First-principles study ofNaAlH4andNa3AlH6complex hydrides

We present a first-principles investigation of the structural properties, electronic structure, and the chemical stability of the complex hydrides $\mathrm{Na}\mathrm{Al}{\mathrm{H}}_4$ and ${\mathrm{Na}}_3\mathrm{Al}{\mathrm{H}}_6$. The calculations are performed within the density functional framework employing norm conserving pseudopotentials. The structural properties of both hydrides compare well with experimental data. A detailed study of the electronic structure and the charge-density redistribution reveal the features of an ionic covalent bonding between Al and H in the ${\left(\mathrm{Al}{\mathrm{H}}_4\right)}^{-}$ and ${\left(\mathrm{Al}{\mathrm{H}}_6\right)}^{-3}$ anionic complexes embedded in the matrix of ${\mathrm{Na}}^{+}$ cations. The orbital hybridization and the characteristics of bonding orbitals within the complexes are identified. The calculated reaction energies of these complex hydrides are in good agreement with the experimentally determined values.