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At least 181 records · Page 10Linked to original sources
PubMed2001

Real time observation of the hydrothermal crystallization of barium titanate using in situ neutron powder diffraction.

The hydrothermal crystallization of barium titanate, BaTiO3, has been studied in situ by time-resolved powder neutron diffraction methods using the recently developed Oxford/ISIS hydrothermal cell. This technique has allowed the formation of the ferroelectric ceramic to be followed in a noninvasive manner in real time and under genuine reaction conditions. In a first set of experiments, Ba(OD)2-8D2O was reacted with two different titanium sources, either crystalline TiO2 (anatase) or amorphous TiO2-H2O in D2O, at 100-140 degrees C and the reaction studied using the POLARIS time-of-flight neutron powder diffractometer, at the ISIS Facility. In a second series of experiments, the reaction between barium chloride and crystalline TiO2 (anatase) in NaOD/D2O was studied at temperatures between 100 and 200 degrees C and at different deuterioxide concentrations using the constant-wavelength D20 neutron powder diffractometer at the Institut Laue Langevin. Quantitative growth and decay curves were determined from analysis of the integrated intensities of Bragg reflections of starting materials and product phases. In both sets of experiments the rapid dissolution of the barium source was observed, followed by dissolution of the titanium source before the onset of crystallization of barium titanate. Using a nucleation-growth model we are able to simulate the growth curve of barium titanate at three temperatures. Our results indicate the predominance of a homogeneous dissolution-precipitation mechanism for the hydrothermal formation of barium titanate, rather than other possible mechanisms that have been discussed in the literature. Analysis of the line widths of the Bragg reflections in the neutron diffraction data indicates that the particle size of the BaTiO3 product phase prepared from the amorphous TiO2-H2O is smaller than that prepared from crystalline TiO2 (anatase).

Journal Article↗
PubMed2003

Subtleties in crystal structure solution from powder diffraction data using simulated annealing: ranitidine hydrochloride.

Recent advances in crystallographic computing and availability of high-resolution diffraction data have made it relatively easy to solve crystal structures from powders that would have traditionally required single crystal samples. The success of direct space methods depends heavily on starting with an accurate molecular model. In this paper we address the applicability of using these methods in finding subtleties such as disorder in the molecular conformation that might not be known a priori. We use ranitidine HCl as our test sample as it is known to have a conformational disorder from single crystal structural work. We redetermine the structure from powder data using simulated annealing and show that the conformational disorder is clearly revealed by this method.

Algorithms↗
PubMed2005

Ab initio structure determination of the high-temperature phase of anhydrous caffeine by X-ray powder diffraction.

The high-temperature phase I of anhydrous caffeine was obtained by heating and annealing the purified commercial form II at 450 K. This phase I can be maintained at low temperature in a metastable state. A powder X-ray diffraction pattern was recorded at 278 K with a laboratory diffractometer equipped with an INEL curved position-sensitive detector CPS120. Phase I is dynamically orientationally disordered (the so-called plastic phase). The Rietveld refinements were achieved with rigid-body constraints. It was assumed that on each site, a molecule can adopt three preferential orientations with equal occupation probability. Under a deep undercooling of phase I, below 250 K, the metastable state enters in a glassy crystal state.

Caffeine↗
PubMed2005

High-throughput phase-diagram mapping via powder diffraction: a case study of HEWL versus pH.

44 samples of tetragonal hen egg-white lysozyme (HEWL) were obtained as a series of polycrystalline precipitates at 277 K and room temperature in the pH range between 6.56 and 3.33. The precipitates were investigated by the collection of high-resolution powder X-ray diffraction data at 295 K, which reveal the tetragonal or orthorhombic forms of lysozyme depending on the temperature and pH of crystallization. The use of a new robotic sample changer greatly facilitated these measurements. LeBail analyses of the powder patterns display a characteristic behaviour for the pH dependence of the tetragonal unit-cell parameters of HEWL crystallized at both temperatures. More detailed analysis shows that molecular replacement can give a suitable starting point for structural refinements, illustrating that powder data can be sufficient for this approach. Pawley or Rietveld refinements that fit a single model to four data sets simultaneously from four samples crystallized at pH values across the range studied benefit from improved powder data quality via the anisotropic changes in the unit cell. The Rietveld analysis gave an average structural model with excellent goodness of fit and stereochemistry.

