Crystalline Geometry Research Articles

Determination of the crystalline structure of nonlinear molecules in a combined quantum chemical and classic lattice potential description

AbstractThe general problem of the prediction of the crystal parameters and molecular structure of nonlinear molecules is taken up in a treatment that couples the molecular electronic structure determination to the surrounding crystalline lattice field. The molecular electronic structure method used may be at any level of sophistication, but we used the Hartree–Fock approximation in this application. A partial charge analysis of the molecular electronic structure is performed to establish the values of atom‐centered partial charges that interact via Coulomb's law, along with a Lennard–Jones potential between all atoms to establish a crystalline lattice field. The Lennard–Jones parameters are taken from the widely used AMBER force field. The lattice field terms are included in the molecular Hamiltonian and exert forces on the nuclei, leading to a somewhat different equilibrium structure than is obtained for the isolated molecule. The positions, orientations, and unit cell parameters are optimized for the lattice field, leading to a full determination of the crystal structure, including the space group. Results for sulfur dioxide, ammonia, and water are reported. In each case an equilibrium crystalline geometry faithfully reproducing the experimentally determined space group is obtained. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003

Fourier transform infrared spectroscopy study of nanostructured nickel oxide

Nanostructured nickel oxide having different average particle sizes ranging from 3 to 16 nm were synthesized and Fourier transform infrared (FTIR) spectra of the samples were recorded in the far infrared (IR) region. The spectra were found to be dominated by surface mode absorptions with no distinct absorption corresponding to the bulk transverse optical mode. IR absorption coefficient, α, for the nanostructured NiO samples were calculated as a function of frequency using a macroscopic approach devised by Fuchs. The effects of crystalline geometry, numerical values of optical constants, filling factor and increased damping on the spectral features of the samples were analyzed. Though the simulations approximately reproduced the occurrence of a shoulder in the experimental spectra, the most intense peak in the simulated spectra was found to be about 50 cm −1 above the corresponding experimentally observed peak. It was shown that the experimentally observed absorption maximum of all the samples were in close agreement with that determined using a microscopic theory based on the rigid ion model. The weak absorption peaks in the frequency region 60–100 cm −1 appearing in the spectra of all the samples were identified as surface induced transverse acoustical modes, ω TA, which became IR active due to the breakdown of translational symmetry in the nanocrystallites.

Vibrational properties of amorphous GaN

The vibrational densities-of-states (DOS) of crystalline hexagonal GaN and a simulated amorphous GaN (a-GaN) structure are calculated. Neutron scattering spectra of simulated amorphous GaN structures are also calculated. Our results for hexagonal GaN yield energies of zone-centered phonons which are consistent with experimental Raman and infrared (IR) absorption studies, and a density of states spectrum, which is in accord with neutron scattering measurements.Our results for amorphous GaN show that the vibrational properties of a-GaN may be attributed to the nearly crystalline nearest-neighbor geometry of the amorphous network.

Gas chromatographic retention behavior of polycyclic aromatic sulfur heterocyclic compounds, (dibenzothiophene, naphtho[ b]thiophenes, benzo[ b]naphthothiophenes and alkyl-substituted derivatives) on stationary phases of different selectivity

Retention indices for 80 polycyclic aromatic sulfur heterocyclic compounds (PASHs) were determined by using high-resolution capillary gas chromatography (GC) with three different stationary phases: a 5% (mole fraction) phenyl-substituted methylpolysiloxane column, a 50% (mole fraction) phenyl-substituted methylpolysiloxane column and a smectic liquid crystalline column. Retention data were obtained for the following PASHs: dibenzothiophene, 28 alkyldibenzothiophenes, naphtho[1,2- b]thiophene, seven of the eight possible methylnaphtho[1,2- b]thiophenes, naphtho[2,1- b]thiophene, all eight methylnaphtho[2,1- b]thiophenes, naphtho[2,3- b]thiophene, the three benzo[ b]naphthothiophene isomers (benzo[ b]naphtho[1,2- d]thiophene, benzo[ b]naphtho[2,1- d]thiophene and benzo[ b]naphtho[2,3- d]thiophene), and all 30 possible methylbenzo[ b]naphthothiophene isomers. Molecular descriptors [length, breadth, thickness ( T), and length-to-breadth ratio ( L/ B)] were calculated for all the compounds studied. Correlations for retention on the liquid crystalline phase and solute geometry ( L/ B and T) ratios were investigated, and the retention was found to be highly correlated with L/ B resulting in correlation coefficients ranging from r=0.68 to r=0.98.

Optical reflectance anisotropy of Ag and Au (110) single crystals

We have developed a theoretical model for describing the electromagnetic response of noble metal surfaces. The model takes into account the d to s-p interband transitions as well the actual crystalline geometry. The surface response is incorporated in a calculation of the optical reflectance, and it yields corrections to Fresnel's formulae which depend on the angle between the polarization vector and the surface principal directions. The anisotropy effect has been confirmed by surface reflectance anisotropy spectroscopy experiments performed on (110) Ag and Au surfaces. We show that the main contribution to the anisotropy is the screened surface local-field effect on resonant dipolar oscillations localized near the surface.

Classical trajectories studies of diet from alkali halides

Abstract Desorption of positive ions in alkali halides resulting from the repulsive environment created by core-hole Auger decay has been previously found not likely due to lattice rearrangement and trapping of the ion. We revisit the problem by studying ion trajectories using classical molecular dynamics in the crystalline (rather than cluster) geometry with careful account of the Madelung energy. We find that the previous findings remain unchanged. In contrast to previous works, we also assume that the positive ion gained substantial amount of kinetic energy at the onset of simulations, crudely mimicking ion-stimulated desorption. Then the ejection of the formed positive halogen ion occurs for initial kinetic energies of the order 2 eV for NaF and 0.65 eV for LiF. Implications for viability of the Knotek-Feibelman mechanism are discussed.

