Showing papers on "Crystal published in 1996"

Control of crystal phase switching and orientation by soluble mollusc-shell proteins

Abstract: IN the initial stages of the biomineralization of abalone shells, a primer layer of oriented calcite crystals grows on a nucleating protein sheet1,2. The deposition of this primer is followed by an abrupt transition to c-axis-oriented crystals of aragonite, another crystalline form of calcium carbonate. The formation of each of the two crystal types is accompanied by the synthesis of specific polyanionic proteins1–3, suggesting that cooperative interactions between these proteins and the inorganic ions during crystal nucleation and growth control the phase of the deposited mineral and that differential expression of the proteins allows the organism to induce phase changes. It is known that soluble shell proteins can control crystal morphology4–10, but it has been suspected that the switch in phase—from calcite to aragonite—might require the deposition of a new nucleating protein sheet. Here we describe in vitro studies of the crystallization of calcium carbonate in the presence of soluble polyanionic proteins extracted from abalone shell. We find that these proteins alone are sufficient to control the crystal phase, allowing us to switch abruptly and sequentially between aragonite and calcite without the need for deposition of an intervening protein sheet. These results show that soluble organic components can exert greater control over hierarchical biomineral growth than hitherto suspected, offering the prospect of similar phase control in materials chemistry.

X-ray and neutron diffraction in nonideal crystals

Abstract: 1. Distribution of the Scattering Intensity. General Aspects.- 1.1 Diffraction Techniques for Analyzing Imperfections in Crystals.- 1.2 Kinematical Theory of Scattering.- 1.2.1 Dynamical and Kinematical Theories.- 1.2.2 X-Ray Scattering Intensity.- 1.2.3 Scattering Cross Section for Thermal Neutrons.- 1.2.4 Applicability Range for the Kinematic Theory.- 1.3 Scattering by Perfect Crystals of Finite Size.- 1.3.1 Intensity Distribution in Reciprocal Lattice Space: Form Function.- 1.3.2 Intensity Distribution in the Debye Diffraction Pattern.- 1.4 Scattering in Undistorted Crystals Containing Microscopic Cavities or Inclusions.- 1.5 Scattering by Crystals Containing Defects of Arbitrary Type. Classification of Defects.- 1.5.1 Analysis of Scattering by Imperfect Crystals.- 1.5.2 Scattering by Crystals with Randomly Distributed Defects.- 1.5.3 Classification of Defects.- 1.5.4 Diffuse Scattering by Crystals Containing First-Class Defects Under Weak Overlap of the Displacement Fields of Individual Defects.- 1.5.5 Approximation of Smoothly Varying Distortions.- 1.5.6 Scattering Intensity with Correlated Arrangement of Defects.- 1.6 Harmonic Analysis of the X-Ray Line Shapes.- 1.6.1 Fourier Coefficients for the Intensity Distributions of X-Ray Lines.- 1.6.2 Limiting Cases of Nondistorted and Large-Size Crystallites.- 1.6.3 Analysis of Crystallite Size and Distortions.- 2. Static Displacements in Crystals with Bounded Defects.- 2.1 Fluctuation Waves of Defects Concentration and Static Displacements.- 2.1.1 Symmetry of Defects.- 2.1.2 The Defect Distribution in Terms of Static Concentration Waves.- 2.1.3 Static Displacement Waves.- 2.2 Macroscopic Theory for the Static Displacement Waves.- 2.2.1 Long-Wavelength Fluctuation Waves and the Free Energy of the Anisotropic Elastic Continuum.- 2.2.2 Amplitudes of the Fluctuation Waves of Static Displacements.- 2.2.3 Fourier Components of the Static Displacements in the Continuum Description.- 2.2.4 Simplifications Introduced by Symmetry.- 2.2.5 Fluctuation Waves in Thin Films.- 2.3 Microscopic Theory for the Static Displacement Waves.- 2.3.1 Free Energy of Distorted Crystal with Bravais Lattice.- 2.3.2 Transition to the Long-Wave Approximation and the Related Force Constants.- 2.3.3 Crystals of Arbitrary Structure.- 2.4 Static Displacement Fields Around Bounded Defects.- 2.4.1 Atom Displacements Far from Defects.- 2.4.2 Atomic Displacements Near Defects, Green Functions and Mean Squares of Static Displacements.- 2.5 Static Distortions in Quasi-One-Dimensional and Quasi-Two-Dimensional Crystals.- 2.5.1 Discreteness of the Lattice and Spatial Dispersion.- 2.5.2 Static Distortion Fields of Defects in Strongly-Anisotropic Crystals.- 3. Positions and Intensities of Regular Reflection Peaks.- 3.1 Shift of X-Ray Lines in Imperfect Crystals and the Determination of Defect Concentrations.