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Direct Methods (direct + methods)
Selected AbstractsDirect methods and the solution of organic structures from powder dataJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2 2007Angela Altomare The electron density map produced after the application of direct methods to powder diffraction data of organic compounds is usually very approximated: some atoms are missed, other atoms are in false positions, some atoms are imperfectly located and the connectivity is quite low. A new procedure able to recover the complete structure model is described. In this procedure, a better interpretation of the map is combined with geometrical techniques for generating new atomic positions. The application of the new procedure may lead to the recovery of the complete crystal structure. [source] Completion of crystal structures from powder data: the use of the coordination polyhedraJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 6 2000Angela Altomare Direct methods applied to powder diffraction data often provide well located heavy atoms and unreliable light-atom positions. The completion of the crystal structure is then not always straightforward and may require a considerable amount of user intervention. The heavy-atom connectivity provided by the trial solution may be used to guess the nature of the coordination polyhedra. A Monte Carlo procedure is described which, in the absence of a well defined structural model, is able to locate the light atoms correctly under the restraints of the experimental heavy-atom connectivity model. The correctness of the final model is assessed by criteria based on the agreement between the whole experimental diffraction pattern and the calculated one. The procedure requires little CPU computing time and has been implemented as a routine of EXPO [Altomare et al. (1999). J. Appl. Cryst.32, 339,340]. The method has proved to be sufficiently robust against the distortion of the coordination polyhedra and has been successfully applied to some test structures. [source] Direct methods and protein crystallography at low resolutionACTA CRYSTALLOGRAPHICA SECTION D, Issue 10 2000Christopher J. Gilmore The tools of modern direct methods are examined and their limitations for solving protein structures discussed. Direct methods need atomic resolution data (1.1,1.2,Å) for structures of around 1000 atoms if no heavy atom is present. For low-resolution data, alternative approaches are necessary and these include maximum entropy, symbolic addition, Sayre's equation, group scattering factors and electron microscopy. [source] Crystal structure of a polar nematogen 4-(trans- 4-undecylcyclohexyl) isothiocyanatobenzeneCRYSTAL RESEARCH AND TECHNOLOGY, Issue 10 2007S. Biswas Abstract Crystal and molecular structures of a nematogenic compound 4-(trans- 4-undecylcyclohexyl) isothiocyanatobenzene (11CHBT) have been determined by direct methods using single crystal X-ray diffraction data. The compound (C24H37N1S1) crystallizes in the monoclinic system with the space group P21/c and Z = 4. The unit cell parameters are a = 5.5539(11) Å, b = 8.1341(10) Å, c = 51.494(5) Å, and (= 91.127(14)0. The structure was refined to Rw = 0.051. The molecule is found to be slightly bow-shaped though the alkyl chain is in all- trans conformation. The phenyl ring and the alkyl chain are planar and the cyclohexyl group is in chair conformation. The isothiocyanato groups are almost linear. Parallel imbricated mode of packing of the molecules is found in the crystalline state which is precursor to the nematic phase structure. There are many van der Waals' interactions particularly in the isothiocyanato benzene part of the molecule. Of the various associated pairs of molecules the one having anti-parallel configuration with overlaps in the isothiocyanato phenyl group probably exists in both the crystalline and the nematic phases. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Crystal structure of 4-(1-methyl-1-mesitylcyclobutane-3-yl)-2-aminothiazoleCRYSTAL RESEARCH AND TECHNOLOGY, Issue 3 2006. Aksoy Abstract The crystal structure of 4-(1-methyl-1-mesitylcyclobutane-3-yl)-2-aminothiazole (C17H22N2S1) has been determined by X-ray crystallographic techniques. The compound crystallizes in the triclinic space group P-1 with Z = 6. The crystal structure was solved by direct methods and refined by full-matrix least squares to a final R-value of 0.052 for 2298 observed reflections [I > 2, ( I ) ]. There are three crystallographically independent molecules, I, II and III. These molecules are held together by intermolecular N-H...N hydrogen bonds. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim [source] Crystal structure of N-[(1Z)-1-(3-methyl-3-phenylcyclobutyl)-2-thiomorpholin-4-ylethylidene] thioureaCRYSTAL RESEARCH AND TECHNOLOGY, Issue 7 2005U. Sar Abstract The crystal structure of N-[(1Z)-1-(3-methyl-3-phenylcyclobutyl)-2-thiomorpholin-4-ylethylidene] thiourea (C18H26N4S2) has been determined by X-ray crystallographic techniques. The compound crystallizes in the orthorhombic space group Pbca, with unit cell parameters: a = 15.692(3), b = 20.803(8), c = 11.979(6)Å, Z = 8, V = 3911(7)Å3. The crystal structure was solved by direct methods and refined by full-matrix least squares to a final R-value of 0.084 for 1447 observed reflections [I > 2, ( I ) ]. In the thiosemicarbazide moiety, the S = C bond length is 1.656(6), N-C-N angle is 115.6(5)°. The crystal structure is stabilized by the intermolecular N-H...S hydrogen bonds. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Crystal structures of two acridinedione derivativesCRYSTAL RESEARCH AND TECHNOLOGY, Issue 3 2005K. Palani Abstract The crystal structures of two acridinedione derivatives, namely 10-(3,4-Dichloro-5-hydroxyphenyl)-3,4,6,7,9,10-hexahydro-1,8(2H, 5H) acridinedione (DHHA, CCDC 206440) and 10-(3,5-Dihydroxy-4-nitrophenyl)-3,4,6,7,9,10-hexahydro-1,8(2H, 5H) acridinedione (DHNA, CCDC206441) are reported here. Both the structures were solved by direct methods and refined by full-matrix least-squares procedures to final R- values of 0.073 and 0.076 respectively. In both the crystal structures, the central pyridine ring in the acridinedione moiety tends to be planar while the outer two rings adopt half-chair (sofa) conformation. The buckling angles 2.2(2)° and 11.0(1)° for DHHA and DHNA show the degree of planarity of the acridinedione moiety. The C-H,O types of hydrogen bonds help to stabilize the molecules in the unit cell in addition to van der Waals forces. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Cytoplasmic localization of oocyte-specific variant of porcine DNA methyltransferase-1 during early developmentDEVELOPMENTAL DYNAMICS, Issue 7 2009Young Sun Jeong Abstract DNA methyltransferase-1 (Dnmt1) is involved in the maintenance of genomic methylation patterns. Rather than full-length Dnmt1, mouse oocytes have a truncated variant called Dnmt1o. Immunofluorescence data showed that Dnmt1o localized to the cytoplasm, but this has not been confirmed using more direct methods. The cytoplasmic localization of Dnmt1o has been assigned to the main cause of global DNA demethylation in early mouse embryos. We studied localization of Dnmt1o in mouse and pig embryos. We identified pig Dnmt1o protein and its transcript with unique 5,-end sequence. Physically separating mouse and pig 2-cell embryos into their nuclear and cytoplasmic components demonstrated that Dnmt1o of both species localized to the cytoplasm. Cloned pig embryos had Dnmt1o as the main form, with no indication of somatic Dnmt1. These findings indicate that Dnmt1o is cytoplasmic during early development; its presence in both pig and mouse embryos further suggests that Dnmt1o is conserved in mammals. Developmental Dynamics 238:1666,1673, 2009. © 2009 Wiley-Liss, Inc. [source] Energy functions for FACTS devices with an energy-storage systemEUROPEAN TRANSACTIONS ON ELECTRICAL POWER, Issue 5 2007V. Azbe Abstract An energy-storage system (ESS) can provide additional capabilities for FACTS devices in dynamic power-flow control, and in this way improve electric-power system stability. In order to assess their influence on the system's dynamic behavior or to determinate the device's control strategy using direct methods, proper energy functions for these devices are needed. In this paper the energy functions for the whole spectrum of FACTS devices with an ESS have been developed. For each of the devices various energy functions were proposed according to the control strategy applied. The energy functions were constructed as additional terms that can be added to any existing structure-preserving energy function (SPEF). Tests within a single-machine infinite-bus system proved the correctness of the proposed energy functions. The application of new energy functions was demonstrated on the problem of transient-stability assessment. Copyright © 2006 John Wiley & Sons, Ltd. [source] Energy functions analysis in voltage collapseEUROPEAN TRANSACTIONS ON ELECTRICAL POWER, Issue 4 2001F. Jurado Time-domain approach examines the behaviour of the system, one determines whether stability has been maintained or lost. In contrast to the time-domain approach, direct methods determine system stability based on energy functions. The basis of direct methods for the stability assessment of a system is knowledge of the stability region. During the last decade, many researches have thoroughly analysed the use of energy functions for the direct stability assessment of networks. Energy function analysis offers a different geometric view of voltage collapse. The Transient Energy Function, a technique based on Lyapunov stability theory and originally developed for direct stability analysis of power systems, has been successfully used as a voltage stability index for collapse studies. In this paper the simulation