Collective surface modes of Ag single crystals

We calculate the electron-energy-loss spectra and the normal surface modes of the low-index faces of Ag single crystals. To this end we develop a model that accounts for the surface crystalline geometry and for both intraband and interband transitions. We obtain surface response functions that display a resonant structure due to face- and orientation-dependent modes, which lead to an anisotropy in the loss spectra for finite momentum transfers. Furthermore, they lead to a large anisotropy in the optical reflectance.

Microstructure of superconductor bulk materials and thin films

The microstructure of single crystals of Y-Ba-Cu-O, Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O prepared by different methods were studied by various physical techniques, in particular by Raman microspectroscopy.Correlations have been established between Raman spectra and structure, allowing the determination of crystalline geometry of the material versus the composition and the preparation parameters.For YBa2Cu3Ox, a relationship: x = 0.025 VH - 5.57 is proposed and permits to evaluate the oxygen content by measuring simply in ZZ scattering geometry the Cu-O stretching mode V'H along the c axis of the crystal. This frequency varies from 465 cm-1 in YBa2Cu3O6 to 503 cm-l in YBa2Cu3O7 (Fig.l).This technique also offers the possibility to follow the structural conversion Y124 into Y123 and TI2212 into TI1212 (Fig.2) by appropriate thermal treatment. The preferentiel departure of the “CuO” or “TIO” chain/layer (Fig.3) is explained by weaker interaction forces between these layers and the adjacent ones, compared to those between the remaining layers. This helps to predict the localization of an eventual intercation in Y123 and TI1212.

Collective oscillations of twin boundaries in high-temperature superconductors as an acoustic analogue of two-dimensional plasmons.

Low-frequency collective oscillations in a superlattice consisting of alternating highly anisotropic layers are considered. Such superstructure may be formed in the ferroelastic near the structural phase transition by alternation of twins. For the surface waves, propagating along the layers, the conditions and the range of existence of those with the dispersion law \ensuremath{\omega}\ensuremath{\sim}${\mathit{k}}^{1/2}$, characteristic for two-dimensional plasmons, have been analyzed for a solid-state system with consideration for elastic anisotropy and retardation of acoustic waves. Such excitations (``dyadons'') were used by Horovitz, Barsch, and Krumhansl [Phys. Rev. B 36, 8895 (1987)] in an attempt to explain the anomalies of low-temperature thermodynamic and kinetic characteristics of high-${\mathit{T}}_{\mathit{c}}$ superconductors. We have shown that the similarity of the densities of the matching phases and the retardation of elastic waves in the crystal narrow the range of existence of dyadons, but the high elastic anisotropy of the solid phases enlarges the range of existence of such excitations in solid-state systems. An example of possible crystalline geometry of the phase matching, for which there arise collective excitations of the type under consideration, is found. For transverse and longitudinal waves propagating across the layers, the existence is proved of low-frequency acoustic branches separated by a wide gap from the nearest optical branches.

Superconductive behavior of quasi-one-dimensionalNb3Se4

Superconductive behavior of ${\mathrm{Nb}}_{3}$${\mathrm{Se}}_{4}$ has been studied in connection with a zigzag chain of Nb. We use diamagnetic supercurrents as a probe for the effect of crystalline geometry, where the actual measurements are ${\ensuremath{\chi}}_{1}^{\ensuremath{'}}\ensuremath{-}i{\ensuremath{\chi}}_{1}^{\ensuremath{'}\ensuremath{'}}$ (a fundamental susceptibility) and $|{\ensuremath{\chi}}_{3}|$ (a third-harmonic susceptibility). ${\ensuremath{\chi}}_{1}^{\ensuremath{'}}$ reflects the Meissner effect. In the transition curves of ${\ensuremath{\chi}}_{1}^{\ensuremath{'}\ensuremath{'}}$ and $|{\ensuremath{\chi}}_{3}|$, we observed a single peak, which is characterized by the unusual growth with the increase of measuring amplitude. We also give the simulated transition curves by means of the weakly coupled chain model. It is interpreted that the amplitude-dependent growth of the peak is due to a disorder in the weak couplings.

New organic molecular materials for nonlinear optics

We present four molecular organic crystals which were recently proposed for applications in nonlinear optics (three-wave-mixing and electrooptic effects) : methyl - (2,4 - dinitrophenyl) - aminopropanoate (MAP), 2 - methyl - 4 - nitroaniline (MNA), 3 - methyl - 4 - nitropyridine - 1 - oxyde (POM) and urea. The nonlinear efficiency of molecular organic crystals is seen to depend on the molecular hyperpolarizability and on the crystalline geometry. Both features are discussed and shown to lead, for crystal point groups of lower symmetry, to new structural relations between crystalline non linear coefficients. Each of the four molecular crystals discussed here are shown to have specific properties connected to different types of applications.

Phonons in amorphous silica

A new theory of lattice vibrations in amorphous silicon dioxide is presented in which the randomness of the solid is treated separately from its chemistry. The theory attributes all measurable properties of phonons in silica to the nearly crystalline nearest-neighbor geometry of the lattice and to the disruptive effects of bondangle disorder. Neutron, infrared, and Raman spectra are calculated and compared with experiment. The theory is an application of the recently developed cluster-Bethe-lattice approach to studying amorphous solids.

Etude des centres paramagnetiques crees par radiolyse dans le K 2CrO 4

Potassium chromate single crystals which have been irradiated at ambiant temperature, have been analyzed by E.S.R. spectroscopy. On the basis of the g values, of comparisons with published data and of crystalline geometry considerations, the nature of some paramagnetic centers could be determined as: CrO 4 −, CrO 4 3, CrO 2 −.