Modeling of Point Defects in Corundum Crystals

Several different approaches including Hartree–Fock ab initio cluster calculations, semiempirical INDO calculations, and atom–atom potentials were used for modeling of the spatial and electronic structure as well as migration mechanisms of both intrinsic defects (self‐trapped and defect‐trapped holes, O and Al vacancies) and impurities (transition‐metal Ions like Co, Fe, Mg, Mn, Ti). The atomic structure of all hole centers is found to be similar to V_K centers in alkali halides (two‐site model); their formation is energetically favorable. The energy required for 60° hole reorientations inside the basic oxygen triangles is found to be similar to both the energy for hops between such triangles and the experimental activation energy for self‐trapped hole migration (0.7 eV). A novel mechanism of hole polaron motion in ionic solids is presented on the basis of quantumchemical cluster calculations. The role of clustering in the solution of impurities is shown to be crucial. Lastly, five kinds of O vacancy hops are simulated. In several cases the activation energy is lowered considerably when the hopping ion is allowed to deviate from a straight path. Theory predicts the lowest activation energy to be 1.85 eV, in excellent agreement with the value observed experimentally below 1550°C. Theoretical predictions of the Arrhenius energy for diffusion at high temperatures are also in excellent agreement with Oishi and Ando's experimental values above 1590°C.