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  • International Union of Crystallography (IUCr)
  • 2000-2004  (1,849)
  • 1950-1954  (276)
  • 2000  (1,849)
  • 1952  (276)
Collection
Publisher
Years
  • 2000-2004  (1,849)
  • 1950-1954  (276)
Year
  • 1
    Electronic Resource
    Electronic Resource
    New methods for diffraction stress measurement: a critical evaluation of new and existing methods (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 1059-1066 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: New methods of diffraction stress analysis of polycrystalline materials, consisting of cubic elastically anisotropic crystallites, are proposed and compared with existing methods. Whereas for the existing methods knowledge of the diffraction elastic constants is presupposed, three new methods are presented that require only knowledge of the (macroscopic) mechanical elastic constants. The stress values obtained with these new methods on the basis of the mechanical elastic constants are more reliable than those obtained with the methods on the basis of the diffraction elastic constants. New and existing methods are illustrated by means of measurements of X-ray diffraction from a magnetron-sputtered TiN layer.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889800004258
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  • 2
    Electronic Resource
    Electronic Resource
    Determination of lattice-transform density profiles for multilayered three-dimensional microcrystals in electron crystallography (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 1102-1112 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Electron crystallography on multilayered three-dimensional microcrystals has been limited in application by the need to define precisely the three-dimensional shape of the diffraction density profiles. A new method is presented here to obtain this profile from experimental spot positions which are shifted in a characteristic way from the expected Bragg positions. While the Bragg positions are defined by the diffraction geometry, the characteristic shift additionally depends on the density profile in Fourier space. In general, these two effects are intermingled. A new correlation approach is presented which uses characteristic shift patterns to separate these effects. This technique also allows the determination of all three crystallographic unit-cell dimensions from a single tilted electron diffraction pattern. It was tested on simulated diffraction patterns and applied to experimental data of frozen hydrated crystals of the protein catalase. Since multilayered catalase crystals with different numbers of crystallographic layers were studied, an inhomogeneous data set had to be evaluated. Processing of such data is now possible using the new correlation approach.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889800005963
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  • 3
    Electronic Resource
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    Data smoothing and distortion of X-ray diffraction peaks. II. Application (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 1136-1142 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Based on the analysis of geometric relations, it is found that the amplitude of the distortion of the full width at half-maximum (FWHM) of a peak after smoothing is the largest among the distortions of the seven parameters of a peak: maximum intensity, FWHM, shape, asymmetry, integral intensity, peak position and integral width. Hence the FWHM distortion is used as a criterion of smoothing. The 1% and 5% error limits of smoothing are calculated. Quadratic and quartic polynomial Savitzky–Golay filters are compared. Aberrations of an X-ray diffraction (XRD) peak caused by using double, matched, 1.5 and 0.7 FWHM filters are described in more detail by means of the analyses of internal and external factors. It is found that the smoothing-induced distortions of an actual XRD peak trend towards those of ideal curves. The distortions are classified according to three types of peak. A possible method to obtain the actual peak parameters, called `system smoothing refinement', is presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889800006944
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  • 4
    Electronic Resource
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    Computing in macromolecular crystallography using a parallel architecture (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 1154-1161 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Despite advances in computer technology, computing in macromolecular crystallography keeps pace in its demand for CPU power. Improvements in CPU speed, together with advances in computing methods that depend on it, often translate into the possibility to solve structures that would otherwise require additional experiments. Programs for data reduction, molecular-replacement programs employing multidimensional searches on a grid in real, Patterson or reciprocal space, and phasing and refinement programs, currently have, among others, the highest requirements for CPU power. For these and other programs, speed-up of calculations as a result of parallel execution on multiprocessor computers is possible. This paper outlines the use of the OpenMP programming interface and reports its successful application for parallelization of ESSENS [Kleywegt & Jones (1997). Acta Cryst. D53, 179–185] and SHELXL [Schneider & Sheldrick (1997). Methods Enzymol. 277, 319–343]. Parallel computing, which is possible as a result of the inherent parallelism of crystallographic algorithms, extends the range of problems in macromolecular crystallography that programs can be applied to and can significantly reduce the time required for progressing from a data set to a refined model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S002188980000697X
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  • 5
    Electronic Resource
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    Development of a graphical interface for the Rietveld refinement program DBWS (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 1189-1189 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889800005410
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  • 6
    Electronic Resource
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    SUBGROUPGRAPH: a computer program for analysis of group–subgroup relations between space groups (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 1190-1191 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889800007135
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  • 7
    Electronic Resource
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    Polar potassium rare earth nitrates K2[RE(NO3)5(H2O)2] (RE = La, Ce, Pr and Nd). II. Linear and nonlinear optical properties (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 380-386 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Linear and nonlinear optical properties of the four isomorphic orthorhombic (point group mm2) potassium rare earth nitrates K2[RE(NO3)5(H2O)2], with RE = La, Ce, Pr or Nd, have been investigated. Refractive indices in a wavelength range from 0.35 to 2.5 µm and the transmission spectra between 0.25 to 3.0 µm were measured. Using the Maker fringe technique, the d_{ijk}^{\rm\, SHG} coefficients of second-harmonic generation for a fundamental of the Nd–YAP laser (λ = 1079.5 nm) were determined for all four compounds. The d_{ijk}^{\rm\, SHG} coefficients of all four substances are about three times larger than those of KDP, as was first published by Ebbers et al. [IEEE J. Quantum Electron. (1993), 29, 497–507] for K2[La(NO3)5(H2O)2]. These nitrates are promising materials for application to nonlinear optics (e.g. second-harmonic generation of Nd laser radiation). The nonlinear optical properties of these compounds seem to have their origin mainly in the nitrate groups, as indicated by a simple model (summing the molecular hyperpolarizabilities of the NO3 group). Neglecting local field effects, the hyperpolarizability of the NO3 group was calculated as β222 = 2.52 × 10−40 m4 V−1.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S002188980000087X
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  • 8
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    The disordered structure of tetraferrocenyl-[3]-cumulene, (Fc)2C=C=C=C(Fc)2, by simulated annealing using synchrotron powder diffraction data (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 1199-1207 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: In the molecular structure of tetraferrocenyl-[3]-cumulene, (Fc)2C=C=C=C(Fc)2, four ferrocene molecules are connected via a linear bridge consisting of four carbon atoms. At room temperature, the crystal structure has space group P21/a (Z = 1) with a = 13.104 (5), b = 6.121 (2), c = 11.194 (4) Å, β = 114.922 (1)° and V = 814.3 (8) Å3. A phase transition during cooling was not observed from room temperature to 75 K. From high-resolution X-ray powder diffraction data, the structure of the room-temperature phase was solved by the method of simulated annealing and refined by the Rietveld method using rigid bodies and restraints. The crystal structure was found to be highly disordered with the molecules occupying two orientations with equal probability and a 50% occupancy of the carbon atoms in the cumulene bridge. The disorder could be modelled by stacking faults in ordered structures. In contrast to other compounds of this class, the ferrocenyl groups are in a syn rather than in an up–down conformation with respect to the cumulene bridge.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889800007470
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  • 9
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    Solving the generalized indirect Fourier transformation (GIFT) by Boltzmann simplex simulated annealing (BSSA) (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 1212-1216 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The structure of colloidal particles can be studied with small-angle X-ray and neutron scattering (SAXS and SANS). In the case of randomly oriented systems, the indirect Fourier transformation (IFT) is a well established technique for the calculation of model-free real-space information. Interaction leads to an overlap of inter- and intraparticle scattering effects, preventing most detailed interpretations. The recently developed generalized indirect Fourier transformation (GIFT) technique allows these effects to be separated by assuming various models for the interaction, i.e. the so-called structure factors. The different analytical behaviour of these structure factors from that of the form factors, describing the intraparticle scattering, allows this separation. The mean-deviation surface is defined by the quality of the fit for different parameter sets of the structure factor. Its global minimum represents the solution. The former non-linear least-squares approach has proved to be inefficient and not very reliable. In this paper, the incorporation of the completely different Boltzmann simplex simulated annealing (BSSA) algorithm for finding the global minimum of the hypersurface is presented. This new method increases not only the calculation speed but also the reliability of the evaluation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889800008372
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  • 10
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    Synchrotron radiation X-ray powder diffractometer with a cylindrical imaging plate (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 1241-1245 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: A synchrotron radiation X-ray powder diffractometer for samples of very small amount has been developed to collect high-quality diffraction patterns under extreme conditions, i.e. at low temperature and/or high pressure. A new cylindrical imaging plate (CIP) is used as a detector, in addition to a conventional flat-type imaging plate (FIP). By using the CIP system, the diffraction data in a diffraction angle range −44 ≤ 2θ ≤ 122° are collected with a dynamic range of about 106. The alignment of the diffractometer, measurement and analysis are automatically operated by a workstation. A performance test shows that the CIP system has spatial resolution of about 0.07° with a dynamic range of 106. The diffraction pattern of a standard sample of Si measured by the CIP system has high quality; the refinement of the structure reaches Rw = 3.68% even in the case of a small amount of sample (about 2 µg) and a short exposure time (60 s). Examples of experiments at low temperatures under ambient and high pressures are also presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889800009286
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  • 11
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    Response to de Graaff & de Gelder's comment on CRUNCH: a phase refinement program for high-resolution macromolecular structures (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 1194-1194 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889800008335
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  • 12
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    A tangent formula derived from Patterson-function arguments. VII. Solution of inorganic structures from powder data with accidental overlap (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 1208-1211 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Accidental overlap constitutes one of the principal limitations for the solution of crystal structures from powder diffraction data, since it reduces the number of available intensities for direct-methods application. In this work, the field of application of the direct-methods sum function is extended to cope with powder patterns with relatively large amounts of accidental overlap. This is achieved by refining not only the phases of the structure factors but also the estimated intensities of the severely overlapped peaks during the structure solution process. This procedure has been specifically devised for inorganic compounds with uncertain cell contents and with probable severe atomic disorder, a situation often found when studying complex minerals with limited crystallinity. It has been successfully applied to the solution of the previously unknown crystal structure of the mineral tinticite. Finally, an estimation of the smallest ratio (number of observations to number of variables) for the procedure to be successful is given.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889800007834
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  • 13
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    Microcrystallography with an X-ray waveguide (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 1231-1240 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: A waveguide microdiffraction setup is described for an undulator beamline at the European Synchrotron Radiation Facility. The composite optics consists of a waveguide, which confines the beam vertically, and a horizontally focusing multilayer mirror. A beam size of about 0.1 × 3 µm (vertical × horizontal) at λ = 0.095 nm has been obtained. The sample stage comprises a three-axis gantry with micrometre precision and a three-axis piezo-scanner with about 0.1 µm repeatability. Diffraction experiments are demonstrated for selected inorganic and polymeric samples. Possibilities for scanning diffractometry and small-angle scattering experiments are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889800009249
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  • 14
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    An imaging-plate detector for small-angle neutron scattering (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 1253-1261 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Small-angle neutron scattering (SANS) measurements have been performed on long-flight-path pinhole-collimation SANS instruments using, as a two-dimensional position-sensitive detector, both a neutron imaging plate, incorporating gadolinium, and a two-step transfer method, with dysprosium foil as the image transfer medium. The measurements are compared with corresponding data taken using conventional position-sensitive gas proportional counters on the SANS instruments in order to assess the viability of the imaging techniques. The imaging plates have pixel sizes of about two orders of magnitude smaller than those of the gas proportional counter. The reduced pixel size provides definite advantages over the gas counter in certain specific situations, namely when limited space necessitates a short sample-to-detector distance, when only small samples (comparable in size to the detector pixels) are available, or when used in conjunction with focusing beam optics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889800009067
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  • 15
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    Peter Goodman (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 1302-1302 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889800011420
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  • 16
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    Extended pseudo-Voigt function for approximating the Voigt profile (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 1311-1316 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The formula of the pseudo-Voigt function expressed by a weighted sum of Gaussian and Lorentzian functions is extended by adding two other types of peak functions in order to improve the accuracy when approximating the Voigt profile. The full width at half-maximum (FWHM) values and mixing parameters of the Gaussian, the Lorentzian and the other two component functions in the extended formula can be approximated by polynomials of a parameter ρ = ΓL/(ΓG + ΓL), where ΓG and ΓL are the FWHM values of the deconvoluted Gaussian and Lorentzian functions, respectively. The maximum deviation of the extended pseudo-Voigt function from the Voigt profile is within 0.12% relative to the peak height when sixth-order polynomial expansions are used. The systematic errors of the integrated intensity ΓG and ΓL, estimated by fitting the extended formula to Voigt profiles, are typically less than 1/10 of the errors arising from the application of the original formula of the pseudo-Voigt approximation proposed by Thompson et al. [J. Appl. Cryst. (1987), 20, 79–83], while the time required for computation of the extended formula is only about 2.5 relative to the computation time required for the original formula.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889800010219
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  • 17
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    Estimating nanoparticle size from diffraction measurements (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 1335-1341 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Nanometre-sized particles are of considerable current interest because of their special size-dependent physical properties. Debye–Scherrer diffraction patterns are often used to characterize samples, as well as to probe the structure of nanoparticles. Unfortunately, the well known `Scherrer formula' is unreliable at estimating particle size, because the assumption of an underlying crystal structure (translational symmetry) is often invalid. A simple approach is presented here which takes the Fourier transform of a Debye–Scherrer diffraction pattern. The method works well on noisy data and when only a narrow range of scattering angles is available.
