Ultra-low diffusion barriers for the ${\rm AlH_3}$-related vacancies in $\gamma$-${\rm NaAlH_4}$

It has been suggested that the diffusion of ${\rm AlH_3}$-related vacancies plays an essential role in the decomposition of ${\rm NaAlH_4}$, a prototypical material for hydrogen storage[1,2]. We find from first-principles calculations that the diffusion barrier for both the neutral ${\rm AlH_3}$ vacancy and the charged ${\rm AlH_4^-}$ vacancy in the newly proposed $\gamma$-phase of ${\rm NaAlH_4}$ [3] is only about 0.1 eV, much lower than the barrier for the diffusion of corresponding vacancies in the conventional $\alpha$-phase 0.5 eV, calculated with the same method. Possible schemes to facilitate the $\alpha\rightarrow\gamma$ phase transformation in order to improve the kinetics of the decomposition reaction of ${\rm NaAlH_4}$ will also be discussed.\\[4pt] [1] H. Gunaydin, K. N. Houk, and V. Ozoli\c{n}\v{s}, Proc Natl Acad Sci USA {\bf 105}, 3673 (2008).\\[0pt] [2] G.~B.~Wilson-Short, A.~Janotti, K.~Hoang, A.~Peles, and C.~G.~Van de Walle, Phys. Rev. B {\bf 80}, 224102 (2009).\\[0pt] [3] B.~Wood and N.~Marzari, Phys. Rev. Lett. {\bf 103}, 185901; {\bf 104}, 019901.