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.
  1. Results and Discussion
  2. 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.