Theoretical study of the vibrational properties of NaAlH4 with AlH3 vacancies.

It has been suggested that the diffusion of AlH3 vacancies plays an essential role in the decomposition of NaAlH4, a prototypical material for hydrogen storage. We find from first-principles calculations that the AlH3 vacancy induces several isolated vibrational modes that are highly localized in the vacancy region with frequencies within the phonon gaps of pure NaAlH4 in both the alpha and gamma phases. Thus, the proposed existence of AlH3 vacancies in the dehydrogenation reaction of NaAlH4 can be possibly confirmed with the experimental detection of these unique vibrational modes associated with the AlH3 vacancy.