We present a theoretical investigation on the properties of interstitial nickel and its related complexes in diamond. The atomic configurations, symmetries, spins, and energetics of interstitial nickel, nickel-boron and nickel-vacancy complexes were investigated by a total energy ab initio methodology. Those results are used to explain certain electrically active centers, commonly found in synthetic diamond.

We carried a theoretical investigation on the electronic and structural properties of titanium impurities in silicon, diamond, and silicon carbide. The calculations were performed using the spin-polarized full-potential linearized augmented plane wave method in the supercell approach. The atomic configurations and transition and formation energies of isolated Ti impurities were computed.

Isolated nickel impurities in diamond have been investigated using the spin-polarized full-potential linearized augmented plane wave total energy method. The electronic and atomic structures, symmetries, transition energies, and formation energies of substitutional and interstitial Ni impurities in diamond were computed. The results were discussed in the context of the electrically active centers in synthetic diamond.

We carried a theoretical investigation on the properties of mercuric iodide in the red tetragonal crystalline phase, and its respective intrinsic defects. Our calculations were performed using the ab initio spin-polarized full-potential linearized augmented plane wave method, and taking into account full atomic relaxation. The results on the structural, electronic, and optical properties were compared to available experimental data.

Iron-acceptor pairs (Fe-A, A = B, Al, Ga, and In) in silicon were investigated using an ionic-based model, which incorporates the valence electron cloud polarization and the lattice relaxation.Our results are generaly in good agreement with the experimental trends among the Fe-A pairs, describing the increase in the pair donor energy level with increasing A principal quantum number and decreasing pair separation distance, and the pair configurational symmetries.

A novel microscopic mechanism for hydrogen-enhanced oxygen diffusion in p-doped silicon is proposed. A path for joint diffusion of O and H is obtained from an ab-initio molecular dynamics "kick" simulation. The migration pathway consists of a two-step mechanism, with a maximum energy of 1.46 eV. This path represents a 0.54 eV reduction in the static barrier when compared with the diffusion of isolated O in Si, in excellent agreement with experiments.