Atomic hydrogen adsorption and incipient hydrogenation of the Mg(0001) surface: A density-functional theory study

TL;DR

This study uses density-functional theory to analyze hydrogen adsorption on Mg(0001), revealing stable adsorption sites, energetics, and the formation of a hydrogenated MgH2-like structure, advancing understanding of magnesium surface hydrogenation.

Contribution

It provides a comprehensive first-principles analysis of hydrogen adsorption sites, energetics, and structural transformations on Mg(0001), including the formation of a MgH2-like phase during surface hydrogenation.

Findings

On-surface fcc site is most stable for hydrogen adsorption.

Hydrogen tends to form islands and clusters at higher coverage.

A stable H-Mg-H sandwich structure resembles MgH2 in electronic properties.

Abstract

We investigate the atomic hydrogen adsorption on Mg(0001) by using density-functional theory within the generalized gradient approximation and a supercell approach. The coverage dependence of the adsorption structures and energetics is systematically studied for a wide range of coverage and adsorption sites. In the coverage range $0<\Theta<1.0, the most stable among all possible adsorption sites is the on-surface fcc site followed by the hcp site, and the binding energy increases with the coverage, thus indicating the higher stability of on-surface adsorption and a tendecy to the formation of H islands (clusters) when increasing the coverage within the region $0<\Theta<1.0. The on-surface diffusion path energetics of atomic hydrogen, as well as the activation barriers for hydrogen penetration from the on-surface to the subsurface sites, are also presented at low coverage. At high coverage of $1.00<\Theta<2.0, it is found that the coadsorption configuration with 1.0 monolayer of H's residing on the surface fcc sites and the remaining monolayer of H's occupying the subsurface tetra-I sites is most energetically favourable. The resultant H-Mg-H sandwich structure for this most stable coadsorption configuration displays similar spectral features to the bulk hydride MgH$_{2}$ in the density of states. The other properties of the H/Mg(0001) system, including the charge distribution, the lattice relaxation, the work function, and the electronic density of states, are also studied and discussed in detail. It is pointed out that the H-Mg chemical bonding during surface hydrogenation displays a mixed ionic/covalent character.