FIRST-PRINCIPLES CALCULATIONS FOR NIOBIUM ATOMS ON A SAPPHIRE SURFACE

We report first-principles, self-consistent calculations of the equili brium structure, electron density, and total energy of a niobium monol ayer on a sapphire (0001) surface. The calculations, which are based o n the density functional theory in the local density approximation, us e 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 su rface, on which the Al atoms relax inwards almost to the level of the surface oxygen, the Nb atoms relax only slightly inwards from the site s where the next Al atoms would sit if the bulk structure were extende d. 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.