Stationary Crystal Research Articles

Can Laue microdiffraction be used to solve and refine complex inorganic structures?

The white-beam Laue diffraction experiment is an attractive alternative to the more conventional monochromatic one for single-crystal structure analysis, because it takes full advantage of the X-ray energy spectrum of a synchrotron source and requires no rotation of the crystal in the beam. Therefore, it could be used for structural characterizations underin situoroperandoconditions. The potential of Laue diffraction was recognized and exploited by the protein community many years ago, and the methodology, which involved positioning and rotating the crystal in the beam, has been successfully applied to the determination of both protein and small-molecule crystal structures. Here, it is proposed that the specificities of Laue diffraction are exploited to study randomly oriented stationary microcrystals of inorganic materials. In order to determine the best strategy for collecting a reasonable quantity of data from stationary crystals, a series of simulations on four model structures for three experimental setups have been performed. It is shown that the structures of the four samples can be solved with the dual-space method inSHELX, even though the data sets are highly incomplete and much of the low-resolution part is missing. The experimental setup and data collection strategy for measuring such microcrystals have been developed on BL12.3.2 at the Advanced Light Source in Berkeley. The intensities of reflections with one and two harmonics can be extracted reliably by exploiting the tunable low-energy threshold of a Pilatus detector. In this way, the number of usable reflections can be increased from 75 to 95%. Such Laue microdiffraction data have been measured and used successfully to refine the structures of the model samples.

Using a non-monochromatic microbeam for serial snapshot crystallography

The new X-ray free-electron laser source (SwissFEL) that is currently being developed at PSI will provide a broad-bandpass mode with an energy bandwidth of about 4%. By using the full energy range, a new option for structural studies of crystalline materials may become possible. The proof of concept of broad-bandpass diffraction presented here is based on Laue single-crystal microdiffraction and the experimental setup on BL12.3.2 at the Advanced Light Source in Berkeley. Diffraction patterns for 100 randomly oriented stationary crystallites of theMFI-type zeolite ZSM-5 were simulated assuming several bandwidths, and the statistical and structural results are discussed. With a 4% energy bandwidth, the number of reflection intensities measured in a single shot is significantly higher than with monochromatic radiation. Furthermore, the problem of partial reflection measurement, which is inherent to the monochromatic mode with stationary crystals, can be overcome.

Crystallization of binary melt with a periodically varying rate

The time dependence of the crystallization rate at crystal pulling to the cold zone with a velocity that has a sinusoidal component has been investigated within a one-dimensional time-dependent model of solidification of a diluted binary melt by the net method. It is shown that the regularities that were obtained previously by the analytical method for pulling rates with small oscillations are repeated in many respects when considering oscillations of larger amplitudes. At significant oscillation amplitudes, the time-averaged values of impurity concentration at the interface for the stationary and vibrational crystallization modes may differ by several dozen percent.

Display strobing: An effective countermeasure against visual blur from whole-body vibration

Crews and equipment in aerospace vehicles, including spacecraft at launch, can be exposed to significant vibration. Prior to this study, we examined the ability of vibrating observers to read alphanumeric symbology on stationary (i.e., non-vibrating) flight-relevant display formats and noted performance degradation with increasing vibration amplitude and decreasing font size. Here we test the efficacy of a display strobing countermeasure for the reading decrements caused by the same 12-Hz whole-body vibration in the surge (chest-to-spine) direction applied in our prior studies. To produce the strobe countermeasure, we triggered the backlight of a stationary liquid crystal diode (LCD) display panel to flash in synchrony with the 12-Hz vibration of the observer’s seat while experimentally varying both the strobe duty cycle and phase angle between the strobe onset and the vibration cycle zero-crossings. Strobing proved an effective countermeasure, restoring reading error rates during 0.7g (6.9m/s2 half-amplitude) whole-body vibration to levels indistinguishable from those achieved under the non-strobed (equivalent luminance) non-vibrating baseline condition and improving response times although not fully to the baseline. While we noted differences in the “preferred” phase angle of individual observers, on average, no overall effect of phase angle was detected. Likewise, no effect was seen for the two duty cycles and their respective equivalent luminance levels. Further studies are needed to determine the efficacy of strobing for multi-axis and multi-frequency vibration, and for displays with moving images.