results are on the IEEE 173-bus test system. [source] Cytolethal distending toxin (CDT): a bacterial weapon to control host cell proliferation?FEMS MICROBIOLOGY LETTERS, Issue 2 2001Jean de Rycke Abstract Cytolethal distending toxins (CDT) constitute a family of genetically related bacterial protein toxins able to stop the proliferation of numerous cell lines. This effect is due to their ability to trigger in target cells a signaling pathway that normally prevents the transition between the G2 and the M phase of the cell cycle. Produced by several unrelated Gram-negative mucosa-associated bacterial species, CDTs are determined by a cluster of three adjacent genes (cdtA, cdtB, cdtC) encoding proteins whose respective role is not yet fully elucidated. The CDT-B protein presents sequence homology to several mammalian and bacterial phosphodiesterases, such as DNase I. The putative nuclease activity of CDT-B, together with the activation by CDT of a G2 cell cycle checkpoint, strongly suggests that CDT induces an as yet uncharacterized DNA alteration. However, the effective entry of CDT into cells and subsequent translocation into the nucleus have not yet been demonstrated by direct methods. The relationship between the potential DNA-damaging properties of this original family of toxins and their role as putative virulence factors is discussed. [source] Structures of Four Crystal Forms of DecaplaninHELVETICA CHIMICA ACTA, Issue 5 2003Christopher Lehmann The glycopeptide antibiotic decaplanin (1; formerly known as MM 47761 and M86-1410) crystallizes in two P21 and two P6122 crystal forms, each with four monomers in the asymmetric unit, with solvent contents varying from 48 to 69%. Although with ca. 600 unique atoms, the structures are larger than typical small molecules, one was solved by direct methods. The other three were solved by typical macromolecular methods: single-wavelength anomalous diffraction (SAD) of the Cl-atoms present naturally in the structure, multiple-wavelength anomalous diffraction (MAD) at the Br absorption edge for a crystal soaked in NaBr solution, and molecular replacement. There is evidence of appreciable radiation damage with loss of 20,30% of the covalent and ionic halogens affecting the synchrotron datasets that may even have unintentionally facilitated the MAD structure solution. The structures contain the dimer units typical of antibiotics related to vancomycin, but, in addition, there are a variety of further intermolecular interactions responsible for the polymorphy leading to intertwined 61 -helices in two of the crystal forms. Except for the sugars and some sidechains, the conformations of the 16 independent monomers are very similar. [source] An efficient out-of-core multifrontal solver for large-scale unsymmetric element problemsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 7 2009J. K. Reid Abstract In many applications where the efficient solution of large sparse linear systems of equations is required, a direct method is frequently the method of choice. Unfortunately, direct methods have a potentially severe limitation: as the problem size grows, the memory needed generally increases rapidly. However, the in-core memory requirements can be limited by storing the matrix and its factors externally, allowing the solver to be used for very large problems. We have designed a new out-of-core package for the large sparse unsymmetric systems that arise from finite-element problems. The code, which is called HSL_MA78, implements a multifrontal algorithm and achieves efficiency through the use of specially designed code for handling the input/output operations and efficient dense linear algebra kernels. These kernels, which are available as a separate package called HSL_MA74, use high-level BLAS to perform the partial factorization of the frontal matrices and offer both threshold partial and rook pivoting. In this paper, we describe the design of HSL_MA78 and explain its user interface and the options it offers. We also describe the algorithms used by HSL_MA74 and illustrate the performance of our new codes using problems from a range of practical applications. Copyright © 2008 John Wiley & Sons, Ltd. [source] EXPO2009: structure solution by powder data in direct and reciprocal spaceJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 6 2009Angela Altomare The program EXPO2009 is the evolution of EXPO2004 [Altomare, Caliandro, Camalli, Cuocci, Giacovazzo, Moliterni & Rizzi (2004). J. Appl. Cryst. 