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    URL: http://dx.doi.org/10.1107/S0021889800010888
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  • 18
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    Resolution calculations for novel neutron beam elements (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 1393-1398 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: It is now possible to build new types of optical elements for neutron beams. These include monochromating crystals with rectangular mosaic profiles, `gradient' monochromating crystals with varying lattice spacing, and collimators with rectangular profiles. Phase-space methods are used to describe the resolution of a combination of two collimators and a monochromating crystal for various combinations of novel beam elements. The description of rectangular-profile elements in this formalism considerably simplifies the normal description of resolution effects and should thus prove useful as a teaching tool.
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    URL: http://dx.doi.org/10.1107/S0021889800013224
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  • 19
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    Kinematical two-dimensional multiple-diffraction intensity profiles. Application to ω–ψ scans obtained with Cu Kα radiation (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 1405-1414 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Expressions for the calculation of multiple-diffraction patterns observed in ω–ψ scans of Bragg reflections are derived within the framework of the kinematical theory, taking into account the divergence and wavelength-spread of the incident beam as well as the mosaicity of the crystal sample. The theoretical results can be applied to X-ray tube radiation as well as synchrotron radiation experiments. A 189.9° experimental Cu Kα \bar2\bar2\bar2 ω–ψ scan of diamond is compared with the corresponding theoretical multiple-diffraction pattern.
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    URL: http://dx.doi.org/10.1107/S002188980001270X
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  • 20
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    Comments on Extinction-corrected mean thickness and integral width used in the program UMWEG98 by Rossmanith (2000) (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 1447-1447 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Comments are made on a paper by E. Rossmanith [J. Appl. Cryst. (2000), 33, 330–333] concerning the use of asymptotic expressions for the extinction-corrected mean thickness.
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    URL: http://dx.doi.org/10.1107/S0021889800010980
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  • 21
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    European Crystallography Prize (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 1450-1450 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
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    URL: http://dx.doi.org/10.1107/S0021889800017660
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  • 22
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    Professor K. Banerjee Endowment Lecture Silver Medal (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 1450-1450 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
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    URL: http://dx.doi.org/10.1107/S0021889800017672
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  • 23
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    Velibor Marinkovic (1929-2000) (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 1450-1450 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889800017659
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  • 24
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    Porous structure of NiO-based xerogels (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 478-482 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: NiO-based xerogels were prepared by sol-gel processing starting from the NiCl2-BuOH-H2O system. Structural features associated to the porosity of xerogels were studied by small-angle X-ray scattering. Xerogels heat-treated at low temperatures (423 and 573 K) are composed of hydrated NiCl2 small crystals and exhibit an essentially single mode nanoporous structure with an average pore radius of ∼75 Å. Xerogels heat-treated at 973 and 1173 K are composed of aggregates of NiO crystals which only contain intergrain mesopores with radii exceeding 200 Å. A mixture of phases is observed in xerogels heat-treated at 773 K. Xerogels with a water concentration of 2 mol/(mol NiCl2) exhibit a narrow nanopore size distribution. Water concentrations higher or lower than 2 mol promote the formation of nanoporous structures with wider size distributions.
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    URL: http://dx.doi.org/10.1107/S0021889899014284
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  • 25
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    Nano-sized ceramics of coated alumina and zirconia analyzed with SANS (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 483-487 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The sintering behaviour of two new types of coated ceramics, made from alumina grains coated with a zirconia shell and from zirconia grains coated with an alumina shell, was analyzed with small angle neutron scattering (SANS). Measurements were performed both for the plain samples, and with contrast variation using D2 O as immersion liquid. The size distribution and the volume fraction of grains and pores were determined from the corrected scattering curves using a direct model fitting, applying two different approaches, a sphere model and a combined sphere/spherical shell model. Results are discussed in context with the macroscopic density of the samples. The sintering behaviour of the two ceramics types was found to be very different.
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    Small-angle neutron scattering investigation of the early stages of decomposition in Ni-rich Ni-Ti (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 488-491 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: In-situ small-angle neutron scattering was performed on polycrystals and single crystals of Ni-10 to 12 at.% Ti at temperatures between 900 and 1240 K. A plateau in the integrated intensity as a function of aging time for polycrystalline Ni-11.3 at.% Ti at 900 K indicates the presence of a precursor state γ'' preceding the formation of the metastable γ' state, with a Ti concentration of 16.5(1.0) at.% in the particles. In the scattering patterns of a Ni-12.0 at.% Ti single crystal aged at 1200 and 1240 K, strong scattering intensities along 〈100〉 related to the coherent γ''/γ' states, and sharp streaks along 〈111〉 related to plates of the stable η phase appear successively. At 1240 K, the presence of the L12-related intensities along 〈100〉 is unexpected, as this temperature lies above the metastable miscibility gap suggested in the literature.
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    Reversion of precipitates in phase separated soda lime silica glass (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 492-495 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The glass with composition 13Na2O-11CaO-76SiO2 (mol%) undergoes subliquidus phase separation via a binodal mechanism. At temperatures below the binodal temperature, Tb = 978 K, the glass separates into two amorphous phases, silica-enriched droplets and a silica-poor matrix. Small-angle X-ray scattering was used to study the formation of the droplet phase at 923 K as well as the process of reversion where the precipitates dissolve after an increase in temperature. After a heat treatment of 48 h at 923 K the system is in the stage of coarsening. However, the equilibrium volume fraction of the droplet phase, we = 0.073, has not yet precipitated. The reversion of the droplets was studied at T = 983 K and T = 963 K. For T 〉 Tb, the precipitates dissolve completely by an interdiffusion process resulting in a decrease of the silica concentration of the droplets, while the phase boundary is preserved over long times. For T 〈 Tb, the reversion process shows two distinct stages. In the first stage, the silica concentration inside the droplets decreases and all precipitates shrink until the concentration in the matrix reaches its new equilibrium value. Afterwards, the largest droplets grow again by slow coarsening.
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    Dynamical scaling property of colloidal aggregation in a zirconia-based precursor sol during gelation (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 496-499 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Zirconia precursor gels, prepared in the zirconium n-propoxide, n-propanol, acetylacetone and water system, are studied by small angle X-ray scattering (SAXS). The SAXS intensity pattern exhibits a pronounced maximum for a scattering vector q different from zero. The intensity maximum of the finite q-vector peak increases during gelation while its position qm moves towards smaller and smaller q—vectors. The SAXS intensity distribution is well accounted for by recent direct numerical calculations of diffusion-limited cluster aggregation (DLCA). The structural aspects of cluster growth can be described by scaling assumptions. The position qm of the intensity maximum and the structure function S(q/qm,t) are shown to scale according to S(q/qm,t) = qm(t)−d F(q/qm) where F(q/qm) is a time-independent scaling function. This scaling requires d=Df, the fractal dimension of the clusters, instead of d=3 for spinodal decomposition. The gel structure is made up of connected fractal clusters, around 50 nm in size, with an apparent fractal dimension of 1.7, resulting from the aggregation of very small primary particles about 2.5 nm in diameter.
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    Concentration and temperature dependence of decomposition in supercooled liquid alloys (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 500-503 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Small-angle neutron scattering experiments were performed on the bulk amorphous alloy Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vit1®, subscripts indicate at.%) and on other alloys, where the (Zr,Ti) and (Cu,Be) contents were varied by following the line in composition space connecting Vit1 and Zr46.8Ti8.2Cu7.5Ni10Be27.5 (Vit4®). The small-angle neutron scattering data of the samples, annealed at temperatures between 603 K and 663 K, show interference peaks, giving evidence for spatially correlated arrangements of inhomogeneities. The Q values of the interference peaks, Qmax, decrease with increasing annealing temperature Ta and, at a given annealing temperature, with composition following the connecting line from Vit1 to Vit4. Down to the glass transition temperature Tg, the data follow a relation 1/L2 ∝ (Ts − Ta) as predicted by the linearized Cahn theory, with L = 2π/Qmax the characteristic wavelength of the decomposition and Ts the apparent spinodal temperature. Below Tg, a different behavior is observed, which may be either due to a change in atomic diffusion or due to an insufficient relaxation of the samples.
