Using first-principles methods a study was made of the interactions between hydrogen impurities and vacancies in hexagonal close-packed Mg and face-centered cubic Al. It was found that single vacancies can, in principle, host up to 9 H atoms in Mg and 10 in Al, not 12 as recently reported in the case of Al. The difference between the present results and the results of previous work was attributed to a more appropriate definition of the trapping energy of hydrogen impurities in vacancies. The concentration of hydrogen-vacancy complexes depends on the amount of hydrogen dissolved in the metal, which in turn was dictated by the hydrogen chemical potential μH. The concentrations of all of the relevant hydrogen-vacancy complexes were evaluated as a function of μH, corresponding to different H loading conditions - ranging from low pressures to high pressures of H2 gas, up to hydrogen plasma conditions. The analysis reveals fundamental differences in the characteristics of the hydrogen-vacancy interaction between Mg and Al. In the case of Al, up to 15% of H atoms were trapped in single vacancies in the form of H-vacancy complexes even for very low values of μH. The trapping effect slows down the diffusion of H atoms in Al by more than an order of magnitude. While interactions between vacancies and single hydrogen atoms were therefore clearly important, interactions with multiple H atoms and related mechanisms (such as hydrogen-induced superabundant vacancy formation) were predicted to occur in Al only at very high values of μH. In the case of Mg, the effects of H trapping in single vacancies were negligible for low values of μH due to the relatively low formation energy of isolated interstitial H. However, vacancies containing multiple H atoms and related mechanisms such as hydrogen-induced superabundant vacancy formation were predicted to occur in Mg at much lower values of μH than in Al. It was estimated that, at room temperature, the critical pressure of an H2 gas to induce hydrogen-enhanced (superabundant) vacancy formation was ∼1GPa in Mg and ∼10GPa in Al

Interactions between Hydrogen Impurities and Vacancies in Mg and Al - a Comparative Analysis Based on Density Functional Theory. L.Ismer, M.S.Park, A.Janotti, C.G.Van de Walle: Physical Review B, 2009, 80[18], 184110