Main Symmetry Axis Research Articles

The distribution of motional correlation times in superionic conductors: nuclear magnetic resonance of tysonite-like

Fluorine mobility at different structural positions in monocrystalline with the tysonite structure is analysed using NMR line-shape analysis. The method is sensitive to ionic exchange with correlation times in the range . For the temperature range between 240 K and 400 K the motion is restricted mainly to the ions in the fluorine layers perpendicular to the main symmetry axis (the sublattice), while ions in the La plane remain immobile. No significant anisotropy of the -ionic diffusion within the layers and along the c-axis is found ( at 400 K). From NMR spectra it is clear that mobility is strongly heterogeneous. The motional disorder can be described well by a broad distribution of correlation times, , which has a shape close to a log - Gaussian function and reflects the potential energy landscape in the superionic state. The variation of the centre position and width of with temperature differs from an Arrhenius law behaviour. Ionic mobility on the microscopical scale, therefore, cannot be considered a process which is activated only thermally. Applying MD techniques shows that the presence of vacancies may lead to pronounced changes of the potential energies, and supports the idea that there is a distribution of activation energies.

Raman bandshape analysis of oxocarbon ions in aqueous solutions

Fourier analysis of Raman bandshapes was undertaken for some modes of the lithium salts of squarate (C 4O 4 2−) and croconate (C 5O 5 2−) ions in saturated aqueous solution. Time correlation functions obtained from the isotropic and anisotropic Raman components of the totally symmetric modes of ring breathing, and the ones obtained from the anisotropic components of the non-totally symmetric modes of CC stretching and ring bending, have similar decay rates for both C 4O 4 2− and C 5O 5 2−, indicating that reorientation is very hindered in these systems for both spinning and tumbling of the main symmetry axis. The experimental vibrational correlation functions were fitted by the Kubo's equation, and are in the intermediate modulation regime, although faster than the ones for other typical ions reported in literature. The small time correlation of vibrational frequency fluctuations (≈ 0.1 ps) together with slow reorientation imply the picture of the ions performing librational motions inside temporary cages formed by water molecules. From the modeling of the vibrational and reorientational correlation functions, the estimated average librational frequency is ca. 80 cm −1. Known models which separate homogeneous and inhomogeneous contributions give no better fitting of the vibrational correlation functions. The homogeneous nature of the vibrational dephasing process is also pointed by the almost perfect Lorentzian shape of the bands and by the calculation of the isotropic components by a memory function approach truncated in second order. Similar results were found after a 1:10 dilution of the saturated solutions, or upon increasing the temperature to 50°C, or yet by using of the sodium salts.

Effect of Moisture Content and Grain Angle on the Ultrasonic Properties of Rubber Wood

The ultrasonic velocity and elastic stiffness constant of rubber wood were studied using ultrasonic technique in three main symmetry axes and an angle rotating from the symmetry axes. The effects of moisture content were also determined in each case. A strong correlation was found between the velocity and the moisture content. As the moisture content increased, the velocity and the stiffness constant were found to decrease. The longitudinal direction showed the highest velocity and hence the elastic stiffness constant. Linear regression equations were obtained between velocity and grain angle with R2 from 0.86 to 0.99.

Improved reflector for interferometric tracking in three dimensions

The paper describes the design and construction of an improved form of cat's-eye retroreflector for use with a 3-D tracking laser interferometer. This reflector can accept an incoming interferometer beam within 60 deg of its main axis of symmetry and can provide a high-quality return beam at ranges up to 25 m.

Molecular Dynamics Simulation Study of Lanthanide Ions Ln3+ in Aqueous Solution. Analysis of the Structure of the First Hydration Shell and of the Origin of Symmetry Fluctuations