Animals↗
PubMed2005

Polysaccharide structures from powder diffraction data: molecular models of arabinan.

X-ray intensity data from a polycrystalline sample of debranched arabinan, [-->5)-alpha-L-Ara(f)-(1-->](n), have been obtained using a powder diffractometer in order to determine its three-dimensional structure. The observed peaks index on a monoclinic cell with a=5.444(7), b=6.395(10), c=8.680(5) A, and gamma=99.6(3) degrees , V=298 A3. One 2-fold helix along the c-axis can be accommodated in the unit cell. Molecular and packing models have been analyzed using the seven C-2'-endo/C-3'-endo allomorphs originally proposed by Radha and Chandrasekaran [Carbohydr. Res. 1997, 298, 105]. The generated powder pattern matches closely with the observed diffraction only for one C-2'-endo model. In this structure, the three main chain conformation angles are in the trans domains, there are no intra-chain hydrogen bonds, and the packing arrangement is stabilized by inter-chain O-3-H...O-2 bonds.

Carbohydrate Conformation↗
PubMed2000

Study of NH4+ in the zeolite phillipsite by combined synchrotron powder diffraction and IR spectroscopy.

The Rietveld refinement on synchrotron data of three natural hydrated NH4+-exchanged phillipsites with different Si/Al ratios allowed acceptable assignment of the NH4+ sites. These phillipsites display the typical monoclinic P2(1)/m cell, whose volume increases with increasing numbers of NH4+ ions, which are refined in the two sites labelled I and II. Some residual Na+ is retained in the site labelled II'. Distortion of the Si/Al tetrahedra, which affects some absorption bands in the IR spectra, seems to be mainly due to the distribution of Al in the framework, while the interaction of NH4+ with the O atoms of the framework seems to play a minor role. NH4+ in both sites acts as a donor in hydrogen bonding with O atoms of the framework and water molecules. NH4+ in site I has three H atoms interacting with O atoms of the framework and a number of interactions with water O atoms. In all three samples, monocentric and dicentric connections are formed. NH4+ in site II has one H atom interacting with O atoms of the framework and a number of interactions with water O atoms. Two monocentric connections and a quadricentric one were found in sample (1), three dicentric connections and a monocentric one in sample (2), and three monocentric connections and a dicentric one in sample (3).

Hydrogen Bonding↗
PubMed2003

Structure determination of enalapril maleate form II from high-resolution X-ray powder diffraction data.

The crystal structure of polymorphic Form II of enalapril maleate, a potent angiotensin-converting enzyme inhibitor, was determined from high-resolution X-ray diffraction data using the direct space method. Enalapril maleate Form II crystallizes in space group P2(1)2(1)2(1), Z = 4, with unit cell parameters a = 33.9898(3) A, b = 11.2109(1) A, c = 6.64195(7) A, and V = 2530.96(5) A(3). By treating the molecules as rigid bodies and using the bond lengths and angles obtained from the X-ray single crystal structures of Form I, which were solved almost 20 years ago, the total degrees of freedom of enalapril maleate were reduced from 25 to 12. This reduction in total degrees of freedom allowed the simulated annealing to complete within a reasonable computation time. In the crystal structure of Form II, the crystal packing, hydrogen-bonding pattern, and conformation of enalapril maleate resemble those in the structure of Form I. The crystal packing and conformation of enalapril maleate in the two polymorphic forms may explain the similarity of the thermal properties, (13)C nuclear magnetic resonance, Fourier transform infrared, and Raman spectra of Forms I and II. In both structures, the conformations of the main peptide chains, which are considered responsible for binding the active angiotensin-converting enzyme sites, remain largely unchanged. Lattice energy calculation showed that Form II is slightly more stable than Form I by 3.5 kcal/mole.

