Figure 1

(Color online) Calculated formation energies of relevant defects and impurities in $\mathrm{Na}\mathrm{Al}{\mathrm{H}}_4$ as a function of Fermi level. The vertical lines denote the Fermi-level position determined by charge neutrality. (a) Hydrogen-related defects. Equilibrium with ${\mathrm{H}}_2$ molecules at $T=0$ is assumed. (b) Ti-related (red) and Zr-related (blue) defects. For Ti and Zr, the chemical potentials were fixed to the energy of the bulk metals and Al-rich conditions were assumed.Reuse & Permissions

Figure 2

(Color online) Lattice relaxations induced by hydrogen-related defects and impurities in $\mathrm{Na}\mathrm{Al}{\mathrm{H}}_4$. The numbers indicate coordination of (distorted) $\mathrm{Al}{\mathrm{H}}_x$ complexes (in green). Background undistorted tetrahedral complexes were removed for clarity. (a) Positively charged hydrogen vacancy $V_{\mathrm{H}}{}^{+}$. Removal of a hydrogen atom (indicated by the open circle) from an $\mathrm{Al}{\mathrm{H}}_4$ complex leads to the formation of a planar $\mathrm{Al}{\mathrm{H}}_3$ complex; the Al atom in this complex moves toward the neighboring $\mathrm{Al}{\mathrm{H}}_4$, partially attracting one of its H atoms and resulting in the formation of a second $\mathrm{Al}{\mathrm{H}}_3$ complex with planar geometry. Structural rearrangements in the case of $V_{\mathrm{H}}{}^{-}$ (not shown here) are much smaller. (b) The positively charged hydrogen interstitial $\left(\mathrm{H}_i{}^{+}\right)$ decays into a ${\mathrm{H}}_2$ molecule and a positively charged hydrogen vacancy. (c) Negatively charged hydrogen interstitial $\mathrm{H}_i{}^{-}$. One Al atom becomes fivefold coordinated, and a distorted $\mathrm{Al}{\mathrm{H}}_4$ is evident. (d) Substitutional Ti on an Al site $\left(\mathrm{Ti}_{\mathrm{Al}}{}^{+}\right)$ causes large local lattice rearrangements involving the surrounding $\mathrm{Al}{\mathrm{H}}_4$ complexes.Reuse & Permissions