Formation and migration of charged native point defects in ${\text{MgH}}_2$: First-principles calculations

Using first-principles calculations we have investigated the possible native point defects in bulk ${\text{MgH}}_2$. 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 ($V_{\text{H}}^{+}$ and $V_{\text{H}}^{-}$). Under extreme H-rich conditions, the lowest-energy defects are $V_{\text{H}}^{+}$, negatively charged hydrogen interstitials $\left({\text{H}}_i^{-}\right)$, and negatively charged Mg vacancies $V_{\text{Mg}}^{2-}$. 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 ${\text{H}}_i^{-}$ and ${\text{H}}_{2i}$, and a highest barrier of 0.63 eV for $V_{\text{H}}^{-}$. 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.

Figure 1

(Color online) Calculated band structure of ${\text{MgH}}_2$. The inset shows the crystal structure of rutile ${\text{MgH}}_2$; large spheres are Mg, small spheres are H atoms.

Figure 2

Formation energies as a function of Fermi level in bulk ${\text{MgH}}_2$ for native point defects, ${\text{H}}_i$, ${\text{H}}_{2i}$, $V_{\text{H}}$, and $V_{\text{Mg}}$, in various charge states under (a) H-poor and (b) H-rich conditions.

Figure 3

(Color online) Structural rearrangements around charged native point defects in ${\text{MgH}}_2$ and schematic diagrams of single-particle states and electron occupations. (a) $V_{\text{H}}^{+}$, (b) $V_{\text{H}}^0$, (c) $V_{\text{H}}^{-}$, (d) ${\text{H}}_i^{+}$, (e) ${\text{H}}_i^{-}$, and (f) $V_{\text{Mg}}^{2+}$.

Figure 4

(Color online) Ball-and-stick model of ${\text{MgH}}_2$; specific hydrogen atoms are labeled to aid in describing diffusion paths of ${\text{H}}_i^{+}$, ${\text{H}}_i^{-}$, $V_{\text{H}}^{+}$, and $V_{\text{H}}^{-}$. Site 1 is the location of a hydrogen interstitial in the negative charge state.