The extent of relaxation of the alpha-Al2O3 (0001) surface and the reliability of empirical potentials

The geometrical structure of the polar, Al-terminated surface of corundum h as been investigated by different theoretical methods ranging from empirica l pair potentials to periodic Hartree-Fock or density functional theory cal culations. The different methods agree in predicting a structure for the bu lk, which is close to the experimental one, and an inward relaxation of mor e than 50% in agreement with experiment. However, a rather strong disagreem ent exists concerning the quantitative amount of relaxation of the surface layers. Accurate density functional theory calculations for slab models con sidering up to 18 layers with 30 atoms in the unit cell strongly suggest th at the degree of relaxation of the outermost Al atomic layer is much larger than the values obtained by other methods. Reasons for the different descr iption are given and implications for the use of pair potentials discussed. (C) 2000 Elsevier Science B.V. All rights reserved.