TRANSFERABLE MODEL FOR THE ATOMISTIC SIMULATION OF AL2O3

We calculate the energies of different solid phases of Al2O3 using a s hell model, a compressible ion model, and a compressible ion model whi ch includes both dipole and quadrupolar polarizability of the O2- ions . Compressible oxygen ions are found to stabilize the corundum structu re with respect to the a form of alumina. The observed corundum struct ure is nevertheless unstable with respect to the bixbyite structure Un til quadripolar polarizability of the oxygen is also included. Entropy differences are estimated with a lattice dynamics calculation and are found to make an insignificant contribution to stabilizing the corund um. We also calculate the relative energies of the corundum and bixbyi te structures using the ab initio pseudopotential method, within the l ocal-density approximation for exchange and correlation. The resulting self-consistent electron density shows graphically how the quadrupola r distortions around the oxygen develop as the basis is made more comp lete (the plane-wave cutoff is increased from 500 to 700 eV), which at the same time brings the energy of the corundum structure below that of the bixbyite structure.