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 (α–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 estimate commonly used for ionic solids. © 1994 John Wiley & Sons, Inc.