3D numerical simulation for single crystal growth of potassium dihydrogen phosphate in a new solution growth system

A new system for improving the growth of KDP crystal with rotary flow field and stationary crystal is proposed. The 3D numerical simulation for flow and mass transfer shows that the rotation rate is the key factor that has a significant effect on the surface supersaturation and surface homogeneity. Especially for the high rotation rate, higher surface supersaturation and better surface homogeneity can be gained. Bulk supersaturation and inlet velocity also affect surface supersaturation and surface homogeneity to a certain extent. The surface shear stress increases with enhancing the rotation rate or the inlet velocity. The optimization for the configuration has also been discussed, which is helpful for the surface homogeneity.

Self-Disproportionation of Enantiomers via Sublimation; New and Truly Green Dimension in Optical Purification

This Review article presents all available in the literature data on optical purification of enantiomerically enriched organic compounds via simple procedure of sublimation. The material is arranged in a chronological order from early publications to current research. Relatively small amount of the results reported in the literature suggest that this area of research is still in its infancy it terms of commonly accepted experimental procedures, generalizations, interpretation or prediction of the stereochemical outcome of sublimation. On the other hand, the available data clearly show the potential of sublimation as most economical and greener alternative to the conventional optical purification techniques such as chromatography on chiral stationary phase or fractional crystallization. Another important goal of this Review is to bring to the attention of scientific community a warning that special care should always be taken in evaluation of the stereochemical outcome of enantioselective reactions. Thus, due to the always different sublimation rates of racemic and enantiomerically pure crystals, such a routine procedure as drying reaction products in vacuum or even removal of solvents under reduced pressure can significantly alter the true enantiomeric composition (% ee) of a compound under study and lead to erroneous results. Keywords: Sublimation, chiral purification, self-disproportionation of enantiomers, non-linear effects, green methodology, optical purification, chromatography, fractional crystallization, enantioselective resctions, biocatalysis

Swimming Photochromic Azobenzene Single Crystals in Triacrylate Solution

Self-motion of a growing single crystal of azobenzene chromophore in triacrylate solution (TA) is investigated in relation to the solid-liquid phase diagram bound by the solidus and liquidus lines. Upon thermal quenching from the isotropic melt to the crystal + liquid gap, various single crystals develop in a manner dependent on concentration and supercooling depth. During the crystal growth, TA solvent is rejected from the growing faceted fronts, enriching with TA in close proximity to the crystal-solution interface. The concentration gradient that formed as the result of TA expulsion induces convective flows in the solution and generates spatial variability of surface tension usually responsible for Marangoni effect. Either or both of these phenomena may have contributed to the observed self-motion including swimming, sinking, and floating of the azobenzene rhomboidal crystal in TA solution. A stationary rhomboidal crystal is also shown to swim upon irradiation with the UV light because of a mechanical torque generated by the trans-cis isomerization. Judging from the sinking or floating behavior of the azobenzene crystal, it may be inferred that the nucleation occurs at the solution-air interface.

Influence of Potential and Non Potential Forces of Interparticle Interaction on Stability of Nuclear Excited Dusty Plasma

AbstractThe highly ordered levitating structures of radioactive dusty particles in nuclear excited plasma oftube capacitor are studied using the Brownian dynamics. Models with potential and non potential forces acting on charged dusty particles have been investigated. Types and stability of obtained dust particle structures are analyzed at atmospheric pressure and normal conditions. The stationary crystal – like structures have been obtained only for potential forces acting on dusty particles, while in model with non potential forces the formation of two slowly rotating in opposite direction highly ordered vortices has been predicted (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Gap solitons in spatiotemporal photonic crystals

We generalize the concept of nonlinear periodic structures to systems that show arbitrary spacetime variations of the refractive index. Nonlinear pulse propagation through these spatiotemporal photonic crystals can be described, for shallow nonstationary gratings, by coupled mode equations which are a generalization of the traditional equations used for stationary photonic crystals. Novel gap soliton solutions are found by solving a modified massive Thirring model. They represent the missing link between the gap solitons in static photonic crystals and resonance solitons found in dynamic gratings.

Resonant generation of surface acoustic waves between moving and stationary piezoelectric crystals

The propagation of surface acoustic waves in a system composed of two piezoelectric crystals moving with respect to each other and separated by a vacuum gap is considered. The waves are localized on different sides of the gap and coupled only through the electrostatic interaction. It is shown that when the velocity of the relative motion of crystals is close to some value, there occurs a wave instability resulting in a resonant generation of these surface waves. The rate of growth of Bleustein-Gulyaev waves in piezoelectric crystals of 6mm symmetry class is determined analytically.

