Ab initio calculations on the Al2O3(0001) surface

We calculate using a density functional pseudopotential method the atomic and electronic structure of the (0001) surface of alpha-alumina (Al2O3). The material is studied in the form of a slab with periodic boundary conditions, containing up to eight layers of the stoichiometric Al2O3 units. Five different terminations of the surface are calculated, representing different surface excesses of oxygen, and their free energies are estimated as a function of oxygen partial pressure. Internal relaxations of the atomic positions are obtained. The aluminium terminated surface, which is stoichiometric, has the lowest surface energy for a wide range of oxygen pressures.