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Powder Diffraction Data (powder + diffraction_data)
Kinds of Powder Diffraction Data Selected AbstractsThe 136-Atom Structure of ZrP2O7 and HfP2O7 from Powder Diffraction Data.CHEMINFORM, Issue 33 2006Graham W. Stinton Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] High-Spin- and Low-Spin-State Structures of [Fe(chloroethyltetrazole)6](ClO4)2 from Synchrotron Powder Diffraction DataCHEMISTRY - A EUROPEAN JOURNAL, Issue 19 2006Eva Dova Dr. Abstract The spin-crossover complex [Fe(teec)6](ClO4)2 (teec = chloroethyltetrazole) exhibits a 50,% incomplete spin crossover in the temperature range 300,30 K. Time-resolved synchrotron powder diffraction experiments have been carried out to elucidate its structural behavior. We report crystal structure models of this material at 300 K (high spin) and 90 K (low spin), as solved from synchrotron powder diffraction data by using Genetic Algorithm and Parallel Tempering techniques and refined with Rietveld refinement. During short synchrotron powder diffraction experiments (five minutes duration) two distinguishable lattices were observed the quantities of which vary with temperature. The implication of this phenomenon, that is interpreted as a structural phase transition associated with the high-to-low spin crossover, and the structural characteristics of the high-spin and low-spin models are discussed in relation to other compounds showing a similar type of spin-crossover behavior. [source] Growth, structure and thermal properties of CuAlxGa1-xSe2 alloysCRYSTAL RESEARCH AND TECHNOLOGY, Issue 10 2006J. Castro Abstract Ingots of the CuAlxGa1-xSe2 (0 , x , 1) alloys system were prepared by direct fusion of the stoichiometric mixture of the elements. The analysis of X-ray Powder Diffraction data showed the presence of one single phase with chalcopyrite tetragonal structure at room temperature for all the studied compositions. The lattice parameters, a and c, and the bond lengths were calculated. The phase transition temperatures were obtained by the onset method from Differential Thermal Analysis measurements performed on samples sealed in evacuated quartz ampoules. Fusion or transition enthalpies were determined from the area of the corresponding DTA peak. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Low temperature powder diffraction and DFT solid state computational study of hydrogen bonding in NH4VO3CRYSTAL RESEARCH AND TECHNOLOGY, Issue 9 2009. Smr Abstract The crystal structure of NH4VO3 was refined by the geometry optimization done by total energy minimization in solid state using DFT/plane waves approach. The lattice parameters were derived by the Le Bail technique from the low temperature X-ray (40-293 K) and synchrotron (100-293 K) powder diffraction data. The structure is formed by the infinite chains of irregular VO4 tetrahedra running approximately parallel to the c -axis, which are interlinked by the ammonium ions placed between them. The ammonium ions link to the [VO4], chains through one linear, one bifurcated and two trifurcated N-H,O hydrogen bonds. Considering their stability there are six distinct N-H,O hydrogen bonds: two strong with the N-H,O bond angles close to the straight, two medium with the bond angles of 123° and 148° and two very bent (105° and 107°) and hence weak hydrogen bonds. There is a reasonable agreement between the energies of the stretching ,(NH) modes estimated using the optimised N,O contact distances and those obtained experimentally. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Crystal structure characterization of the quaternary compounds CuFeAlSe3 and CuFeGaSe3CRYSTAL RESEARCH AND TECHNOLOGY, Issue 5 2009G. E. Delgado Abstract The crystal structure of the chalcogenide compounds CuFeAlSe3 and CuFeGaSe3, belonging to the system I-II-III-III3, were characterized using X-ray powder diffraction data. Both compounds crystallize in the tetragonal space group P42c (N° 112), Z = 1, with unit cell parameters a = 5.609(1) Å, c = 10.963(2) Å for CuFeAlSe3 and a = 5.6165(3) Å, c = 11.075(1) Å for CuFeGaSe3. These compounds are isostructural with CuFeInSe3, and have a normal adamantane structure. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Studies on multiphased mixed crystals of NaCl, KCl and KICRYSTAL RESEARCH AND TECHNOLOGY, Issue 1 2009M. Priya Abstract Multiphased mixed crystals of NaCl, KCl and KI were grown by the melt method, for the first time. Densities and refractive indices of all the grown crystals were determined and used for the estimation of the composition in the crystal. Atomic absorption spectroscopic measurements were done to estimate the metal atom contents in the crystal. Lattice parameters and thermal parameters (Debye-Waller factor, mean square amplitude of vibration, Debye temperature and Debye frequency) were determined from the X-ray powder diffraction data. DC and AC electrical measurements were done at various temperatures ranging from 40 to 150°C. Activation energies were also estimated. The observed lattice parameters showed that the system exhibits three phases each nearly corresponds to NaCl, KCl and KI. The thermal and electrical parameters show a highly nonlinear bulk composition dependence. Results are reported. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Crystal structure refinement of the ternary compound Cu2SnTe3 by X-ray powder diffractionCRYSTAL RESEARCH AND TECHNOLOGY, Issue 4 2008G. E. Delgado Abstract The ternary compound Cu2SnTe3 crystallizes in the Imm2 (Nº 44) space group, Z = 2, with a = 12.833(4) Å, b = 4.274(1) Å, c = 6.043(1) Å, V = 331.5(1) Å3. Its structure was refined from X-ray powder diffraction data using the Rietveld method. The refinement of 25 instrumental and structural variables led to Rp = 10.2%, Rwp = 11.8%, Rexp = 7.7%, RB = 10.6%, S = 1.6 and ,2 = 2.6, for 5501 step intensities and 163 independent reflections. This compound is isostructural with Cu2GeSe3, and consists of a three-dimensional arrangement of slightly distorted CuTe4 and SnTe4 tetrahedra connected by common corners. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Synthesis and crystal structure determination of two dispiro compounds from laboratory x-ray powder diffraction dataCRYSTAL RESEARCH AND TECHNOLOGY, Issue 2 2008A. M. Moustafa Abstract The crystal structures of the dispiro compounds 1,3,4,8,10,11-Hexaphenyl-13-methyl-1,2,8,9,13-pentaazadispiro[4.1.4.3]tetradeca-2,9-dien-6-one (3a) and 4,11-Bis(4-methoxyphenyl)-13-methyl-1,3,8,10-tetraphenyl-1,2,8,9,13-pentaazadispiro-[4.1.4.3]tetradeca-2,9-dien-6-one (3b) have been determined at room temperature from X-ray powder diffraction data using the method of simulated annealing as implemented in the programs DASH and TOPAS. Subsequent Rietveld refinements using the data collected to 1.5 Å resolution yielded R-Bragg values of 2.2% for (3a) and 3.7% for (3b). It was found, that both compounds crystallize in the monoclinc space group P 21/n with lattice parameters of a = 17.1656(5) Å, b =13.8128(3) Å, c = 16.1016(5) Å, and , = 103.7330(2)° for (3a) and a = 17.2529(8) Å, b = 13.8729(5) Å, c =16.1287(10) Å, and , = 103.6910(3)° for (3b). Both compounds exhibit a distorted hexagonal close type of packing (hcp) of the molecular centers of gravity. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Growth and characterization of FexMn1-xIn2Se4 (0 , x , 1) single crystalsCRYSTAL RESEARCH AND TECHNOLOGY, Issue 10-11 2005G. Attolini Abstract Single crystals of Mn1-xFexIn2Se4 with 0 , x , 1.0 were grown by chemical vapor transport technique using I2 as transporting agent. The resulting crystals appeared as brilliant black plates. X-ray powder diffraction data could be indexed following the hexagonal structure with space group Rm for the low Fe concentration samples, while the high concentration sample is associated to the space group P63mc. Both polytypes have similar parameters, however the c parameter reduces as the Fe concentration increases, therefore the number of anion slabs decreases. The characterization of the crystals was based on morphological observations and optical absorption as a function of temperature. The obtained energy gap for the samples at different Fe concentrations suggested that they behave as direct energy gap semiconductors. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Local 3D real space atomic structure of the simple icosahedral Ho11Mg15Zn74 quasicrystal from PDF dataCRYSTAL RESEARCH AND TECHNOLOGY, Issue 12 2003S. Brühne Abstract We present a new complementary strategy to quasicrystalline structure determination: The local atomic structure of simple icosahedral (si) Ho11Mg15Zn74 [a(6D) = 5.144(3)Å in a sphere of up to r = 17Å was refined using the atomic pair distribution function (PDF) from in-house X-ray powder diffraction data (MoK,1, Qmax = 13.5Å,1; R = 20.4%). The basic building block is a 105-atom Bergman-Cluster {Ho8Mg12Zn85}. Its center is occupied by a Zn atom , in contrast to a void in face centred icosahedral (fci) Ho9Mg26Zn65. The center is then surrounded by another 12 Zn atoms, forming an icosahedron (1st shell). The 2nd shell is made up of 8 Ho atoms arranged on the vertices of a cube which in turn is completed to a pentagon dodecahedron by 12 Mg atoms, the dodecahedron then being capped by 12 Zn atoms. The 3rd shell is a distorted soccer ball of 60 Zn atoms, reflecting the higher Zn content of the si phase compared to the fci phase. In our model, 7% of all atoms are situated in between the clusters. The model corresponds to a hypothetical 1/1-approximant of the icosahedral (i) phase. The local coordinations of the single atoms are of a much distorted Frank-Kasper type and call to mind those present in 0/1-Mg2Zn11. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Synthesis of the Clathrate-II K8.6(4)Ge136 by Oxidation of K4Ge9 in an Ionic LiquidEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 17 2009Arnold M. Guloy Abstract The new clathrate-II K8.6(4)Ge136 has been synthesized by mild oxidation of K4Ge9 in the ionic liquid n -dodecyltrimethylammonium chloride (DTAC)/AlCl3 at 300 °C and subsequent annealing at 370 °C. Refinement of the crystal structure from X-ray powder diffraction data revealed the composition K8.6(4)Ge136 [space group Fdm, a = 15.302(1) Å], which was also confirmed by energy-dispersive X-ray spectrometry (EDXS), transmission electron microscopy, and scanning electron microscopy on the bulk material. K atoms preferably occupy the larger Ge28 cages rather than the Ge20 cages in the clathrate-II structure. K8.6(4)Ge136 is metastable and was found to decompose exothermically at 471 °C. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Facile Synthesis of Bastnaesite -Type LaF[CO3] and Its Thermal Decomposition to LaOF for Bulk and Eu3+ -Doped SamplesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 3 2009Oliver Janka Abstract Rare-earth metal(III) oxide fluorides with the composition MOF (M = La,Lu) seem to offer promising host lattices for luminescence applications by doping these materials with trivalent lanthanoid cations. Unfortunately, there was no simple and practicable way to synthesize the compounds in phase-pure quality with classical solid-state chemistry reactions. However, by using the rare-earth metal(III) fluoride oxocarbonates, MF[CO3], as solution-born precursor materials that crystallize with a bastnaesite -type structure, easy access is now possible by mild thermal decomposition. Synthetic details, investigations on the phase purity and the presence of the oxocarbonate anion [CO3]2, proved by IR measurements as well as from X-ray powder diffraction data are given in this paper for the example of bulk LaF[CO3] and Eu3+ -doped samples. The latter (LaF[CO3]:Eu3+) shows a bright orange-red luminescence that is stronger than that of the product of its thermal decomposition: trimorphic LaOF:Eu3+.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Identification of single photoswitchable molecules in nanopores of silica xerogels using powder diffractionJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 5-1 2010Antonio Cervellino Single photoswitchable molecules of (CN3H6)2[Fe(CN)5NO] (GuNP) are embedded into nanopores of an SiO2 xerogel. It is shown that it is possible to identify the structural motif (`fingerprint') of the embedded complex by analyzing neutron powder diffraction data in a limited Q range (Q < 37,nm,1) using the Debye approach. The structural study reveals that the pores are occupied by GuNP monomers with a fill factor of 60,80%. The mutual arrangement of the anion and cations in the GuNP monomer is slightly changed (,1% elongation), while the bond lengths within the anion and cation are changed by less than 0.2% with respect to the single-crystalline form of GuNP. [source] Fityk: a general-purpose peak fitting programJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 5-1 2010Marcin Wojdyr Fityk is portable, open-source software for nonlinear curve fitting and data analysis. It specializes in fitting a sum of bell-shaped functions to experimental data. In particular, it enables Pawley refinement of powder diffraction data and size,strain analysis. [source] The crystal structure of perdeuterated methanol hemiammoniate (CD3OD·0.5ND3) determined from neutron powder diffraction data at 4.2 and 180,KJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2 2010A. D. Fortes The crystal structure of perdeuterated methanol hemiammoniate, CD3OD·0.5ND3, has been solved from neutron powder diffraction data collected at 4.2 and 180,K. The structure is orthorhombic, space group Pn21a (Z = 4), with unit-cell dimensions a = 12.70615,(16), b = 8.84589,(9), c = 4.73876,(4),Å, V = 532.623,(8),Å3 [,calc = 1149.57,(2),kg,m,3] at 4.2,K, and a = 12.90413,(16), b = 8.96975,(8), c = 4.79198,(4),Å, V = 554.656,(7),Å3 [,calc = 1103.90,(1),kg,m,3] at 180,K. The crystal structure was determined by ab initio methods from the powder data; atomic coordinates and isotropic displacement parameters were subsequently refined by the Rietveld method to Rp, 2% at both temperatures. The crystal structure comprises a three-dimensionally hydrogen-bonded network in which the ND3 molecules are tetrahedrally coordinated by the hydroxy moieties of the methanol molecule. This connectivity leads to the formation of zigzag chains of ammonia,hydroxy groups extending along the c axis, formed via N,D···O hydrogen bonds; these chains are cross-linked along the a axis through the hydroxy moiety of the second methanol molecule via N,D···O and O,D···O hydrogen bonds. This `bridging' hydroxy group in turn donates an O,D···N hydrogen bond to ammonia in adjacent chains stacked along the b axis. The methyl deuterons in methanol hemiammoniate, unlike those in methanol monoammoniate, do not participate in hydrogen bonding and reveal evidence of orientational disorder at 180,K. The relative volume change on warming from 4.2 to 180,K, ,V/V, is + 4.14%, which is comparable to, but more nearly isotropic (as determined from the relative change in axial lengths, e.g.,a/a) than, that observed in deuterated methanol monohydrate, and very similar to what is observed in methanol monoammoniate. [source] PSSP, a computer program for the crystal structure solution of molecular materials from X-ray powder diffraction dataJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2 2010Silvina Pagola This work describes