Formation and migration of charged native point defects in MgH2: First-principles calculations

Using first-principles calculations we have investigated the possible native point defects in bulk MgH2. Due to the interest in this material for hydrogen storage, we have paid particular attention to hydrogen-related defects that are likely to be involved in the absorption and release kinetics of hydrogen. We have considered neutral and charged defects and calculated formation energies as a function of Fermi-level position and hydrogen chemical potential. In the absence of impurities, we find that under extreme H-poor conditions the lowest-energy defects are positively and negatively charged hydrogen vacancies (V-H(+) and V-H(-)). Under extreme H-rich conditions, the lowest-energy defects are V-H(+), negatively charged hydrogen interstitials (H-i(-)), and negatively charged Mg vacancies V-Mg(2-). The defects are characterized by unusually large local structural rearrangements. The hydrogen-related defects are also highly mobile, with a lowest migration barrier of less than 0.10 eV for H-i(-) and H-2i, and a highest barrier of 0.63 eV for V-H(-). By combining the calculated formation energies with migration barriers, we find that the lowest activation energy for self-diffusion is about 1.48 eV under H-poor conditions. The consequences of these results for the hydrogenation and dehydrogenation kinetics are discussed.