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RESEARCH PRODUCT

Modeling of Point Defects in Corundum Crystals

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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[sub K] centers in alkali halides (two-site model); their formation is energetically favorable. The energy required for 60[degree] 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 quantum-chemical 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 1,550 C. Theoreticalmore » predictions of the Arrhenius energy for diffusion at high temperatures are also in excellent agreement with Oishi and Ando's experimental values above 1,590 C.« less