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    Comparison of different methods in microstructural characterization of porous silicon (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 504-506 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Results obtained using the Brunauer-Emmett-Teller method, small-angle x-ray scattering and wide angle x-ray diffraction in the study of porous silicon are compared. The BET method seems to fail when the porosity of samples is smaller than 50%, giving unrealistically large values for the specific surface area, but giving results similar to SAXS when the porosity is larger than 50%. In the comparison of the WAXD and SAXS data quite large differences between the average particle size and chord length were observed in low-porosity samples. The possible origin of the differences is discussed.
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    Agglomeration of nano-MgAl2O4 spinel from microemulsion and CVD techniques (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 507-510 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: MgAl2O4 nanoparticles have been prepared using a microemulsion synthesis technique (ME) and a chemical vapour deposition (CVD) approach. In the wet-chemical route, the chain length of the surfactant molecules used to stabilise the microemulsion as well as the annealing conditions (temperature and time) after preparation were varied. For CVD samples, the influence of the target temperature and the consecutive annealing temperature were investigated. A pinhole small-angle X-ray scattering camera was employed to determine the size and size distribution of primary particles as well as agglomerates. ME material is primarily obtained in the form of amorphous non-agglomerated particles; CVD raw material is crystalline and agglomerated. Upon annealing, agglomerates of ME material form and then grow faster than the primary particles do; in CVD material the agglomerates do not grow. Agglomeration is suppressed if the CVD process is carried out at elevated target temperature.
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    Human tissue analysis by small-angle X-ray scattering (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 511-514 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Molecular structure of pathologically changed biological tissue differs greatly from the normal one. It gives the opportunity to diagnose different diseases by using methods of structural tissue analysis. The present paper is based on Ultra-Low Angle X-ray (ULAX) coherent scattering data from 1000 paired samples of mammary gland tissue dissected during the surgical treatment from 500 patients with different types of breast tumours (malignant or benign) and tumour-like formations (fibrocystic change). The medical data (histological and histostereometrical analysis) and the digital scattering information have been compared by regression analysis to produce the scattering pattern recognition code that demonstrates the high sensitivity of the ULAX method.
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    Structural studies of the 30S subunit of ribosomes Thermus thermophilus by small-angle neutron and X-ray scattering (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 515-518 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Three different small-angle neutron scattering facilities, the "YUMO" camera at JINR (Dubna, Russia), the V4 camera at HMI (Berlin, Germany) and D11 camera at ILL (Grenoble, France), were used in the investigation of 30S ribosomal subunit from Thermus thermophilus. Parallel X-ray measurements on X33 camera at DESY (Hamburg, Germany) were used to control the identity of deuterated particles. The neutron and x-ray contrast variation data were interpreted by direct method using spherical harmonics. Structural models of the 30S ribosomal subunit from Thermus thermophilus and its RNA -rich core with the resolution 3.5 nm were constructed. A comparison between the 30S Thermus thermophilus and 30S E.coli models shows that primary difference between them consists in the special distributions of the protein component whereas the special distributions of the RNA component coincide well in both models.
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    In situ structure determination of the ribosomal protein L14 in the large 50S subunit of Escherichia coli (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 524-525 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
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    SAXS, SANS and X-ray crystallography as complementary methods in the study of biological form and function (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 519-523 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: X-ray crystallography is the single most powerful method for the determination of accurate and precise macromolecular structures. The study of macromolecular motions, however, is, despite some successes with time-resolved crystallography not within the realms of the method. Further, the study of large macromolecular complexes sometimes poses tremendous problems of various kinds for x-ray crystallography and many laboratories approach large structures by solving individual domains. We herein present two examples employing small-angle x-ray and/or neutron scattering (SAXS, SANS) to study (i) conformational changes, and (ii) protein-lipid complexes complementing information from x-ray crystallography. The structural changes of creatine kinase upon substrate binding were investigated as an example of conformational changes not comprehensible from crystallography in the absence of crystal structures of the substrate-ligated enzyme. We also present studies of apolipoprotein E, reconstituted with lipid, in which a combined approach using SAXS/SANS, x-ray crystallography and other techniques yields a model of the complex not accessible by either of the methods alone.
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    The diameter of the DNA superhelix decreases with salt concentration: SANS measurements and Monte Carlo simulations (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 526-529 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: We have measured the static form factor of superhelical DNA by SANS in dilute aqueous solution as a function of salt concentration. Theoretical static form factors were calculated by Monte Carlo simulations. Simulated and measured form factors are in very good agreement and can be interpreted in terms of the superhelix diameter, which decreases from (16.0±0.9) nm at 10 mM to (9.0±0.7) nm at 100 mM Na+ concentration. This result contradicts a lateral collapse of the DNA superhelix in physiological salt concentrations as recently found by cryo-electron microscopy.
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    Advanced solution scattering data analysis methods and their applications (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 530-534 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: A method for ab initio low-resolution shape and internal structure retrieval from contrast variation solution scattering data is described. The method uses a multiphase model of a particle build from densely packed dummy atoms and employs simulated annealing to find a compact interconnected configuration of phases that fits the available experimental data. In the particular case of a single phase particle (shape determination) the method is compared to another ab initio method using low resolution envelope functions. Examples of the shape determination of several proteins from experimental X-ray scattering data are presented.
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    Small-angle neutron scattering of aggregated whey protein colloids with an exocellular polysaccharide (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 540-543 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: We have investigated the scattering properties of mixtures of aggregated-whey-protein colloid (AWC) particles and an exocellular polysaccharide (EPS) from a lactic acid bacterium. Exceeding a certain concentration leads to a phase separation, which can be explained by an effective depletion attraction between the AWCs due to non-adsorbing EPS. The measured scattering intensity of a AWC/EPS mixture shows that the structure factor of the AWC particles increases at low wave vectors with increasing EPS concentration. The measurements can be described qualitatively in terms of the structure factor, calculated using integral theory including an expression for the depletion interaction potential.
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    Solution structure of detergent micelles at conditions relevant to membrane protein crystallization (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 577-581 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: In this study small angle neutron scattering was used to characterize the formation of micelles in aqueous solutions of the detergents DMG and SPC as a function of detergent concentration and ionic strength of the solvent. The effects on the micelle structure of the additives glycerol and PEG, alone as well as in combination typical for membrane protein crystallization, were also explored. This study suggests that the micelles are rod-like in form at the concentrations studied. The size of the micelles was observed to increase with increasing ionic strength but decrease with the addition of glycerol or PEG.
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    Measurement of sugar depletion from uncharged lamellar phases by SANS contrast variation (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 569-573 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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    Notes: We applied small-angle neutron scattering (SANS) contrast variation to samples where a microphase separation occurred. The samples contain multilamellar vesicles in equilibrium with excess "solvent" that produce a very common powder pattern in SANS: a Debye-Sherrer ring produced by the regular bilayer packing superposed to a sharply decaying Porod behaviour. These two features of the SANS pattern have distinct contrast match points (CMP). We exploit here the small angle signal to determine the partition of sugars between two coexisting microphases. The net result is an exclusion of small sugar molecules from the liquid crystalline domains of the sample. We discuss this exclusion in relation with the observed maximum swelling, headgroup hydration and bilayer softening induced by the presence of the sugar molecules.
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    Combining USAXS with osmotic pressure measurements: a tool for investigating the equation of states of diluted charged colloids (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 582-586 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: We have developed a cell allowing in situ measurement of the osmotic pressure of solutions under Ultra Small-Angle X-ray Scattering (USAXS) examination. The coupled set-up made of a Bonse/Hart camera and a deionisation close circuit allows the combined determination of the structure factor and the equation of state of colloidal suspensions. A precise control of ions concentration inside the reservoir allows to vary the ionic strength of the suspensions from the very pure case of colloidal crystals to sols with tunable screening of electrostatic repulsive interactions. We describe in detail the experimental set-ups and the data treatment methodology that we have been developing in the last years to allow detailed measurements of interactions and correlations between spherical particles. A review of our recent results obtained on the model system of negatively charged parabromostyrene latex particles forming solutions, crystals and microcrystals, is given as an illustration.
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    Analysis of the volume transition in thermosensitive core-shell particles by synchrotron small-angle X-ray scattering (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 574-576 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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    Notes: We discuss the analysis of the volume transition in thermosensitive colloidal core-shell particles by synchrotron small-angle X-ray scattering. The latex-particles are dispersed in water and consist of a solid poly(styrene) core (diameter: 107 nm) and a shell of a thermosensitive crosslinked polymer chains. The thermosensitive shell is built up from poly(N-isopropylacrylamide) chains (PNIPA) crosslinked by N,N'-methylenbisacrylamide (BIS). The scattering intensities measured as function of temperature may be decomposed into a part deriving from the overall structure and a part originating from the fluctuations within the network. The analysis of the overall structure leads to the volume fraction of the swollen network at different temperatures. The intensity caused by the fluctuations of the network becomes the leading contribution at high scattering angles. It may be described by a Lorentzian below and above the volume transition. The analysis demonstrates that critical fluctuations of the network around the transition temperature are fully suppressed. This finding is explained by the strong steric constraint of the network by its confinement within a shell of colloidal dimensions: i) The swelling and shrinking can only take place along the radial direction, and ii) the chains are bound to the solid surface of the cores which remains constant during the transition.
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    Scattering from biopolymers with helical symmetry in solution (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 659-663 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Small angle X-ray fibre diffraction on aligned elongated macromolecules has been a routine tool for the structure of materials that either can not be crystallized or in cases in which one wants to study the structure of the molecule as close as possible to those occurring in solution. One of the problems with this approach is that quite often it is not possible to obtain completely aligned samples but only samples in which there is an angular distribution around the main orientation axis. This has as a consequence that atlarger scattering angles the contributions from different layer lines start to overlap. A possible method to overcome this problem can be found in combining results from small angle solution scattering with small angle fibre diffraction from molecules aligned with their long axis parallel with respect to the X-ray beam. In the latter case one only observes the projection of the molecule on the basal plane. We show how this method can be applied to hydrated microtubules.