Spontaneous generation of vortex crystals from forced two-dimensional homogeneous turbulence

The long-term limit of statistically stationary two-dimensional turbulence is shown to depend on the form of the large-scale forcing, in agreement with previous results. That effect is studied systematically by continuously varying the forcing from deterministic to Brownian in direct numerical simulations in doubly periodic boxes. As expected, this switches on or off the enstrophy cascade and the presence of strong coherent structures, but the transition is not monotonic. Under intermediate forcing conditions, the flow evolves to a stationary vortex crystal with triangular lattice, which appears to be stable and to last indefinitely. Deterministic forcings frustrate crystallization through the formation of fast-moving dipoles, and very random ones melt the crystal. The dispersion properties of the different regimes are studied, and it is shown that efficient particle dispersion depends on the presence of multiscale turbulence. The relation with other two-dimensional systems is discussed.

First results from the high-resolution mouseSPECT annular scintillation camera

High-resolution single-photon emission computed tomography (SPECT) imaging in small animals tends to use long imaging times and large injected doses due to the poor sensitivity of single pinhole gamma cameras. To increase sensitivity while maintaining spatial resolution, we designed and constructed a multipinhole collimator array to replace the parallel hole collimators of a Ceraspect human SPECT brain scanner. The Ceraspect scanner is composed of an annular NaI(TI) crystal within which the eight pinhole collimators (1-mm-diameter holes) rotate while projecting nonoverlapping images of the object onto the stationary annular crystal. In this manner, only one-eighth of a collimator rotation is required to acquire a full circle orbit tomographic data set. The imaging field of view (FOV) has a diameter of 25.6 mm in the transverse direction, which is sufficient to encompass a mouse in the transverse direction. The axial FOV is 25.6 mm at the center of the FOV and 13.9 mm at the edge of the transverse FOV. Data are currently acquired in step-and-shoot mode; however, the system is capable of list mode acquisition with the collimator continuously rotating. Images are reconstructed using a cone-beam ordered subsets expectation maximization method. The reconstructed spatial resolution of the system is 1.7 mm and the sensitivity at the center of the FOV is 13.8 cps/microCi. A whole-body bone scan of a mouse injected with [Tc-99 m]MDP clearly revealed skeletal structures such as the ribs and vertebral bodies. These preliminary results suggest that this approach is a good tradeoff between resolution and sensitivity and, with further refinement, may permit dynamic imaging in living animals.

Dynamically excited single-component ion Coulomb crystals in linear Paul traps

This paper briefly discusses a few experiments regarding dynamically excited crystalline structures in a linear Paul trap. The dynamically excited structures are created either by applying a torque to an initially stationary crystal by utilizing the radiation pressure force from a near-resonant laser beam or by operating the trap at parameters where parametric resonances melt normal stationary crystals.

Colloidal crystal motion in electric fields

Abstract We report on the short and long time flow behaviour of colloidal crystals subjected to a homogeneous electrical square wave a.c.-field in closed cells of rectangular cross section. Local velocities are determined from Laser Doppler velocimetry, sample structure and morphology were observed by time resolved polarisation microscopy. We observe a complex time dependence of the flow behaviour. After each field reversal we find a transition from an initial parabola-like flow to a final plug-like flow. On the time scale of minutes to hours channel narrowing occurs, i.e. flow initially restricted by the cell walls becomes restricted to the cell centre by wall based stationary crystals. Within the narrowed channel, however, a qualitatively similar time dependence of flow is observed.

Non-stationary Coulomb crystals in linear Paul traps

Ion Coulomb crystals form when trapped ions are cooled below a certain criticaltemperature. In radiofrequency (RF) traps, such crystals are usually stationary(i.e. non-rotating) with shapes similar to those of cold, homogeneously chargedliquids when the number of ions is sufficient for three-dimensional structures toappear. ‘Dynamically stable’ structures can, however, also be created either bydirectly applying a torque to an initially stationary crystal or by choosing trapparameters just within the stability region where stationary crystals will melt dueto parametric resonances between the RF trap field and ion plasma modes.

Stability of Coulomb crystals in a linear Paul trap with storage-ring-like confinement.

