Molecular dynamics study of high-pressure alumina polymorphs with a tight-binding variable-charge model

Abstract

A tight-binding variable-charge model aimed at performing large-scale realistic simulations of bulk, surfaces and interfaces of aluminum oxides have been developed. This model is based on the charge equilibration (QEq) method and explicitly takes into account the mixed iono–covalent character of the metal–oxygen bond by means of a tight-binding analytical approach in the second-moment approximation of the electronic structure. The parameters of the model were optimized to reproduce structural and energetic properties of the α-Al2O3 corundum structure at room temperature and pressure. The model exhibits a good transferability between five alumina polymorphs: corundum, Rh2O3(II)-type, perovskite (Pbnm), CaIrO3-type and U2S3-type structures. In this paper, we present results obtained by molecular dynamics for pressure ranging from 0 to 500GPa. First, static relaxations reproduce satisfactorily experimental and ab initio results concerning the stability domains and transitions from corundum to Rh2O3(II) and then to CaIrO3-type structure when pressure increases. At higher pressure, the transition from CaIrO3 to U2S3 structure is also observed at a pressure significantly lower than that given ab initio. Molecular dynamics confirm these results and also predict a phase transition at about 400GPa from corundum to a triclinic structure.