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PubMed2004

Characterization of the thermally induced topochemical solid-state transformation of NH4[N(CN)2] into NCN[double bond]C(NH2)2 by means of X-ray and neutron diffraction as well as Raman and solid-state NMR spectroscopy.

The mechanism of the solid-solid transformation of NH(4)[N(CN)(2)] into NCN[double bond]C(NH(2))(2), which represents the isolobal analogue of Wöhler's historic conversion of ammonium cyanate into urea, has been investigated by temperature-dependent single-crystal and powder X-ray diffraction, neutron powder diffraction, and Raman and solid-state NMR spectroscopy as well as thermoanalytical measurements. The transformation of the ionic dicyanamide into its molecular isomer upon controlled thermal treatment was found to proceed topochemically in the solid state with little molecular motion, giving rise to a single-crystal to single-crystal transformation which manifests itself by a defined metric relation between the unit cells of the two isomers. The exothermic phase transition is thermally activated and was observed to commence at temperatures > or =80 degrees C. The pronounced temperature dependence of the onset of the transformation may be assessed as an indication for the metastability of ammonium dicyanamide at elevated temperatures. Thermal analyses reveal a decrease in the reaction enthalpy (56-13 kJ mol(-1)) at higher heating rates and an average mass loss of 10% gaseous ammonia. Evidence was found for crucial mechanistic steps of the transformation, which is likely to proceed via proton transfer from the ammonium ion to one of the terminal nitrogen atoms of the anion. The protonation is followed by nucleophilic attack of the in situ generated ammonia at the electrophilic nitrile carbon. The proposed mechanistic pathway is based on the results of combined Raman and solid-state NMR spectroscopic as well as neutron powder diffraction measurements.

Journal Article↗
PubMed2005

High-temperature, high-pressure hydrothermal synthesis, crystal structure, and solid-state NMR spectroscopy of Cs2(UO2)(Si2O6) and variable-temperature powder X-ray diffraction study of the hydrate phase Cs2(UO2)(Si2O6) x 0.5H2O.

A new uranium(VI) silicate, Cs2(UO2)(Si2O6), has been synthesized by a high-temperature, high-pressure hydrothermal method and characterized by single-crystal X-ray diffraction and solid-state NMR spectroscopy. It crystallizes in the orthorhombic space group Ibca (No. 73) with a = 15.137(1) A, b = 15.295(1) A, c = 16.401(1) A, and Z = 16. Its structure consists of corrugated achter single chains of silicate tetrahedra extending along the c axis linked together via corner-sharing by UO6 tetragonal bipyramids to form a 3-D framework which delimits 8- and 6-ring channels. The Cs+ cations are located in the channels or at sites between channels. The 29Si and 133Cs MAS NMR spectra are consistent with the crystal structure as determined from X-ray diffraction, and the resonances in the spectra are assigned. Variable-temperature in situ powder X-ray diffraction study of the hydrate Cs2(UO2)(Si2O6) x 0.5H2O indicates that the framework structure is stable up to 800 degrees C and transforms to the structure of the title compound at 900 degrees C. A comparison of related uranyl silicate structures is made.

Journal Article↗
PubMed2004

Rationalizing the structural properties of bupivacaine base--a local anesthetic--directly from powder X-ray diffraction data.

Bupivacaine belongs to a family of 1-alkyl-2',6'-pipecoloxylidides, which has shown promise as reversible action potential blockers that can introduce prolonged local anesthetic effects. The crystal structure of the free-base form of bupivacaine has been determined directly from powder X-ray diffraction data using the Genetic Algorithm technique for structure solution, followed by Rietveld refinement. This work further emphasizes the scope and utility of ab initio structure solution directly from powder X-ray diffraction data for tackling structural problems within the biomedical field, leading to opportunities for the investigation of structure-property relationships.

Anesthetics, Local↗
PubMed2006

Structure-reactivity correlations for solid-state enantioselective photochemical reactions established directly from powder X-ray diffraction.