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    The internal structure of single carbon fibers determined by simultaneous small- and wide-angle scattering (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 695-699 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Simultaneous small-angle scattering and wide-angle diffraction using a synchrotron radiation microbeam was applied for the first time to investigate single carbon fibers in a position-resolved way. Taking into account the exact X-ray beam profile and examining the fibers in two scattering geometries allowed a discrimination between different models for the internal arrangement of carbon layers and pores. For a fiber based on polyacrylnitrile the carbon layers were randomly oriented within the fiber cross section, whereas in a mesophase-pitch based fiber the layers were arranged in a radial structure.
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    Synchrotron studies of polymers at DND-CAT (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 704-708 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Two examples are presented that illustrate the capabilities of DND-CAT instrumentation for the study of the effects of processing on polymers. Firstly, a thermoplastic elastomer, Hytrel®, was stretched while 2-D data were collected simultaneously. The Hytrel® data show that the yield point of the stress-strain curve is associated with the sudden appearance of a four-point pattern. At higher deformations, strain-induced crystallization and the destruction of the hard segment domains lead to a substantial decrease of the contrast as monitored by the SAXS invariant. Prior to breakage, the extent and intensity of an equatorial streak develops as the material fibrillates. Secondly, SAXS and WAXS data were collected from quenched and annealed Poly(ethylene terephthalate) (PET) samples mounted on a DSC cell, to characterize the pre-melting shoulder in this material. Results show that substantial melting and re-crystallization occurs within the range of this shoulder in the quenched sample.
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    Small and wide-angle X-ray scattering study of metallocene isotactic poly(propylene) (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 714-717 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: We report a study of metallocene isotactic poly(propylene) (m-iPP) which crystallizes into α and γ crystal modifications. Simultaneous in-situ small- and wide-angle X-ray scattering (SAXS and WAXS) were used to study kinetics during crystallization. Both techniques provide information about time development of crystallinity, while WAXS gives also kinetics of formation of α and γ crystals. During the earliest stages of crystal formation, the SAXS Bragg peak occurs simultaneously, or slightly lags, the appearance of crystalline WAXS reflections. We conclude crystallization occurs by a nucleation and growth process in this m-iPP.
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    Droplet interfacial structure studied by SANS contrast variation (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 749-752 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Details of structure and composition at interfaces may be obtained by Small Angle Neutron Scattering (SANS) contrast variation. At the curved oil/surfactant/water interfaces of microemulsion droplets useful results may be obtained by simultaneous fits of core-shell models to SANS data measured at different scattering contrasts. Results for small ethyl octanoate/dimethyldodecylamineoxide/water droplets, where the oil core penetrates the surfactant layer, will illustrate this method. For block-copolymer micelles with deuterated poly(lactic acid) core, the composition profiles of thicker poly(ethylene glycol) shells were obtained using a model with a vertical step and two diffuse shells. One of the three water contrasts used was close to the "mean contrast" match point for the whole micelle, as opposed to "shell" contrast, providing a peak in the form factor very sensitive to the model used.
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    The new SANS instrument at the Swiss spallation source SINQ (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 804-806 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: With the start-up of the neutron spallation source SINQ at PSI an instrument for small angle neutron scattering became operational and is open for the user community. The instrument is equipped with state-of-the-art components and compares well with the world's largest and most powerful facilities of this kind. Great emphasis was put on providing a flexible, universal multi-user facility which guaranties a comfortable and reliable operation. In the present paper, the layout of the instrument is presented, the main components are described, and the performance is illustrated by selected examples
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    sasCIF: an extension of core Crystallographic Information File for SAS (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 812-816 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Data acquisition packages developed at different small angle scattering facilities use different formats both for raw and processed data storage. To facilitate the data exchange between laboratories, a consensus in the small angle scattering community has been reached on an ASCII format for one-dimensional data which includes a self-describing header containing relevant information about the sample and instrumental conditions followed by raw or reduced data in a tabular form. This format called sasCIF was implemented as an extension of core CIF (Crystallographic Information File) dictionary.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889800001357
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    Evaluation of double-crystal SANS data influenced by multiple scattering (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 824-828 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Evaluation of small-angle neutron scattering (SANS) data is often complicated by multiple scattering effects if large particles of relatively high volume fraction have to be studied and dilution or contrast reduction is impossible. The use of pin-hole SANS instruments is often limited due to the contradictory requirements of high resolution and short wavelength needed to keep scattering contrast as low as possible. Double crystal (DC) SANS diffractometers of Bonse-Hart and bent-crystal type are useful alternatives in such cases, as they permit reaching very high resolution with thermal neutrons. A method for SANS data evaluation suited to DC instruments is presented. It includes the common scheme of the indirect Fourier transformation method, but takes multiple scattering into account. The scattering medium is described by the frequency function g(x) defined as the cosine Fourier transform of slit-smeared data. Although a simplistic model of polydisperse spheres is used to represent g(x), resulting g(x) function and some integral parameters are independent of this model. Tests on simulated data show, that the method reproduce well true values of microstructural parameters, though systematic errors are observed in the cases when the unscattered part of incident beam completely disappears. If the scattering power 〈N〉 is known and kept fixed during fitting, then other parameters are reproduced well also in the regime of strong multiple scattering. The evaluation procedure permits simultaneously fitting to several sets of data measured for different Q-regions, resolutions and sample thicknesses. It has proved to provide reliable results for particle sizes ranging from about 100 Å to several microns and 〈N〉 〈 10.
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    Distance distribution function of interacting uniform spheres (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 839-842 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The scattering intensity equals the Fourier-transform of the distance distribution function p(t) = t2Δ\tilde{\rho}2(t) of the sample, where Δ\tilde{\rho}2(t) is the autocorrelation function of the scattering contrast Δρ(r). Δ\tilde{\rho}2(t) carries information both on the structure of the elementary scatterers and on their spatial distribution. In the present paper a method is described how to separate the two types of information in a system of interacting hard spheres.
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    Use of Cr Kα radiation to enhance the signal from anomalous scatterers including sulfur (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 876-881 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The anomalous signals from scatterers such as sulfur (S) and arsenic (As) were compared in diffraction data sets collected from an X-ray source with three different targets, Au, Cu and Cr, on a multi-target rotating anode. HIV-1 integrase crystals served as the test case for this study. The crystalline specimen of HIV-1 integrase contains in each protein molecule two As atoms, each covalently bound to a cysteine S atom, and two additional S atoms derived from methionine. It was found that the Cr Kα radiation gave the clearest peaks in anomalous difference Fourier maps, although the signal-to-noise ratios of the anomalous signal for the Cu Kα and Cr Kα data were similar but better than that for Au Lα. This result was in spite of the fourfold higher flux from the Cu anode versus the Cr anode. For all three X-ray wavelengths, anomalous difference Fourier maps calculated with bias-removed phases derived from the known atomic model revealed clear peaks at the two As sites. However, only in the map calculated using the Cr Kα data were both peaks of the expected ellipsoidal shape, enveloping the As atom and the adjacent S atom. None of the S sites was apparent in difference maps calculated using the Au Lα data. The ability to enhance the S-derived anomalous signal using Cr Kα radiation has particularly useful applications in the structure determination of proteins, for example in resolving ambiguities in the chain tracing of a protein with numerous disulfide bonds and in assigning amino acid identities. Additionally, anomalous difference Patterson maps calculated from the Cr Kα data were sufficiently clear to identify the As-related peaks. These results form the groundwork for in-house phase determination with the multi-wavelength anomalous diffraction method.
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    Whole-profile structure solution from powder diffraction data using simulated annealing (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 899-908 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Techniques and methods to facilitate the solution of structures by simulated annealing have been developed from the starting point of a space group and lattice parameters. The simulated-annealing control parameters have been systematically investigated and optimum values characterized and determined. Most significant is the inclusion of electrostatic-potential penalty functions in a non-linear least-squares Rietveld refinement procedure. The long-range electrostatic potentials are calculated using a general real-space summation which can be used for all space groups. In addition, a general weighting scheme for penalty functions negates the need to determine weighting schemes experimentally. Also investigated and improved is the non-linear least-squares minimization procedure used in the refinement of structural parameters. The behaviour and success of the techniques have been tested on X-ray diffraction powder data against the known structures of AlVO4 in P1 with 18 atoms in the asymmetric unit, K2HCr2AsO10 in P31 with 15 atoms in the asymmetric unit excluding hydrogen, and [Co(NH3)5CO3]NO3.H2O in P121 with 15 atoms in the asymmetric unit excluding hydrogen.
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    Displacements from Cu4Ti in a Cu–Ti single crystal using small-angle and diffuse X-ray scattering: a synchrotron radiation study (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 928-937 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Single crystals of Cu–2.5 at.% Ti have been studied by small-angle and large-angle X-ray scattering to determine the displacements induced by the formation and the coarsening of ordered Cu4Ti precipitates. The variation of the atomic scattering factor of Cu near its absorption edge was used to confirm the nature of the precipitates and to show that the scattering appearing near Bragg peaks was due to the displacements of the atoms from their ideal positions, induced by the formation of the precipitates, and not to the segregation of the Ti atoms. The displacement field was found to be dependent on the precipitate mean size, with a single component aligned along a soft 〈100〉 direction: it is composed of a region of dilatation, where the Cu-atom positions follow the increase of the atomic distance induced by the Cu4Ti precipitate formation, followed by a region of contraction. This effect is responsible for the pile-up of the precipitates along the soft directions: the interaction between precipitates is repulsive at long distances and attractive at shorter distances.
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    Effect of a crystallite size distribution on X-ray diffraction line profiles and whole-powder-pattern fitting (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 964-974 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: A distribution of crystallite size reduces the width of a powder diffraction line profile, relative to that for a single crystallite, and lengthens its tails. It is shown that estimates of size from the integral breadth or Fourier methods differ from the arithmetic mean of the distribution by an amount which depends on its dispersion. It is also shown that the form of `size' line profiles for a unimodal distribution is generally not Lorentzian. A powder pattern can be simulated for a given distribution of sizes, if it is assumed that on average the crystallites have a regular shape, and this can then be compared with experimental data to give refined parameters defining the distribution. Unlike `traditional' methods of line-profile analysis, this entirely physical approach can be applied to powder patterns with severe overlap of reflections, as is demonstrated by using data for nanocrystalline ceria. The procedure is compared with alternative powder-pattern fitting methods, by using pseudo-Voigt and Pearson VII functions to model individual line profiles, and with transmission electron microscopy (TEM) data.
