AVS 49th International Symposium
    Surface Science Tuesday Sessions
       Session SS3-TuA

Invited Paper SS3-TuA3
Calculations of the Free Energy and Structure of Alpha-Alumina (0001)

Tuesday, November 5, 2002, 2:40 pm, Room C-112C

Session: Metal/Oxide Surfaces
Presenter: M. Finnis, Queens University, UK
Authors: M. Finnis, Queens University, UK
A. Marmier, Queens University, UK
Correspondent: Click to Email
We report a study of the excess free energy of this surface using two different shell models (S1 and S2) and an ab initio (AI) model in which the total energy was obtained from self-consistent pseudopotential calculations using the local density approximation. We apply the quasi-harmonic approximation to estimate the phonon contribution to the free energy. The surface energies with S1 and S2 bracket the AI result. They also show that only two k-points are needed to sample the phonons in the Brillouin zone with good accuracy. A six layer slab is thick enough for obtaining all contributions to the surface free energy, which converges faster with slab thickness than the mean square amplitudes of atomic vibration, and in fact the latter diverges for a finite slab. Anharmonicity is insufficient to account for the discrepancy beween calculated and experimental surface relaxations. A stoichiometric Al-terminated surface is predicted to be stable over all but very low (close to decomposition of the oxide) and very high (> 1 atmosphere) oxygen partial pressures, in agreement with experiment.