the computer program PSSP (powder structure solution program) for the crystal structure solution of molecular solids from X-ray powder diffraction data. This direct-space structure solution program uses the simulated annealing global optimization algorithm to minimize the difference between integrated intensities calculated from trial models and those extracted in a Le Bail fit of the experimental pattern, using a cost function for dealing with peak overlap through defined intensity correlation coefficients, computationally faster to calculate than Rwp. The methodology outlined is applicable to organic solids composed of moderately complex rigid and flexible molecules, using diffraction data up to relatively low resolution. PSSP performance tests using 11 molecular solids with six to 20 degrees of freedom are analyzed. [source] PeckCryst: a program for structure determination from powder diffraction data using a particle swarm optimization algorithmJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 6 2009Zhen Jie Feng PeckCryst has been developed for the solution of molecular crystal structures from powder diffraction data using a particle swarm optimization (PSO) algorithm. In order to speed up the calculation process, a modified Bragg R factor is used as the evaluation function for the PSO algorithm. The effectiveness of the program has been tested by solving four known structures from powder diffraction data. A Python script is also provided for convenient repetitive running of PeckCryst. The distributed PeckCryst program is freely available from the authors upon request, and runs on Linux and Windows (32- and 64-bit) platforms. [source] ReX: a computer program for structural analysis using powder diffraction dataJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3 2009Mauro Bortolotti Multi-platform software has been developed for the analysis of powder diffraction data, with particular focus on structure solution. The program provides a Rietveld optimization engine, with the possibility of refining parameters describing both the sample and the instrument model. Geometric constraints such as rigid fragments and torsion angles can be defined for the atomic structure, to reduce the number of degrees of freedom of the model. An innovative hierarchical description of the asymmetric unit has been adopted, which allows, in principle, the definition of arbitrarily complex geometric relationships. Additionally, global optimization algorithms may be used in place of the standard nonlinear least squares, when particularly challenging problems are being faced. [source] A differential thermal expansion approach to crystal structure determination from powder diffraction dataJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 6 2008P. Fernandes Differential thermal expansion over the range 90,210,K has been applied successfully to determine the crystal structure of chlorothiazide from synchrotron powder diffraction data using direct methods. Key to the success of the approach is the use of a multi-data-set Pawley refinement to extract a set of reflection intensities that is more `single-crystal-like' than those extracted from a single data set. The improvement in reflection intensity estimates is quantified by comparison with reference single-crystal intensities. [source] QUALX: a computer program for qualitative analysis using powder diffraction dataJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 4 2008Angela Altomare QUALX is a new computer program for phase identification using powder diffraction data. It uses the Powder Diffraction File database, where a search for the phase best matching the experimental powder diffraction pattern is carried out. The program is characterized by a high level of automation: the traditional steps aimed at interpreting the experimental pattern before the search (background estimation, peak search, peak intensity evaluation) are executed automatically. The search may be carried out via constraints on compound name and/or chemical elements. In addition, several graphical options requested interactively enable the user to perform zero point correction evaluation, K,2 stripping and smoothing. The program, written in Fortran95 and C++, runs on PCs under the Windows XP operating system. It is supported by a very effective graphical interface. [source] Advances in space-group determination from powder diffraction dataJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 4 2007Angela Altomare The space-group determination process by powder diffraction data is not straightforward. The low accuracy of the reflection intensities may invalidate the calculation of the probability associated to each extinction group that is compatible with the crystal system determined in the indexation step. Here the combination of the z statistics with two new algorithms is reported: the first checks the quality of each 2, interval in order to omit doubtful z estimates from the calculations; the second creates a list of reflections with peaks that weakly overlap with any other peak, in order to check if any of them violates the extinction rules