The crystal structure and thermodynamic stability of a number of proposed models for the inclusion of Ti in NaAlH 4 have been calculated by employing density functional theory in the generalized gradient approximation. It was shown that the least unfavorable location of Ti is close to the surface, replacing Al in the host lattice. Intricate complexation is simulated around the included Ti atom, and the preferred coordination number of H around Ti is eight. The Ti content was varied by the supercell approach, and even at 3 mol % Ti the resulting cell parameters were predicted to be significantly changed from the pure alanate. In addition, the most stable configurations were found to be thermodynamically metastable compared to the pure alanate and Ti standard state phases, and an ordered doped phase with significant bulk Ti content is thus ruled out by this study. It is proposed that Ti most probably works as a catalyst, situated at the surface of the alanate.

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The alanates (complex aluminohydrides) have relatively high gravimetric hydrogen density and are among the most promising solid-state hydrogen-storage materials. In this work, the crystal structure and electronic structure of pure and mixed-alkali alanates were calculated by ground-state density-functional band-structure calculations. The results are in excellent correspondence with available experimental data. The properties of the pure alanates were compared, and the relatively high stability of the Li3AlH6 phase was pointed out as an important difference that may explain the difficulty of hydrogenating lithium alanate. The alkali alanates are nonmetallic with calculated band gaps around 5 eV and 2.5–3 eV for the tetra- and hexahydrides. The bonding was identified as ionic between the alkali cations and the aluminohydride complexes, while it is polar covalent within the complex. A broad range of hypothetical mixed-alkali alanate compounds was simulated, and four were found to be stable compared to the pure alanates and each other: LiNa2AlH6, K2LiAlH6, K2NaAlH6, and K2.5Na0.5AlH6. No mixed-alkali tetrahydrides were found to be stable, and this was explained by the local coordination within the different compounds. The only alkali alanate that seemed to be close to fulfilling the international hydrogen density targets was NaAlH4.

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There has been confusion which external reference should be used for predicting ground state Ti substitution enthalpies in NaAlH4: element gaseous atomic states or standard states. It is proposed instead to use all the relevant product phases for the Ti-enhanced sodium alanate system as internal reference states. The results are similar to when external reference element standard states are used: Ti doping is unstable in NaAlH4, and substitution on the Al sublattice is the least unfavorable. Substitution near the surface is more favorable than in the bulk, thus metastable Ti substitution may occur close to interfaces, grain boundaries, or defects.