A prerequisite for the development of structure-reactivity correlations for photoreactive crystalline materials is to have detailed knowledge of the structural properties of the reactant crystalline phase. In some cases, however, the materials of interest can be prepared only as microcrystalline powders and are not amenable to structural characterization by single-crystal X-ray diffraction. This paper demonstrates the utility of modern powder X-ray diffraction techniques for obtaining structural understanding in such cases, leading to the development of structure-reactivity correlations. In particular, a series of three photoreactive organic salts are considered, which undergo the same photochemical asymmetric reaction but with high enantiomeric excess in two cases and low enantiomeric excess in the other case. The structural properties of the three salts determined from powder X-ray diffraction data are shown to provide a direct rationalization of these observations.

Journal Article↗
PubMed2003

Synthesis of face-centered tetragonal FePt nanoparticles and granular films from Pt@Fe2O3 core-shell nanoparticles.

This paper describes a new approach for making face-centered tetragonal (fct) FePt nanoparticles with a diameter of 17 nm and granular films from Pt@Fe2O3 core-shell nanoparticle precursors. The core-shell nanoparticles were converted to fct FePt through a reduction and alloy formation process at enhanced temperatures. The Fe and Pt elemental analysis was conducted on both individual nanoparticles and granular films using energy-dispersive X-ray (EDX) spectroscopy. Our convergent evidence from selected area electron diffraction (SAED), powder X-ray diffraction (PXRD), and EDX analysis indicates that the final products are fct FePt alloys. The fct FePt films have coercivities of 8.0-9.1 kOe at 5 K and 7.0 kOe at 300 K measured by a SQUID magnetometer. These values depend on the conversion temperatures of Pt@Fe2O3 nanoparticles. Unlike the previously synthesized disordered face-centered cubic (fcc) FePt nanoparticles with diameters of 4-6 nm (Sun, S. H.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989), the FePt nanoparticles presented in this work not only possess the preferred fct phase but also are in a size range that is expected to be ferromagnetic and have high coercivity, which is important to the practical applications in ultrahigh density data storage media and magnetic nano devices.

Journal Article↗
PubMed2006

Micronization and microencapsulation of felodipine by supercritical carbon dioxide.

Felodipine (FLD) is a poorly water-soluble drug. To improve its dissolution rate, the rapid expansion of supercritical solutions (RESS) technique was used to prepare micronized FLD drug particles, which were encapsulated in poly-(ethylene glycol) 4000 (PEG 4000). The physical properties of the encapsulated drug particles were characterized by a variety of analytical methods, including optical light microscopy, scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and powder X-ray diffraction (powder-XRD) and the dissolution behaviour of FLD was studied in the microparticles. The supercritical condition of micronized FLD occurred at a relatively high pressure and moderate temperature. FLD-PEG 4000 microparticles compared well with micronized FLD. RESS was effective in reducing the particle size of FLD; spot-shaped micronized FLD and popcorn-shaped FLD-PEG 4000 microparticles were observed. The particulate properties of the microparticles included a narrow distribution and uniform size. Thermodynamic analysis showed an implantation interaction between FLD and PEG 4000 molecules, but no polymorphism in the micronized FLD or FLD-PEG 4000 microparticles. FLD-PEG 4000 microparticles had a significantly faster drug dissolution rate than micronized FLD. These data show that RESS can be used to prepare FLD-PEG 4000 microparticles with small particle size (2-6 microm) and enhanced dissolution rate.

Carbon Dioxide↗
PubMed2001

Superconductivity of ternary silicide with the AlB(2)-type structure Sr(Ga(0.37),Si(0.63))(2).

