Density functional theory studies of the hydrogenation properties of Mg and Ti

Absorption energies of hydrogen in Mg and Ti as a function of the hydrogen concentration were calculated using density functional theory. We investigated hydrogen absorbed in metal hosts with different structures (fcc, hcp, and bct for Mg; hcp and fcc for Ti). The most stable configurations were determined for different hydrogen concentrations. Rutile and fluorite structures are found to be the most stable for Mg and Ti hydrides, respectively. Preference of hydrogen filling up the interstices of the metal hosts, and crystal lattice transformations and distortions were also investigated. Hydrogen atoms prefer to pair up and form clusters in Mg; but hydrogen atoms like to occupy sites which are apart as far as possible in Ti. The differences in the hydrogenation behavior of Mg and Ti were compared and analyzed using the electron density. The hydrogenation behaviors can be related to bonding characteristic of Mg and Ti hydrides. Mg hydride is more ionic than Ti hydride.

Figure 1

fcc unit cell for Mg and Ti with four metal atoms. The big spheres represent metal and the small spheres represent hydrogen atoms. Labels a through d are positions of octahedral sites, and e to l are positions of tetrahedral sites.

Figure 2

hcp unit cells for Mg and Ti with four metal atoms as used in the calculations. The crystallographic unit cell is doubled in the direction of the $a$ axis. The big spheres represent metal and the small spheres represent hydrogen atoms. Label a through d are positions of octahedral sites, and e to l are positions of tetrahedral sites. The crystallographic $c$ axis is in the vertical direction.

Figure 3

bct unit cell for Mg with four metal atoms as used in the calculations. The crystallographic unit cell is doubled in the direction of the $a$ axis. The big spheres represent metal and the small spheres represent hydrogen atoms. The crystallographic $c$ axis is in the vertical direction.

Figure 4

Hydride formation energy (a) and volume (b) of Mg hydride (four metal unit cell) as a function of hydrogen concentration. The volume is in ${\text{Å}}^3$.

Figure 5

Formation energy (a) and volume (b) of Ti hydride (four metal unit cell) as a function of hydrogen concentration. The “O” and “T” indicate that hydrogen atoms absorb first in octahedral and tetrahedral sites, respectively. The volume is in ${\text{Å}}^3$.

Figure 6

Valence electron density for Mg (top) and Ti (bottom) fluorite hydride in cross sections parallel to (100). On the left are electron densities in a plane with only metal atoms; on the right are electron densities in a plane with only hydrogen atoms. The contour lines are drawn from 0 to 1.72 (1.74) for Mg (Ti) hydride at $0.22e/{\text{Å}}^3$ intervals.