FIRST-PRINCIPLES CALCULATIONS FOR NIOBIUM ATOMS ON A SAPPHIRE SURFACE

We report first-principles, self-consistent calculations of the equilibrium structure, electron density, and total energy of a niobium monolayer on a sapphire (0001) surface. The calculations, which are based on the density functional theory in the local density approximation, use norm-conserving pseudopotentials and a basis of plane waves. We work with a slab which is three Al-O-Al layers in thickness and we assume that the surface Al atoms are replaced by Nb. All the atomic positions are relaxed to minimize the total energy. In contrast to the clean surface, on which the Al atoms relax inwards almost to the level of the surface oxygen, the Nb atoms relax only slightly inwards from the sites where the next Al atoms would sit if the bulk structure were extended. The Nb atoms are partially ionized and there is strong directional bonding, due to the hybridization of d-orbitals on Nb with the oxygen p-orbitals. The work of adhesion is 13 eV per Nh atom.