We have analyzed molecular dynamics simulations of tripositive lanthanide ions Ln{sup 3+} in aqueous solution. We were able to extract in detail their angular structure and their dynamic behavior along the series of Ln{sup 3+} ions. For a heavy lanthanide ion (Yb{sup 3+}) the eight water molecules of the first hydration shell form a well-defined square antiprism, whereas for a nine-coordinate light lanthanide ion (Nd{sup 3+}) the first hydration shell adopts the tricapped trigonal prism geometry. In the middle of the series both geometries coexist. Both the square antiprism and the tricapped trigonal prism rearrange via 90{degree} pseudorotations of the main symmetry axis. The pertaining transition state of lowered symmetry is a dodecahedron for the Yb{sup 3+} octaaqua complex and a capped square antiprism for the Nd{sup 3+} enneaaqua complex. The lifetime of a square antiprism between two pseudorotations is 11 ps but amounts to only 2 ps for a tricapped trigonal prism. The lifetime of a square antiprism from the simulation of Yb{sup 3+} (CN = 8) is in quantitative agreement with the correlation time for the fluctuation of the zero-field splitting from experimental EPR spectra of the Gd{sup 3+} octaaqua complex. On the basis of our MD-simulations wemore » can go beyond these general notions and propose a model with 90{degree} pseudorotations of the coordination polyhedron as the principal mechanism for distortions of the first hydration shell. 26 refs., 11 figs.« less

Seismic anisotropy of shales

The seismic properties and preferred clay mineral orientation of a suite of shales are investigated using laboratory velocity measurements as a function of confining pressure, X ray diffraction techniques, and electron microprobe backscatter (BSE) imaging. The velocity measurements indicate that these shales are transversely isotropic with the main symmetry axis perpendicular to bedding. Anisotropy, at elevated pressures caused mainly by preferred orientation of clays (illite) parallel to bedding, ranges from 20% (Vp) and 19% (Vs) for a sample of New Albany Shale to 30% (Vp) and 35% (Vs) for a sample of Chattanooga Shale. The degree of clay mineral alignment in the shales is constrained by “orientation indices” produced using simple X ray diffraction techniques. A strong positive correlation is found between the degree of preferred orientation, as expressed in the orientation indices, and seismic anisotropy. BSE images of the shale fabrics confirm in a qualitative manner the results of the X ray study. To investigate wave propagation in the shales, elastic constants of each sample are calculated and used to produce phase and group (wave) velocity surfaces, which describe variation in velocity as a function of angle to the bedding normal. The calculated velocity surfaces, constrained by independent velocity measurements, display a lack of shear wave splitting at “near‐normal incidence” in even the most anisotropic shales. For the highly anisotropic Chattanooga shales, group velocity surfaces differ significantly from corresponding phase velocity surfaces.

Elastic constants and velocity surfaces of indurated anisotropic shales

The velocities of two Devonian-Mississippian shales have been measured to confining pressures of 200 MPa in a laboratory study of anisotropy and wave propagation. Both samples were found to be transversely isotropic at elevated pressures with the main symmetry axis perpendicular to bedding. The elastic constants of the shales were used to calculate phase and group velocity surfaces as a function of angle to the bedding normal. Multiple velocity measurements in non-symmetry directions, not undertaken in previously published studies of shales, have been used to confirm features observed on calculated velocity surfaces. It is demonstrated that velocities measured in non-symmetry directions are phase velocities. Group velocities were found to be significantly lower than the corresponding phase velocities of the shales due to their high anisotropies. Shear wave splitting was found to be negligible for propagation directions within approximately 30° of the bedding normals.

NMR study of the geometric confinement effects on the anisotropic rotational diffusion of acetonitrile-d3

The NMR spin-lattice relaxation times, T 1 , of deuteron and nitrogen-14 of a symmetric-top molecule of acetonitrile-d 3 confined to Sol-gel porous silica glasses were measured as a function of pore size over the temperature range 233-308 K. In agreement with our earlier results for other polar liquids we found the two-state fast-exchange model to be valid for both deuteron and nitrogen-14 spin-lattice relaxation data of acetonitrile-d 3 . The classical rotational diffusion equation was used to calculate the rotational diffusion constants, D∥ and D⊥, corresponding to motion about tge main symmetry axis and motion about tge axis perpendicular to tge main symmetry axis, respectively

Adsorption of β-cyclodextrin at the polycrystalline gold electrode-aqueous solution interface

