The diffusion of metal species was suggested to be a possible kinetic bottleneck in Ti-doped materials. An approach was outlined here 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. This approach was applied to the diffusion of metal defects that were predicted to exist in large concentrations. It was found that, among the metal defects that existed in the largest concentrations, a neutral AlH3 vacancy was the most mobile in NaAlH4 (ΔHmig = 0.34eV, D0 = 1.30 x 10-2cm2/s, DT=400K = 7.55 x 10-7cm2/s) and that a negatively charged Na vacancy was the most mobile in Na3AlH6 (ΔHmig = 0.33eV, D0 = 6.67 x 10-3cm2/s, DT=400K = 4.96 x 10-7cm2/s). At 400K, the calculated diffusion rates were an order of magnitude lower for charged AlH4 vacancies in NaAlH4 (ΔH = 0.44eV, D0 = 1.41 x 10-2cm2/s, DT=400K = 3.92 x 10-8cm2/s) and charged Na vacancies in NaAlH4 (ΔH = 0.43eV, D0 = 2.96 x 10-3cm2/s, DT=400K = 1.19 x 10-8cm2/s).

Vacancy Diffusion in NaAlH4 and Na3AlH6. K.J.Michel, V.Ozoliņš: Journal of Physical Chemistry C, 2011, 115[43], 21465-72