bluuebunny/crossref_metadata_2025 huggingface dataset:
High-resolution direct observation of the carbon-cage structure of C60 buckyballs
Authors: Narlikar A. V. , Samanta S. B. , Dutta P. K. , Grigoryan L. S. , Majumdar A. K.
Published: July 2002

In C60 films containing higher fullerene derivatives and having electrically conducting islands, it has been possible for the first time to observe, using high resolution scanning tunnelling electron microscopy (STM), the individual carbon cage of C60 buckyballs forming a f.c.c. lattice on a silver-coated glass substrate. The observed images of the molecule are surprisingly distinct and not smeared out, indicating their ambient-temperature reorientational motion to be pinned. The possible causes of the freezing are: (i) the presence of higher fullerene derivatives; (ii) changes in the electronic structure of the films due to interaction and proximity of Ag atoms; and (iii) the pinning of the molecules by the presence of large electric field gradients between the STM tip and the sample surface, a factor which, in addition, is considered responsible for the distortion observed in the buckyball images.
In-plane lattice-parameter and crystallite-size determination in a turbostratic graphite-like structure
Authors: Andreev Yu. G. , Lunström T.
Published: July 2002

A detailed recipe is given for the refinement of in-plane lattice-parameter and crystallite-size values of a turbostratic graphite-like structure through the profile fitting of the diffraction peaks from random layers. Folding of the theoretical line shape with the experimentally measured instrumental function provides convenient correction for systematic broadening. The devised technique is applied to the analysis of turbostratic hexagonal boron nitrides.
A High-Temperature Furnace for In Situ Small-Angle Neutron Scattering during Ceramic Processing
Authors: Kerch H. M. , Burdette H. E. , Long G. G.
Published: July 2002

The design and operation of a new research furnace, optimized for small-angle neutron scattering experiments at a steady-state nuclear reactor with a cold source, are presented. The apparatus enables the study of thermal processing of materials, such as the sintering behavior in ceramics, through the measurement of their microstructural parameters (e.g. pore sizes, pore volumes and pore surface areas) during thermal treatment. Small-angle neutron scattering measurements can be performed in situ in an oxidizing, reducing or neutral environment at temperatures up to 2000 K. Representative data, obtained from the sintering of a silica gel, are presented, including analysis, which offers the first quantitative measure of the evolution of the surface areas, the pore sizes and the pore-size distribution within a real material during densification.
Crystallization in a chamber furnace
Authors: Cabrić B. , Kovacević M. , Pavlović T.
Published: July 2002

A model of an air cooler (`cold finger') in a laboratory chamber furnace, for simultaneous regulation of different crystallization rates in a series of Tamman test tubes with the purpose of obtaining crystals, is given.
The Use of a Multi-Double-Crystal Diffractometer to Investigate Nickel Domains
Authors: Treimer W. , Höfer A. , Strothmann H.
Published: July 2002

The investigations of the domain structure in Ni single crystals is best performed with a double-crystal diffractometer. Conventional small-angle instruments do not have the necessary angular resolution to distinguish satellites of a main peak due to spin-dependent refraction of unpolarized neutrons by Bloch walls in Ni. With the help of a double-crystal diffractometer operating with lamellae crystals as monochromator and analyzer, the angular resolution is maintained but the intensity enhanced by a factor of 5.4. Within a series of measurements, it was possible to investigate Ni domains due to the high angular resolution of the new multi-double-crystal instrument and this improved intensity. The first results of the domain structure in (110) Ni single crystals and a good estimation of the Bloch wall thickness are given.
Small-Angle X-ray Scattering Study on Growth of AgCl Crystallites in Photochromic Glasses
Authors: Takatohi U. E. , Bittencourt D. R. S. , Watanabe S.
Published: July 2002

Reversible changes in the optical properties of photochromic glasses are observed owing to the presence of small silver halide crystals inside the glassy matrix. These crystals grow during the glass heat-treatment processing. Samples with molar composition of 40SiO2.10Al2O3.16.1K20.33.9B2O3, doped with AgCl and CuO, were produced and submitted to different heat treatments: (i) for 0.5 h at temperatures from 753 to 893 K and (ii) at 873 K for periods of time from 0.25 to 1.25 h. Small-angle X-ray scattering (SAXS) was used to characterize the samples. The samples heat treated between 843 and 893 K presented an increasing growth rate of the Guinier radius (Rg
). Samples heat treated at a fixed temperature of 873 K and different time t showed a law Rg