- 3.1.1 Influence of Defects on X-Ray Line Positions and Estimated Crystal Sizes.- 3.1.2 Studies of Vacancies in Crystals.- 3.1.3 Complexes in Solid Solutions and Their Effect on the Lattice Parameters.- 3.1.4 Dilation Effects Caused by Dislocation Loops.- 3.2 Regular Reflection Intensities in Perfect Crystals.- 3.2.1 Intensity Attenuation Factors.- 3.2.2 Debye-Waller Factor in Perfect Harmonic Crystals.- 3.2.3 Chain-Like and Layered Crystals.- 3.2.4 Effect of Anharmonicity on the Debye-Waller Factor.- 3.3 Effect of Static Displacements on Intensities of Regular Reflections.- 3.3.1 Debye-Waller Factor Due to Static Displacements.- 3.3.2 Effects in Crystals Containing Particles of a New Phase or Dislocation Loops.- 3.3.3 Layered and Chain-Like Crystals.- 3.3.4 Concentrated Solutions.- 3.3.5 Experimental Results on Regular Reflection Intensities in Imperfect Crystals.- 3.4 Effect of Thermal Vibrations in Imperfect Crystals.- 3.4.1 Crystals with Low Defect Concentrations.- 3.4.2 Concentrated Solutions.- 3.5 Debye-Waller Factors in Dynamical Diffraction Effects.- 3.5.1 Anomalous Transmission.- 3.5.2 X-Ray Fluorescence.- 3.5.3 Spatial Intensity Oscillations.- 3.5.4 Critical Potentials.- 4. Diffuse Scattering of X-Rays and Neutrons by Crystal Defects.- 4.1 Weakly Distorted Crystals.- 4.1.1 Scattering by Single Defects.- 4.1.2 Scattering Intensity Near Reciprocal Lattice Points: Symmetry of Defects and Force Dipole Tensors.- 4.1.3 Scattering Intensity Distribution at Large Distances from Reciprocal Lattice Points and Determination of the Defect Configuration and the Force Field.- 4.1.4 Diffuse Scattering and the Correlation in Defect Positions.- 4.1.5 Experiments on Scattering by Point Defects in Irradiated Crystals and Dilute Solutions.- 4.1.6 Scattering by Self-localized Electrons.- 4.1.7 Diffuse Scattering Representation in Various Experimental Techniques.- 4.2 Effects of Groups of Point Defects, New-Phase Particles, or Small-Radius Dislocation Loops.- 4.2.1 Scattering by Large Bounded Defects in Weakly Distorted Crystals.- 4.2.2 Diffuse Scattering by Weakly Distorted Crystals with Particles of a Second Phase and Ageing of Solutions.- 4.2.3 Diffuse Scattering by Small-Radius Dislocation Loops in Strained and Irradiated Materials.- 4.3 Intensity Distribution for Scattering by Strongly Distorted Crystals with Finite Defects.- 4.3.1 Change in Scattering Intensity Distribution with Increasing Defect Strength.- 4.3.2 Integrated Intensity from Strongly Distorted Crystals.- 4.3.3 Intensity Distribution in the Reciprocal Space.- 4.3.4 The Debye Diffraction Pattern.- 4.3.5 Experiments on Strongly Distorted Ageing Alloys and Irradiated Materials.- 4.3.6 Nonrandom Arrangement of Finite Defects.- 4.4 Strongly Anisotropic Crystals.- 4.4.1 Quasi-Two-Dimensional Crystals.- 4.4.2 Quasi-One-Dimensional Crystals.- 4.5 Effect of Finite Defects in Thin Films and Surface Layers on X-Ray Scattering.- 4.5.1 Scattering Intensity for Imperfect Finite Crystals.- 4.5.2 Diffuse Scattering by Defects in Thin Films.- 4.5.3 Broadening of Regular Reflection Peaks in Free Films with a Large Surface Area.- 4.5.4 Diffuse Scattering by Defects in a Thin Surface Layer.- 5. Scattering of X-Ray and Neutrons in Crystals with Dislocations.- 5.1 Broadening of Peaks by Randomly Distributed Defects of the Second Class.- 5.1.1 Linear Dislocations.- 5.1.2 Large-Radius Dislocation Loops.- 5.1.3 Dislocation Dipoles.- 5.1.4 Stacking Faults and Split Dislocations.- 5.2 Effect of Nonrandom Dislocation Arrangement on Scattering Intensity Distribution.- 5.2.1 Scattering by Crystals with Dislocation Walls and a Dislocation Description for the Effects Caused by Blocks and Cells.- 5.2.2 Correlation in the Uniform Dislocation Ensemble and in Crystals with Nonuniform Dislocation Arrangement.- 5.3 Diffraction Methods of Investigation of Dislocation Ensembles.- 5.3.1 Determination of Dislocation Density.- 5.3.2 Correlation and Inhomogeneity in Dislocation Arrangement.- 5.3.3 Dislocations in Narrow Small-Angle Walls (Boundaries) and Excess Dislocations of a Given Sign.- 5.3.4 Diffraction Techniques for Analyzing the Grain Boundaries.- Appendices.- A. Cumulant Expansion.- B. Equations for Amplitudes of Static Displacement Waves for Various Crystal and Defect Symmetries.- D. Mean Squares of Static Displacements in fee Crystals.- for Strongly Deformed Crystals Containing Limited-Size Defects.- F. Calculation of T(?) for Homogeneous Dislocation Ensemble.- References.

