Point-defect kinetics in α- and γ-MgH2

The kinetics of hydrogen desorption from storage materials in principle depend on the crystalline phase of the material. In MgH2, desorption rates may be higher in the crystalline γ phase compared to the equilibrium bulk α phase. It has been suggested [R. A. Varin, T. Czujko, Z. Wronski, Nanotechnol., 17 (15) (2006) 3856-3865] that this effect is responsible for enhanced desorption from ball-milled MgH2, since smaller particles contain a higher proportion of the metastable γ phase. We investigate hydrogen transport kinetics in these phases of MgH2 by using first-principles calculations based on density functional theory. Imposing charge neutrality, we find that the formation energy of hydrogen vacancies in γ-MgH2 is smaller by 0.032 eV compared to α-MgH2. Our calculations of migration barriers show that the only relevant point defect for mass transport in both crystal structures is the positively charged hydrogen vacancy, and that its lowest migration barrier in γ-MgH2 is 0.02 eV lower than in α-MgH2. We conclude that hydrogen vacancies exist in higher concentrations and are also more mobile in the γ phase than in the α phase, thus explaining the faster dehydrogenation kinetics of γ-MgH2.