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Powder Pattern (powder + pattern)
Kinds of Powder Pattern Selected AbstractsCorrelating sideband patterns with powder patterns for accurate determination of chemical shift parameters in solid-state NMRMAGNETIC RESONANCE IN CHEMISTRY, Issue 10 2008M. S. Ironside Abstract Powder patterns and sideband patterns have different strengths when it comes to using them to determine chemical shift parameters. Here, we show that chemical shift parameters can be determined with high accuracy by analysing the correlation pattern from a 2D experiment which correlates a powder pattern in the indirect dimension with a sideband pattern in the direct dimension. The chemical shift parameters so determined have greater accuracy than those obtained by analysing a sideband or powder pattern alone, for the same signal-to-noise ratio. This method can be applied for both resolved correlation patterns and to cases where two components share similar isotropic chemical shifts. The methodology is demonstrated in this paper, both theoretically and experimentally, on the 31P signals of the bis-phosphonate drug, pamidronate. Copyright © 2008 John Wiley & Sons, Ltd. [source] Simultaneous refinement of structure and microstructure of layered materialsJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 1 2004Matteo Leoni The recursive description of stacking in layered crystals, originally developed by Treacy et al. [Proc. R. Soc. London Ser. A (1991), 433, 499,520] and implemented in the DIFFaX code, is enclosed in a non-linear least-squares minimization routine and combined with additional models (of specimen-related broadening and instrumental broadening) to allow the simultaneous refinement of both structural and microstructural parameters of a layered crystal. This implementation is named DIFFaX+. As examples, the refinements both of a simulated pattern of diamond, showing fault clustering, and of the observed powder pattern of a synthetic stoichiometric nanocrystalline chrysotile are reported. [source] Effect of a crystallite size distribution on X-ray diffraction line profiles and whole-powder-pattern fittingJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3-2 2000J. I. Langford A distribution of crystallite size reduces the width of a powder diffraction line profile, relative to that for a single crystallite, and lengthens its tails. It is shown that estimates of size from the integral breadth or Fourier methods differ from the arithmetic mean of the distribution by an amount which depends on its dispersion. It is also shown that the form of `size' line profiles for a unimodal distribution is generally not Lorentzian. A powder pattern can be simulated for a given distribution of sizes, if it is assumed that on average the crystallites have a regular shape, and this can then be compared with experimental data to give refined parameters defining the distribution. Unlike `traditional' methods of line-profile analysis, this entirely physical approach can be applied to powder patterns with severe overlap of reflections, as is demonstrated by using data for nanocrystalline ceria. The procedure is compared with alternative powder-pattern fitting methods, by using pseudo-Voigt and Pearson VII functions to model individual line profiles, and with transmission electron microscopy (TEM) data. [source] MarqX: a new program for whole-powder-pattern fittingJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 1 2000Y. H. Dong MarqX is a computer program for the modelling of powder diffraction data. It can be used for an unconstrained profile fitting (pattern decomposition, PD) or constrained modelling of the whole powder pattern (Pawley method, PM), for single- as well as multiple-phase samples. The program output includes: lattice parameters or peak positions (for PM and PD, respectively), width and shape of the diffraction peak (in terms of half width at half-maximum and mixing parameter of a pseudo-Voigt function), corrected for the instrumental broadening component, intensity, peak area and profile asymmetry. In addition, errors on the goniometer zero and shift in sample position with respect to the goniometric axis can also be modelled, together with distance and relative intensity of the spectral components of the X-ray beam (e.g.K,1 and K,2). Specific output files are provided for line-profile analysis, including the Williamson,Hall plot and Warren,Averbach method. [source] Correlating sideband patterns with powder patterns for accurate determination of chemical shift parameters in solid-state NMRMAGNETIC RESONANCE IN CHEMISTRY, Issue 10 2008M. S. Ironside Abstract Powder patterns and sideband patterns have different strengths when it comes to using them to determine chemical shift parameters. Here, we show