37, 1025,1028]. EXPO2009 performs all the steps of ab initio structure solution by powder data: indexing, space-group determination, estimation of the reflection integrated intensities, structure solution by direct/Patterson methods and/or by a direct-space/hybrid approach, and model refinement by the Rietveld technique. New procedures have been introduced in EXPO2009 for enhancing the structure solution process, particularly in the case of low-resolution data and/or organic compounds, when traditional approaches like direct methods may fail. The EXPO2009 graphical interface has been optimized and made very user friendly. [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] The revenge of the Patterson methods.JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 5 2007In the present paper, the third and last of a series (the first two papers were dedicated to the crystal structure solution of proteins), the Patterson superposition method, based on the use of the symmetry minimum function, has been applied to powder diffraction patterns. The method has been modified to take into account the special challenges of this kind of data and to optimize the performance of the approach. The new algorithms have been implemented in a computer program and applied also to single-crystal data of small and medium-size crystal structures. The experimental results have been compared with those obtained via direct methods, so enabling the role and the perspectives of these two approaches in the global phasing problem to be established, no matter what the experimental technique (powder or single-crystal diffraction) or the size of the structures (small, medium or macro-molecules). [source] The revenge of the Patterson methods.JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2 2007The Patterson techniques, recently developed by the same authors for the ab initio crystal structure solution of proteins, have been applied to single and multiple anomalous diffraction (SAD and MAD) data to find the substructure of the anomalous scatterers. An automatic procedure has been applied to a large set of test structures, some of which were originally solved with remarkable difficulty. In all cases, the procedure automatically leads to interpretable electron density maps. Patterson techniques have been compared with direct methods; the former seem to be more efficient than the latter, so confirming the results obtained for ab initio phasing, and disproving the common belief that they could only be applied to determine large equal-atom substructures with difficulty. [source] Direct methods and the solution of organic structures from powder dataJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2 2007Angela Altomare The electron density map produced after the application of direct methods to powder diffraction data of organic compounds is usually very approximated: some atoms are missed, other atoms are in false positions, some atoms are imperfectly located and the connectivity is quite low. A new procedure able to recover the complete structure model is described. In this procedure, a better interpretation of the map is combined with geometrical techniques for generating new atomic positions. The application of the new procedure may lead to the recovery of the complete crystal structure. [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] New opportunities in biological and chemical crystallographyJOURNAL OF SYNCHROTRON RADIATION, Issue 1 2002John R. Helliwell Banerjee [Proc. R. Soc. (1933), 141, 188,193] offered a new way of approaching the crystallographic phase problem which not only broke new ground beyond the `trial and error' structure solution method of that time but also heralded the extremely powerful direct methods of crystallography of the modern era from the 1970s onwards in chemical crystallography. Some 200000 crystal structures are known today. More complex crystal structures such as proteins required new experimental and theoretical methods to solve the phase problem. These are still evolving, and new methods and results involving synchrotron radiation at softer X-ray wavelengths (2,Å) are reported. In addition, an overview is given of the new opportunities that are possible for biological and chemical crystallography, especially via harnessing synchrotron radiation and neutron beams. [source] Synthesis and Crystal Structure of a New Layered Carbide ZrAl4C4JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2008Tomoyuki Iwata A new ternary layered carbide, ZrAl4C4, has been synthesized and characterized by X-ray powder diffraction. The crystal structure was successfully determined using direct methods and further refined by the Rietveld method. The crystal is trigonal (space group P3m1, Z=2) with lattice dimensions a=0.332471(3) nm, c=2.19717(2) nm, and V=0.210330(3) nm3. The final reliability indices calculated from the Rietveld refinement were Rwp=6.56% (S=1.58), Rp=4.92%, RB=1.90%, and RF=0.98%. The compound shows an intergrowth structure with NaCl-type [Zr2C3] thin slabs separated by Al4C3 -type [Al8C7] layers. [source] Direct measurement of the transverse and longitudinal 15N chemical shift anisotropy,dipolar cross-correlation rate constants using 1H-coupled HSQC spectraMAGNETIC RESONANCE IN CHEMISTRY, Issue 10 2003Jennifer B. Hall Abstract We describe direct methods for the measurement of the transverse and longitudinal 15N chemical shift anisotropy,dipolar cross-correlation rates based on comparison of the 15N doublet components observed in 1H-coupled 1H,15N HSQC-type spectra. This allows the determination of the cross-correlation rates with no need for correction factors associated with other methods. The signal overlap problem of coupled HSQC spectra is addressed by using the IPAP scheme (Ottiger M, Delaglio F, Bax A. J. Magn. Reson. 