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Cell Constants (cell + constant)
Selected AbstractsA Chemical Co-Reduction Route to Synthesize Nanocrystalline Vanadium CarbideJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2006Cun Li Nanocrystalline vanadium carbide (VC) was synthesized via a chemical co-reduction process, in which VCl4 and CCl4 were used as the vanadium source and the carbon source, respectively, and metallic Na as a reductant in an autoclave at 500°C for 12 h. X-ray powder diffraction indicated that the product was an NaCl type of VC with a cell constant a=4.171 Å. A transmission electron microscopy image showed the VC particles were 10,40 nm in size. X-ray photoelectron spectrum and Raman spectrum showed the surface covered with oxide and graphite. The formation of nanocrystalline VC was discussed based on thermodynamics. [source] Structure of the quaternary alloy Zn0.6Mn0.4In2S4 from synchrotron powder diffraction and electron transmission microscopyJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 1 2006Asiloé J. Mora The aim of the present work was to determine the structure of the quaternary alloy Zn0.6Mn0.4In2S4 and to locate the Mn2+. This was accomplished by means of powder synchrotron X-ray diffraction, high-resolution microscopy and convergent-beam electron diffraction (CBED). The powder X-ray diffraction pattern was indexed in a rhombohedral cell, with cell constants a = 3.875,(2), c = 37.208,(4),Å, and possible space groups Rm or R3m. Rietveld refinements using different cationic arrangements in these space groups were performed. A model in space group R3m, in which the tetrahedral and octahedral sites were occupied by different proportions of Zn, Mn and In atoms, gave the best result. The Rietveld refinement of this model led to figures of merit Rwp = 9.8%, Rp = 9.1% and ,2 = 11.1. Selected-area electron diffraction patterns and high-resolution transmission electron micrographs along [001] reveal the rhombohedral configuration. CBED patterns perpendicular to [001], showing the distinctive 3m symmetry, confirmed space group R3m and the breaking of the centrosymmetry of the parent compound, ZnIn2S4. [source] FBR: a robust method to determine the basis matrix of the Bravais lattice from oscillation imagesJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3-2 2000Klaus Döhring The FBR (Fourier basis reconstruction) method described in this paper has been designed to determine the basis matrix of the Bravais lattice with respect to the laboratory frame of reference and without prior knowledge of cell constants, particularly for protein crystals of comparatively low quality. It is based on Fourier analysis of a three-dimensional intensity distribution in reciprocal space, which is directly obtained from observed intensity distributions, provided that they are recorded by the rotation method using a fixed X-ray wavelength, resulting in a direct-space determination of the basis vectors. After a description of the motivation and theory behind the method, it is tested by application to numerically generated images of a virtual sample crystal and to experimental data of a lysozyme crystal with well known cell constants. Finally, FBR is applied to a set of images of bacteriorhodopsin crystals suffering from strong anisotropic spot broadening; this case provided the original motivation for the present work. [source] Structural transformations in organic crystals during photochemical reactionsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 10 2004Ilona Turowska-Tyrk Abstract In the 1980s and 1990s, x-ray studies of the photochemical reaction course in crystals dealt with the analysis of changes in cell constants or movements of atom groups inside molecules. This review presents the results of crystallographic studies on the monitoring of the behaviour of whole molecules in organic crystals during photochemical reactions. Papers on this subject started to appear only a few years ago. The studies showed quantitatively that reactant and product molecules do not take a fixed position in a crystal during the reaction. The product molecules move smoothly to a position assumed in the pure product crystal and the reactant molecules move from a position occupied in the pure reactant crystal. Moreover, with the reaction progress the adjacent reactant molecules gradually come closer and change their mutual orientation to resemble the product. The analysis of the photoreaction kinetics in crystals is also presented. Copyright © 2004 John Wiley & Sons, Ltd. [source] Scaling of one-shot oscillation images with a reference data setJOURNAL OF SYNCHROTRON RADIATION, Issue 1 2004Kunio Hirata By combining a least-squares procedure with the program MOSFLM, a program SCLONE has been developed which processes diffraction images that do not contain serial oscillation images and may have a few or no full reflections. After each image was processed by MOSFLM, the partialities and structure amplitudes of the reflections were estimated using a least-squares method to refine the scaling factor, the relative temperature factor, the mosaic spread, cell constants, and missetting angles for each independent image. The SCLONE calculation significantly improved the quality of the intensities from the reflections obtained by the initial MOSFLM calculation and crystal structural refinement confirmed the improvement. The SCLONE calculation indicated that the reflection of the present crystal had a rocking curve that was steeper at the middle of the profile and more gradual at both ends of the profile than that assumed in the program MOSFLM. [source] Monitoring structural transformations in crystals.ACTA CRYSTALLOGRAPHICA SECTION B, Issue 6 2007Structural changes proceeding in a crystal during the Yang photocyclization of the salt 