3 = kt + c. Variations in the optical absorbance at 280 nm and the additional absorbance spectra of samples exposed to light showed correlation with the SAXS results.
Simultaneous SAS and 100 Experiments on Phase Decomposition and Reversion in Al–Li Binary Alloys
Authors: Okuda H. , Tanaka I. , Osamura K. , Osawa M. , Ito K. , Amemiya Y.
Published: July 2002

Phase decomposition and reversion processes in Al–Li binary alloys have been studied by synchrotron-radiation small-angle/100 scattering experiments. The microstructure and its evolution obtained from small-angle scattering (SAS) and 100 profiles during phase decomposition and reversion are discussed. For the coarsening and reversion processes where a well defined interface between the δ′ precipitates and the matrix can be expected, the information obtained from the SAS and 100 profiles was essentially the same. On the other hand, the structural information they convey can be different in the early stage of phase decomposition. The interpretation of the SAS and 100 intensities by means of an extension of the two-phase model has been examined.
New Developments in Instrumentation for X-ray and Neutron Fibre Diffraction Experiments
Authors: Shotton M. W. , Pope L. H. , Forsyth V. T. , Denny R. C. , Archer J. , Langan P. , Ye H. , Boote C.
Published: July 2002

New instrumentation has been developed for use in X-ray and neutron fibre diffraction studies to provide automated control of sample position, orientation and ambient relative humidity. A sample mount system has been constructed for the Keele fibre diffraction camera on beamline 7.2 at the Daresbury Laboratory synchrotron radiation source. This allows the sample position and orientation to be controlled remotely, without disturbing the sample environment or the detector, and permits the implementation of efficient data-collection strategies. The system can be used to record entire datasets without incurring any loss of data in the meridional region of the diffraction pattern and has also been used to record three-dimensional datasets from fibres exhibiting `double orientation'. The design incorporates a translation feature so that the effects of radiation damage during data collection can be minimized. An automated humidity control system has also been constructed and can be used in either X-ray or neutron fibre diffraction studies where the water content of samples is a critical parameter and careful control of the relative humidity of the sample environment is required. This system allows the relative humidity to be controlled according to a pre-programmed set of instructions and alleviates some of the common problems that are encountered with conventional methods that require the use of saturated salt solutions. The system is currently installed on instrument D19 at the Institut Laue–Langevin, Grenoble. Results illustrating the application of these facilities in high-angle fibre diffraction studies of DNA are presented.
Combination of energy minimizations and rigid-body Rietveld refinement: the structure of 2,5-dihydroxybenzo[de]benzo[4,5]imidazo[2,1-a]isoquinolin-7-one
Authors: Schmidt Martin U. , Dinnebier Robert E.
Published: July 2002

The crystal structure of the yellow pigment 2,5-dihydroxybenzo[de]benzo[4,5]imidazo[2,1-a]isoquinolin-7-one (C18H10N2O3) was determined from powder data. The crystal structure was solved by minimizing the intermolecular energy starting from random packings. Subsequently, the structure was refined by rigid-body Rietveld analysis, using synchrotron powder data. The refinement included several intramolecular degrees of freedom. The compound crystallizes inPna21,Z\,=\,4, with lattice parametersa\,=\,13.2759 (3),b\,=\,20.9561 (5),c\,=\,4.7798 (1) Å, andV\,=\,1329.79 (5) Å3. The crystal consists of planar molecules, connected by hydrogen bonds of the types O–H...OH and O–H...N, which form a three-dimensional hydrogen-bond network.
SECTION– a computer program for the graphic display of cross sections through a unit cell
Authors: Barbour Leonard J.
Published: July 2002

SECTIONis a 32-bit Microsoft Windows-based program that displays cross-sectional slices through a packed crystal structure. Unit-cell dimensions as well as the unique atomic positions and symmetry operations are read fromSHELXinstruction files.
An improved small-angle X-ray scattering analysis of δ′ precipitation in an Al–Li alloy for growth kinetic studies
Authors: Tsao Cheng-Si , Lin Tsang-Lang
Published: July 2002

