Vacancy Diffusion in NaAlH4 and Na3AlH6

In recent years there has been considerable interest in sodium alanate as a prototypical complex hydride for solid-state hydrogen storage. Much effort has gone into understanding the rate-limiting processes in its hydrogen release and absorption reactions. The diffusion of metal species has been suggested as a possible kinetic bottleneck in Ti-doped materials. In this paper, we outline an approach for calculating the diffusivity of defects in complicated lattices using a combination of first-principles density-functional theory calculations and stochastic kinetic Monte Carlo methods. We apply this methodology to the diffusion of metal defects in NaAlH4 and Na3AlH6 that have been predicted to exist in large concentrations. We find that of the metal defects that exist in the largest concentrations, a neutral AlH3 vacancy is the most mobile in NaAlH4 (Delta H-mig = 0.34 eV, D-0 = 1.30 x 10(-2) cm(2)/s, and D-T=400K = 7.55 x 10(-7) cm(2)/s) and that a negatively charged Na vacancy is the most mobile in Na3AlH6 (Delta H-mig = 0.33 eV, D-0 = 6.67 x 10(-3) cm(2)/s, and D-T=400K = 4.96 x 10(-7) cm(2)/s). At T = 400 K, the calculated diffusion rates are an order of magnitude lower for charged AlH4 vacancies in NaALH(4) = 0.44 eV, Do = 1.41 x 10(-2) cm(2)/s, and D-T=400K = 3.92 x 10(-8) cm(2)/s) and charged Na vacancies in NaAlH4 (AH = 0.43 eV, D-0 = 2.96 x 10(-3) cm(2)/s, and D-T=400K = 1.19 x 10(-8) cm(2)/s). This information is necessary for understanding the kinetics of mass transport during the hydrogen release and absorption reactions of NaAlH4.