6,6-diethyl-5-oxo-5,6,7,8-tetrahydronaphthalene-2-carboxylate with (1S)-1-(4-methylphenyl)ethylamine were monitored by means of X-ray structure analysis. The course of the photoreaction was evaluated on the basis of the geometrical parameters for the pure reactant crystal. Variations in the cell constants, the product content, the geometry of the reaction centre, the orientation of molecular fragments and the geometry of hydrogen bonds were described and analyzed. It was found that the cell volume increased until 56% product content and decreased thereafter. The distance between the directly reacting C atoms was constant, ,,3.0,Å, until ,,75% reaction progress. Analysis of the distance between atoms that would participate in the formation of the second (unobserved) enantiomorph excluded the formation of such an isomer. Molecular fragments varied their orientation during the photoreaction, and the largest change was observed for the carboxylate group despite its participation in strong hydrogen bonds. The geometry of the hydrogen bonds changed during the photoreaction. The largest change was 0.17,Å for the D...A distance and 13° for the D,H...A angle. A comparison of the intra- and intermolecular parameters for the studied salt with data for other compounds undergoing the Yang photocyclization in crystals revealed a diversity of structural changes brought about by this type of photochemical reaction. [source] Monitoring structural transformations in crystals.ACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2006Structural changes taking place in a crystal during an intramolecular photochemical reaction [the Yang photocyclization of the ,-methylbenzylamine salt with 1-(4-carboxybenzoyl)-1-methyladamantane] were monitored step-by-step using X-ray structure analysis. This is the first example of such a study carried out for an intramolecular photochemical reaction. During the photoreaction, both the reactant and product molecules change their orientation, but the reactant changes more rapidly after the reaction is about 80% complete. The distance between directly reacting atoms in the reactant molecule is almost constant until about 80% reaction progress and afterwards decreases. The torsion angle defined by the reactant atoms that form the cyclobutane ring also changes in the final stages of the photoreaction. These phenomena are explained in terms of the influence of many product molecules upon a small number of reacting molecules. The adamantane portion shifts more than the remaining part of the anionic reactant species during the reaction, which is explained in terms of hydrogen bonding. The structural changes are accompanied by changes in the cell constants. The results obtained in the present study are compared with analogous results published for intermolecular reactions. [source] Monitoring structural transformations in crystals.ACTA CRYSTALLOGRAPHICA SECTION B, Issue 5 2003The structural changes in a crystal of 5-benzylidene-2-(4-chlorobenzyl)cyclopentanone during [2,+,2]-photodimerization were monitored by means of X-ray diffraction. It was observed that the monomers moved gradually from the position occupied in the crystal at the initial stage of the photoreaction and the dimers moved towards the position assumed at the final step. The movements of the molecules possess a rotational component. Moreover, with the progress of the phototransformation the monomers in the reacting pair gradually move closer and change their relative orientation to resemble more the product molecule. The behaviour of the molecules and also the variation of the cell constants for the studied compound were compared with data for 5-benzylidene-2-benzylcyclopentanone. [source] Diastereomers (RC,SP)- and (RC,RP)- S -methyl P -(3-azidopropyl)- N -[(1R)-1-phenylethyl]phosphonamidothioateACTA CRYSTALLOGRAPHICA SECTION C, Issue 4 2009Lilu Guo Diastereoisomers of the title organophosphorus compound, C12H19N4OPS, denoted RCSP, (I), and RCRP, (II), were structurally characterized and compared. Asymmetric phosphorus compounds are of interest with regard to the use of these systems as possible protein probes via the stereoselective delivery of an azide group tethered to the P atom into key protein regions. The diastereomers were produced in a 1:1 mixture and isolated by chromatography. Although both isomers crystallize in the same space group with superficially similar cell constants, conformational and packing differences are pronounced. Despite the conformational differences, strong intermolecular hydrogen bonding links both isomers into chains parallel to the a axis [N...O = 2.8609,(18) and 2.966,(3),Å in (I) and (II), respectively], with C,H..., interchain interactions of ca 3.5,Å. [source] Atmospheric Pressure CVD of Molybdenum Diselenide Films on Glass,CHEMICAL VAPOR DEPOSITION, Issue 11 2006D. Boscher Abstract Atmospheric pressure (AP)CVD of molybdenum diselenide films on glass substrates was achieved by reaction of diethyl selenide or di- tert -butylselenide with MoCl5 at 500,650,°C. X-ray diffraction (XRD) showed that the MoSe2 films were crystalline with a mixed 2Hb/3R polytype stacking and typical cell constants of a,=,3.28, c,=,6.48,Å (c -axis is,×,2 for the 2Hb form and,×,3 for the 3R form). Energy dispersive X-ray analysis (EDAX) gave a Mo/Se ratio close to 1:2 for films formed at 600,650,°C, those formed at lower temperatures contained some chlorine. The films were brown in appearance, were adhesive, passed the Scotch tape, test but could be scratched with a steel scalpel. Scanning electron microscopy (SEM) showed that the films were composed of needlelike agglomerates which became longer and thinner with increasing deposition temperature. The films showed high absorbance in the visible spectrum but were more transparent in the near-infrared. [source] | |||||||||||||