The adsorption of β-cyclodextrin, β-CD, on a polycrystalline gold electrode has been studied in NaClO 4, LiClO 4 and Na 2SO 4 dilute solutions by impedance measurements. The adsorption of β-CD is similar to that for other organic compounds such as dimethylsulphoxide. β-CD is a strong adsorbate, half coverage (θ/2) being reached in solutions containing less than 5×10 −5 mol dm −3. It was shown that the main symmetry axis of the adsorbed β-CD molecule is oriented perpendicular to the electrode surface. The adsorption of β-CD on the polycrystalline gold electrode follows a Langmuir isotherm with \\ ̄ gD G max o = −34.70 ± 0.57 kJ mol −. The observed behaviour has been compared with the literature data for β-CD adsorption on the Hg electrode.

Characterization of the overall and internal dynamics of short oligonucleotides by depolarized dynamic light scattering and NMR relaxation measurements

The dynamics of three synthetic oligonucleotides d(CG)4, d(CG)6, and d(CGCGTTGTTCGCG) of different length and shape were studied in solution by depolarized dynamic light scattering (DDLS) and time-resolved nuclear Overhauser effect cross-relaxation measurements. For cylindrically symmetric molecules the DDLS spectrum is dominated by the rotation of the main symmetry axis of the cylinder. The experimental correlation times describe the rotation of the oligonucleotides under hydrodynamic stick boundary conditions. It is shown that the hydrodynamic theory of Tirado and Garcia de la Torre gives good predictions of the rotational diffusion coefficients of cylindrically symmetric molecules of the small axial ratios studied here. These relations are used to calculate the solution dimensions of the DNA fragments from measured correlation times. The hydrodynamic diameter of the octamer and dodecamer is 20.5 +/- 1.0 A, assuming a rise per base of 3.4 A. The tridecamer, d(CGCGTTGTTCGCG), adopts a hairpin structure with nearly spherical dimensions and a diameter of 23.0 +/- 2.0 A. The DDLS relaxation measurements provide a powerful method for distinguishing between different conformations of the oligonucleotides (e.g., DNA double-helix versus hairpin structure). Furthermore, the rotational correlation times are a very sensitive probe of the length of different fragments. The NMR results reflect the anisotropic motion of the molecules as well as the amount of local internal motion present. The experimental correlation time from NMR is determined by the rotation of both the short and long axes of the oligonucleotide.(ABSTRACT TRUNCATED AT 250 WORDS)

Zeeman spectroscopy of the defect-induced bound-exciton lines in GaAs grown by molecular-beam epitaxy.

A Zeeman spectroscopic investigation of the ``defect-induced'' bound-exciton lines in GaAs grown by molecular-beam epitaxy is presented. The observed magnetic properties are shown to be consistent with exciton recombination at either single acceptor pairs, or double acceptor--isoelectronic-center pairs. The initial state of the recombination has hole angular momentum quantum number ${m}_{J}$=\ifmmode\pm\else\textpm\fi{}(3/2 and an electron with ${m}_{J}$=\ifmmode\pm\else\textpm\fi{}1/2, whilst the final state is magnetically inactive, as expected for two holes paired off in the ${m}_{J}$=\ifmmode\pm\else\textpm\fi{}1/2 acceptor ground state. The symmetry of the defects giving rise to the principal sharp lines is found to be rhombic I. Study of the anisotropy of the Zeeman splittings demonstrates preferential incorporation of the defects with main symmetry axes along [110] in the (1\ifmmode\bar\else\textasciimacron\fi{}10) plane, as opposed to [1\ifmmode\bar\else\textasciimacron\fi{}10] in the (110) plane, thus giving rise to the remarkable polarization of the lines reported previously. Less strongly polarized lines are found to have main symmetry axes along 〈111〉 directions, but again with preferential incorporation in (1\ifmmode\bar\else\textasciimacron\fi{}10) planes. The transitions are shown to arise from \ensuremath{\sigma} oscillators at the low-symmetry defects.