Single Scattering Properties of Atmospheric Ice Crystals

Abstract: Simulations of scattering and polarization properties for randomly oriented polyhedral ice crystals are presented based on the geometric optics and the far-field diffraction approximation. Particle shapes range from various hexagonal symmetric particles to highly complex shaped deterministic and random fractals. All calculations are performed at a wavelength of 0.55 µm. Hexagonal symmetric particles show several narrow scattering peaks besides the well known 22° and 46° halos. Column-like ice crystals provide neutral points (NP) at larger scattering angles than plate-like ice crystals. The ranges of NPs for column-like and plate-like crystals are separated at a scattering angle of about 156°, which may allow a polarimetric distinction between these two crystal types. The effects of particle size are studied by applying observationally derived aspect-ratio parameterizations to the individual particle types. Differences in the asymmetry parameter versus size relations for column-like particle types...

Two-dimensional arrays of colloidal gold particles : A flexible approach to macroscopic metal surfaces

Abstract: Covalent attachment of nanometer-scale colloidal Au particles to organosilane-coated substrates is a flexible and general approach to formation of macroscopic Au surfaces that have well-defined nanostructure. Variations in substrate (glass, metal, Al2O3), geometry (planar, cylindrical), functional group (−SH, −P(C6H5)2, −NH2, −CN), and particle diameter (2.5−120 nm) demonstrate that each component of these assemblies can be changed without adverse consequences. Information about particle coverage and interparticle spacing has been obtained using atomic force microscopy, field emission scanning electron microscopy, and quartz crystal microgravimetry. Bulk surface properties have been probed with UV−vis spectroscopy, cyclic voltammetry, and surface enhanced Raman scattering. Successful application of the latter two techniques indicates that these substrates may have value for Raman and electrochemical measurements. The assembly method described herein is compared with previous methods for controlling the na...