of the extinction symbol. The new approach has been applied to a large set of test structures and proved to be much more efficient than the procedure based only on the z statistics. [source] Chlorartinite, a volcanic exhalation product also found in industrial magnesia screedJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 5 2006Kunihisa Sugimoto The volcanic exhalation product chlorartinite, [Mg2(CO3)(H2O)(OH)]Cl·H2O, has recently been found to be a minor, and in some samples a major, component of magnesia floors for industrial use. In order to be able to perform quantitative phase analysis using the Rietveld method, its crystal structure was determined from high-resolution synchrotron powder diffraction data by the global optimization technique of simulated annealing and Rietveld refinement. The final Rp and Rwp values are 5.23% and 6.56%, respectively. Chlorartinite crystallizes in the rhombohedral space group R3c (No. 161), with a = 23.14422,(16),Å, c = 7.22333,(5),Å, V = 3350.84,(5),Å3, Z = 18. The building units of chlorartinite consist of MgO6 octahedra forming 15-membered puckered rings which are interconnected by CO3 triangular moieties. The rings are stacked to form a honeycomb-like three-dimensional framework structure with large isolated channels. Within the channels, free chlorine atoms and disordered water molecules are located. [source] The combined use of Patterson and Monte Carlo methods for the decomposition of a powder diffraction patternJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2 2006Angela Altomare The success of ab initio crystal structure solution by powder diffraction data is strictly related to the quality of the integrated intensity estimates. A new method that is able to improve the pattern decomposition step has been developed. It combines the inversion of a suitably modified Patterson map with the use of the Hamming codes [13,10] and [40,36] in order to explore more decomposition trials. The new approach has been introduced in EXPO2005, an updated version of EXPO2004, and successfully applied to a set of known organic and inorganic structures. [source] Space-group determination from powder diffraction data: a probabilistic approachJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 6 2004Angela Altomare Experimental powder diffraction diagrams, once indexed and decomposed into single diffraction intensities, can be submitted to statistical analysis for the determination of space-group symmetry. A new algorithm is illustrated, which is able to provide, on a quantitative basis, a probability value for each extinction symbol compatible with the previously established lattice symmetry. The algorithm has been implemented in EXPO2004 [Altomare, Caliandro, Camalli, Cuocci, Giacovazzo, Moliterni & Rizzi (2004). J. Appl. Cryst.37, 1025,1028] and has been successfully tested using a large set of experimental data. [source] Observation of orientational disorder in the hexagonal stuffed tridymite Sr0.864Eu0.136Al2O4 by the maximum-entropy methodJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 5 2004H. Yamada The crystal structure of a strontium europium aluminate, Sr0.864Eu0.136Al2O4, with a novel hexagonal form was investigated by a combination of Rietveld analysis and the maximum-entropy method (MEM) with synchrotron X-ray powder diffraction data. The electron density image calculated by the MEM/Rietveld method revealed that the apical oxygen ion in the AlO4 tetrahedron has a broad distribution corresponding to an extraordinarily large atomic displacement parameter. This structure could be expressed by a split-atom model, with which the Rietveld refinement gave Rwp = 2.99% and RB = 4.16%. Subsequently, MEM-based pattern fitting (MPF) decreased the R factors to Rwp = 2.81% and RB = 2.34% and the electron density image clearly showed that the apical oxygen ions of the AlO4 tetrahedra are split over three sites around a threefold axis involving an elongated distribution of the residual O ions along the c axis. These results suggest that AlO4 tetrahedra in Sr0.864Eu0.136Al2O4 are orientationally disordered. [source] On the equivalence of the Rietveld method and the correlated integrated intensities method in powder diffractionJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 4 2004William I. F. David The Rietveld method is the most straightforward and statistically correct approach for the refinement of crystal structure parameters from powder diffraction data. The equivalent two-stage approach, involving the refinement of structural parameters based on integrated intensities extracted using the Pawley method, is extremely useful in circumstances such as the global optimization methods of structure determination, where a great many refinements need to be performed very quickly. The equivalence is emphasized in a simple mathematical relationship between the goodness of fits obtained in Rietveld, Pawley and correlated integrated intensities refinements. A rationale is given for determining the estimated standard deviations for structural variables from powder diffraction data. [source] Nanocrystalline domain size