A ternary silicide Sr(Ga(0.37),Si(0.63))(2) was synthesized by a floating zone method. Electron diffraction and powder x-ray diffraction measurements indicate that the silicide has the AlB(2)-type structure with the lattice constants of a = 4.1427(6) A and c = 4.7998(9) A, where Si and Ga atoms are arranged in a chemically disordered honeycomb lattice and Sr atoms are inercalated between them. The silicide is isostructural with the high-temperature superconductor MgB(2) reported recently. Electrical resistivity and dc magnetization measurements revealed that it is a type-II superconductor with onset temperature of 3.5 K.

Journal Article↗
PubMed2006

Formation of methane hydrate from polydisperse ice powders.

Neutron diffraction runs and gas-consumption experiments based on pressure-volume-temperature measurements are conducted to study the kinetics of methane hydrate formation from hydrogenated and deuterated ice powder samples in the temperature range of 245-270 K up to high degrees of transformation. An improved theory of the hydrate growth in a polydisperse ensemble of randomly packed ice spheres is developed to provide a quantitative interpretation of the data in terms of kinetic model parameters. This paper continues the research line of our earlier study which was limited to the monodisperse case and shorter reaction times (Staykova et al., 2003). As before, we distinguish the process of initial hydrate film spreading over the ice particle surface (stage I) and the subsequent hydrate shell growth (stage II) which includes two steps, i.e., an interfacial clathration reaction and the gas and water transport (diffusion) through the hydrate layer surrounding the shrinking ice cores. Although kinetics of hydrate formation at stage II is clearly dominated by the diffusion mechanism which becomes the limiting step at temperatures above 263 K, both steps are shown to be essential at lower temperatures. The permeation coefficient D is estimated as (1.46 +/- 0.44) x 10(-12) m2/h at 263 K with an activation energy Q(D) approximately 52.1 kJ/mol. This value is close to the energy of breaking hydrogen bonds in ice Ih and suggests that this process is the rate-limiting step in hydrate formation from ice in the slower diffusion-controlled part of the reaction.

Journal Article↗
PubMed2004

Mg(1 + x)Ir(1 - x) (x = 0, 0.037 and 0.054), a binary intermetallic compound with a new orthorhombic structure type determined from powder and single-crystal X-ray diffraction.

The new binary compound Mg(1 + x)Ir(1 - x) (x = 0-0.054) was prepared by melting the elements in the Mg:Ir ratio 2:3 in a sealed tantalum tube under an argon atmosphere in an induction furnace (single crystals) or by annealing cold-pressed pellets of the starting composition Mg:Ir 1:1 in an autoclave under an argon atmosphere (powder sample). The structure was independently solved from high-resolution synchrotron powder and single-crystal X-ray data: Pearson symbol oC304, space group Cmca, lattice parameters from synchrotron powder data a = 18.46948 (6), b = 16.17450 (5), c = 16.82131 (5) A. Mg(1 + x)Ir(1 - x) is a topologically close-packed phase, containing 13 Ir and 12 Mg atoms in the asymmetric unit, and has a narrow homogeneity range. Nearly all the atoms have Frank-Kasper-related coordination polyhedra, with the exception of two Ir atoms, and this compound contains the shortest Ir-Ir distances ever observed. The solution of a rather complex crystal structure from powder diffraction, which was fully confirmed by the single-crystal method, shows the power of powder diffraction in combination with the high-resolution data and the global optimization method.

Alloys↗
PubMed2003

A glass capillary cell for in situ powder X-ray diffraction of condensed volatile compounds. Solid HCFC-123a and HCFC-124.

A rotating glass capillary cell with a gas handling system has been built to allow in situ studies by powder X-ray diffraction. The cell can be used to condense volatile compounds, or to follow solid-state chemical reactions under vacuum or at gas pressures up to around 7 x 10(5) Pa. Using the cell, cooled by a stream of helium gas, the solid phases of 1,2-dichlorotrifluoroethane (HCFC-123a) and 2-chloro-1,1,1,2-tetrafluoroethane (HCFC-124) have been investigated using powder synchrotron X-ray radiation. These were found to have disordered hexagonal structures, with a = 4.018 (5), c = 6.553 (1) A and a = 4.048 (1), c = 6.625 (1) A, respectively, at 64 K.