Electron paramagnetic resonance and polarized optical absorption spectra of Ni2+ in synthetic forsterite

The investigated Ni doped forsterite was grown with the floating zone technique. The EPR spectra were taken at room temperature using both 9.5 and 35 GHz. All specimens show EPR signals resulting from Mn2+ at M2 and Fe3+ at M1, M2, and Si positions. Ni2+ EPR signals are observed at 35 GHz but not at 9.5 GHz. The Ni2+ spectra are described by the spin Hamiltonian $$H = \beta SgB + D\left( {S_{\text{z}}^{\text{2}} - \left( {S + 1} \right)S/3} \right) + E\left( {S_{\text{x}}^{\text{2}} - S_{\text{y}}^{\text{2}} } \right)$$ with S=1. The EPR parameters obtained are: D=−39.56 cm−1, E=−0.58 cm−1, gx=2.194, gy=2.160, gz=2.188. Polarized optical absorption spectra of Ni2+ were taken at room and liquid nitrogen temperature covering the range from 5500 to 42000 cm−1. The spectra are nearly independent of the polarization of light. Both EPR and optical spectra reveal that Ni2+ is ordered into M1. The local symmetry of the Ni(M1) complex is described by the point group D2h with the main symmetry axis along a. Heating experiments in air up to 1000° C and three days indicated no change in the Ni ordering in this synthetic sample.

Unidirectional Crystal Growth and Crystal Anisotropy

In unidirectional crystal growth ordered or chaotic cellular patterns develop depending on the angle between the growth direction and the crystal main symmetry axis [⟨100⟩ direction in our experiment]. The three possible bifurcations for a cell are discussed.

Calculation of Grüneisen Constants in Si

AbstractLattice dynamical calculations of the Grüneisen constants for general strain components are performed using the bond charge model of Weber with additional assumptions on anharmonic forces. The model is adjusted to fit data available at the Γ, X, and L points from various experimental techniques. The results along the main symmetry axes indicate that the Grüneisen constants depend sensitively on those model parameters which were originally determined from the phonon dispersion in the harmonic approximation. It is concluded that for a consistent description of phonon dispersion and anharmonic properties the model has to be revised.

Electron-spin resonance of a complexPb+(6p1)defect in alkali halides

The electron-spin-resonance spectra are analyzed of a ${\mathrm{Pb}}^{+}$ defect produced by a long x-ray irradiation below 150 K in ${\mathrm{Pb}}^{2+}$-doped KCl and RbCl. At liquid-helium temperatures the main symmetry axis of the center is tipped a few degrees away from 001> in a {110} plane. At higher temperatures the center exhibits a librational motion which averages out the tipping effects but leaves the spectra with a small orthorhombic component. A model is proposed which is in agreement with these facts and with the behavior of the hyperfine components: A substitutional ${\mathrm{Pb}}^{+}$ possessing weak molecular bonds with two adjoining substitutional ${\mathrm{Cl}}^{\ensuremath{-}}$ ions is perturbed by an interstitial ${\mathrm{Cl}}^{\ensuremath{-}}$ ion in a next-nearest interstitial tetrahedral site. The analysis of the hyperfine data confirms a general property of the $n{p}^{1}(n=4,5,6)$ heavy metal ions and atoms: The electron-spin density at the nucleus is large and negative.

Structure of the Benzene-Argon Complex in the Gas Phase at 3 Kelvin

Abstract The structure of the benzene-argon complex is determined from a high resolution band contour of the ν6 transition of the complex measured in a hypersonic jet. It is shown that the argon is located on the main symmetry axis of the benzene molecule. The distance from the center of the benzene ring is found to be 3.4 Å.

Bulk magnetization study of a DyNi 5 single crystal

Magnetization and susceptibility measurements were performed on a single crystal of DyNi 5 along the three main symmetry axes of the ortho-hexagonal cell. Below its ordering temperature ( T c = 11.6 K), b and c are respectively the easy and hard magnetization axes. The strong anisotropy originates from the crystalline electric field acting on the 4 f electrons of the Dy 3+ ions. A small magnetization is induced on nickel atoms by the applied field and the exchange interactions with the dysprosium atoms. The crystal field parameters, the molecular field coefficients and the susceptibility of nickel atoms are determined from the experimental data.

