  First-principles density functional theory studies have been carried out for
native defects and transition-metal (Ti and Ni) impurities in lithium alanate
(LiAlH$_{4}$), a potential material for hydrogen storage. On the basis of our
detailed analysis of the structure, energetics, and migration of lithium-,
aluminum-, and hydrogen-related defects, we propose a specific atomistic
mechanism for the decomposition and dehydrogenation of LiAlH$_{4}$ that
involves mass transport mediated by native point defects. We also discuss how
Ti and Ni impurities alter the Fermi-level position with respect to that in the
undoped material, thus changing the concentration of charged defects that are
responsible for mass transport. This mechanism provides an explanation for the
experimentally observed lowering of the temperature for the onset of
decomposition and of the activation energy for hydrogen desorption from
LiAlH$_{4}$.
