NaAlH4 is a prototypical complex metal hydride that offers a reasonable hydrogen capacity and exhibit a reversible hydrogen reaction at favorable temperature and pressure ranges. The conventional phase (a-phase) of NaAlH4 has a well-characterized body-centered tetragonal structure with the I41/a space group. A new orthorhombic ?-phase was proposed recently that has the space group Cmcm with the a to ? transformation predicted to occur at around room temperature. It is commonly accepted that the dehydrogenation process involves motion of defects, and that the AlH3-related ones are most probable. In order to better understand this process, we have performed first-principles calculations to study the energetics of the motion of these defects. We find that the diffusion barrier is only about 0.1 eV for both the charged AlH4 vacancy and charge-neutral AlH3 vacancy in ?-NaAlH4, considerably lower than the values in the conventional a-phase. These results suggest that the reaction kinetics could be significantly improved if one can force a a ? ? transformation by means of, for example,  nano-confinement.

Ultra-low diffusion barriers for AlH<sub>3</sub>-related defects in γ-NaAlH<sub>4</sub>

245th National Meeting (2013)

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Ultra-low diffusion barriers for AlH3-related defects in γ-NaAlH4

Ultra-low diffusion barriers for AlH₃-related defects in γ-NaAlH₄