Density and temperature effects on the molecular reorientation and vibrational relaxation in liquid methyl iodide

The reorientational correlation function for liquid methyl iodide has been measured by analysis of the Raman 526 cm−1 ν3 (a1) band as a function of pressure up to 2.5 kbar within the temperature range 0 to 90°C. These reorientational correlation functions have been obtained from the anisotropic component of the vibrational Raman scattered light using a method of Fourier deconvolution. Density and viscosity of methyl iodide over the same range of temperatures and pressures have also been determined. In order to obtain information about the effects of density and temperature on the internal C3 rotation of the methyl group the NMR deuteron spin-lattice relaxation times in liquid methyl iodide-d3 have been measured under the same experimental conditions. In addition the vibrational relaxation rates have been determined from the isotropic Raman band shapes. The Raman experiments enable us to calculate the reorientational correlation function describing the reorientational motion of the CH3I molecules about an axis perpendicular to the main symmetry axis for the following cases: (i) constant density ρ-variable temperature; (ii) constant T-variable ρ; and (iii) constant pressure-variable T. Activation energies and activation volumes for the reorientation process are also discussed. All experimental results show unambiguously the large effects of density on molecular motions and the importance of constant density experiments. Assuming a rotational diffusion process the Raman data furnish the rotational diffusion constant D⊥. By using this D⊥ and the experimental NMR deuteron spin-lattice relaxation times in CD3I we calculate the rotational diffusion constant, D∥, characterizing the internal rotation of the methyl group about the symmetry axis. In order to interpret the experimental data we propose that the deuteron field gradient deviates from the C–D bond axis. Both the rotational diffusion model and the Langevin model treatments of the internal rotation indicate very strong inertial effects in the methyl group rotation. As the density changes the rotation of the methyl group about the C3 axis remains largely unaffected. In this study it has also been possible to obtain the vibrational relaxation rate 1/τvib from the isotropic component of the vibrational Raman band shape. The experimental results can be summarized as follows: (i) as ρ increases the relaxation rate increases; (ii) as T increases at constant ρ, 1/τvib remains approximately constant, and (iii) at constant pressure increasing T causes a significant decrease in the vibrational relaxation rate. The experimental data are discussed in terms of a binary collision model as developed by Litovitz and the time between collisions is estimated from the Enskog theory, the cell model, and the J-diffusion model. Based on our earlier results we assume a temperature dependence of the effective hard sphere diameter in order to explain the experimental trends in the vibrational relaxation rate. On the basis of this study one arrives at a conclusion of general validity that any serious experimental attempt to characterize the molecular dynamics in a liquid must include the separation of the effects of density and temperature on the molecular motions.

Lowest-order anharmonic thermal scattering of X-rays

The lowest-order term due to anharmonicity in the thermal diffuse scattering of X-rays by some cubic crystals has been investigated. This term can be regarded as an interference between harmonic one- phonon and two-phonon scattering. The resultant intensities have been evaluated along several lines in reciprocal space, including the three main symmetry axes, for a number of face centred cubic alkali halides and for aluminium. The calculations were based on an anharmonic interaction restricted to nearest neighbour short range terms for the alkali halides with next nearest neighbour interactions included for aluminium. Significant intensity changes are found for all materials investigated.

Energy Bands of Hexagonal II-VI Semiconductors

Energy bands for hexagonal ZnO and ZnS have been calculated along the main symmetry axes of the hexagonal Brillouin zone with the relativistic mass velocity and Darwin correction considered. The band structure of ZnO differs from the ZnS band structure in that $d$ bands occur closely below the upper valence bands, and $p$-like conduction bands lie 17 eV above the valence bands. Thus, the ZnO band structure exhibits a very broad lowest conduction band. For hexagonal ZnS, the relativistic corrections increase the separation between $p$-like valence and conduction bands. This effect is discussed in connection with relativistic band calculations for cubic ZnS along the $\ensuremath{\Lambda}$ axis, and a revised interpretation of higher interband transitions is given. At k = 0, full relativistic calculations yielded good agreement with experimental values for crystal-field and spin-orbit splittings for ZnS, CdS, and CdSe, and gave a negative spin-orbit splitting for ZnO. Deformation potentials for CdS were calculated in good accord with experiment.