal positions. Additionally, we see
chemical fluctuations take place at temperatures above 600
K. We observe the reaction of two amide anions exchanging
hydrogen to form ammonia and an imide anion: 2NH
2
-
→ NH
3
+NH
2-
. We also find interesting structural and kinetic
properties on the cation sublattice: the Li
+
sublattice disor-
ders before the B, N, and H sublattices, and we find indica-
tions of high Li
+
diffusion in the liquid. Finally, we find that
PHYSICAL REVIEW B 80, 224201 2009
1098-0121/2009/8022/2242018 ©2009 The American Physical Society 224201-1