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.

The structural and electronic properties of the neutral and positively charged oxygen vacancies (F and F + centres) in the bulk and on the (001) surfaces of SrTiO3 crystal are examined within the hybrid Hartree-Fock and density functional theory (HF-DFT) method based upon the linear combination of atomic orbital (LCAO) approach. A comparison of the formation energy for surface and bulk defects indicates a perceptible propensity for the segregation of neutral and charged vacancies to both SrO and TiO2 surface terminations with a preference in the latter case which is important for interpretation of space charge effects at ceramic interfaces. It is found that the vacancies reveal more shallow energy levels in the band gap on surfaces rather than in the bulk, in particular, on the TiO2 surface. The charged F + centre has significantly deeper energy levels both in bulk and on the surfaces, as compared with the neutral F centre.

The structural and electronic properties of Cu 2 O have been investigated using the periodic Hartree-Fock method and a posteriori density-functional corrections. The lattice parameter, bulk modulus, and elastic constants have been calculated. The electronic structure of and bonding in Cu 2 O are analyzed and compared with x-ray photoelectron spectroscopy spectra, showing a good agreement for the valence-band states. To check the quality of the calculated electron density, static structure factors and Compton profiles have been calculated, showing a good agreement with the available experimental data. The effective electron and hole masses have been evaluated for Cu 2 O at the center of the Brillouin zone. The calculated interaction energy between the two interpenetrated frameworks in the cuprite structure is estimated to be around Ϫ6.0 kcal/mol per Cu 2 O formula. The bonding between the two independent frameworks has been analyzed using a bimolecular model and the results indicate an important role of d 10 -d 10 type interactions between copper atoms. ͓S0163-1829͑97͒01735-9͔

We compare two approaches to the atomic, electronic, and magnetic structures of LaMnO 3 bulk and the ͑001͒, ͑110͒ surfaces-hybrid B3PW with optimized LCAO basis set ͑CRYSTAL-2003 code͒ and GGA-PW91 with plane-wave basis set ͑VASP 4.6 code͒. Combining our calculations with those available in the literature, we demonstrate that combination of nonlocal exchange and correlation used in hybrid functionals allows to reproduce the experimental magnetic coupling constants J ab and J c as well as the optical gap. Surface calculations performed by both methods using slab models show that the antiferromagnetic ͑AF͒ and ferromagnetic ͑FM͒ ͑001͒ surfaces have lower surface energies than the FM ͑110͒ surface. Both the ͑001͒ and ͑110͒ surfaces reveal considerable atomic relaxations, up to the fourth plane from the surface, which reduce the surface energy by about a factor of 2, being typically one order of magnitude larger than the energy difference between different magnetic structures. The calculated ͑Mulliken and Bader͒ effective atomic charges and the electron density maps indicate a considerable reduction of the Mn and O atom ionicity on the surface.

For ab initio simulations on hexagonal single-wall BN and TiO 2 nanotubes (SW NTs), we have applied the formalism of line symmetry groups describing one-periodic (1D) nanostructures with rotohelical symmetry. Both types of NTs can be formed by rolling up the stoichiometric nanosheets of either (i) a (0001) monolayer of BN hexagonal phase or (ii) a three-layer (111) slab of fluorite-type TiO 2 phase. Optimized parameters of the atomic and electronic structure of corresponding slabs and nanotubes have been calculated using hybrid LCAO method as implemented in CRYSTAL code. Their band gaps (∆ε gap ) and strain energies (E strain ) have been analyzed as functions of NT diameter (D NT ). For hexagonal BN and TiO 2 nanotubes, certain qualitative similarities between the ∆ε gap (D NT ) or E strain (D NT ) functions exist despite the different chemical nature.

No abstract