Density functional calculations of Ti-enhanced NaAlH4 -: art. no. 054103

The crystal structure and thermodynamic stability of a number of proposed models for the inclusion of Ti in NaAlH4 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.