First-principles calculations have been performed on lithium borohydride LiBH4 using the ultrasoft pseudpotential method, which is a potential candidate for hydrogen storage materials due to its extremely large gravimetric capacity of 18 mass % hydrogen. We focus on an orthorhombic phase observed at ambient conditions and predict its fundamental properties; the structural properties, electronic properties, dielectric properties, vibrational properties, and the heat of formation. The calculation gives a nearly ideal tetrahedral shape for BH4 complexes, although the recent experiment suggests that their configuration is strongly distorted [J-Ph. Soulié et al., J. Alloys Compd. 346, 200 (2002)]. Analyses for the electronic structure and the Born effective charge tensors indicate that Li atoms are ionized as Li+ cations. The internal bonding of [BH4]- anions is primarily covalent. The high-frequency dielectric permittivity tensor ε∞ is predicted as almost isotropic, but the static dielectric permittivity tensor ε0 as considerably anisotropic. The Γ-phonon eigenmodes can be classified into three groups, namely, the librational modes involving the displacements of Li+ cations (less than 500 cm-1), and the internal B-H bending and stretching modes of [BH4]- anions (around 1100 and 2300 cm-1, respectively). The molecular approximation fairly reproduces the phonon frequencies in the latter two groups, implying the strong internal bonding of BH4 complexes. The librational modes have significant contributions to the large anisotropies of ε0. The agreement of the heat of formation with the experimental value is reasonably good.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - 2004 Jun|
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics