Abstract:
To cope with the energy crisis and global warming issues, researcher are
rendering their efforts and paying their attentions to analyze and
fabricate hydrogen storage devices. In this regard, we report a
comprehensive study on the structural, vibrational, and optoelectronic
properties of Lithium Borohydride (LiBH4), a hydrogen
storage material. For this purpose, calculations of structural
properties have been made using the local, non-local and hybrid
functionals within the framework of density functional theory (DFT). The
lattice constants for the orthorhombic phase are determined by applying
LDA, PBE and HSE06 density functionals and their results are compared
with available experimental and theoretical studies. In order to
determine IR and Raman active modes of vibrations, vibrational
spectroscopy has been utilized through Density Functional Perturbation
Theory (DFPT) approach. Li, B and H atoms are noticed to be contributing
in the modes of vibrations between different ranges of frequencies,
i.e., 0 to 400 cm-1, 1100 to 1300
cm-1 and 2250 -2400 cm-1. The
respective values of band gaps are found to be 6.35 eV, 6.81 eV and 7.58
eV for LDA, PBE and HSE06 functionals, respectively, leading to indicate
insulating nature of LiBH4 which makes it a promising
candidate for applications in optoelectronic devices. The mechanical
analysis reveals that LiBH4 is a brittle material. The optical
properties such as dielectric constant, refractive index, reflectivity,
absorptivity, conductivity and loss function are also calculated with
the aid of well-recognized relation of Kramer-Kronig. The plasma
frequency is noted at the highest peak (13.7 eV) of the energy loss
function.
Keywords : optoelectronic devices, hydrogen storage, vibrational
spectroscopy, IR and Raman modes, electronic band gap, loss function.
Introduction
Over the next 30 years, two major challenges would be facing by this era
of the Science and Technology: (a) curtailing supply of the fossil fuels
and (b) destructive impact of the growth rate of global warming on the
climate changes. That is why, researchers of the day are being motivated
to render their optimal efforts and paying their full attentions to
manufacture hydrogen storage devices [1-2]. The bountiful amount of
natural water, available in the universe, is a fundamental and
inexhaustible source of hydrogen which could be utilized as an
alternative and renewable source of energy. Nonetheless, it has been a
vital challenge for the researchers as well as industrial community of
the current era to mystified, predict and explore novel combinations of
materials which is ought to be advantageous in devising such an
appropriate device wherein hydrogen could be stored more efficiently and
largely for its mobile applications [3-4]. Among various capable
hydrogen storage materials, LiBH4 is one to be
considered for such hydrogen storage devices. This material was studied
by Wang et al. [5] to examine the phase transitions occurring
at pressures of 1.64 GPa and 2.83 GPa along with its electronic
properties through the first principles approach. Setten et al.[6] have reported that the optical response of the simple and
complex hydrides is found to be similar, despite the fact that their
electronic band gaps are of different values. Xiaohua et al.[7] have investigated the dehydrogen characteristics of
LiBH4 modified by Mg and showed that Mg can destabilize
LiBH4 by reducing the energy cost required for
H-desorption. Miwa et al. [8] studied its structural,
electronic, vibrational and heat of formation along with its vibrational
modes of vibrations. Ikeshoji et al. [9] reported the ionic
conductivity of Li+ ions in LiBH4system, wherein, activation energy and diffusion coefficient were
estimated by vesting theoretical as well as experimental studies. Songet al. [10] have studied the metal borohydride ammonia borane
complex using LiBH4 and
NH3BH3. They found both of these systems
to be thermodynamically stable. Benzidi et al. [11] have
compared the vibrational and thermodynamical properties of
LiBH4 in orthorhombic and hexagonal phases, wherein,
they reported that orthorhombic phase of LiBH4 is more
stable thermodynamically. Khalil et al. [12] recently studied
the spectroscopic and thermodynamical properties of a similar hydride,
MgH2 in the presence of catalysts and found that
hydrogen release temperature is reduced due to reaction of metal hydride
with catalysts. Ghellab et al. [13] reported the phase
transition in LiBH4 system at high values of pressure
based on density functional theory calculations using the WIEN2K code.
Although few studies display the vibrational and optical behavior of
LiBH4, nevertheless, for better insights into it, the
critique on various modes of vibrations and optical response of
LiBH4 through CASTEP simulation code is still
unaddressed. Therefore, we are motivated to study this imperative
compound by utilizing various local, non-local and hybrid functionals
for better and improved electronic properties as well as insight into
the vibrational characteristics. The results are discussed and compared
for their suitability in utilizing them for the hydrogen storage and
other optoelectronic applications.
Research Methodology
All calculations are carried out using density functional theory as
implemented in CASTEP simulation code [14]. The norm-conserving
pseudopotentials [15-17] have been used to model electron-ion
interaction along with plane-wave cut-off energy of 884 eV to converge
the LiBH4 system in order to attain the structural
properties. The Monck-Pack [18] grid of the values of 8×4×8 was
selected to sample the Brillouin zone. The conjugate gradient method
[19] was selected to relax the ionic positions, cell volume and
lattice parameters of the systems until the Hellmann Feynman forces
[20] found smaller than 0.02 eV/Å and the energy convergence
criteria was met at 10 μeV. The most popular functionals LDA, GGA-PBE
and HSE06 [21-23] are used to apply exchange correlation potentials
in solving the Kohn-Sham [24-25] equations for the structural and
electronic properties. The vibrational properties are calculated using
the density functional perturbation theory [26]. Kramer-Kronig
[27-28] relations have been adopted to determine the optical
properties of LiBH4. The GGA-PBE functional was adopted
for determining vibrational and optical properties.
- Results and Discussion
- Structural Properties
The geometrically optimized crystal structure of a unit cell of
LiBH4 is shown in figure 1. Red color balls represent
lithium atoms, green color ones indicate boron atoms and blue color ones
define hydrogen atoms. There are a total of 24 atoms in a primitive unit
cell of LiBH4, which are further split into 16 hydrogen
atoms, 4 lithium atoms and 4 atoms of boron as obvious from figure 1.
The optimized cell angles and lattice constants determined through
CASTEP code are summarized in the table-1. It is noticeable that present
outcomes regarding cell angles and lattice constants obtained through
GGA-PBE functional are found to be in more-closer agreement with the
former experimental results [29] as compared to these determined
through LDA and HSE06 functionals (see Table-1). The highest deviation
in lattice constants is noted for HSE06 functionals, which is
underestimating the values of lattice constants for
LiBH4 system.