Hartree-fock ab-initio characterization of ionic crystal surfaces with a slab model. The (0001) face of α-Al2O3

Abstract

The adequacy of a slab model in ab-initio calculations of ionic crystal surfaces is critically discussed. The role of electrostatic terms is first analysed through the examples of the (001) and (011) faces of MgO, and the (0001) face of α-alumina. It is shown that if a few repeat units parallel to the exposed face are included in the slab, the electrostatic field due to the missing semi-infinite crystal is negligible at the opposite face of the slab, provided that the repeat unit has no net dipole perpendicular to the face. As a consequence, all important electrostatic effects are accurately taken into account in a slab model. For testing “chemical” effects at the two slab endings and their possible interdependence, calculations have been effected for α-alumina slabs of different thickness, using an Hartree-Fock periodic approach, and adopting a minimal STO-3G basis set for the involved atoms. It is shown that the distance of surface A9 atoms from the layer of underlying O atoms is considerably decreased (by more than 0.2 Å) with respect to the unrelaxed value. This relaxation entails important changes in the surface formation energy and in the location of surface states, but these effects are almost independent of slab thickness. It is argued that the thin film model is adequate for the study of ionic crystal surfaces, and improving the quality of the basis set is more important than increasing the slab thickness beyond a few layers.