Theoretical simulations of the radiation-induced defect processes in insulating materials

The results of two basic kinds of computer simulations of radiation-induced processes in insulating materials, one based on quantum-mechanical and pair-potential (atomistic) approaches, and the other a phenomenological theory of diffusion-controlled reactions, are presented. It is shown that, by combining different techniques (atom-atom potentials and semi-empirical quantum chemical methods) the optimized geometry and the electronic structure of a family of hole centres in crystalline corundum (α-Al₂O₃) could be found. Their energetics are analyzed; V^2-, V^-V^-_Mg hole centres all have a common basic element, namely the diatomic molecule O₂^3-, which is responsible for their similar absorption energies. Our calculations provide evidence for the existence of a small-radius two-centre polaron (self-trapped hole, or STH) with an optical absorption energy of around 2.9 eV. The strong covalent bonding of the two O atoms sharing a hole makes this centre analogous to the V_K centre in alkali halides. The calculated activation energy for the STH hops (≈ 0.9 eV) between different O triangles is close to the experimental.