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    OASIS: a computer program for breaking phase ambiguity in one-wavelength anomalous scattering or single isomorphous substitution (replacement) data (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 980-981 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The phase problem is reduced to a sign problem once the anomalous-scatterer or the replacing-heavy-atom sites are located. OASIS adopts the CCP4 format [Collaborative Computational Project, Number 4 (1994). Acta Cryst. D50, 760–763]. It applies a direct-method procedure to break the phase ambiguity intrinsic to one-wavelength anomalous scattering (OAS) or single isomorphous replacement (SIR) data.
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    Ionization chambers for monitoring the position and tilt of X-ray beams (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 986-987 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: An air-filled ionization chamber with a three-way split anode can be used to measure the tilt and the translation of a collimated X-ray beam with respect to the chamber axis.
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    GraphEnt: a maximum-entropy program with graphics capabilities (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 982-985 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: A maximum-entropy formalism aimed at the production of a `maximally noncommittal' map is a standard method in fields of science like radioastronomy, but a rare exception in both X-ray crystallography and electron microscopy (or crystallography). This is rather unfortunate, given the wealth of information that a maximum-entropy map can reveal, especially when the map itself is the end product (for example, low-resolution electron or potential density maps, Patterson functions, deformation maps). The program GraphEnt attempts to automate the procedure of calculating maximum-entropy maps, with emphasis on the calculation of difference Patterson functions for macromolecular crystallographic problems, while providing a useful graphical output of the current stage of the calculation.
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    A simple technique to control macromolecular crystal nucleation efficiently using a standard vapour-diffusion setup (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 988-989 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Excessive nucleation often leads to a large number of small macromolecular crystals that are not useful for X-ray diffraction analysis. Crystals with dimensions suitable for data collection can be reproducibly obtained by releasing, for a discrete amount of time, the vapour pressure in both hanging- and sitting-drop experiments, set up at lower precipitant concentrations than those required for crystallization.
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    Tcl/Tk-based programs. IV. CALCRYS: crystallographic calculator (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 992-992 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
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    ORTHON: transformation from triclinic axes and atomic coordinates to orthonormal ones (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 994-994 
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    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
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    Incommensurately modulated phase of Rb2CoBr4 at 295 and 200 K (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 17-21 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The crystal structure of Rb_2CoBr_4 at 295 and 200 K has been determined. At these temperatures Rb_2CoBr_4 exhibits an incommensurately modulated structure with wavevector {\bf q} = (1/3+\delta){\bf a}^*. At room temperature only the average structure was refined. Lattice parameters are a = 9.732 (3), b = 13.328 (4), c = 7.654 (3) Å, space group Pnam. The R(F) value was 0.0414 for 286 observed reflections (0.0778 for all 477 reflections). At 200 K the lattice parameters are a = 9.691 (4), b = 13.278 (5), c = 7.630 (6) Å, superspace group P:Pnam:\overline{1}ss. Main reflections and satellite reflections of first order were measured. The refinement converged at R(F) = 0.052 for 309 observed reflections (255 main reflections and 54 satellites) and 0.2971 for all reflections (1849; 695 main reflections and 1154 satellites). Amplitudes and phases of the modulation function as well as bond distances show close relationships to those observed in the incommensurately modulated phase of Rb_2ZnBr_4.
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    Conformational preferences in 2-nitrophenylthiolates: interplay between intra- and intermolecular forces; structures of (E)-1-(4-methyl-2-nitrobenzenethiolato)-2-phenylethene, S-(2-nitrophenyl)benzenecarbothiolate and 1-(2-nitrophenylthio)-2,5-pyrrolidinedione (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 58-67 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: In (E)-1-(4-methyl-2-nitrobenzenethiolato)-2-phenylethene, C15H13NO2S (1) (orthorhombic Pbca), the nitro group is almost coplanar with the adjacent aryl ring, but the dihedral angles between the nitro-aryl and styryl fragments is ∼121°. The molecules are linked by paired C—H...O hydrogen bonds in a chain of rings. In S-(2-nitrophenyl)benzenecarbothiolate, C13H9NO3S (2) (monoclinic P21/a), the nitro group is rotated by 33.0 (2)° out of the plane of the adjacent aryl ring and the thiobenzoate group is strongly twisted away from the plane of the disubstituted aryl ring. The molecules of (2) are linked into chains by C—H...O hydrogen bonds, and each chain is linked to two neighbouring chains by means of aromatic π...π stacking interactions. In 1-(2-nitrophenylthio)-2,5-pyrrolidinedione, C10H8N2O4S (3) (monoclinic P21/a), the nitro group is again almost coplanar with the adjacent aryl ring, but the pyrrolidinedione unit is almost orthogonal to the O2NC6H4SN plane. There are three types of C—H...O hydrogen bond in the structure, and these link the molecules into a two-dimensional net. The conformations of these molecules have been investigated by SCF calculations and two energy minima have been identified for each: the molecules of (1) and (3) adopt conformations in their crystals which are close to those at the overall energy minima calculated for isolated molecules, while molecules of (2) adopt a conformation in the crystal close to that calculated for the local energy minimum. Comparisons are made with the structures of some related compounds and it is concluded that, while the nature of the two conformational minima is determined by intramolecular forces, the choice between them is determined primarily by intermolecular forces.
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    Correlation of structural and physico-chemical parameters with the bioactivity of alkylated derivatives of indole-3-acetic acid, a phytohormone (auxin) (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 94-111 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: As part of molecular recognition studies on the phytohormone indole-3-acetic acid (IAA) a series of alkylated IAAs has been examined. Phenyl-ring substitution (alkyl = methyl and ethyl) at positions 4-, 6- or 7- as well as pyrrole substitution at the 2- site resulted in the six compounds which are analyzed: 2-Me-IAA, 4-Me-IAA, 6-Me-IAA, 7-Me-IAA, 4-Et-IAA and 6-Et-IAA. The structure–activity relationships investigated include those between the geometrical parameters of the molecular structures determined by X-ray analysis, the growth-promoting activities in the Avena coleoptile straight-growth bioassay and relative lipophilicities calculated from retention times on a reversed-phase HPLC column and from RF values in reversed-phase TLC. Lipophilicities are correlated with the moments of inertia, average polarizability, molecular mass, and the van der Waals radii of the ring substituents. The influence of substitution on the electronic properties of the indole ring and its geometry is discussed on the basis of the UV and 1H NMR spectra.
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    Novel infinite three-dimensional network of neutral fullerene molecules in (C60)8(twin-TDAS)6 (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 497-500 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The crystal structure of (C60)8(twin-TDAS)6 [twin-TDAS = C4S6N4 = 3,3′,4,4′-tetrathiobis(1,2,5-thiadiazole)] has been redetermined at low temperature in the correct space group [I23, a = 18.849 (1) Å]. Structural analysis reveals a novel three-dimensional close-contact network of C60 molecules with tetrahedral holes similar to pristine C60.
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    Crystal chemistry of some synthetic 2-oxa-steroids: conformation, packing motifs and isostructurality (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 512-525 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The crystal structures of six synthetic 2-oxa-steroids (A-ring lactone steroids) have been determined by single-crystal X-ray diffraction. The conformation and hydrogen bonding in these oxa-steroids is compared with packing motifs in the natural steroids and the anabolic agent, Anavar®. O—H...O hydrogen bonding with lactone carbonyl O is the preferred arrangement in molecules with a C—OH group. The donor H atoms of A, B and D rings participate in C—H...O interactions with lactone carbonyl O and D-ring hydroxyl/ketone O acceptor atoms. The conformation of the lactone ring in these analogues is different from the natural androgens because replacement of the C2-methylene group by an O atom changes the geometry of the A ring. Two structurally related lactone steroids provide the first example of O—H...O/C—H...O interaction mimicry and furthermore the two components form a binary solid solution. The O—H...O and C—H...O hydrogen bonds in 2-oxa-steroid crystal structures are analysed and the observed preferences discussed in terms of geometric and chemical factors.
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    Towards a unified description of the AMOB2O5 (A = K, Rb, Cs, Tl; M = Nb, Ta) family of compounds (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 558-564 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Single-crystal X-ray diffraction data (Mo Kα radiation) are used to re-refine the structure of RbNbOB2O5, rubidium niobium oxo pyroborate. The structure is refined as an incommensurate modulated structure with superspace group symmetry Pmn21(0,0.4,0)s and lattice parameters a = 7.406 (2), b = 3.939 (2) and c = 9.475 (2) Å. Refinement on 3242 unique reflections converged to R = 0.031, while a previous conventional superstructure refinement led to R = 0.090. This lowering of the R factor goes hand-in-hand with a substantial reduction in the number of refined parameters. The refinement strongly suggests that the structure is effectively incommensurately modulated, despite an apparently rational magnitude of the primary modulation wavevector and overlap of satellite reflections.
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    Structure of uranium(VI) oxide dihydrate, UO3·2H2O; synthetic meta-schoepite (UO2)4O(OH)6·5H2O (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 577-583 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The structure of uranium oxide dihydrate, also known as meta-schoepite (UO2)4O(OH)6·5H2O, has been determined from a synthetic single crystal. The structure, at 150 K, space group Pbcn, lattice constants a = 14.6861 (4), b = 13.9799 (3) and c = 16.7063 (5) Å, consists of layers of stoichiometry (UO2)4O(OH)6, formed from edge-sharing UO7 pentagonal bipyramids, interleaved with hydrogen-bonded water molecules. Three of the layer hydroxyl groups are linked through hydrogen bonding to single water molecules and the three remaining OH units form interactions with water molecules that each act as acceptors in two hydrogen bonds. One of the water molecules in the inter-layer region is disordered over two symmetry-related sites and forms hydrogen-bonded interactions only within the layer and with the uranyl O atoms. The relationship of the structure of meta-schoepite to that of schoepite is discussed in detail.
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    Analysis of LiKSO4 crystals in the temperature range from 573 to 943 K (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 607-617 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The structural phases observed in LiKSO_4 crystals due to thermotropic transitions have been studied for more than a century. Nowadays many different phases are referenced, but some of the results are still controversial. Structural studies by single-crystal X-ray diffraction from room temperature to 803 K are presented here. Phase II (708 \lt T \lt 943 K) is extensively discussed on the basis of ordered and disordered models, using harmonic and anharmonic atomic displacements, and considering a twinned crystal composed of three orthorhombic domains. Analyses of the same phase at different temperatures determine the best structure model.