that chemical shift parameters can be determined with high accuracy by analysing the correlation pattern from a 2D experiment which correlates a powder pattern in the indirect dimension with a sideband pattern in the direct dimension. The chemical shift parameters so determined have greater accuracy than those obtained by analysing a sideband or powder pattern alone, for the same signal-to-noise ratio. This method can be applied for both resolved correlation patterns and to cases where two components share similar isotropic chemical shifts. The methodology is demonstrated in this paper, both theoretically and experimentally, on the 31P signals of the bis-phosphonate drug, pamidronate. Copyright © 2008 John Wiley & Sons, Ltd. [source] X-ray powder diffraction and electron diffraction studies of the thortveitite-related L phase, (Zn,Mn)2V2O7ACTA CRYSTALLOGRAPHICA SECTION B, Issue 2 2009Kevin M. Knowles The phase designated ,-Zn3(VO4)2 reported as a minor second phase in zinc oxide-based varistor materials doped with vanadium oxide and manganese oxide is shown to be the L phase, (Zn1,,,xMnx)2V2O7 (0.188 < x < 0.538), in the pseudo-binary Mn2V2O7,Zn2V2O7 system. Analysis of X-ray powder diffraction patterns and electron diffraction patterns of this phase shows that the previously published a, c and , values for this thortveitite-related phase are incorrect. Instead, Rietveld refinement of the X-ray powder pattern of the L phase shows that it has a monoclinic C lattice with Z = 6, with a = 10.3791,(1), b = 8.5557,(1), c = 9.3539,(1),Å and , = 98.467,(1)°. Although prior convergent-beam electron diffraction work of `,-Zn3(VO4)2' confirmed the C Bravais lattice, the space group was found to be Cm rather than C2/m, the difference perhaps arising from the inability of the X-rays to detect small displacements of oxygen. Attempts to refine the structure in Cm did not produce improved R factors. The relationship between the crystal structure of the L phase and the high-temperature C2/m,,-Zn2V2O7 thortveitite-type solid solution is discussed. [source] Rietveld refinement of a wrong crystal structureACTA CRYSTALLOGRAPHICA SECTION B, Issue 6 2007Christian Buchsbaum Rietveld refinements are generally used to confirm crystal structures solved from powder diffraction data. If the Rietveld refinement converges with low R values and with a smooth difference curve, and the structure looks chemically sensible, the resulting structure is generally considered to be close to the correct crystal structure. Here we present a counter example: The Rietveld refinement of the X-ray powder pattern of ,-quinacridone with the crystal structure of ,-quinacridone gives quite a smooth difference curve; the resulting crystal structure looks reasonable in terms of molecular conformation, molecular packing and intermolecular hydrogen bonds. However, neither the lattice parameters, the molecular packing nor the conformation of the molecules show any similarity with the actual structure, which was determined from single-crystal data. This example shows that a successful Rietveld refinement is not always final proof of the correctness of a crystal structure; in special cases the resulting crystal structure may still be wrong. [source] Intermolecular dihydrogen- and hydrogen-bonding interactions in diammonium closo -decahydrodecaborate sesquihydrateACTA CRYSTALLOGRAPHICA SECTION C, Issue 1 2010Teshome B. Yisgedu The asymmetric unit of the title salt, 2NH4+·B10H102,·1.5H2O or (NH4)2B10H10·1.5H2O, (I), contains two B10H102, anions, four NH4+ cations and three water molecules. (I) was converted to the anhydrous compound (NH4)2B10H10, (II), by heating to 343,K and its X-ray powder pattern was obtained. The extended structure of (I) shows two types of hydrogen-bonding interactions (N,H...O and O,H...O) and two types of dihydrogen-bonding interactions (N,H...H,B and O,H...H,B). The N,H...H,B dihydrogen bonding forms a two-dimensional sheet structure, and hydrogen bonding (N,H...O and O,H...O) and O,H...H,B dihydrogen bonding link the respective sheets to form a three-dimensional polymeric network structure. Compound (II) has been shown to form a polymer with the accompanying loss of H2 at a faster rate than (NH4)2B12H12 and we believe that this is due to the stronger dihydrogen-bonding interactions shown in the hydrate (I). [source] Variable-Temperature Powder X-ray Diffraction of Aromatic Carboxylic Acid and Carboxamide CocrystalsCHEMISTRY - AN ASIAN JOURNAL, Issue 4 2007L. Sreenivas Reddy Abstract The effect of temperature on the cocrystallization of benzoic acid (BA), pentafluorobenzoic acid (FBA), benzamide (BAm), and pentafluorobenzamide (FBAm) is examined in the solid state. BA and FBA formed a 1:1 complex 1 at ambient temperature by grinding with a mortar and pestle. Grinding FBA