An improved method for small-angle X-ray scattering (SAXS) data analysis is developed to reconstruct the free-form particle size distribution of δ′ precipitation in an Al–Li alloy. This improved method consists of four iterative steps; the interparticle interference is also included. The indirect transform method (ITM) plus a hard-sphere (HS) model which considers the depleted zones are used in the analysis of δ′ precipitation in an Al–Li alloy. Two parameters, namely the hard-sphere volume fraction, ηHS, and the ratio of hard-sphere radius to the particle radius,RHS/R, which determine the structure factor of the interparticle effect, are iteratively calculated using the monodisperse assumption and Gaussian size distribution. These two parameters are finally used in reconstructing the particle size distribution by the ITM + HS method. This method is tested by analysing simulated SAXS data and shows a better agreement than found in similar studies. This improved method is applied to analyse a set of experimental SAXS intensities from δ′ (Al3Li particles) precipitation in an Al–9.7 at.% Li alloy. The monodisperse results are compared with the polydisperse ITM + HS results. The current ITM + HS method fits the SAXS data better than the other methods. The variations of average radii with aging time were found to follow the kinetic power law. The SAXS results are used to investigate the theoretical kinetic model of the volume-fraction effect on late-stage coarsening (Ostwald ripening). By comparing both experimentally obtained asymptotic size distributions of δ′ particles as well as coarsening rate constants with those predicted by the various kinetic models, the modified Lifshitz–Slyozov–Wagner (MLSW) theory is found to be in better agreement with the experimental results than the other theories.
Optimization of resolution–intensity balance in angular-dispersive powder diffractometry at a synchrotron radiation source
Authors: Grabcev B. , Tirziu A.
Published: July 2002

A unique resolution function of angular-dispersive synchrotron radiation powder diffractometers, properly parameterized to be valid for any multicrystal variant of this instrument, has been derived paying special attention to the absolute correctness of the formulae by avoiding any unknown proportionality factors. Moreover, the concept of experimental performance criterion has been proposed in an attempt to introduce a kind of `worth-scale' for the results of measurements performed with a certain instrumental configuration. Both entities were used in computer simulations to determine the most convenient configuration of instrumental parameters that, apart from the fulfilment of the experiment goal, ensure an adequate counting rate. Several case studies using different instrumental layouts, different single-crystal monochromators and samples of different low symmetries and unit-cell volumes were considered. The results offer reliable information that may be useful for designing diffractometers as well as for conducting experiments.
Morphological and synchrotron X-ray microdiffraction studies of large columnar CVD diamond crystallites
Authors: Lang A. R. , Makepeace A. P. W. , Butler J. E.
Published: July 2002

Optical microscopic and goniometric measurements were combined with microradiography, diffraction-pattern analysis and topography to study a 2 mm thick [001]-texture CVD (chemical vapour deposition) diamond film that had developed a coarse-grained structure composed of separate columnar crystallites. Individual columns were capped by large (001) facets, with widths up to 0.5 mm, and which were smooth but not flat, whereas the column sides were morphologically irregular. The refractive deviation of X-rays transmitted through the crystallites was exploited for delineating facet edges, thereby facilitating the controlled positioning of small-cross-section X-ray beams used for recording diffraction patterns from selected volumes in two representative crystallites. Their structure consisted of a [001]-axial core column surrounded by columns in twin orientation with respect to the core. The diamond volume directly below the (001) facets was free from low-angle boundaries, and no dislocation outcrops on the facets were detected. Significant elastic deformation of this volume was only present close to the facet periphery, where misorientations reached a few milliradians. Lattice imperfection was high in the twins, with ∼1° misorientations.
Structural evolution during micelle polymerization
Authors: Kline Steven
Published: July 2002

The evolution of microstructure during the free-radical polymerization of the surfactant cetyltrimethylammonium 4-vinylbenzoate has been measured using small-angle neutron scattering. The polymerization of the micelles was performed online, collecting SANS data continuously during the polymerization. Unlike microemulsion polymerizations, this micellar polymerization is seen to proceed through a highly turbid phase with local, short-range order at a length scale of approximately 50 Å. A simple structural model has been proposed to quantify the structural changes during the course of the reaction. The ordered structure is postulated to be an ordered bundle of cylindrical micelles, which subsequently breaks up into the final equilibrium structure of discrete, polymerized cylinders.
Nanopowder diffraction analysis beyond the Bragg law applied to palladium
Authors: Kaszkur Zbigniew
Published: July 2002

