Quantum chemical simulations of hole self-trapping in corundum

Microscopic quantum chemical calculations and simulations based on atom-atom potentials have been undertaken for hole self-trapping in pure corundum ( alpha -Al 2 O 3 ) crystals. A comparison of different modes of ionic relaxation during hole trapping has shown that the inward Jahn-Teller 40% displacement of two O ions accompanied by the 20% outward displacement of the two nearest Al ions is energetically the most favourable. Eighty per cent of the hole density is concentrated on these two O ions, thus confirming that a small-radius two-site polaron model similar to that for alkali halides (the V K centre), is applicable here. The calculated absorption energy of the STH (2.9 eV) is close to that observed experimentally.