and BAm together resulted in partial conversion into the 1:1 adduct 2 at 28,°C and complete transformation into the product cocrystal at 78,°C. Further heating (80,100,°C) and then cooling to room temperature gave a different powder pattern from that of 2. BAm and FBAm hardly reacted at ambient temperature, but they afforded the 1:1 cocrystal 3 by melt cocrystallization at 110,115,°C. Both BA+FBAm (4) and BA+BAm (5) reacted to give new crystalline phases upon heating, but the structures of these products could not be determined owing to a lack of diffraction-quality single crystals. The stronger COOH and CONH2 hydrogen-bonding groups of FBA and FBAm yielded the equimolar cocrystal 6 at room temperature, and heating of these solids to 90,100,°C gave a new crystalline phase. The X-ray crystal structures of 1, 2, 3, and 6 are sustained by the acid,acid/amide,amide homosynthons or acid,amide heterosynthon, with additional stabilization from phenyl,perfluorophenyl stacking in 1 and 3. The temperature required for complete transformation into the cocrystal was monitored by in,situ variable-temperature powder X-ray diffraction (VT-PXRD), and formation of the cocrystal was confirmed by matching the experimental peak profile with the simulated diffraction pattern. The reactivity of H-bonding groups and the temperature for cocrystallization are in good agreement with the donor and acceptor strengths of the COOH and CONH2 groups. It was necessary to determine the exact temperature range for quantitative cocrystallization in each case because excessive heating caused undesirable phase transitions. [source] Crystal structure prediction of organic pigments: quinacridone as an exampleJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 1 2007N. Panina The structures of the ,, , and , polymorphs of quinacridone (Pigment Violet 19) were predicted using Polymorph Predictor software in combination with X-ray powder diffraction patterns of limited quality. After generation and energy minimization of the possible structures, their powder patterns were compared with the experimental ones. On this basis, candidate structures for the polymorphs were chosen from the list of all structures. Rietveld refinement was used to validate the choice of structures. The predicted structure of the , polymorph is in accordance with the experimental structure published previously. Three possible structures for the , polymorph are proposed on the basis of X-ray powder patterns comparison. It is shown that the , structure in the Cambridge Structural Database is likely to be in error, and a new , structure is proposed. The present work demonstrates a method to obtain crystal structures of industrially important pigments when only a low-quality X-ray powder diffraction pattern is available. [source] Rietveld quantitative amorphous content analysisJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2 2001A. G. De La Torre A procedure for Rietveld quantitative amorphous content analysis (RQACA) is outlined, in which the effects of systematic errors in the powder patterns are studied. The method derives the amorphous content from the small overestimation of an internal crystalline standard in a Rietveld refinement of an appropriate mixture. Of several standards studied, Al2O3 gave the best results. The statistical analysis of standard mixtures with a known amount of amorphous content indicated that this is a precise and accurate tool. It enables the measurement of the amorphous content with an accuracy close to 1%. Sample preparation and Rietveld analysis need to be optimized in order to minimize the systematic errors. The analysis of samples with phases displaying strong preferred orientation effects gives very high errors in the amorphous content. Samples with different absorption coefficients have also been studied in order to evaluate the importance of microabsorption. This plays an important role but it can be adequately corrected if the absorption coefficients of the standard and the sample are not very different. RQACA has been applied to tricalcium silicate, C3S, which is the main component of Portland cement. The average amorphous content of C3S, after microabsorption correction using two standards of higher and lower absorption coefficients, was found to be 19%. [source] Synthesis and Characterization of Mixed-Metal Oxide Nanopowders Along the CoOx,Al2O3 Tie Line Using Liquid-Feed Flame Spray PyrolysisJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2006Jose Azurdia We report here the use of liquid-feed flame spray pyrolysis (LF-FSP) to produce a series of nanopowders along the CoOx,Al2O3 tie line. The process is a general aerosol combustion synthesis route to a wide range of lightly agglomerated oxide nanopowders. The materials reported here were produced by