1998; 131: 373). The methods proposed here use a conventional t1 evolution period, which allows one to minimize the truncation artifacts observed in a constant-time-type experiment (Hall JB, Dayie K, Fushman D. J. Biomol. NMR 2003; 26: 181). Applications of these measurements to the B3 domain of protein G are discussed. Copyright © 2003 John Wiley & Sons, Ltd. [source] Dynamic Newton,gradient-direction-type algorithm for multilayer structure determination using grazing X-ray specular scattering: numerical simulation and analysisACTA CRYSTALLOGRAPHICA SECTION A, Issue 1 2009F. N. Chukhovskii A new dynamic iterative algorithm code for retrieving macroscopic multilayer structure parameters (the layer thickness and complex refraction index for each layer, the surface roughness and the interface roughness between the layers) from specular scattering angular scan data is proposed. The use of conventional direct methods, particularly the well known Newton algorithm and gradient-direction-type algorithm operating dynamically to minimize the error functional in a least-squares fashion, is explored. Such an approach works well and seems to be effective in solving the inverse problem in the high-resolution X-ray reflectometry (HRXR) method. In order to demonstrate some features of the proposed iterative algorithm, numerical calculations for retrieving three-layer structure parameters are carried out using simulated HRXR angular scan data. The calculations indicate clearly that the dynamic iterative algorithm is convergent and capable of yielding the true solution. It is important that the performance coefficient for successful iterative cycles for the absolute minimization of the HRXR error functional is quite high even if the initial values of the search parameters are chosen rather far from the true values. It is particularly noteworthy that the relative number of successful iterative cycles is of the order of 90,40% when only moderately accurate initial parameter values, varying by ±10,40% from the true values, are presumed. [source] Structure of Ti2P solved by three-dimensional electron diffraction data collected with the precession technique and high-resolution electron microscopyACTA CRYSTALLOGRAPHICA SECTION A, Issue 2 2003Xiaodong Zou The crystal structure of Ti2P has been analysed using electron diffraction and high-resolution electron-microscopy techniques. A new unit cell was found, the compound is hexagonal with a = 19.969,(1) and c = 3.4589,(1),Å. The structure was first solved in space group in projection using direct methods on electron diffraction data from the [001] zone axis. A three-dimensional solution was obtained using again direct methods but on a three-dimensional set of electron diffraction data recorded with the precession technique. Ti2P is a distorted Fe2P structure and, based on high-resolution images, it is possible to explain that the tripling of the unit cell is due to the ordering of P vacancies that reduces the symmetry to . [source] Structure of nanocrystalline anatase solved and refined from electron powder dataACTA CRYSTALLOGRAPHICA SECTION A, Issue 4 2002T. E. Weirich Energy-filtered Debye,Scherrer electron powder data have been successfully employed to determine the structure of nanocrystalline anatase (TiO2). The performed structure analysis includes determining the unit cell, space group, solving the structure via direct methods from extracted intensities and refining the structure using the Rietveld technique. The refined structural parameters for space group I41amd are a = 3.872,(2), c = 9.616,(5),Å with titanium at 0.5,0.75,0.375 and oxygen at 0.5,0.75,0.1618,(6). The obtained structure indicates low internal stress as judged from the almost regular geometry of the TiO6 building blocks. Striking resemblance with the anatase structure determined previously by Burdett, Hughbanks, Miller, Richardson & Smith [J. Am. Chem. Soc. (1987). 