Experimental investigation of the melting transition of the plasma crystal.

Abstract: Measurements of the melting transition of a Coulomb crystal consisting of dust particles immersed in an rf parallel plate discharge in helium were performed. The dust crystal is shown to be solid at higher gas pressure (120 Pa) and low discharge power (10--20 W). Reducing the gas pressure or increasing the discharge power leads to fluid states of the dust ensemble. Even gaslike states are observed at low pressures of about 40 Pa. The transition is attributed to an increasing effective particle temperature. The phase transition is compared with one-component-plasma and Yukawa models, and with basic predictions of theories for two-dimensional melting. \textcopyright{} 1996 The American Physical Society.

Photonic band structure of fcc colloidal crystals.

Abstract: Polystyrene colloidal crystals form three dimensional periodic dielectric structures which can be used for photonic band structure measurements in the visible regime. From transmission measurements the photonic band structure of an fcc crystal has been obtained along the directions between the $L$ point and the $W$ point. Kossel line patterns were used for locating the symmetry points of the lattice for exact positioning and orientation of the crystals. In addition, these patterns reveal the underlying photonic band structure of the crystals in a qualitative way.

Fluctuation formula for elastic constants

Abstract: In thermal motion, the instantaneous stresses and strains are correlated throughout phase space---a fluctuation equation for the elastic constants is presented here that takes advantage of this fact. Results obtained with a nearest-neighbor Lennard-Jones fcc crystal indicate that the equation is considerably more efficient than the Parrinello-Rahman fluctuation formula. \textcopyright{} 1996 The American Physical Society.

High‐Resolution Structure (1.33 Å) of a HEW Lysozyme Tetragonal Crystal Grown in the APCF Apparatus. Data and Structural Comparison with a Crystal Grown under Microgravity from SpaceHab‐01 Mission

Abstract: Crystals of tetragonal hen egg-white lysozyme were grown using Advanced Protein Crystallization Facility (APCF) apparatus under a microgravity environment (SpaceHab-01 mission) and ground control conditions. Crystals were grown from NaCl as a crystallizing agent at pH 4.3. The X-ray diffraction patterns of the best diffracting ground- and space-grown crystals were recorded using synchrotron radiation and an image plate on the W32 beamline at LURE. Both ground- and space-grown crystals showed nearly equivalent maximum resolution of 1.3–1.4 A. Refinements were carried out with the program X-PLOR with final R values of 18.45 and 18.27% for structures from ground- and space- grown crystals, respectively. The two structures are nearly identical with the root-mean-square difference on all protein atoms being 0.13 A. Some residues of the two refined structures show multiple alternative conformations. Two ions were localized into the electron-density maps of the two structures: one chloride ion at the interface between two symmetry-related molecules and one sodium ion stabilizing the loop Ser60–Leu75. The sodium ion is surrounded by six ligands which form a bipyramid around it at distances of 2.2–2.6 A.

The vacuum ultraviolet phase‐matching characteristics of nonlinear optical KBe2BO3F2 crystal

Abstract: The nonlinear optical crystal KBe2BO3F2 has been used to produce vacuum ultraviolet second‐harmonic generation with an output range of 200–184.7 nm. The Sellmeier equation further indicates that this new crystal can generate the sixth harmonic from Nd‐based laser systems.

Ab Initio Theory of NMR Chemical Shifts in Solids and Liquids.

Abstract: We present a theory for the {ital ab} {ital initio} computation of NMR chemical shifts ({sigma}) in condensed matter systems, using periodic boundary conditions. Our approach can be applied to periodic systems such as crystals, surfaces, or polymers and, with a supercell technique, to nonperiodic systems such as amorphous materials, liquids, or solids with defects. We have computed the hydrogen {sigma} for a set of free molecules, for an ionic crystal LiH, and for a H-bonded crystal HF, using density functional theory in the local density approximation. The results are in excellent agreement with experimental data. {copyright} {ital 1996 The American Physical Society.}