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    Aplanarity of CO3 groups: a theoretical investigation (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 648-653 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Density functional theory-based calculations have been used to demonstrate that the aplanarity of CO_{3} groups in some carbonates such as dolomite, CaMg(CO_3)_2, aragonite, CaCO_3, and norsethite, BaMg(CO_3)_2, is a ground-state property. This distortion stabilizes dolomite by ∼500 J mol^{-1}. Up to at least 6 GPa, the aplanarity of CO_3 groups in dolomite is independent of pressure. In aragonite the aplanarity increases slightly on increasing pressure, while a significant tilting of the CO_3 groups occurs. The calculations do not support previous findings of anomalously low values for the pressure derivative of the bulk moduli, B^\prime, of aragonite and dolomite. Instead, the computed pressure dependences of the unit-cell volumes correspond to B^\prime = 5.0 (5) for aragonite and B^\prime = 4 (1) for dolomite, when fitted with a third-order Birch–Murnaghan equation-of-state.
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    Determining the crystal structure of twinned 2-methylpyrazine (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 677-681 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Although systematic absences and symmetry relations among reflections pointed to space group I4122 (one molecule in the asymmetric unit), a direct methods solution could only be obtained in I4¯ (two molecules in the asymmetric unit). Refinement in I4¯ was unsatisfactory until merohedral twinning was taken into account. The resulting molecular dimensions are in excellent agreement with analogous values in the literature. The molecular arrangement is described.
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    Non-centrosymmetric racemates: space-group frequencies and conformational similarities between crystallographically independent molecules (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 715-719 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: DL-Allylglycine (DL-2-amino-4-pentenoic acid, C5H9NO2) yields crystals with Pca21 symmetry and two crystallographically independent yet pseudo-inversion-related enantiomers. The distribution among the common space groups of other crystalline racemates with more than one molecule in the asymmetric unit has been established. The conformational similarities between crystallographically independent enantiomers in 114 non-centrosymmetric racemates were quantified using the r.m.s. deviation for a molecular superposition. The analysis shows that in the majority of crystals the conformations of the crystallographically independent molecules are very similar with mean r.m.s. deviation = 0.190 Å. In almost 80% of the structures the mean r.m.s. deviations is in the interval 0–0.2 Å. It is estimated that racemates constitute 23% of the centrosymmetric organic structures in the Cambridge Structural Database.
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    Electron density and electrostatic properties of two peptide molecules: tyrosyl-glycyl-glycine monohydrate and glycyl-aspartic acid dihydrate (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 728-737 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The electron density and electrostatic properties of Tyr-Gly-Gly and Gly-Asp molecules have been determined from high-resolution X-ray diffraction data at 123 K. Topological properties of the charge distribution are discussed and compared with those derived from other experimental studies on peptide molecules, and the characteristics of the (3,−1) critical points of the C=O, C—N, C—C bonds are analysed. Crystal data for Tyr-Gly-Gly: C13H17N3O5·H2O, Mr = 313, orthorhombic, P212121, Z = 4, T = 123 ± 2 K; lattice parameters: a = 7.984 (2), b = 9.535 (3), c = 18.352 (5) Å, V = 1397.1 (6) Å3, Dx = 1.49 g cm−3, μ = 1.2 cm−1 for λMo = 0.7107 Å. Crystal data for Gly-Asp: C6H10N2O5·2H2O, Mr = 212, orthorhombic, P212121, Z = 4, T = 123 ± 2 K; lattice parameters: a = 9.659 (1), b = 9.672 (1), c = 10.739 (1) Å, V = 1003.3 (4) Å3, Dx = 1.40 g cm−3, μ = 1.3 cm−1 for λMo = 0.7107 Å.
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    Architecture of crystal structures from square planes (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 750-765 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The crystal structures of ordered b.c.c. (body-centered cubic), f.c.c. (face-centered cubic) or primitive cubic alloys A_xB_y and related NaCl, ZnS or CaF_2 derivative structures are characterized by the self-coordination numbers T_1, T_2 of the A atoms with A atoms. Structures with identical T_1 and T_2 values for all A atoms are at the corners of T_1 and T_2 structure maps, and can be analyzed for attractive or repulsive interactions of A atoms. Most observed structures are at the borders of the structure map and can be obtained by ∼10 different combinations of structural units. The different combination mechanisms explain e.g. the shear structures of CuAu II or \rm Nb_2O_5 and the occurrence of vacancies in NaCl-related structures like NbO.
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    Synchrotron X-ray analysis of RbTiOAsO4 (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 785-792 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Structure factors for rubidium oxotitanium arsenate, RbTiOAsO4, were measured at 293 K with focused synchrotron X-radiation [λ = 0.7500 (9) Å] using a fast avalanche photodiode counter. The accurate synchrotron single-crystal data are of sufficient quality and resolution to detect the splitting positions of the Rb cations at room temperature. Strong accumulation of the Δρ difference charge density near the Rb atoms at a distance of ∼0.5 Å in the −c direction can be attributed to the partial occupancy of additional sites related by pseudosymmetry. This type of static and/or dynamic disorder is temperature-dependent and seems to be universal for the KTiOPO4 family of compounds. The best modelling of the experimental data was obtained with the Rb atom in split positions described within the harmonic approximation and the multipole functions for the other atoms. The Δρ density features in the Ti—O and As—O covalent bonds can be related to the linear and non-linear susceptibility using bond-polarization theory. The charge-density maps reflect the anisotropy of non-linear susceptibility, which is larger for directions where locally antisymmetric components of Δρ are strong.
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    Two-dimensional packing and pseudosymmetry (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 828-832 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: It is possible to understand, or at least rationalize, the occurrence of pseusosymmetry elements in some crystals as a consequence of a higher symmetry in a two-dimensional layer being obscured by the three-dimensional packing of the layers. Six examples, including both simple compounds and co-crystals and involving six different symmetries, are described.
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    Graph-set and packing analysis of hydrogen-bonded networks in polyamide structures in the Cambridge Structural Database (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 857-871 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The hydrogen-bond networks and crystal packing of 81 unique secondary di- and polyamides in the Cambridge Structural Database are investigated. Graph-set analysis, as implemented in the RPluto program, is used to classify network motifs. These have been rationalized in terms of the relative dispositions of the amide groups. Peptide and retropeptides exhibit significant conformational flexibility, which permits alternative hydrogen-bonding patterns. In peptides, dihedral angles of −ψ ∼ φ ∼ 105° allow an antiparallel ladder arrangement, containing rings of either the same or alternating sizes. For retropeptides, and diamides with an odd number of CH2 spacers, this conformation leads to a parallel ladder with rings of equal size. If φ approaches −60° and ψ 180°, ladders adopt a helical twist, and if the conformation is distorted further, a three-dimensional network is usually adopted. Diamides with aromatic or an even number of CH2 spacers generally form either antiparallel ladders or sheets, although some exhibit both polymorphs. Symmetry relationships within and between hydrogen-bonded chains, ladders and sheets in the crystal packing have also been analysed. Polyamides form considerably more complex networks, although many of the structural motifs present in the diamides occur as components of these networks.
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    Conformational transformation coupled with the order–disorder phase transition in 2-methyl-1,3-cyclohexanedione crystals (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 872-881 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The half-chair conformation of the dynamically disordered molecular ring of 2-methyl-1,3-cyclohexanedione, C_7H_{10}O_2, transforms to a sofa below T_c = 244 K, when the crystal undergoes a continuous phase transition induced by the onset of halting large-amplitude vibrations of methylene groups C(4)H_2 and C(5)H_2. The temperature dependence of the crystal structure has been investigated by X-ray diffraction. The Ibam symmetry of the crystal reduces below T_c to space group Pccn. The mechanism of the phase transition and of the conversion of the ring conformation is discussed.
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    Diffraction des rayonnements. Par Jean Protas. Paris: Dunod Editeur, 1999. Pp. xiv + 308. Prix FF 235. ISBN 2 10 004144 4. (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 920-920 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Structure and complex twinning of dysprosium disilicate (Dy2Si2O7), Type B (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 940-946 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Dysprosium disilicate (Dy2Si2O7) is triclinic with a = 6.6158 (2), b = 6.6604 (2), c = 12.0551 (4) Å, α = 94.373 (2), β = 90.836 (2), γ = 91.512 (2)°, V = 529.4 (1) Å3, space group P\overline 1, Z = 4 and Dx = 6.156 g cm−3. The structure (single-crystal X-ray, R = 0.033, wR = 0.041) is built from a linear triple tetrahedral group [Si3O10] and isolated [SiO4] tetrahedron cross-linked by Dy3+ in one sixfold and three eightfold coordinated positions, and corresponds to the presently revised type B structure of Ho2Si2O7. The formation of the unusual linear triple tetrahedral group in the type B structure allows for a more continuous transition in the mean size of REE3+On (REE = rare earth element) polyhedra in REE disilicates through the 4f transition metal series. The crystal of Dy2Si2O7 investigated was complexly twinned such that the diffraction pattern was also consistent with a larger dimensionally monoclinic unit cell (a = 22.5354, b = 14.2102, c = 6.6158 Å, β = 91.788°), which resulted in an apparent superstructure of the type B structure in space group C1¯. Lattice coincidence with the type B unit cell appears to have been maintained during crystal synthesis and quenching by the complex sector-zoned growth twin.
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    X-ray diffraction study of copper(I) thiourea complexes formed in sulfate-containing acid solutions (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 993-997 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The formation of three different copper(I) thiourea complexes in sulfate-containing acid solutions was observed. The ratio between CuI and thiourea (tu) in these complexes depends on the amount of thiourea and copper sulfate in the solution. The crystal and molecular structure of a new complex, [Cu_2(tu)_6](SO_4)\cdotH_2O, was determined, and the formation and structures of [Cu_2(tu)_5](SO_4)\cdot3H_2O and [Cu_4(tu)_7](SO_4)_2\cdotH_2O were confirmed. The compound [Cu_2(tu)_6](SO_4)\cdotH_2O crystallizes in the P\overline 1 space group, with a = 11.079 (2), b = 11.262 (1), c = 12.195 (2) Å, \alpha = 64.84 (1), \beta = 76.12 (1), \gamma = 66.06 (1)°, and Z = 2. The Cu-thiourea complex is arranged as a CuI tetranuclear ion, [Cu_4(tu)_{12}]^{4+}, sited on a crystallographic inversion center. All copper ions are in a tetrahedral coordination with thiourea ligands and located at alternate sites on an eight-membered, crown-like ring.
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    Topology of molecular packings in organic crystals (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 1035-1045 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Using the methods of coordination sequences and of molecular Voronoi–Dirichlet polyhedra, the topological properties of molecular packings and molecular coordination numbers (MCNs) were determined in the crystal structures of 33 575 monosystem organic compounds within the first three coordination spheres. Numerous examples of disagreement between the topology of molecular packing and the system of intermolecular contacts in a crystal structure were found. It is concluded that within the first coordination sphere most of the molecules tend to arrange with MCN = 14, obeying the model of the thinnest covering of space, but molecular packings as a whole tend to be constructed according to one of the close packings.
