First-principles study on lithium borohydride LiBH₄

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.