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 MgH2(110) would be energetically favorable.