First-principles study on lithium borohydride LiBH4 -: art. no. 245120

First-principles calculations have been performed on lithium borohydride LiBH4 using the ultrasoft pseudopotential 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. Soulie , 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 epsilon(infinity) is predicted as almost isotropic, but the static dielectric permittivity tensor epsilon(0) as considerably anisotropic. The Gamma-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 epsilon(0). The agreement of the heat of formation with the experimental value is reasonably good.