Relationship between melting and amorphization of ice

Abstract: THE discovery1 in 1984 that an ice crystal can be transformed by pressure to an amorphous phase has since been followed by other examples of pressure-induced amorphization2. This transition, like melting, involves loss of long-ranged order, prompting the question of whether the two transitions are related. Here I describe experiments probing this relationship for a form of crystalline ice (denoted Ih) which is melted and amorphized by pressure. To avoid the complication of crystal–crystal transformations interrupting the melting process I use an ice emulsion, in which the very small particle size (about 5μm) suppresses nucleation of other crystal phases. As the temperature is decreased, I see a smooth crossover from (pressure-induced) equilibrium melting to sluggish amorphization at around 140–165 K. In this temperature range, ice Ih became 'supercompressed' before melting to a highly viscous liquid which seemed to be related to an imperfectly relaxed amorphous ice. Below about 140 K, ice Ih was transformed to an unrelaxed phase apparently related to the high-density amorphous form of ice. This sequence of transitions can be viewed as a crossover from a two-phase melting process (which is determined by the relative free energies of the solid and liquid phases) towards a one-phase amorphization process (where the transition is induced by a mechanical instability limit of the solid).

Gold Is Smaller than Silver. Crystal Structures of (Bis(trimesitylphosphine)gold(I)) and (Bis(trimesitylphosphine)silver(I)) Tetrafluoroborate

Abstract: Although it may have gone largely unnoticed, there is considerable confusion in handbooks of physical data as well as in chemistry textbooks and periodic tables concerning the relative sizes of silver and gold atoms. Values quoted for the ionic or covalent radii for the most common oxidation state +1 are either approximately equal for the two metals or larger for gold than for silver.1-4 The Pauling covalent radii for the two metals are essentially equal, which is due to the fact that the “metallic radius” in the close-packed cubic lattices happens to be virtually the same [the lattice constants are 4.0862 (Ag) and 4.07825 A (Au),5 and the nearest-neighbor interatomic distances are 2.889 (Ag-Ag) and 2.884 A (Au-Au) for coordination number 12].6 On the other hand, recent theoretical calculations including relativistic and correlation effects consistently predict that gold should be significantly smaller than silver, a phenomenon which is generally referred to as the “relativistic contraction”.7-9 In more qualitative terms, the concept of the “Lanthanide contraction”,7,8 employed successfully for other radius discontinuities in the periodic table, also points in the same direction.10 It appears, however, that no attempt has been made to settle this simple question by an experiment which can give unambiguous results. The most straightforward approach to this problem would be a comparison of metal-to-ligand bond lengths in a set of complexes involving (a) the same ligands and counterions, (b) the same coordination number and geometry, (c) an isomorphous crystal lattice, and (d) equal experimental conditions. More often than not these conditions are not fulfilled, since Ag(I) and Au(I) cations form compounds which differ significantly in their basic structure,11,12 such that a direct comparison is not meaningful. We have now discovered that the pair of title compounds meets all criteria on which to base the desired direct comparison, and from accurate single-crystal work we find that gold(I) is indeed much smaller than silver(I), by almost 0.1 A.12 The two reference complexes are readily prepared from AgBF4 and two equiv of Mes3P in dichloromethane (95% yield, mp 193 °C) or from equimolar quantities of (Mes3P)AuCl, Mes3P, and AgBF4 in CH2Cl2 (98% yield, mp 232 °C), respectively. The products can be obtained as large, transparent, isomorphous crystals (trigonal, space group P3121, Z ) 3), which are stable to air, moisture, and light at ambient temperature.13 Their analytical and spectroscopic data are in full agreement with the proposed compositions.14 Selected crystal data for [(Mes3P)2M]BF4 presented in Table 1 show the close crystallographic resemblance of the two unit cells, which suggest a very similar crystal field environment for the individual components. Both compounds are ionic in the crystal with no significant sub-van der Waals contacts between the ions. The cations have a crystallographically imposed 2-fold axis passing through the metal atom perpendicular to the P-M-P axis and relating the two phosphine ligand propellers, which thus have the same directionality (leftor right-handed propellers).14 The metal atoms are essentially linearly two-coordinate with bond angles which deviate from perfect collinearity by less than 0.3°. Selected distances and angles are compared in Table 2. The structure of the cation of the gold complex is shown in Figure 1, and Figure 2 offers a superposition of the structures of the gold and silver complexes. It is obvious from this diagram that there is almost perfect agreement of all details except for the M-P distance, which is smaller for M ) Au than for M ) Ag by 0.09(1) A. Assuming a covalent radius of

