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Crystal Faces (crystal + face)
Selected AbstractsCold pressing of copper single crystals for a large-area doubly focusing monochromatorJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3 2004D. F. R. Mildner The design for a large-area doubly focusing neutron monochromator consists of a large number of small square copper single crystals mounted onto thin aluminium blades that both buckle and rotate independently. This avoids the need for large individual alignment mechanisms for each crystal that introduce unacceptably high levels of background. However, it does require that the crystals be oriented such that the diffracting crystallographic planes are parallel to the crystal face. Cold pressing broadens the natural narrow mosaic of the virgin crystal discs to increase the diffracted intensity. This introduces anisotropy into the crystal that determines its orientation in the final monochromator. The alignment procedure used for each crystal before cutting out the square tile in the correct orientation is described. A few crystals are characterized in detail by neutron diffraction at various stages of the operation, revealing the variation in the mosaic width and the angular position of the reciprocal-lattice vector as a function of the azimuthal angle by rotating the crystal about the normal to its face. The twofold symmetry of the mosaic width of the pressed crystal is modulated by the 2, periodicity introduced by the precession of the reciprocal-lattice vector around the crystal face normal. Satisfactorily aligned crystals have a variation in the angular position for diffraction within the allowed tolerance. [source] A study of sulfamerazine single crystals using atomic force microscopy, transmission light microscopy, and Raman spectroscopyJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 9 2005Xiaoping Cao Abstract Sulfamerazine (SMZ) Form I and II single crystals were prepared from aqueous dispersions of SMZ bulk samples and studied using several microscopic and spectroscopic techniques. Transmission light microscopy and Raman spectroscopy were used to observe and identify single crystals. The results indicated that Form I single crystals tended to be rectangular laths while Form II ones tended to be hexagonal laths. Surface morphology of individual single crystals was further investigated by atomic force microscopy (AFM). AFM images revealed a smooth top surface, a uniform height, and sharp edges for both forms of single crystals. Both height and phase images showed crystalline terraces with different step heights for the top surface of Form I. Surface properties of single crystals were evaluated using AFM force measurements. Experimental results indicated that the top surface of Form I single crystals was more hydrophilic than that of Form II. Theoretical calculations predicted a dominant crystal face of (020) for the Form I single crystals and (002) for the Form II ones. The correlations between calculation predictions and experimental results were discussed. © 2005 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 94:1881,1892, 2005 [source] Polarization dependences of absorption and luminescence spectra on each crystal face of ,-quaterthiophene and ,-quinquethiophenePHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 6 2010Shin Tanaka Abstract Polarization dependence of the absorption spectra (ABS) and the luminescence spectra in ,-quaterthiophene (4T) and ,-quinquethiophene (5T) has been measured directly with near normal incident light on the ab, bc, and ca crystal planes of their thick single crystals (SCs). Vibronic excitons (Frenkel excitons) have been observed on each crystal plane of 4T and 5T. The polarizations of the ABS are consistent with the prediction obtained from the MOPAC calculations of the transition dipole moment directions of the molecules in the SCs. Excited electron relaxes to the lowest excited state which is composed of the LUMO states. [source] In situ measurement of growth kinetics of {100} KDP crystal faces in the presence of polyphosphate impuritiesCRYSTAL RESEARCH AND TECHNOLOGY, Issue 7 2008Bing Liu Abstract The face growth rate and critical supersaturation of {100} face were in situ measured using the laser-polarization-interference technique in the presence of potassium pyrophosphate, trimetric sodium phosphate and sodium hexametaphosphate impurities. The polyphosphate impurities inhibit the growth rate of prismatic faces. The face growth rate as a function of supersaturation at different impurity concentrations, as well as critical supersaturation as a function of impurity concentrations, was found in good agreement with a two-dimensional nucleation model in the pure system and Kubota and Mullin's model in the presence of impurities. The average distance L between active sites available for impurity adsorption as well as the edge free energy was calculated. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Rapid crystal growth without inherent supersaturation induced by nanoscale fluid flows?CRYSTAL RESEARCH AND TECHNOLOGY, Issue 1 2006M. J. Jones Abstract Crystal growth is a process that only takes place under non-equilibrium conditions and a necessary prerequisite is that the crystal is exposed to a phase that is supersaturated in the material the crystal is composed of, be it a solution, a vapour or a supercooled melt. In industrial mass crystallization the growth rate for a population of crystals (in suspension growth processes [1]) rarely exceeds mean linear velocities of 10 -7 ms -1. Here we present a mass crystallization process which is accompanied by rapid crystal growth several orders of magnitude faster and into a region of solution that is without inherent supersaturation. The material investigated is a solid hydrate that exhibits a solution mediated phase transition to its anhydrous form in the presence of methanol [2]. The phase transition is initiated simply by placing an amount of hydrate crystals into the solvent and is characterized by the rapid emergence of needle-shaped crystals. The needles emanate from the crystal faces of the hydrate crystals and grow into the solution, which is nominally free of the substance to be crystallized. The high growth rate of the crystals, which of the order of up to 10 -4 ms -1 is surprising. Although rapid needle growth has been observed before [3-9], to date a satisfactory explanation for needles growing under the abovementioned conditions is still outstanding. Based upon the topology of the crystals we propose a tentative mechanism for this phenomenon capable of explaining the unusually rapid growth and highlight those questions that need addressing in order to verify this mechanism. X-ray powder diffraction is used to characterize the crystal phase of the needles; confocal fluorescence microscopy reveals that the needles are hollow. The width of these needles is between 0.5 and 5 ,m, their length appears to be limited only by the amount of hydrate available for their formation. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Retardation of setting of plaster of Paris by organic acids: Understanding the mechanism through molecular modelingJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 12 2004Jörg-Rüdiger Hill Abstract To develop an understanding of the action of specific formulations, the growth of gypsum crystals under the influence of retardation agents (tartaric and citric acid) has been studied using molecular modeling. Surface energies of gypsum and plaster crystal faces were calculated using established protocols. The crystal morphology predicted for gypsum crystals in the absence of retardation agents is in excellent agreement with experiment. The simulations show that only in an alkaline environment is the crystal morphology of gypsum changed by retardation agents. The simulations provide a detailed description of retardation, for example, the specific mechanisms by which tartaric and citric acid retard setting of gypsum and how they differ. At high pH meso, D(,), and L(+) tartaric acid inhibit both the growth of gypsum and the dissolution of plaster while at low pH tartaric acid and citric acid will principally inhibit the growth of gypsum. The simulations provide a molecular rationalization for a range of experimental observations and a basis for the selection of alternate retardation agents. © 2004 Wiley Periodicals, Inc. J Comput Chem 25: 1438,1448, 2004 [source] Crystal growth rate dispersion modeling using morphological population balanceAICHE JOURNAL, Issue 9 2008Cai Y. Ma Abstract Crystal growth in solution is a surface-controlled process. The variation of growth rates of different crystal faces is considered to be due to the molecular arrangement in the crystal unit cell as well as the crystal surface structures of different faces. As a result, for some crystals, the growth rate for a specific facet is not only a function of supersaturation, but also dependent on some other factors such as its size and the lattice spread angle. This phenomenon of growth rate dispersion (GRD) or fluctuation has been described in literature to have attributed to the formation of some interesting and sophisticated crystal structures observed in experimental studies. In this article, GRD is introduced to a recently proposed morphological population balance model to simulate the dynamic evolution of crystal size distribution in each face direction for the crystallization of potash alum, a chemical that has been reported to show GRD phenomenon and sophisticated crystal structures. The GRD is modeled as a function of the effective relative supersaturation, which is directly related to crystal size, lattice spread angle, relative supersaturation, and solution temperature. The predicted results clearly demonstrated the significant effect of GRD on the shape evolution of the crystals. © 2008 American Institute of Chemical Engineers AIChE J, 2008 [source] Template-Directed Control of Crystal MorphologiesMACROMOLECULAR BIOSCIENCE, Issue 2 2007Fiona C. Meldrum Abstract Biominerals are characterised by unique morphologies, and it is a long-term synthetic goal to reproduce these synthetically. We here apply a range of templating routes to investigate whether a fascinating category of biominerals, the single crystals with complex forms, can be produced using simple synthetic methods. Macroporous crystals with sponge-like morphologies identical to that of sea urchin skeletal plates were produced on templating with a sponge-like polymer membrane. Similarly, patterning of individual crystal faces was achieved from the micrometer to nanometer scale through crystallisation on colloidal particle monolayers and patterned polymer thin films. These experiments demonstrate the versatility of a templating approach to producing single crystals with unique morphologies. [source] Morphology and the strength of intermolecular contacts in protein crystalsACTA CRYSTALLOGRAPHICA SECTION D, Issue 8 2003Yoshiki Matsuura The strengths of intermolecular contacts (macrobonds) and the areas occupied by each contact on the molecular surface were estimated in four polymorphic modifications of lysozyme crystals based on the bond strengths between individual atomic pairs belonging to the molecules in contact. It has been shown that the periodic bond chains of these macrobonds account for the morphology of protein crystals. The Coulombic contribution to the macrobond strength has also been estimated. Making use of the contact strengths and taking into account bond hydration, crystal,water interfacial energies were also estimated for different crystal faces. The areas of all contacts are mapped on the molecular surface, making use of a polar-coordinate representation of the contact. Comparing the locations of the intermolecular contacts in the different polymorphic crystal modifications, it is shown that these contacts can form a wide variety of patches on the molecular surface. The patches are located practically everywhere on the surface except for the inside of a concave active site. It is also shown that the contacts, which frequently involve water molecules, are formed by specific intermolecular hydrogen bonds on a background of non-specific attractive electrostatic interactions. Typical values of the macrobond strength are compared with the strength of association in other protein-complex systems. [source] | |||||||||||||