We present the results of room-and low-temperature measurements of second-order Raman scattering for perfect GaN and AlN crystals as well as the Raman-scattering data for strongly disordered samples. A complete group-theory analysis of phonon symmetry throughout the Brillouin zone and symmetry behavior of phonon branches, including the analysis of critical points, has been performed. The combined treatment of these results and the lattice dynamical calculations based on the phenomenological interatomic potential model allowed us to obtain the reliable data on the phonon dispersion curves and phonon density-of-states functions in bulk GaN and AlN. ͓S0163-1829͑98͒06840-4͔

Periodic density functional theory (DFT) based on plane waves (PW) and Hartree−Fock (HF) based on the linear combination of atomic orbitals (LCAO) calculations using slabs separated by vacuum gaps were carried out to model the H2O−TiO2 (rutile) (110) interface. Positions of all atoms were allowed to relax except atoms in the central layer of the slab. Both associative and dissociative adsorption mechanisms were considered for half-monolayer and monolayer coverages. Five different orientations of H2O molecules on the TiO2 surface were studied to determine the most energetically favorable water positions for the associative mechanism. Two slab thicknesses (three Ti layers and five Ti layers) were chosen to test the effect of slab depth on calculated surface structures and adsorption energies. Results indicate that associative adsorption is favorable by −8 to −20 kJ/mol/H2O depending on the slab thickness for full-monolayer coverage. Embedded cluster HF calculations were also performed for comparison. Adsorption energies of H2O in the embedded cluster case are much more favorable for the associative mechanism. The role of H-bond formation on the adsorption energies and structures is discussed.

The atomic and electronic structure, formation energy, and the energy barriers for migration have been calculated for the neutral O vacancy point defect ͑F center͒ in cubic SrTiO 3 employing various implementations of density functional theory ͑DFT͒. Both bulk and TiO 2 -terminated ͑001͒ surface F centers have been considered. Supercells of different shapes containing up to 320 atoms have been employed. The limit of an isolated single oxygen vacancy in the bulk corresponds to a 270-atom supercell, in contrast to commonly used supercells containing ϳ40-80 atoms. Calculations carried out with the hybrid B3PW functional show that the F center level approaches the conduction band bottom to within ϳ0.5 eV, as the supercell size increases up to 320 atoms. The analysis of the electronic density maps indicates, however, that this remains a small-radius center with the two electrons left by the missing O ion being redistributed mainly between the vacancy and the 3d͑z 2 ͒ atomic orbitals of the two nearest Ti ions. As for the dynamical properties, the calculated migration energy barrier in the low oxygen depletion regime is predicted to be 0.4 eV. In contrast, the surface F center exhibits a more delocalized character, which leads to significantly reduced ionization and migration energies. Results obtained are compared with available experimental data.