aerosolizing ethanol solutions of alumatrane [Al(OCH2CH2)3N] and a cobalt precursor, made by reacting Co(NO3)2·6H2O crystals with propionic acid. The compositions of the as-produced nanopowders were controlled by selecting the appropriate ratios of the precursors. Nine samples with compositions (CoO)y(Al2O3)1,y, y=0,1 along the CoOx,Al2O3 tie line were prepared and studied. The resulting nanopowders were characterized by X-ray fluorescence, BET, scanning electron microscopy, high-resolution transmission electron micrographs, X-ray diffraction (XRD), thermogravimetric analysis (TGA), and FTIR. The powders typically consist of single-crystal particles <40 nm diameter and specific surface areas (SSAs) of 20,60 m2/g. XRD studies show a gradual change in powder patterns from ,-Al2O3 to Co3O4. The cobalt aluminate spinel phase is observed at stoichiometries (21 and 37 mol%) not seen in published phase diagrams, likely because LF-FSP processing involves a quench of >1000°C in microseconds frequently leading to kinetic rather than thermodynamic products. Likewise, the appearance of Co3O4 rather than CoO as the end member in the tie line is thought to be a consequence of the process conditions. TGA studies combined with diffuse reflectance FTIR spectroscopic studies indicate that both physi- and chemi-sorbed H2O are the principal surface species present in the as-processed nanopowders. The only sample that differs is Co3O4, which has some carbonate species present that are detected and confirmed by a sharp mass loss event at ,250°C. The thermal behavior of the high cobalt content samples differs greatly from the low cobalt content samples. The latter behave like most LF-FSP-derived nanopowders exhibiting typical 1%,4% mass losses over the 1400°C range due mostly to loss of water and some CO2. The high cobalt content samples exhibit a sharp mass loss event that can be attributed to the decomposition of Co3O4 to CoO. [source] Correlating sideband patterns with powder patterns for accurate determination of chemical shift parameters in solid-state NMRMAGNETIC RESONANCE IN CHEMISTRY, Issue 10 2008M. S. Ironside Abstract Powder patterns and sideband patterns have different strengths when it comes to using them to determine chemical shift parameters. Here, we show that chemical shift parameters can be determined with high accuracy by analysing the correlation pattern from a 2D experiment which correlates a powder pattern in the indirect dimension with a sideband pattern in the direct dimension. The chemical shift parameters so determined have greater accuracy than those obtained by analysing a sideband or powder pattern alone, for the same signal-to-noise ratio. This method can be applied for both resolved correlation patterns and to cases where two components share similar isotropic chemical shifts. The methodology is demonstrated in this paper, both theoretically and experimentally, on the 31P signals of the bis-phosphonate drug, pamidronate. Copyright © 2008 John Wiley & Sons, Ltd. [source] A solid-state NMR investigation of the structure of nanocrystalline hydroxyapatiteMAGNETIC RESONANCE IN CHEMISTRY, Issue 6 2006Christian Jäger Abstract Nanocrystalline hydroxyapatite (HAp) prepared by a precipitation route was investigated. The X-ray diffraction (XRD) powder patterns of the elongated nanocrystals with a typical diameter of about 10 nm and length of 30,50 nm (by transmission electron microscopy (TEM)) revealed the presence of HAp with significantly broadened XRD reflections. However, Ca deficiency was found, as the Ca/P ratio was 1.5 only (so-called calcium-deficient hydroxyapatite (CDHA)), and not 1.67. This Ca deficiency of nanocrystalline HAp is explained using NMR. It is shown unambiguously that (i) the nanocrystals consist of a crystalline core and a (disordered) surface region with a relative phosphate content of about 1:1, (ii) the crystalline core is HAp, and (iii) the surface region is dominated by hydrogen phosphate anions (with no hydroxyapatite-like structural motif) and structural water (hydrate). From the relative phosphate content and taking into account the crystal shape, the thickness of the surface layer along the main crystal axis could be estimated to be about 1 nm, and the average chemical composition of the surface layer has been determined. Finally, a Ca/P ratio of 1.52 was estimated from the NMR data that compares well with the value of 1.51 from chemical analysis. The important consequences are that the surface of nanocrystalline HAp has nothing in common with the bulk composition and that the chemistry of such materials (e.g. the binding of protein molecules to phosphate surfaces) must be reconsidered. Copyright © 2006 John Wiley & Sons, Ltd. [source] Experimental and predicted crystal structures of Pigment Red 168 and other