Journal Article↗
PubMed1996

Quantitation of cefepime.2HCl dihydrate in cefepime.2HCl monohydrate by diffuse reflectance IR and powder X-ray diffraction techniques.

The identification, characterization and quantitation of crystal forms is becoming increasingly important within the pharmaceutical industry. Multi-disciplinary, physical analytical techniques are necessary for this task. In this work, diffuse reflectance mid-infrared (IR) and powder X-ray diffraction (XRD) analyses were used to identify two different hydrated forms of cefepime.2HCl, a cephalosporin. Characterization of the mono- and dihydrate forms led to separate IR and XRD quantitative assays for the determination of dihydrate content in cefepime.2HCl monohydrate bulk material. For the IR assay, a working range of 1.0-8% (w/w) was established with a minimum quantifiable level (MQL) of 1.0% (w/w) and a limit of detection (LD) of 0.3% (w/w) dihydrate in monohydrate material. The XRD assay displayed a working range of 2.5-15% (w/w) with an MQL of 2.5% (w/w) and an LD of 0.75% (w/w). Cross validation was performed between the two techniques, with a good correlation displayed for each assay as compared with the known concentrations and as compared with each other. In addition, a full evaluation of potential assay errors was made.

Cefepime↗
PubMed2002

Intermolecular recognition and crystal packing in molybdenum and tungsten coordination polymers as deduced from powder X-ray diffraction data.

The molecular conformations and packing of [(tBuCO2)3M2(mu-X)M2(O2CtBu)3], where M = Mo and W, and X = oxalate and perfluoroterephthalate, determined in the solid-state from powder X-ray diffraction analysis, reveal one-dimensional coordination polymers involving pivalate-oxygen to metal interactions (X = perfluoroterephthalate), and oxalate--as well as pivalate-oxygen to metal bonds (X = oxalate), and allows explanation of the unusual state-dependent chromic properties of these compounds.

Journal Article↗
PubMed2006

Quantification of olanzapine polymorphs using powder X-ray diffraction technique.

Accurate quantification of crystalline phases present in drug materials is becoming increasingly important, due to stringent regulatory concerns about polymorph characterization and control in drug substances and products. In the present study, a quantification method for polymorphic forms of olanzapine (OLZ) has been developed using powder X-ray diffraction (PXRD). Preferred orientation has been reported to be the major source of error in PXRD analysis, therefore, prior to development of a quantification method, pure polymorphic forms (I and II) of different size ranges were analyzed. Preferred orientation effect was found to decrease on using sieve fraction BSS # 120/240 for form I. In order to obtain good peak resolution in optimum time, the step time and step size were varied so as to optimize the scan rate. Among the five combinations selected, step size of 0.05 degrees with step time of 5s demonstrated identification of four characteristic peaks of form I in form II in 62 min. A calibration curve was constructed in the range of 0-100% (w/w) using the characteristic peak of form I at 18.48 degrees 2theta (I/I(0) 78.8%). The PXRD assay was reproducible and precise and displayed a LOD of 0.40% (w/w) and LOQ of 1.22% (w/w). Validation results showed excellent correlation between actual and predicted concentrations with R(2) 0.9999.

Antipsychotic Agents↗
PubMed1999

Influence of particle size, air flow, and inhaler device on the dispersion of mannitol powders as aerosols.

