First-principles study of hydrogen absorption on Mg(0001) and formation of magnesium hydride

The hydrogen absorption on Mg(0001) surfaces, penetration into the subsurface region, and transition toward magnesium hydride up to six monolayer absorption is studied by density-functional theory calculations. The favorable absorption sites are identified, and average absorption energies and their dependence on the coverage are calculated and analyzed by projected density of states. It is found that at lower absorption (less than one monolayer), H atoms prefer to adsorb at on-surface fcc sites, and the bonding strength increases with the absorption due to the enhanced hybridization between H and Mg substrates. We find that the H absorption in the subsurface region is energetically unfavorable until full monolayer H absorption on the Mg(0001) surfaces. After this, H absorption in the subsurface region becomes energetically and kinetically favorable, and forms a stable locale H-Mg-H trilayer (so-called surface hydride with local uptake of two monolayers). It is found that the H-Mg-H trilayers interact weakly with Mg substrates underneath and grow steadily by stacking with each other at constant average absorption energy. The H-Mg-H trilayers is proposed to be the precursors of the formation of magnesium hydride. It is found that when the number of the H-Mg-H trilayers is over three (six monolayer for overall uptake), the transition to the ${\text{MgH}}_2\left(110\right)$ would be energetically favorable.

Figure 1

Hydrogen absorption sites on Mg(0001). The upper plot is the top view of Mg(0001) surface and different absorption sites are indicated: a, b, c, and d represent on-surface top, bridge, fcc, and hcp hollow sites, respectively. The lower three plots are the side views for possible subsurface H sites below the first Mg layer, which are (e) octa, (f) tetraI, and (g) tetraII, respectively. The dashed line shows the surface unit cell used in the calculation.

Figure 2

(Color online) Calculated absorption energy for H on fcc (square) and hcp (circle) sites at coverage less than 1 ML and favorable structures with presence of both on-surface and subsurface hydrogen fcc/tetraII and hcp/octa (see details in the main context) at coverage from 1 to 2 ML. The solid lines are used to guide the eye.

Figure 3

(a) PDOS for Mg atoms for clean surface, and absorbed H atoms (solid line) and surface Mg atoms $3s$ (dash line) and $3p$ (dot line) bands at (b) 0.25, (c) 0.50, (d) 0.75, and (e) 1.00 ML, respectively.

Figure 4

The schematic model for the (a) fcc/octa, (b) fcc/tetraI, (c) fcc/tetraII, and (d) hcp/octa at ${\theta }_{\text{total}}=2\text{ML}$, the upper parts are the top views and the lower parts are the side views. The gray gradient is used to denote the distance to surface, the lighter the closer to the surface. Small circles are H atoms, while big circles are Mg atoms. The dashed line shows the surface unit cell used in the calculation

Figure 5

PDOS for on-surface fcc H atom (H1, dash-dot-dash line) and subsurface tetraII H atom (H2, dash-dot-dot-dash line) and surface Mg atoms (solid line) for fcc/tetraII configuration at overall absorption of (a) 1.25, (b) 1.5, (c) 1.75, and (d) 2 ML, respectively.

Figure 6

Energy (per Mg atom) required to lift off a complete Mg layer from (a) Mg(0001) substrate underneath for clean surface, (b) for H-Mg-H (fcc/tetraII) trilayer with ${\theta }_{\text{total}}=2\text{ML}$, and (c) for fcc/tetraII-octa configuration with ${\theta }_{\text{total}}=3\text{ML}$. The big circles represent Mg atoms and the small circles represent H atoms. The gray gradient shows how far the atoms are from the surface.

Figure 7

PDOS of H-Mg-H trilayers with coverage of 2, 3, and (a) in rutile ${\text{MgH}}_2$, and of surface H with coverage of 1 ML and (b) the third H layer where the total coverage is 3 M.

Figure 8

Average absorption energies of H-Mg-H trilayers (square) and ${\text{MgH}}_2\left(110\right)$ orientated rutile layers (circle) as a function of numbers of layers in the stack. The dotted line indicates the calculated formation energy of bulk ${\text{MgH}}_2$, which is the limit approached by the rutile curve for the infinite thick slab.