Effect of chromium on the electronic structure of the cementite Fe3C

The effect of alloying of the cementite Fe₃C with chromium on the band structure, atomic interactions, electric field gradients, and asymmetry parameters for iron nuclei is investigated using the self-consistent full-potential linear muffin-tin orbital (FPLMTO) method. An increase in the cohesive energy for the Fe₃C-Cr system indicates an enhancement of the atomic interactions in the lattice of the cementite alloyed with chromium. It is found that the substitution of chromium for iron in the FeII positions containing eight equivalent iron atoms is energetically most favorable.     

Electronic, energy, and thermal properties of the Möbius strip and related ring nanostructures of NbS3

The electronic structure and energy characteristics of the Möbius strip and related ring nanostructures of niobium trisulfide NbS₃ are investigated using the density functional tight-binding (DFTB) method. The results obtained are analyzed and compared with those obtained for the crystal and planar ribbon structures of NbS₃. The formation of NbS₃ ring structures leads to a considerable increase in the density of Nb 4d states at the Fermi level. The simulation of the behavior of the ring structures in the temperature range T = 0–700 K demonstrates that the Möbius strip has a lower thermal stability as compared to the NbS₃ ring.     

Résolution à l'infini et courbes polaires affines

Let f be a polynomial map of two variables and let l be a linear form. The affine polar curve of f, for the direction l, is the singular locus of the map $\Phi =(l,f)$. We introduce two resolutions at infinity: the small resolution Π and the total resolution ${\Pi }_{\mathrm{tot}}$. Let $L_{\infty }$ be the line at infinity. The task of this Note is to determine the intersection locus of the strict transform of the polar curve with ${\Pi }^{\mathrm{?1}}(L_{\infty })$ and ${\Pi }_{\mathrm{tot}}^{\mathrm{?1}}(L_{\infty })$. To cite this article: D. Ivanovski, C. R. Acad. Sci. Paris, Ser. I 343 (2006).     

Magnetic properties and electronic structure of the LaGaO3 perovskite doped with nickel

Solid solutions of the composition LaGa_{1 ? x} Ni _x O₃ (0.01 ≤ x ≤ 0.10) are synthesized, and their magnetic and electrical properties are investigated. It is established that the ground state of the Ni(III) atoms is the low-spin state ² E _g ; however, in the temperature range under investigation, there occurs the ² E _g ? ⁴ T _1g spin equilibrium. An increase in the nickel concentration leads to an increase in the electron conduction of the solid solutions. The band structure of the LaGa_0.5Ni_0.5O₃ model compound is calculated using the ab initio full-potential linearized augmented-plane-wave method within the generalized gradient approximation (FLAPWGGA). It is shown that the dominant role in the variations observed in the magnetic and electrical properties of the nonmagnetic semiconductor LaGaO₃ upon doping with nickel is played by the Ni 3d(e _g ↑, ↓) states.     

Band structure and properties of polymorphic modifications of lower tungsten carbide W<Subscript>2</Subscript>C

The structural and electronic properties are investigated and the relative stabilities of all the known polymorphic modifications (α, β, γ, ?) of the lower tungsten carbide W₂C are numerically estimated using the ab initio full-potential linearized augmented plane wave (FLAPW) method within the generalized gradient approximation (GGA) of the local spin density. The equilibrium parameters of the crystal lattices, the band structures, and the total and partial densities of states are determined for the first time within a unified approach. The energies of formation of the α, β, γ, and ? polymorphic modifications of the lower tungsten carbide [in the reactions W₂C ? 2W + C (graphite)] are calculated and used to discuss their relative stabilities.     

Elastic parameters of single-crystal and polycrystalline wurtzite-like oxides BeO and ZnO: Ab initio calculations

The structural and elastic parameters (elastic constants; bulk, shear, and Young’s moduli; Poisson’s ratios; and Lamé coefficients) for ideal wurtzite-like beryllium and zinc monoxides are calculated by the full-potential linearized augmented-plane-wave (FLAPW) method with the WIEN2k code. These parameters are approximated to those for polycrystalline oxides BeO and ZnO in the framework of the Voigt-Reuss-Hill model. The results of calculations are compared with the available experimental data and used to estimate numerically the velocities of sound and the Debye temperatures for BeO and ZnO polycrystals.     

Nearest-neighbour-interaction model in the coupled-optical-phonon-mode theory of the infrared dispersion in monoclinic crystals: Application to Tutton salt single crystal

The coupled-optical-phonon-mode theory of Barker and Hopfield of two mode interaction in isotropic crystals has been extended to monoclinic crystals. The analytical expressions for the calculation of the dielectric tensor elements in the ac crystal plane have been derived. It has been shown that the interaction dielectric model is a generalized expression of the dielectric tensor for monoclinic case when no interaction between modes is present. Also, the results of Barker and Hopfield are obtained from this more general theory, when an isotropic case is considered. In order to be able to investigate real crystals, meaning extending the interactions to a large number of pairs of modes, but at the same time to make the fitting procedure possible, a model dielectric function taking into account the interaction between modes with closest frequencies has been derived. The validity of the model obtained has been tested on a Tutton salt single crystal of K₂Co(SO₄)₂·6H₂O. The recorded spectra from the ac crystal plane were fitted in order to obtain best fit parameters. The comparison between the experimentally recorded spectra and the model reflectance function give good results and verify this model to be applicable.     

Magnetization of beryllium oxide in the presence of nonmagnetic impurities: Boron,carbon, and nitrogen

The electronic and magnetic states of a nonmagnetic insulator, namely, beryllium oxide, doped with nonmagnetic 2p elements (boron, carbon, and nitrogen) are studied using the density functional theory. The spin polarization of the 2p impurity states, as well as the transition of the doped BeO:(B,C,N) systems to the states of semiconducting or half-metallic magnets, is observed. The prospects for creating new magnetic materials by doping nonmagnetic insulators with nonmagnetic p impurities are discussed.     

The perturbation theory model of a spherical oscillator in electric field and the vibrational stark effect in polyatomic molecular species

The effect of external electrostatic fields on the spherical oscillator energy states was studied using stationary perturbation theory. Besides the spherical oscillator with ideal symmetry, also a variety of the deformed systems were considered in which the deformations may be induced by the external fields, but also by the short-range crystal lattice forces. The perturbation theory analysis was carried out using the field-dependent basis functions. Predicted spectral appearances and band splittings due to the deformations and external field influences were shown to be helpful in interpreting the experimental spectra of molecular oscillator possessing subsets of mutually orthogonal triply degenerate normal modes (such as, e.g. tetrahedral species). To verify the results of the perturbation theory treatments, as well as to provide a further illustration of the usefulness of the employed technique, a numerical HF/aug-cc-pVTZ study of the vibrational states of methane molecule in external electrostatic field was performed.     

Scattering and bound states of a relativistic neutral spin-1/2 particle in a magnetic field

By using the Dirac equation we have investigated the scattering and bound states of a relativistic spin-1/2 particle in the system ofN magnetic barriers (or magnetic wells) andN magnetic delta functions. Using theM-matrix method we have studied tunnelling with spin and band structure.