Adsorption of monolayers of P2Mo18O626- and deposition of multiple layers of Os(bpy)32+-P2Mo18O626- on electrode surfaces

Abstract: The unusually strong, irreversible adsorption of monolayer quantities of the P2Mo18O626- anion on glassy carbon, highly ordered pyrolytic graphite, indium tin oxide, and gold-coated quartz electrodes was examined by electrochemical, spectroelectrochemical, and quartz crystal microgravimetric techniques. A very simple method was developed to deposit multiple layers of composites consisting of alternating layers of the P2Mo18O626- anion and large, multiply-charged cations such as Os(bpy)32+. The resulting deposit is stable and exhibits electroactivity from both of the components that reflects the structure and ionic conductivity of the solid multilayer.

Alignment and instability of dust crystals in plasmas.

Abstract: Charged dust particulates, forming a layered crystal in the electrode sheath of a rf discharge, are known to show vertical alignment and an onset of characteristic oscillations below a threshold of neutral gas density. Here forces on the particulates due to the formation of positive space-charge clouds below the dust particles are calculated from Monte Carlo calculations of the ion motion in the sheath. The forces are shown to be attractive and nonreciprocal for the different crystal layers. From the Monte Carlo results an analytical lattice model is derived that quantitatively explains the experimental findings. \textcopyright{} 1996 The American Physical Society.

Spectroscopic data of the 1.8, 2.9 and 4.3µm transitions in dysprosium-doped Ga:La:S glass

Abstract: Infrared emission at 1.8, 2.9, and 4.3 µm is measured in dysprosium-doped gallium lanthanum sulfide (Ga:La:S) glass excited at 815 nm. Emission cross sections were calculated by Judd-Ofelt analysis, the Fuchtbauer-Ladtenburg equation, and the theory of McCumber. The sigma tau value for the 4.3-µm transition is ~4000 times larger in the Ga:La:S glass than in a dysprosium-doped LiYF4 crystal, which has lased on this transition. The large sigma tau value and the recently reported ability of Ga:La:S glass to be fabricated into fiber form show the potential for an efficient, low-threshold mid-infrared fiber laser. The fluorescence peak at 4.3 µm coincides with the fundamental absorption of atmospheric carbon dioxide, making the glass a potential laser source for gas-sensing applications.

Structure and stability of the plasma crystal

Abstract: The plasma crystal formed by monodisperse particles trapped in the sheath of an rf discharge is known to show vertically aligned structures. Here, oscillations of the aligned particles are found below a threshold value of gas density as a precursor of the melting transition. Attractive forces due to the formation of a positive space-charge region below the upper particle are calculated from Monte Carlo simulations of ion trajectories in the sheath. The alignment as well as the oscillations of the plasma crystal are explained by a simple model based on the asymmetry of the forces.

Study of localized and extended excitons in 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) I. Spectroscopic properties of thin films and solutions

Abstract: Excitonic transitions responsible for low energy absorption and fluorescence in dilute solutions and thin films of the organic molecule 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) have been investigated. Combining the results of concentration-dependent solution and thin film absorption and fluorescence with results on thin film exciton diffusion lengths (presented in the following paper, Paper II), we proposed a comprehensive model for the low energy excitonic states in this archetype molecular crystal. We unambiguously identify an extended free charge transfer state at E F CT = 2.23±0.03 eV which has previously been observed to exhibit effects of quantum confinement in ultrathin layers grown under ultrahigh vacuum. This state self-traps at an energy of E ST CT = 2.11±0.04 eV due to the strong exciton-phonon coupling characteristic of this closely packed organic crystal.

