We use density-functional theory to investigate the bulk and surface properties of ${\mathrm{MgB}}_2$. The unique bonding structure of ${\mathrm{MgB}}_2$ is investigated by Bader's atoms-in-molecules, charge density difference, and occupancy projected band structure analyses. Oxygen adsorption on the charge-depleted surfaces of ${\mathrm{MgB}}_2$ is studied by a surface potential energy mapping method, reporting a complete map including low-symmetry binding sites. The B-terminated ${\mathrm{MgB}}_2$(0001) demonstrates reconstruction of the graphenelike B layer, and the reconstructed geometry exposes a threefold site of the subsurface Mg, making it accessible from the surface. Detailed reconstruction mechanisms are studied by simulated annealing method based on ab initio molecular dynamics and nudged elastic band calculations. The surface clustering of B atoms significantly modifies the B $2p$ states to occupy low energy valence states. The present paper emphasizes that a thorough understanding of the surface phase may explain an apparent inconsistency in the experimental surface characterization of ${\mathrm{MgB}}_2$. Furthermore, these results suggest that the surface passivation can be an important technical challenge when it comes to development of a superconducting device using ${\mathrm{MgB}}_2$.

• Received 23 December 2017

DOI:https://doi.org/10.1103/PhysRevB.97.195416