A method of powder diffraction analysis is proposed that is applicable to transition metal nanoparticles and which has been applied to palladium. From results of simulations it is shown that: (a) the overall average lattice constant of small transition metal clusters decreases with decreasing size of the cluster as a result of surface contraction; (b) the shift of a diffraction peak position driven by chemisorption increases monotonically with the decrease of cluster size and can be used for the estimation of cluster size; (c) the peak broadening of small clusters may be larger than that resulting from their size; the additional contribution originates from a high density of defects; (d) the extent of the microstrain may be evaluated from the peak broadening and the rate of attenuation of peak intensity; (e) the intensity scattered by small palladium clusters indicates significant surface disorder that may vary during a change of gas atmosphere and during chemisorption of oxygen at the surface.
Analysis of the volume transition in thermosensitive core-shell particles by synchrotron small-angle X-ray scattering
Authors: Seelenmeyer S. , Deike I. , Dingenouts N. , Rosenfeldt S. , Norhausen Ch. , Ballauff M. , Narayanan T.
Published: July 2002

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.
Scattering from biopolymers with helical symmetry in solution
Authors: Bras W. , Diakun G.P. , Denny R.C. , Ferrero C. , Levine Y.K. , Dìaz F.
Published: July 2002

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.
Sample geometry effects on incoherent small-angle scattering of light water
Authors: Carsughi F. , May R. P. , Plenteda R. , Saroun J.
Published: July 2002

Light water is frequently used as a standard for calibrating small-angle neutron scattering (SANS) data. The intensities collected for 1 and 2 mm of light water in standard quartz cells may differ by up to about 50% due to the presence of multiple and inelastic scattering [Rennie & Heenan (1993).Proceedings of ISSI Meeting, Dubna,pp. 254–260, Report E3-93-65; Teixeira (1992).Structure and Dynamics of Strongly Interacting Colloids and Supramolecular Aggregates in Solution, edited by S. H. Chen, pp. 625–658. Dordrecht: Kluwer Academic Publishers]. Multiple scattering increases with the thickness of the sample. The use of only elastically scattered neutrons may lead to an absolute intensity of the SANS data of about a factor of 2 higher than that obtained by taking into account all of the neutrons on the detector [Ghosh & Rennie (1990).Inst. Phys. Conf. Ser.107, 233–244]. However, it is shown here that the scattering intensities collected with different ratios of sample-to-beam dimension do present large differences as a function of sample thickness. In particular, ratios smaller and larger than unity are considered and the results are discussed and compared with Monte Carlo simulations.
Dynamical scaling property of colloidal aggregation in a zirconia-based precursor sol during gelation
Authors: Lecomte A. , Dauger A. , Lenormand P.
Published: July 2002

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 q
m 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 q
m of the intensity maximum and the structure function S(q/q
m,t) are shown to scale according to S(q/q

m
,t) = q
m(t)−d

F(q/q
m) where F(q/q
m) is a time-independent scaling function. This scaling requires d=D
f, 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.
Comparison of different methods in microstructural characterization of porous silicon
Authors: Salonen J. , Björkqvist M. , Laine E.
Published: July 2002

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.
In-situ small-angle scattering study on the formation of a nanocrystalline soft-magnetic alloy
Authors: dos Santos D. R. , Torriani I. L. , Silva F. C. S. , Knobel M.
Published: July 2002

A detailed study is presented on the nanocrystallization of the amorphous alloy Fe86Zr7Cu 1B6 (indices indicate at. %). Melt-spun ribbons were rapidly annealed by Joule heating, and the electrical resistance showed strong variations during thermal treatment. X-ray diffraction patterns indicate that these variations are related to the nucleation and growth of α-Fe nanocrystals, and from peak profile analysis we obtained the average grain size and crystalline volume fraction for different annealing currents. The disorder-order transition was studied by in-situ small-angle X-ray scattering during conventional furnace treatments. SAXS intensity evolution for different temperatures, both below and above the crystallization temperature of the alloy, showed that a fast atomic rearrangement leads to the formation of atomic clusters before crystallization. The evolution of the size distribution function of these clusters as a function of time and temperature was obtained assuming a polydisperse system of spherical particles.
An examination of the relationship of the Δω,Δ2Θ intensity distribution to crystal substructure (mosaic distribution) and shape
Authors: McL Mathieson A. , Stevenson A. W.
Published: July 2002