109, 3639,3646] from neutron diffraction on coarse-grained material gives strong support for the correctness of the structure determined here. The result of the present study shows that the methods originally developed for determining structures from X-ray powder data work equally well with data from electron powder diffraction. This may open the window for structural investigations on the vast number of nanocrystalline materials and thin films whose structures are difficult to determine by X-ray diffraction since they are frequently only available in small quantities. [source] Statistical dynamical direct methods.ACTA CRYSTALLOGRAPHICA SECTION A, Issue 3 2001The triplet distribution used for kinematical diffraction is extended to the complex case appropriate for dynamical transmission electron diffraction. It is demonstrated that this gives good results if the distributions are handled statistically rather than relying upon single triplet relationships. As a consequence, conventional statistical direct methods will yield a reasonable approximation to the effective dynamical potential for thicknesses when kinematical theory is not appropriate. The recovered effective dynamical potential may be similar to the kinematical potential, but does not have to be and in general will not be. [source] Two new silicate structures based on a rhodesite-type heteropolyhedral microporous frameworkACTA CRYSTALLOGRAPHICA SECTION B, Issue 2 2010Marcella Cadoni Two new members of the mero-plesiotype rhodesite series [Sr2Na2(Si8O19)·4H2O, abbreviated as TR09; SrNa4(Si8O19)·4H2O, TR10] have been hydrothermally synthesized in Teflon-lined autoclaves at 503,K and structurally characterized using X-ray diffraction single-crystal data. The crystal structures were solved by direct methods and refined to R = 0.021 [TR09; 3317 reflections with Io > 2,(Io)] and R = 0.033 [TR10; 5007 reflections with Io > 2,(Io)]. Both structures are based on a rhodesite-type microporous heteropolyhedral framework, where two types of channels are within the double silicate layer that alternates with an `octahedral' O sheet. The large Sr2+ cation constrains to the roughly ellipsoidal shape of the channels. The H2O molecules are located both in the O sheets and in the channels, where they are loosely hydrogen bonded. The crystal-chemical features that allow flexibility to the rhodesite-type microporous heteropolyhedral framework and make it interesting for possible technological applications are discussed. [source] Structure solution of the new titanate Li4Ti8Ni3O21 using precession electron diffractionACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2010Mauro Gemmi A sample having stoichiometry Li[Ti1.5Ni0.5]O4 has been synthesized to obtain a spinel structure. The resulting crystalline powder revealed a multiphase nature with spinel as the minor phase. The main phase is a new trigonal phase having a = 5.05910,(1), c = 32.5371,(1),Å. The structure has been solved by direct methods working on a three-dimensional set of intensities obtained from a precession electron-diffraction experiment, and refined on synchrotron powder diffraction data in the space group . The model consists of hexagonal layers of edge-sharing octahedra occupied either by the heavy cations Ti and Ni, or preferentially by Li. On the basis of cation-site occupancies the stoichiometry becomes Li4Ti8Ni3O21, which is compatible with the microanalysis results. [source] Copper(II) chloride and bromide complexes with 2-methyl-2H -tetrazol-5-amine: an X-ray powder diffraction studyACTA CRYSTALLOGRAPHICA SECTION C, Issue 4 2010Ludmila S. Ivashkevich The complex catena -poly[[dibromidocopper(II)]-bis(,-2-methyl-2H -tetrazol-5-amine)-,2N4:N5;,2N5:N4], [CuBr2(C2H5N5)2]n, (I), and the isotypic chloride complex catena -poly[[dichloridocopper(II)]-bis(,-2-methyl-2H -tetrazol-5-amine)-,2N4:N5;,2N5:N4], [CuCl2(C2H5N5)2]n, (II), were investigated by X-ray powder diffraction at room temperature. The crystal structure of (I) was solved by direct methods, while the Rietveld refinement of (II) started from the atomic coordinates of (I). In both structures, the Cu atoms lie on inversion centres, adopting a distorted octahedral coordination of two halogen atoms, two tetrazole N atoms and two 5-amine group N atoms. Rather long Cu,Namine bonds allow consideration of the amine group as semi-coordinated. The compounds are one-dimensional coordination polymers, formed as a result of 2-methyl-2H -tetrazol-5-amine ligands bridging via a tetrazole N atom and the amine N atom. In the polymeric chains, adjacent Cu atoms are connected by two such bridges. [source] CdBiO2Cl: synthesis and powder structure solutionACTA CRYSTALLOGRAPHICA SECTION C, Issue 12 2001Sergei D. Kirik The title compound, cadmium bismuth dioxide chloride, CdBiO2Cl, was obtained as a white powder by reaction of solid BiOCl with CdO at 973,K. Ab initio crystal structure determination was carried out using X-ray powder diffraction techniques, including direct methods for atom location and Rietveld fitting for the final refinement. Being monoclinic, the crystal structure can be related to tetragonal Sillen layered phases. The main structural elements present are CdBiO2+ pleated metal,oxygen layers alternating with Cl layers along the c axis, whereas along the b axis, all atoms are on mirror planes. The formation of a strong Cd,Cl bond draws the layers together, causing layer deformation and a monoclinic distortion in the layer arrangement. [source] | ||||||||||||||||||||||||||||||||||