First-principles calculations, based upon density-functional theory, of the vacancymediated

hydrogen migration energy in bulk NaH and near to the NaH(001) surface were performed. The estimated rate of vacancy-mediated hydrogen

transport, obtained within a hopping diffusion model, was consistent with the

reaction rates of H-D exchange in nano-NaH at the relatively low temperatures

observed in recent neutron studies of TiCl3-doped NaAlH4. The formation energy

for hydrogen vacancies and interstitials in NaH, in all relevant charged states, was

obtained. Such formation energies were too high to lead to the abundant hydrogen

concentrations seen experimentally. Ab initio calculations of the NaCl/NaH

interface were presented in order to provide an insight into the mechanism which

might lead to high hydrogen concentrations. It was shown that the formation of an

fcc-Na interlayer, during the growth of NaH on top of NaCl, was plausible,

providing a source of vacancies and leading to fast hydrogen transport. The low

interface energies for NaCl//NaH were consistent with the easy growth of NaH

crystallites on NaCl nucleation centers, which could therefore act as grain refiners.

Hydrogen Vacancies Facilitate Hydrogen Transport Kinetics in Sodium Hydride

Nanocrystallites. S.Singh, S.W.H.Eijt: Physical Review B, 2008, 78[22], 224110