The basic Δω, Δ2θ technique for examining single-crystal Bragg reflections [Mathieson (1982). Acta Cryst. A38, 378-387] has recently been improved [Mathieson & Stevenson (1984). Aust. J. Phys.
37; 657-665], by using a simple experimental modification which reduces the source component to a minor (angular) role, thereby making the extraction (deconvolution) of the remaining components more accurate. The application of this new technique in the determination of reflectivity (rocking) curves for imperfect crystals has been demonstrated [Mathieson & Stevenson (1985). Acta Cryst. A41, 290-296]. In the present case, the examination of individual reflections from a small single crystal of CuInSe2 reveals that the improved technique is capable (i) of identifying, by its locus extension in Δω, Δ2θ space, diffraction from one side of the specimen crystal to the other (in the diffraction plane), even for a crystal of average dimension ~0.06 mm, and (ii) of estimating the reflectivity curve for different parts of the crystal. A series of model eases is discussed, to clarify the interpretation of observed two-dimensional intensity distributions. While considered here in relation to a small crystal, this technique is applicable to extended-plate crystals (in transmission mode) by a selected-area procedure.
X-ray particle-size broadening
Authors: Rao S. , Houska C. R.
Published: July 2002

X-ray diffraction profiles and Fourier coefficients are given for particles distributed according to experimentally verified size distributions. Calculations are based upon the log normal distribution of sphere diameters and intercept lengths in addition to a normal distribution of column heights. It is found that the diffraction profile is not sensitive to the fine details of the distribution but rather the mean column height and the column-height variation coefficient. Errors in particle-size determinations will result from an improper choice of the variation coefficient. Two simplified models are given that describe the diffraction profiles for a large range of variation coefficients.
Histogram matching as a new density modification technique for phase refinement and extension of protein molecules
Authors: Zhang K. Y. J. , Main P.
Published: July 2002

A new density modification technique - histogram matching - is being developed with encouraging results. Its application to the known structure of pig 2Zn insulin refines the 6500 1.9 Å MIR phases from a mean error of 60° to one of 46°. With these refined phases as a starting point for phase extension to 1.5 Å, the mean error for the final 13 000 phases is 46°. The original 1.9 Å phases continue to refine during the phase extension to a final mean error of 40°. A comparison is made with similar calculations already published.
Direct phase determination for the molecular envelope of tryptophanyl-tRNA synthetase from Bacillus stearothermophilus by X-ray contrast variation
Authors: Carter C. W. , Crumley K. V. , Coleman D. E. , Hage F. , Bricogne G.
Published: July 2002

Monoclinic crystals of Bacillus stearothermophilus tryptophanyl-tRNA synthetase grown in the presence of substrate trytophan (space group P21) display evidence of a low-resolution trigonal space group (P321). The origin and averaging transformations for the local 32 point group of this unusually clear sixfold non-crystallographic symmetry may be inferred without prior estimation of the electron density. This local symmetry was exploited in conjunction with solvent density contrast variation to determine the shape of the molecular envelope. X-ray intensities measured from crystals equilibrated in mother liquors of three different electron densities were used to estimate three parameters for each reflection: the modulus of the envelope transform, |G

h
]; and components, X

h
and Y

h
, relative to G

h
, of the structure-factor vector for the transform of intramolecular density fluctuations. The moduli {|G

h
|} behave somewhat like structure-factor amplitudes from small-molecule crystals, and estimation of their unknown phases was successfully carried out by statistical direct methods. Reflections to 18 Å resolution, which obey rather well the symmetry of space group P321, were merged to produce an asymmetric unit in that space group. |G