PURPOSE: To study the effect of particle size, air flow and inhaler type on the dispersion of spray dried mannitol powders into aerosols. METHODS: Mannitol powders were prepared by spray drying. The solid state properties of the powders were determined by laser diffraction, X-ray powder diffraction, scanning electron microscopy, freeze fracture, Karl Fischer titration and gas pycnometry. The powders were dispersed using Rotahaler and Dinkihalerg, connected to a multistage liquid impinger at different air flows. RESULTS: Three crystalline mannitol powders with primary particle size (MMD) 2.7, 5.0, 7.3 microm and a similar polydispersity were obtained. The particles were spherical with a density of 1.5 g/cm3 and a moisture content of 0.4 wt.%. At an air flow of 30 L/min all the powders were poorly dispersed by both inhalers. With the Rotahaler increasing the flow (60-120 L/min) increased the fine particle fraction (FPF) in the aerosols for the 2.7 microm powder, and decreased the FPF for the 7.3 microm powder; whereas the FPF for 5.0 microm powder was unaffected. With the Dinkihaler, all the powders were near complete dispersion at > or = 60 L/min. CONCLUSIONS: The FPF in the mannitol powder aerosols was determined by an interplay of the particle size, air flow and inhaler design.

Aerosols↗
PubMedDate not supplied

A combined 14N/27Al nuclear magnetic resonance and powder X-ray diffraction study of impurity phases in beta-sialon ceramics.

Beta-sialons are ceramic phases occurring in the SiO(2)-Si(3)N(4)-AlN-Al(2)O(3) system. A series of samples with differing compositions has been investigated by magic-angle spinning nuclear magnetic resonance (NMR) spectroscopy and powder X-ray diffraction (XRD). Although the constituent nitrogen nuclei occupy positions of low symmetry in the beta-sialon structure, 14N NMR spectra could be recorded for the samples examined. The origin of the 14N signal could be traced to the presence of an aluminium nitride (AlN) impurity phase with the help of 27Al NMR and XRD results. Similarly, the existence of Al(2)O(3) grains could be readily detected for a number of samples. Thus, the combination of 14N and 27Al NMR is shown to be an especially effective tool in identifying and characterizing impurity phases in sialon ceramics, complementing the results obtained from standard XRD analysis.

Journal Article↗
PubMed2005

Structural understanding of a molecular material that is accessed only by a solid-state desolvation process: the scope of modern powder X-ray diffraction techniques.

Many molecular materials cannot be prepared as a "pure" (nonsolvate) crystalline phase by conventional crystal growth from solution due to the facile formation of solvate structures. In such cases, it may be possible to obtain the pure phase by a solid-state desolvation process, although such processes are generally associated with loss of crystal integrity, yielding a microcrystalline powder of the pure phase. This paper demonstrates the utility of modern powder X-ray diffraction techniques for obtaining structural understanding in such cases, focusing on a particular member of a structural family that is of wider relevance within the context of crystal engineering and design.

Journal Article↗
PubMed2000

Structure determination of phi-Bi8Pb5O17 by electron and powder X-ray diffraction

The triclinic crystal structure of phi-Bi8Pb5O17, a ionic fast conductor material, has been determined by the synergy of both electron and powder X-ray crystallography. The heavy atom positions were found by direct methods on electron diffraction data and the structure was completed by iterative use of a priori information in direct methods and difference Fourier maps on both types of data. Structure refinement was performed by the Rietveld method on powder X-ray data. The results suggest that phi-Bi8Pb5O17 is an ordered phase, with Bi and Pb atoms occupying different sites of the lattice, at variance with the other structural phases known for similar composition in the Bi-Pb-O phase diagram, which are solid solutions characterised by a wide compositional range.

Journal Article↗
PubMed2002

Structure prediction as a tool for solution of the crystal structures of metallo-organic complexes using powder X-ray diffraction data.

A simulated-annealing direct-space approach has been applied to predict the crystal structures of a series of metallo-organic complexes containing Zn, Cu and Ni. The prediction methodology generates a set of energetically reasonable crystal structures among which the actual structure is present, but it is not always possible to specify unambiguously the known crystal structure solely on the basis of energy. In each case, however, the ambiguity may be resolved by recourse to laboratory powder X-ray diffraction (PXRD) data. In this manner, structure prediction is shown to be a powerful tool for structure solution using PXRD data, with the additional advantage that indexing of the PXRD profile is not required at the outset.

Journal Article↗