Metallization of aluminum hydride at high pressures: A first-principles study

We have used first-principles density-functional-theory electronic structure methods and a random searching technique to identify stable high pressure phases of aluminum hydride $\left(\mathrm{Al}{\mathrm{H}}_3\right)$. We find a transition from the insulating low-pressure $\alpha$ phase to an insulating layered structure of space group $Pnma$ at $34\mathrm{GPa}$, and a transition to a semimetallic $Pm\overline{3}n$ phase at $73\mathrm{GPa}$. These phases are predicted to be stable against dehydridation (the evolution of ${\mathrm{H}}_2$ molecules), and they could be formed at pressures easily attainable within diamond-anvil-cell experiments.

Figure 1

(Color online) Enthalpy per $\mathrm{Al}{\mathrm{H}}_3$ unit as a function of pressure for the low enthalpy phases found in the structure search. The dotted lines indicate dehydridation into Al, AlH, or $\mathrm{Al}{\mathrm{H}}_2$, and ${\mathrm{H}}_2$.

Figure 2

(Color online) The structures of the $Pnma$ phase at $50\mathrm{GPa}$ (top), and the $Pm\overline{3}n$ phase at $80\mathrm{GPa}$ (bottom). The Al atoms are shown as dark spheres and the H atoms as light spheres. For both structures, bonds have been drawn between atoms separated by less than $1.75\mathrm{\AA }$. The layers and the Al-H-Al bridges of the $Pnma$ phase are clearly visible. The H chains and body-centered cubic lattice of Al atoms of the $Pm\overline{3}n$ can also be seen.

Figure 3

(Color online) Band structure of the $Pnma$ phase of $\mathrm{Al}{\mathrm{H}}_3$ at $80\mathrm{GPa}$, calculated at the PBE-GGA level. The thick lines indicate the H-derived electronic bands, and the thin lines indicate the mainly Al-derived bands. The bands with energies below zero are occupied, and the higher energy bands are unoccupied.

Figure 4

(Color online) Band structure of the $Pm\overline{3}n$ phase of $\mathrm{Al}{\mathrm{H}}_3$ at $80\mathrm{GPa}$, calculated at the PBE-GGA level. The thick lines indicate the H-derived electronic bands, and the thin lines indicate more diffuse, mainly Al-derived, bands. The Fermi level is at zero energy, so that the H-derived states near $M$ are unoccupied, while the mainly Al-derived states at $R$ are occupied.

Figure 5

(Color online) The electronic density of states of the $Pm\overline{3}n$ phase of $\mathrm{Al}{\mathrm{H}}_3$ at $80\mathrm{GPa}$. The Fermi energy is shown as a dotted vertical line at zero energy.

Figure 6

(Color online) Charge density slices for the $Pm\overline{3}n$ phase at $80\mathrm{GPa}$, with the H atoms shown as light spheres and the Al atoms as dark spheres. Top: total valence charge density. Middle: charge density from the unoccupied highest-energy H-derived state at the $M$ point (thick line in Fig. 4 at $0.6\mathrm{eV}$). Bottom: charge density from the Al-derived occupied state at the $R$ point (thin line in Fig. 4 at an energy of $-0.9\mathrm{eV}$).