QUANTUM-CHEMICAL SIMULATIONS OF HOLE SELF-TRAPPING IN SEMI-IONIC CRYSTALS

A novel formalism is presented for reliable calculations of the energetics of hole self-trapping in semi-ionic solids with mixed valence bands. Unlike previous model-Hamiltonian-type approaches, it is based on self-consistent quantum chemical INDO simulations of the atomistic and electronic structure of a self-trapped hole, making no a priori assumptions about a particular form of its localization (if any). This formalism is applied to the problem of hole self-trapping in corundum crystals (alpha - Al2O3). The hole self-trapping is found to be energetically favorable in the form of a diatomic O2 molecule with strong covalent bonding quite similar to the self-trapped hole (V(K)-center) in alkali halides. The so-called localization energy (i.e., the energy that is required to localize the Bloch-like wave packet of the free hole on the molecule, as the first stage of further trapping) is essentially less than one-half of the upper valence band width, which is the commonly used for ionic solids. (C) 1994 John Wiley & Sons, Inc.