distributions from powder diffraction dataJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 4 2004Matteo Leoni The need for an a priori domain size distribution is one of the main limitations of existing line profile analysis methodologies. A numerical modification of the whole-powder-pattern modelling algorithm is proposed, to allow the refinement of a general domain size distribution from powder diffraction data. The shape of domains has to be inferred for the specimen under study. The algorithm is robust enough to unveil fine details in the refined distribution, as witnessed by the results of tests performed both on simulated and on real patterns of nanocrystalline ceria. [source] Structure determination of diclofenac in a diclofenac-containing chitosan matrix using conventional X-ray powder diffraction dataJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2 2004Nongnuj Muangsin The structure determination of diclofenac embedded in a diclofenac-containing chitosan matrix using conventional X-ray powder diffraction data is demonstrated. It reveals that sodium diclofenac, the starting material in the preparation of a controlled-release diclofenac-containing chitosan matrix, changes to diclofenac acid in space group C2/c in the matrix. Simple methods were employed for handling the sample to obtain X-ray powder diffraction data of sufficiently high quality for the determination of the crystal structure of diclofenac embedded in chitosan. These involved grinding and sieving several times through a micro-mesh sieve to obtain a suitable particle size and a uniformly spherical particle shape. A traditional technique for structure solution from X-ray powder diffraction data was applied. The X-ray diffraction intensities were extracted using Le Bail's method. The structure was solved by direct methods from the extracted powder data and refined using the Rietveld method. For comparison, the single-crystal structure of the same drug was also determined. The result shows that the crystal structure solved from conventional X-ray powder diffraction data is in good agreement with that of the single crystal. The deviations of the differences in bond lengths and angles are of the order of 0.030,Å and 0.639°, respectively. [source] Cationic disorder, microstructure and dielectric response of ferroelectric SBT ceramicsJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3-2 2003Ch. Muller Polycrystalline samples of SrBi2Ta2O9 (acronym SBT) have been prepared by means of solid-state reaction either using a classical route or by mechanochemical activation. For each compound, a structural analysis of the ferroelectric orthorhombic phase (space group A21am) has been performed from the fitting of neutron and/or X-ray powder diffraction data using the Rietveld method. A cationic disorder on Bi3+ and Sr2+ crystallographic sites has been revealed, the Sr atoms occupying the Bi sites and vice versa. From diffraction peak broadening analyses of high-resolution synchrotron X-ray diffraction data, it has been shown that the two grinding methods (manual or mechanical) induce local strains, the average apparent strain being three times larger for the mechanically ground sample. In order to link microstructure and ferroelectric properties, the dielectric constant has been measured as a function of the temperature. It appears that the position and the shape of the dielectric anomaly strongly depend upon the composition and the route used to elaborate the powders. More exactly, for the mechanically ground powder, the large apparent strain, probably correlated to the strong cation exchange revealed by the structural refinement, leads to a significantly enhanced dielectric response. [source] FOX, `free objects for crystallography': a modular approach to ab initio structure determination from powder diffractionJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 6 2002Vincent Favre-Nicolin A new program has been developed for ab initio crystal structure determination from powder diffraction data (X-ray and neutron). It uses global-optimization algorithms to solve the structure by performing trials in direct space. It is a modular program, capable of using several criteria for evaluating each trial configuration (e.g. multi-pattern). It is also modular in the description of the crystal content, with the possibility of describing building blocks in the sample, such as polyhedra or molecules, and with automatic adaptive handling of special positions and sharing of identical atoms between neighbouring building blocks. It can therefore find the correct structure without any assumption about the connectivity of the building blocks and is suitable for any kind of material. Several optimization algorithms (simulated annealing, parallel tempering) are available, with the possibility of choosing the convergence criterion as a combination of available cost functions. This program is freely available for Linux and Windows platforms; it is also fully `open source', which, combined with an object-oriented design and a complete developer documentation, ensures its future evolution. [source] | |||||||||||||