Abstract
First-principles calculations have been performed on lithium borohydride using the ultrasoft pseudopotential method, which is a potential candidate for hydrogen storage materials due to its extremely large gravimetric capacity of 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 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 cations. The internal bonding of anions is primarily covalent. The high-frequency dielectric permittivity tensor is predicted as almost isotropic, but the static dielectric permittivity tensor as considerably anisotropic. The -phonon eigenmodes can be classified into three groups, namely, the librational modes involving the displacements of cations (less than ), and the internal B-H bending and stretching modes of anions (around 1100 and , respectively). The molecular approximation fairly reproduces the phonon frequencies in the latter two groups, implying the strong internal bonding of complexes. The librational modes have significant contributions to the large anisotropies of . The agreement of the heat of formation with the experimental value is reasonably good.
- Received 27 January 2004
DOI:https://doi.org/10.1103/PhysRevB.69.245120
©2004 American Physical Society