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    Structures and phase transitions of the A7PSe6 (A = Ag, Cu) argyrodite-type ionic conductors. III. α-Cu7PSe6 (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 972-979 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The crystal structure of the third polymorph of the Cu7PSe6 argyrodite compound, α-Cu7PSe6, heptacopper phosphorus hexaselenide, is determined by means of single-crystal diffraction from twinned crystals and X-ray powder diffraction, with the help of extensive NMR measurements. In the low-temperature form, i.e. below the last phase transition, α-Cu7PSe6 crystallizes in orthorhombic symmetry, space group Pna21, with a = 14.3179 (4), b = 7.1112 (2), c = 10.1023 (3) Å, V = 1028.590 (9) Å3 (deduced from powder data, T = 173 K) and Z  = 4. Taking into account a twinning by reticular merohedry, the refinement of the α-Cu7PSe6 structure leads to the residual factors R = 0.0466 and wR = 0.0486 for 127 parameters and 3714 observed, independent reflections (single-crystal data, T = 173 K). A full localization of the Cu+d10 element is reached with one twofold-, one threefold- and five fourfold-coordinated Cu atoms. The observation of two phase transitions for Cu7PSe6, to be compared with only one for Ag7PSe6, is attributed to the d10 element stability in a low coordination environment, copper being less prone to lower coordination sites than silver, especially at low temperature.
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    Modulated structure of La2Co1.7 from neutron and X-ray diffraction data (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 959-971 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: An La2Co1.7 crystal was investigated by single-crystal neutron and X-ray diffraction. The neutron measurement was performed with a Laue white-beam technique at 15 K and room temperature, using a large position-sensitive detector. The X-ray measurements were obtained at room temperature from a CCD detector. The average structure of La2Co1.7 is hexagonal with cell parameters a = 4.885 (1), c = 4.273 (2) Å and space group P63/mmc. The satellites are located at the vertices of small hexagons perpendicular to the c axis. The modulated crystal was indexed assuming a sixfold twinned 3 + 1 dimensional structure with q = (α, 0, γ). The structure was solved in the pseudoorthorhombic cell, with a = 8.461 (1), b = 4.885 (1), c = 4.273 (2) Å, in the superspace group C2/m(α, 0, γ). Owing to space requirements, the Co atoms cannot fit precisely into the octahedral sites of the La h.c.p. (hexagonal close packing). Instead, the Co atoms adopt a different periodicity, which is not commensurate with the periodicity of the La atoms. Two structure models have been refined in order to describe this behaviour, one using the sawtooth function for the positional modulation of cobalt and the other describing the structure as a composite system. The chemical composition calculated from the composite model is La2Co1.8 (1) with the estimated standard deviation arising from the variation of q for different samples. In both models lanthanum is incommensurately modulated, while the position of cobalt seems not to be affected by any relative periodic displacement.
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    Dopant positions in strontium/chromium- and barium-doped KTP, determined with synchrotron X-radiation (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 980-987 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Structure factors for strontium/chromium- (Sr/Cr) and barium- (Ba) doped potassium titanyl phosphate (KTiOPO4, KTP) were measured with focused synchrotron X-radiation [0.75000 (9) Å] using a fast avalanche photodiode counter. Space group Pna21, Z = 8, a = 12.786 (2), b = 6.3927 (8), c = 10.5585 (9) Å, T = 293 (1) K, R = 0.028 (SrCrKTP); a = 12.851 (6), b = 6.418 (3), c = 10.620 (5) Å, T = 120 (1) K, R = 0.031 (BaKTP). The refinement of the dopant positions showed that Ba2+ is positioned in the larger of the two K cavities of KTP, while the smaller Sr2+ ion is located in both. Split positions are found for the strontium dopant in both cavities and they are located in the positive c direction from the potassium cation. The chromium dopant has two different oxidation states, namely +III and +VI; in both states the dopant is located inside the TiO6 octahedra. The two structures show slightly less distorted TiO6 octahedra than pure KTP.
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    Supramolecular structure of 1H-pyrazoles in the solid state: a crystallographic and ab initio study (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 1018-1028 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The secondary structure of 1H-unsubstituted pyrazole derivatives bearing only one hydrogen donor group and one or more acceptor groups has been analyzed in terms of some descriptors representing the substituents at C3 and C5. The substituent at C4 appears to affect mainly the tertiary or quaternary structure of these compounds. The proposed semi-quantitative model, which explains most hydrogen-bonded motifs as a combination of the effects of substituents at C3 and C5, has also been examined as a function of the steric and polarizability effects of these substituents represented by molar refractivity. The model also applies to other five-membered rings (1,2,4-triazoles, 1,2,4-diazaphospholes and 1,2,4-diazaarsoles). Furthermore, ab initio calculations at RHF/6-31G* have been performed to discover the relative stability of three of the four hydrogen-bond patterns displayed by several symmetrical pyrazoles (dimers, trimers, tetramers). The fourth motif, catemers, has only been discussed geometrically.
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    Adducts of meso and racemic 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane with trigonally trisubstituted benzene carboxylic acids: supramolecular structures in one and two dimensions (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 1054-1062 
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    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The meso and racemic forms of 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, C16H36N4 (tet-a and tet-b, respectively), form adducts with trigonally trisubstituted benzene carboxylic acids; tet-a–3,5-dinitrobenzoic acid (1/2) (1), tet-a–5-hydroxyisophthalic acid–water (1/1/1) (3) and tet-b–5-hydroxyisophthalic acid–water (1/1/1) (4) are all salts, [C16H38N4]2+·2[C7H3N2O6]− (1) and [C16H38N4]2+·[C8H4O5]2−·H2O (3) and (4). The conformations of the [(tet-a)H2]2+ and [(tet-b)H2]2+ cations are entirely different: [(tet-a)H2]2+ is precisely centrosymmetric in (1) and approximately so in (3), while [(tet-b)H2]2+ has approximate C2 symmetry in (4). In each salt the cation forms two intramolecular N—H...N and four intermolecular N—H...O hydrogen bonds. In (1) the supramolecular structure is one-dimensional, a C22(13)[R24(16)] chain of rings. Compounds (3) and (4) crystallize in space groups P212121 and P21/c, respectively, but the supramolecular structures are very similar: in each, the anions and the water molecules form a C(7)[R33(13)] chain of rings, generated in (3) by a 21 axis and in (4) by a glide plane. These chains are linked, in both (3) and (4), by cations to form sheets. Adjacent meso cations in (3) are related by a 21 axis and adjacent chiral cations in (4) are related by a glide plane.
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    Halogen trimer synthons in crystal engineering: low-temperature X-ray and neutron diffraction study of the 1:1 complex of 2,4,6-tris(4-chlorophenoxy)-1,3,5-triazine with tribromobenzene (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 1080-1084 
    Details
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The title complex has been studied using low-temperature X-ray (150 K) and neutron (100 K) diffraction. Molecules of the triazine host form a two-dimensional hexagonal network mediated by trigonally symmetric Cl3 synthons having Cl...Cl interactions of 3.441 (3) Å, a C—Cl...Cl angle of 165° and a Cl...Cl—C angle of 105°, close to the ideal values of 180 and 90°, respectively. The guest molecules are of an appropriate size to fit the hexagonal networks and interact with the host via C—H...π (phenyl) and C—Br...π (phenyl) interactions which stabilize the overall structure. Both C-donor bond vectors are directed more closely towards the mid-point (X) of an individual aromatic bond, rather than the ring centroid, with H...X 2.817 (9) Å and C—H...X 174.0 (9)°, and Br...X 3.353 (4) Å and C—Br...X 158.1 (2)°.
    Type of Medium: Electronic Resource
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    Scintag X'TRA: new X-ray diffraction system (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 1113-1113 
    Details
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108768100016992
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  • 90
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    X-ray study of deformation density and spontaneous polarization in ferroelectric NaNO2 (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 11-16 
    Details
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The deformation electron density of ferroelectric sodium nitrite has been determined from X-ray diffraction data at 30 K, using Hirshfeld deformation functions. Owing to the strong correlation between odd terms of the deformation coefficients, constraints were imposed in the refinement. The net charges for Na, N and O atoms were estimated to be 0.27, 0.20 and −0.24 e, respectively. The calculated spontaneous polarization using these net charges and atomic dipole terms, 7.8 µC cm−2, is much closer to the recently measured value, 12 µC cm−2, as compared with the value calculated from the formal point charges (74 µC cm−2).
    Type of Medium: Electronic Resource
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  • 91
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    Octahedral deformations and cationic displacements in the ferroelectric PbHf0.8Ti0.2O3: a neutron powder diffraction study from 10 to 770 K (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 27-38 
    Details
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Neutron powder diffraction data, collected over the temperature range 10–770 K, have been analysed in order to make a detailed characterization of the sequence of phase transitions occurring in the Hf-rich ferroelectric PbHf0.8Ti0.2O3, titanium hafnium lead oxide. Over the whole temperature range this compound undergoes two phase transitions, which involve cationic displacements and octahedral deformations (tilt and/or distortion) leading to strongly distorted perovskite-type structures. The first transition appears around 415 K between two ferroelectric rhombohedral phases: a low-temperature nonzero-tilt phase FRL (space group R3c) and an intermediate zero-tilt phase FRH (space group R3m). The second one, detected around 520 K, is associated with a ferroelectric to-paraelectric transition between the FRH phase and the PC cubic phase (space group Pm3¯m). From high-resolution neutron powder diffraction data (diffractometer 3T2-LLB, Saclay, France, λ = 1.2251 Å), the crystallographic structure of the three successive phases has been accurately determined at the following temperatures: T = 10 K (FRL): space group R3c, Z = 6, ahex = 5.7827 (1), chex = 14.2702 (4) Å, Vhex = 413.26 (2) Å3; T = 150 K (FRL): space group R3c, Z = 6, ahex = 5.7871 (1), chex = 14.2735 (4) Å, Vhex = 413.98 (3) Å3; T = 290 K (FRL): space group R3c, Z = 6, ahex = 5.7943 (1), chex = 14.2742 (5) Å, Vhex = 415.04 (3) Å3; T = 440 K (FRH): space group R3c, Z = 6, ahex = 5.8025 (1), chex = 14.2648 (4) Å, Vhex = 415.94 (3) Å3; T = 520 K (PC): space group Pm3¯m, Z = 1, acub = 4.1072 (2) Å, Vcub = 69.29 (1) Å3. In addition, a neutron powder thermodiffractometry experiment, performed between 290 and 770 K (diffractometer D1B-ILL, Grenoble, France, λ = 2.533 Å), has been used to study in situ the temperature-induced phase transitions. From sequential Rietveld refinements, the temperature dependence of the cation displacements and the rotation and/or distortion of oxygen octahedra was derived.