Sensitivity of cirrus cloud albedo, bidirectional reflectance and optical thickness retrieval accuracy to ice particle shape

Abstract: We examine the sensitivity of cirrus cloud albedo and bidirectional reflection function to particle shape using the phase functions of liquid water spheres, regular hexagonal ice crystals, and random-fractal ice particles calculated at a nonabsorbing visible wavelength of 0.63 μm. Accurate multiple-scattering calculations for plane-parallel clouds show that hexagonal ice crystal clouds have systematically larger planetary and global albedos than liquid water clouds of the same optical thickness. There is accumulating evidence that the idealized phase function of regular hexagonal crystals, which causes pronounced halos, is not necessarily the best representation of the range of reflectance characteristics of the majority of ice clouds. A more typical representation of the scattering phase function for ice clouds that are composed of a complex set of crystal shapes and sizes may be obtained using a model of randomly shaped irregular particles. Even larger cloud albedos are obtained for the random-fractal particle model because of its smaller asymmetry parameter. Our computations also show that a larger planetary albedo does not always imply a larger reflectance and that the relative brightness of ice versus liquid water clouds is highly scattering-geometry dependent. Use of the wrong particle shape model (crystal instead of water droplet and vice versa) in retrieving cloud optical thickness from bidirectional reflectance measurements can result in an underestimation or overestimation of the true optical thickness by a factor that can exceed 3. At some scattering geometries, use of the wrong model can give an unrealistically large optical thickness or no solution at all. Overall, bidirectional reflectance differences between random-fractal and regular hexagonal particle shapes are significantly smaller than those between either ice crystal and liquid water spheres, except at the back scattering direction.

Modeling cirrus clouds. Part II: Treatment of radiative properties

Abstract: A new radiation scheme, suitable for two-stream radiation transfer models, was developed for cirrus clouds. Analytical expressions were derived for the extinction and absorption coefficients and the asymmetry parameter. These are functions of the ice particle size distribution parameters, ice particle shapes, and wavelength. The ice particle shapes considered were hexagonal plates and columns, bullet rosettes, and planar polycrystals. These appear to be the principal crystal types found in cirrus clouds. The formulation of radiative properties accounts for the size distribution projected area and the distance radiation travels through ice particles. For absorption, refraction and internal reflection of radiation were parameterized. By assuming an idealized cirrus cloud, the dependence of the single scatter albedo, reflectance, and emissivity on wavelength, ice particle shape, and size distribution was demonstrated. Reflectance and emissivity exhibited a strong dependence on ice particle shape, wi...

Electrofreezing of Liquid Water: A Microscopic Perspective

Abstract: Using molecular dynamics simulations performed with the TIP4P and SPC/E pair potentials we examine the process of field-induced crystallization (electrofreezing) of supercooled liquid water. In the presence of an electric field the transformation of a liquid into a crystalline solid is observed for a narrow range of density and temperature on a time scale of a few hundred picoseconds. The resulting crystal has the structure of polar cubic ice I. TIP4P water appears to be more susceptible to nucleation than SPC/E water. For a field of 0.5 V/A TIP4P water undergoes crystallization at densities between 0.94 and 0.96 g/cm3 at a temperature of 250 K. Intermediate structures emerging during the electrofeeezing of water closely resemble a low-density amorphous ice. We observe formation of a high-density amorphous component at densities above those favoring the crystallization of the cubic ice. Application of an electric field to low-density water samples (0.90−0.92 g/cm3 for TIP4P water at 250 K) produces low-de...