Angiotensin-Converting Enzyme Inhibitors↗
PubMed1997

Novel Alkali-Metal Coordination in Phenoxides: Powder Diffraction Results on C(6)H(5)OM (M = Li, Na, K, Rb, Cs).

We report the ab initio structure solutions of C(6)H(5)OM (M = K, Rb, Cs) by high-resolution powder X-ray diffraction. The compounds, which are of interest for reactions of the Kolbe-Schmitt type, are isostructural. The crystal structures are orthorhombic, space group Pna2(1), Z = 12, with lattice parameters (a, b, c in Å) 14.1003(1), 17.9121(1), and 7.16475(1) for the K compound, 14.4166(2), 18.2028(2), and 7.4009(1) for the Rb compound, and 14.8448(2), 18.5070(2), and 7.6306(1) for the Cs compound. They have a chain structure [M([6])] along the crystallographic c axis. This is a very unusual arrangement in which two different alkali-metal coordination spheres are observed: a distorted octahedron and a 3-fold oxygen coordination. In the latter, the 3-fold-coordinated unsaturated alkali metals additionally show weak interactions with phenyl rings. We also give powder patterns for the compounds with M = Li, Na. The former crystallizes in the monoclinic space group P2(1)/a with lattice parameters a = 22.594 Å, b = 4.7459 Å, c= 10.053 Å, and beta = 97.82 degrees with Z = 8, but no structure solution was possible. The powder pattern for the Na phenolate is in agreement with the earlier single-crystal structure.

Journal Article↗
PubMed2004

An X-ray powder diffraction study of the spin-crossover transition and structure of bis(2,6-dipyrazol-1-ylpyrazine)iron(II) perchlorate.

The crystal structure of the iron(II) spin-crossover compound [Fe(C(10)H(8)N(6))(2)](ClO(4))(2) in the high-spin state has been solved from powder X-ray diffraction data using the DASH program and refined using Rietveld refinement. The thermal spin transition has been monitored by following the change in unit-cell parameters with temperature. The title compound has been found to undergo a crystallographic phase change, involving a doubling of the crystallographic a axis, on undergoing the spin transition.

Journal Article↗
PubMed2003

Solid-state supramolecular organization, established directly from powder diffraction data, and photoluminescence efficiency of rigid-core oligothiophene-S,S-dioxides.

The "rigid-core" material 3,5-dimethyl-2,3'-bis(3-methylthiophene)-dithieno[3,2-b:',3'-d]thiophene-4,4-dioxide (DTTOMe4) has the highest photoluminescence ever reported for thiophene-based molecules in the solid state. We report the structure of this material, determined directly from powder X-ray diffraction data using the Genetic Algorithm method for structure solution, followed by Rietveld refinement, and the structural properties are discussed in relation to the structures of the corresponding subsystems DTTO and DTTOMe. While the crystal structures of the latter compounds contain cofacial dimers, the crystal structure of DTTOMe4 comprises layers of molecules aligned in an antiparallel fashion. Intermediate neglect of differential overlap with single configuration interaction (INDO/SCI) calculations on the intermolecular interactions in the three crystal structures show that the different solid-state photoluminescence efficiencies of DTTOMe4, DTTOMe, and DTTO cannot be correlated with the different types of dipole-dipole alignment in the solid state. Instead, photoluminescence efficiencies correlate well with the rate of formation of nonradiatively decaying charge-transfer pairs upon photoexcitation. Because of larger intermolecular distances in DTTOMe4, the photoluminescence is less effectively quenched by charge-transfer processes than in DTTOMe and DTTO.

Journal Article↗
PubMed1999

Reverse Monte Carlo modelling of neutron powder diffraction data.

A new reverse Monte Carlo (RMC) method for modelling both lattice and magnetic disorder in powder crystalline materials by direct calculation of the structure factor has been developed. The method, the program and the basic theory are described in some detail. Initial results from modelling the lattice and magnetic structure of MnO around the Néel temperature are also presented.

Journal Article↗