dihalogenated anthanthronesACTA CRYSTALLOGRAPHICA SECTION B, Issue 5 2010Martin U. Schmidt The crystal structures of 4,10-dibromo-anthanthrone (Pigment Red 168; 4,10-dibromo-dibenzo[def,mno]chrysene-6,12-dione), 4,10-dichloro- and 4,10-diiodo-anthanthrone have been determined by single-crystal X-ray analyses. The dibromo and diiodo derivatives crystallize in P21/c, Z = 2, the dichloro derivative in , Z = 1. The molecular structures are almost identical and the unit-cell parameters show some similarities for all three compounds, but the crystal structures are neither isotypic to another nor to the unsubstituted anthanthrone, which crystallizes in P21/c, Z = 8. In order to explain why the four anthanthrone derivatives have four different crystal structures, lattice-energy minimizations were performed using anisotropic atom,atom model potentials as well as using the semi-classical density sums (SCDS-Pixel) approach. The calculations showed the crystal structures of the dichloro and the diiodo derivatives to be the most stable ones for the corresponding compound; whereas for dibromo-anthanthrone the calculations suggest that the dichloro and diiodo structure types should be more stable than the experimentally observed structure. An experimental search for new polymorphs of dibromo-anthanthrone was carried out, but the experiments were hampered by the remarkable insolubility of the compound. A metastable nanocrystalline second polymorph of the dibromo derivative does exist, but it is not isostructural to the dichloro or diiodo compound. In order to determine the crystal structure of this phase, crystal structure predictions were performed in various space groups, using anisotropic atom,atom potentials. For all low-energy structures, X-ray powder patterns were calculated and compared with the experimental diagram, which consisted of a few broad lines only. It turned out that the crystallinity of this phase was not sufficient to determine which of the calculated structures corresponds to the actual structure of this nanocrystalline polymorph. [source] Features of the secondary structure of a protein molecule from powder diffraction dataACTA CRYSTALLOGRAPHICA SECTION D, Issue 7 2010Sebastian Basso Protein powder diffraction is shown to be suitable for obtaining de novo solutions to the phase problem at low resolution via phasing methods such as the isomorphous replacement method. Two heavy-atom derivatives (a gadolinium derivative and a holmium derivative) of the tetragonal form of hen egg-white lysozyme were crystallized at room temperature. Using synchrotron radiation, high-quality powder patterns were collected in which pH-induced anisotropic lattice-parameter changes were exploited in order to reduce the challenging and powder-specific problem of overlapping reflections. The phasing power of two heavy-atom derivatives in a multiple isomorphous replacement analysis enabled molecular structural information to be obtained up to approximately 5.3,Å resolution. At such a resolution, features of the secondary structure of the lysozyme molecule can be accurately located using programs dedicated to that effect. In addition, the quoted resolution is sufficient to determine the correct hand of the heavy-atom substructure which leads to an electron-density map representing the protein molecule of proper chirality. [source] Application of molecular replacement to protein powder data from image platesACTA CRYSTALLOGRAPHICA SECTION D, Issue 4 2009Jennifer A. Doebbler Macromolecular structures can be solved via molecular replacement from powder diffraction data collected not only on multi-analyzer diffractometers but also on image plates. Diffraction peaks recorded on image plates are generally broader than those collected using an array of crystal analyzer detectors, but the image-plate data often allow the use of powder data to lower d -spacings. Owing to the high incidence of overlaps in powder patterns, which is especially evident for larger structures, a multi-pattern Pawley refinement is necessary in order to distinguish intensity peaks. This work utilized various salt concentrations to produce small lattice distortions, which resulted in shifts of Bragg peak positions, in a suite of five powder patterns. Using reflection structure factors obtained from this combined refinement, the structure of hen egg-white lysozyme was determined by molecular replacement using the 60% identical human lysozyme (PDB code 1lz1) as the search model. This work also expands upon previous work by presenting a full-scale multi-species analysis combined with an investigation of the sensitivity with regard to discrimination between incorrect fold types. To test the limits of this technique, extension to higher molecular-weight structures is ongoing. [source] | ||||||||||