    Type of Medium: Electronic Resource
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  • 92
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    Two different three-dimensional hydrogen-bonded framework structures in two hydrated adducts meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane–terephthalic acid–water (1/1/4) and (1/1/6) (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 85-93 
    Details
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Co-crystallization of meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, C16H36N4, with terephthalic acid (1,4-benzenedicarboxylic acid), C8H6O4, from ethanol yields the tetrahydrate (C16H36N4).(C8H6O4).(H2O)4 (1), meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane–terephthalic acid–water (1/1/4), while similar co-crystallization from methanol yields the hexahydrate (C16H36N4).(C8H6O4).(H2O)6 (2), meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane–terephthalic acid–water (1/1/6). Both compounds are salts, [C16H38N4]2+.[C8H4O4]2−.4H2O (1) and [C16H38N4]2+.[C8H4O4]2−.6H2O (2): in (2) both ionic components lie across centres of inversion and in both salts there are two protons held within the N4 cavity of the cation. In the structure of (1), the cations and anions are linked into chains by means of paired N—H...O hydrogen bonds, and the water molecules are linked into continuous chains by means of O—H...O hydrogen bonds, in which each of the H atoms is independently disordered over two equally populated sites. The water chains link the ionic chains into a continuous three-dimensional framework by means of further O—H...O hydrogen bonds. In (2) the cations and anions are linked by single N—H...O hydrogen bonds, again into chains: at the same time the anions and the water molecules are linked by multiple O—H...O hydrogen bonds into continuous two-dimensional nets, which are linked by the cations, by means of N—H...O hydrogen bonds into a three-dimensional framework, entirely different from that in (1).
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  • 93
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    N-n-Alkyl N,N-dimethylammonioacetic acid bromides: the first complete series of crystal and molecular structure determinations of an amphiphilic compound with alkyl chain lengths n = 1,..., 16 (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 124-131 
    Details
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The molecular and crystal structures of 16 N-n-alkyl N,N-dimethylammonioacetic acid bromides with chain lengths between n = 1 and n = 16 have been determined. All compounds from n = 5 to n = 16 form bilayers with interdigitated chains. The even-numbered chains display the chain packing type M2II. The chain packing of the odd-numbered chain compounds is less regular. The head groups of all compounds are connected via electrostatic N+...Br− interactions, and by OH...Br− hydrogen bonds. The compounds with short chains are packed in different ways. Their molecular conformation depends on the crystal packing.
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  • 94
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    Structure of the defect perovskite Ce1/3NbO3: a redetermination by electron and neutron powder diffraction (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 183-188 
    Details
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The crystal structure of the defect perovskite Ce1/3NbO3, cerium niobium oxide, has been re-examined by neutron powder and electron diffraction. The results of a powder neutron Rietveld refinement indicate that the structure is monoclinic: space group P2/m with Z = 4, a = 5.5267 (3), b = 7.8824 (2), c = 5.5245 (3) Å, β = 90.294 (1)°, V = 240.67 (2) Å3 at 298 K with χ2 = 2.570. Previous reports have described the Ce1/3NbO3 structure in a smaller (V/2) orthorhombic cell based solely upon X-ray powder diffraction data. The presence of weak reflections in the electron diffraction pattern provides conclusive evidence for a monoclinic superstructure of the orthorhombic cell. While these superlattice reflections are barely detectable with X-ray radiation, they are clearly visible in the neutron diffraction experiments. The superlattice reflections are shown to arise from a tilting of the NbO6 octahedra which results in the reduction of symmetry from orthorhombic to monoclinic. It is also found that the Ce3+ and Nb5+ cations are displaced from the centres of their respective polyhedra to accommodate the bond-valence requirements of the crystal structure. It is likely that distortions of this type are present in other Ln1/3NbO3 and Ln1/3TaO3 defect perovskites.
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    Dimorphism and inclusion compounds of N,N′-di(benzenesulfonyl)-p-phenylenediamine (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 234-244 
    Details
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Crystal structures of a new polymorph of N,N′-(p-phenylene)bis(benzenesulfonamide) (C18H16N2O4S2) and of two inclusion compounds with acetone [(CH3)2CO] and dimethylsulfoxide [2(CH3)2SO], respectively, have been determined at 150 K. For a more reliable comparison, the structure of the already known polymorph of N,N′-di(benzenesulfonyl)-p-phenylenediamine has been redetermined under identical conditions. In addition, the phase transformation behavior has been examined by differential scanning calorimetry and the crystallization conditions of both polymorphs including their formation by weathering of the inclusion compounds were investigated.
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    URL: http://dx.doi.org/10.1107/S0108768199005790
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  • 96
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    Direct observation of deuterium migration in crystalline-state reaction by single-crystal neutron diffraction. II. 3–1 Photoisomerization of a cobaloxime complex (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 245-253 
    Details
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Single crystal neutron diffraction analysis of photo-exposed(3-cyanopropyl-d2α,α)-[(R)-1-phenylethylamine-d11]bis(dimethylglyoximato-d14)cobalt(III) was carried out in order to clarify the mechanism of the crystalline-state photoisomerization of the 3-cyanopropyl group bonded to the Co atom in some cobaloxime complexes. Before irradiation the two H atoms bonded to the C1 atom of the 3-cyanopropyl group were exchanged with the D atoms such as —CH2CH2CD2CN. On exposure to a xenon lamp, the cell dimensions of the crystal were gradually changed. After 7 d exposure the change became insignificantly small. The structure was analyzed by neutron diffraction. The 3-cyanopropyl group was transformed to the 1-cyanopropyl group such as —CD(CN)C(H1/2,D1/2)2CH3 with retention of the single-crystal form. This indicates that one of the D atoms bonded to C1 migrates to either position bonded to C2. The other atoms of the complex remained unchanged. These results indicate that photoisomerization proceeded in two steps: the 3-cyanopropyl group was isomerized to the 2-cyanopropyl group in the first place and then the 2-cyanopropyl group was transformed to the 1-cyanopropyl group. Moreover, it was made clear that the second-step isomerization was irreversible, since one of the D atoms was retained. The disordered structure at C2 is estimated to be caused by the interconversion between the 1-cyanopropyl group produced and its dehydrogenated olefin after the photoisomerization.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108768199012008
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    Bulk modulus and high-pressure crystal structures of tetrakis(trimethylsilyl)methane C[Si(CH3)3]4 determined by X-ray powder diffraction (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 310-316 
    Details
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The pressure dependence of the crystal structure of cubic tetrakis(trimethylsilyl)methane C[Si(CH3)3]4 (TC) (P 〈 16.0 GPa, T = 298 K) is reported using high-resolution angle-dispersive X-ray powder diffraction. The compound has crystal structures with the molecules in a cubic-close-packed (c.c.p.) arrangement. It shows three phase transitions in the measured pressure range. At ambient conditions, TC has space group Fm{\bar 3}m (Z = 4) with a = 12.8902 (2) Å, V = 2141.8 (1) Å3 (phase I). Between 0 and 0.13 GPa TC exhibits a first-order phase transition into a structure with space group Pa{\bar 3} (phase II). A second first-order phase transition occurs between 0.2 and 0.28 GPa into a structure with space group P213 (phase III). Under non-hydrostatic pressure conditions (P 〉 10  GPa) a transformation is observed into a c.c.p. structure that is different from the face-centred-cubic structure at ambient conditions. A non-linear compression behaviour is observed, which could be described by a Vinet relation in the range 0.28–4.8 GPa. The extrapolated bulk modulus of the high-pressure phase III was determined to be K0 = 7.1 (8) GPa. The crystal structures in phase III are refined against X-ray powder data measured at several pressures between 0.49 and 4.8 GPa, and the molecules are found to be fully ordered. This is interpreted to result from steric interactions between neighbouring molecules, as shown by analysing the pressure dependence of intramolecular angles, torsion angles and intermolecular distances. Except for their cell dimensions, phases I, II and III are found to be isostructural to the corresponding phases at low temperatures.
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    The weak hydrogen bond in structural chemistry and biology. By Gautam R. Desiraju and Thomas Steiner. IUCr Monographs on Crystallography, Vol. 9. Oxford: Oxford University Press/International Union of Crystallography, 1999, pp. xiv + 507. Price »85.00. ISBN 0 19 850252 4. (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 333-334 
    Details
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108768199016900
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    Modulated structure of Ag2SnO3 studied by high-resolution electron microscopy (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 363-368 
    Details
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The modulated structure of Ag2SnO3, disilver tin trioxide, was investigated by high-resolution electron microscopy and electron diffraction along four different directions. Electron diffraction showed an incommensurate one-dimensional modulated structure with a modulation wavevector of 1/6.4a*. High-resolution images showed a large number of superstructure domains with the size range 10–100 nm and orientations related by hexagonal rotation. The modulation was determined to be displacements along the c axis of the Ag atoms both in octahedral and linear coordination. An approximate structure model with a commensurate sixfold superstructure, with an orthorhombic cell (P212121, a = 2.922, b = 1.267, c = 0.562 nm), is proposed. Calculated images and electron diffraction patterns, based on this model, agree well with experimental observations.
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    URL: http://dx.doi.org/10.1107/S0108768199013695
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  • 100
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    A new structure type in the hexagonal perovskite family; structure determination of the modulated misfit compound Sr9/8TiS3 (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 409-418 
    Details
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Sr9/8TiS3, strontium titanium sulfide, a new phase in the hexagonal perovskite-like SrxTiS3 system, has been prepared and its structure solved from single-crystal X-ray data within the (3 + 1)-dimensional [(3 + 1)D] formalism. Sr9/8TiS3 crystallizes with trigonal symmetry [R3¯m(00γ)0s superspace group], with the following lattice parameters: as = 11.482 (3), cs = 2.9843 (8) Å, q = 0.56247 (7)c* and Vs = 340.7 (3) Å3. The structure was considered as commensurate [R3¯c three-dimensional (3D) space group], but refined within the (3 + 1)D formalism to a residual factor R = 2.79% for 64 parameters and 1084 independent reflections. Original crenel functions were used for the sulfur and strontium description. The structure is different from that of the hexagonal perovskite-like oxide counterparts. The main difference is related to the presence of a new type of polyhedron in the [MS3] transition metal chains, intermediate between the octahedra classically found in such chains and the trigonal prismatic sites encountered in the oxides.
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    URL: http://dx.doi.org/10.1107/S0108768100002160
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