Electro-optical switching characteristics of volume holograms in polymer dispersed liquid crystals

Abstract: Electrically switchable volume holograms lead to the possiblity of real-time electro-optical control of diffractive optic components. We report here on the development of a novel photopolymer-liquid crystal composite material system for writing in a fast single step, high diffraction efficiency volume holograms, capable of switching in applied electric fields of low voltage. Switching of a first-order Bragg diffracted beam into the zero-order with an applied field of ~10 V/µm was observed. With the addition of a surfactant to our pre-polymer syrup, we observed lowering of the switching fields to ~5 V/µm. We report response times for switching and relaxation in the order of microseconds. Low voltage, high resolution scanning electron microscopy studies show that the Bragg gratings formed consist of periodic polymer dispersed liquid crystal planes. The addition of surfactant leads to formation of very uniform small (20–40 nm) nematic droplets. A simple model based on the shape of the liquid crystal droplets...

Diffraction of Visible Light by Ordered Monodisperse Silica−Poly(methyl acrylate) Composite Films

Abstract: Composites of ordered colloidal crystals of amorphous monodisperse silica particles in amorphous poly(methyl acrylate) (PMA) films selectively Bragg diffract visible light. The 153 nm diameter colloidal silica particles coated with 3-(trimethoxysilyl)propyl methacrylate form colloidal crystals in methyl acrylate (MA) monomer, and the crystal order is locked into place by polymerization of the MA. Bragg diffraction from the films is detected spectrophotometrically as narrow peaks of low percent transmission of visible light normal to the film plane. The diffraction wavelength is tuned by varying the d spacing of the crystal lattice and by varying the Bragg angle. Variation in the lattice spacing is achieved via the particle size or particle concentration, uniaxial stretching of the composite, and swelling the composite with monomers such as styrene or MA followed by photopolymerization of the imbibed monomers. Films that diffract from 434 to 632 nm have been prepared.

The use of BaZrO3 crucibles in crystal growth of the high-Tc superconductors Progress in crystal growth as well as in sample quality

Abstract: Home-made BaZrO3 crucibles have been used as a crucible material for solution growth of single crystals of REBa2Cu3−δ (RE - Y, Er, Dy, Pr) and of Y1−xPrxBa2Cu3O7−δ single crystals (0 ≤ x ≤ 1). As we recently reported [Physica C 245 (1995) 245], this new crucible material does not react with the melts commonly used as flux [ A. Erb, PhD. thesis, Universitat Karlsruhe (1994) ]. As a consequence, the composition of the melt is not altered during crystal growth and the impurity content in the single crystals is much smaller than for crystals grown in other crucible materials. The absence of corrosion also makes the crystal growth far more efficient; thus it was worthwhile to grow crystals using high-purity (5N) starting materials, in order to further enhance their quality. The high quality of the 123 crystals grown in BaZrO3 crucibles is reflected in their physical properties. As an example, the first observation of the flux lattice with STM experiments on the 123 superconductors has been performed on an uncleaved crystal grown in BaZrO3 without any surface conditioning, thus revealing superconducting behaviour up to the uppermost layer of the crystal. Another example is furnished by low-temperature specific heat measurements: besides a very narrow superconducting transition, these measurements show the absence of the upturn usually attributed to flux inclusions and hence to magnetic ordering of the flux component BaCuO2 These results prove that the quality of 123 single crystals has been considerably improved by using BaZrO3 crucibles.

Modified Calcite Deposition Due to Ultrathin Organic Films on Silicon Substrates

Abstract: In order to study the effect of organic surface chemistry on calcite nucleation, attachment, and growth, calcium carbonate was precipitated in the presence of various ultrathin-film organosilane-modified silicon wafers. The chemistry of the aminosilane surfaces was systematically changed by the coupling of various acidic molecules, without creating a geometric lattice of acidic functional groups. Optical microscopy, scanning electron microscopy with image analysis, and X-ray scattering were employed to characterize crystallite density and orientation normal to the surface. Calcite grown on amino-modified surfaces was produced with the equilibrium rhombohedral habit and had the 〈104〉 orientation. Surfaces of the silicon oxide, carboxylate, iminodiacetate, or phosphoramidate tended to favor the orientation of surface crystals along 〈001〉 or near the 〈001〉 axes of the crystal. Primarily this is a result of the affinity of the surface for cations, but functional-group-mediated ion ordering and/or stereochemic...