We report experiments on the stability of ion Coulomb crystals in a linear Paul trap with storage-ring-like confinement. The transverse dynamics of charged particles in a trap of this type is analogous to that of a fast beam traveling through a channel with periodic, magnetic alternating gradient confinement. The experimentally observed stability conditions for stationary crystals comply remarkably well with current theory of crystalline plasmas and beams.

New model for the vapor growth of hexagonal ice crystals in the atmosphere

We present a new microphysical model for the vapor growth and aspect ratio evolution of faceted, hexagonal ice crystals in the atmosphere. Our model is based on a novel, efficient numerical method for solving Laplace's equation for steady state diffusion on the surface of a three‐dimensional hexagonal prism, and also takes into account the surface kinetic processes of crystal growth. We do not include ventilation, so our model is limited to stationary crystals or falling crystals smaller than 100 μm. We calculate a self‐consistent solution for the distribution of the supersaturation and the condensation coefficient on each crystal face, for several different assumptions regarding the crystal growth mechanism and ice surface properties. We use this model to predict the aspect ratios expected for faceted ice crystals over a range of temperatures and supersaturations, as well as to estimate the conditions for which faceted growth becomes unstable and the crystals become hollowed or dendritic. We compare these predictions to observed features of ice cloud crystals to infer some microphysical characteristics of ice crystals and their temperature dependence. We also compare our predicted mass growth rates with those of the capacitance model for spheres and ellipsoids to look at the effects of shape and surface kinetics. Finally, we insert the single‐particle code into a simple parcel cloud model to investigate the feedbacks between crystal surface kinetics, shape, and the thermodynamic properties of clouds.

Explosive crystallization mechanism of ultradisperse amorphous films

The explosive crystallization of germanium ultradisperse amorphous films has been studied experimentally. We show that crystallization may be initiated by local heating at the small film thickness but is realized spontaneously at large thickness. The fractal pattern of the crystallization phase is discovered to be inherent in the phenomena of diffusion-limited aggregation. It is shown that in contrast to the ordinary crystallization mode, the explosive one is connected with the instability which is caused by self-heating. A transition from the first mechanism to the second one is modelled by the Lorenz system. The process of explosive crystallization is represented on the basis of the self-organized criticality conception. The front movement is described as the effective diffusion in the ultrametric space of hierarchically subordinated avalanches, corresponding to the explosive crystallization of elementary volumes of ultradisperse powder. The expressions for the stationary crystallization heat distribution and the steady-state heat current are obtained. The heat needed for initiation of the explosive crystallization is obtained as a function of the thermometric conductivity. The time dependence of the spontaneous crystallization probability in a thin film is examined.

Ventilation Coefficients for Falling Ice Crystals in the Atmosphere at Low–Intermediate Reynolds Numbers

The ventilation coefficients for columnar, hexagonal plate, and broad branch ice crystals falling in air are computed by first solving numerically the convective diffusion equation for water vapor density to obtain its profile around these ice crystals and then determining the total vapor flux on the surface of the crystal. The ratio of this flux to the flux on a stationary crystal gives the ventilation coefficient. The local flow velocity profiles around the falling crystals necessary for specifying the convective term in the convective diffusion equation were obtained previously by numerically solving the unsteady Navier–Stokes equations subject to appropriate crystal-shaped boundary conditions. Ventilation coefficients obtained in this way are illustrated as a function of the Schmidt and Reynolds numbers and are also fitted by empirical expressions. Applications of these ventilation coefficients are discussed.

On the divergence of rays Bragg diffracted by perfect stationary crystals illuminated with collimated polychromatic radiation

The properties of three diffraction geometries, viz symmetric Bragg-case reflection from a crystal surface, symmetric Laue-case transmission through a parallel-sided crystal and asymmetric Bragg-case reflection from a crystal surface, have been examined under the conditions that the crystal (which is assumed to be perfect) is held in fixed orientation relative to an ideally collimated incident beam of white X-rays. Symmetric Bragg-case reflection has the valuable property of invariant direction of reflected rays for all orders of reflection excited, and for strong and weak reflections alike. For the symmetric Laue case, and for the asymmetric Bragg case approximately, for small asymmetry, expressions are given for the angular range of diffracted rays, scaled in terms of the extinction distance of the reflection concerned. The findings have practical applications in the design and interpretation of continuous-radiation topographic experiments.