h
| values for the 34 strongest of these were phased using the small-molecule direct-methods package MITHRIL [Gilmore (1984). J. Appl Cryst.
17, 42-46]. The best phase set was expanded back to the P21 lattice and negative density was truncated to generate initial phases for all reflections to 18 Å resolution. Phase refinement by iterative imposition of the local 32 symmetry produced an envelope with convincing features consistent with known properties of the enzyme. The envelope implies that the tryptophanyl-tRNA synthetase dimer is an elongated structure with an axial ratio of about 4: 1, in which the monomers have two distinct domains of unequal size. The smaller of these occurs at the dimer interface, and resembles the nucleotide binding portion of the tyrosyl-tRNA synthetase. It may therefore contain the amino-terminal one hundred or so residues, including all three cysteines, previously suggested to comprise a nucleotide-binding domain in the tryptophanyl enzyme. A purely crystallographic test of the overall features of this envelope was carried out by transporting it to a tetragonal crystal form of the same protein in which the asymmetric unit is a monomer. The small domain fits snugly inside three mercury and one gold heavy-atom binding sites for this crystal form; and symmetry-related molecules provide excellent, but very different, lattice contacts in nearly all directions.
Surface structure analysis by transmission electron diffraction: effects of the phases of structure factors
Authors: Takayanagi K.
Published: July 2002

To solve the problems related to the phases of the structure factors in surface structure analysis by transmission electron diffraction, the dynamical diffraction in the bulk crystal underneath the surface layer is considered by the many-beam theory. In the case of normal beam incidence, the intensities of superlattice reflections from surfaces and/or adsorbed layers are used to discriminate whether the surface structure is substitutional or displacive in type. In the case where only two beams are strongly excited in the bulk crystal, the phases of the structure factors of the surface layer are determined by the rocking curve of the superlattice reflections.
Simulation and application of the distorted ZOLZ patterns from dislocations in Si
Authors: Lu G. , Wen J. G. , Zhang W. , Wang R.
Published: July 2002

The previously reported distorted zeroth-order fringes in a bright-field Tanaka pattern from a dislocated region in silicon have been computer simulated and the experimental and the many-beam calculated patterns agree well. Calculations are carried out for nine distinct cases of edge, screw and 60° dislocations in a silicon crystal. The general usefulness of the distortion of the ZOLZ pattern in determining geometrical properties of a dislocation is discussed.
A multisolution method of phase determination by combined maximization of entropy and likelihood. VI. The use of error-correcting codes as a source of phase permutation and their application to the phase problem in powder, electron and macromolecular crystallography
Authors: Gilmore Christopher , Dong Wei , Bricogne Gérard
Published: July 2002

The use of error-correcting codes as a source of efficient designs of phase permutation schemes is described. Three codes are used, all taken from the Bricogne BUSTER program [Bricogne (1993). Acta Cryst. D49, 37–60]: the Hamming [7, 4, 3], the Nordström–Robinson (16, 256, 6) and the Golay [24, 12, 8] or its punctured [23, 12, 7] form. These are used in a maximum-entropy–likelihood phasing environment to carry out phase permutation of basis-set reflections instead of the usual quadrant permutation or magic integer approaches. The use of codes in this way inevitably introduces some errors in the phase choices, but for most structures this is not significant especially when the gain in sampling efficiency is considered. For example, the Golay [24, 14, 8] allows the permutation of 24 centric phases in such a way that only 4096 phase sets are produced instead of 224 = 16 777 216, and one of these sets has, at most, only four wrong phases. The method is successfully applied to three powder diffraction data sets of increasing complexity, and with increasing degrees of overlap {Mg3BN3, Sigma-2 ([Si64O128]\,\cdot\,4C10H17N) and the NU-3 zeolite}, a sparse electron diffraction data set for buckminsterfullerene, C60, and the small protein molecule crambin at 3 Å resolution where 42 reflections are phased with a U-weighted mean phase error of 58.5°.
Quasiperiodic patterns generated by mixing lattices derived from a dodecahedral star and an icosahedral star
Authors: Soma T. , Watanabe Y.
Published: July 2002

A 3D quasiperiodic pattern by projection from an nD lattice can be defined by an orthonormal n × n
lattice matrix which produces basis vectors in pattern space with a prescribed arrangement and basis vectors in perpendicular or test space satisfying the quasicrystallographic condition. A 16 × 16 lattice matrix is derived which produces basis vectors in pattern space as a combination or mixing of dually positioned dodecahedral star and icosahedral star. It is shown that the mixed star constitutes a eutactic star. Since the module generated by the mixed eutactic star is totally irrational, patterns generated by projection using the lattice matrix are quasicrystallographic and the equilateral pattern is generated for 1: 1 mixing.
Wavelength-modulated diffraction: a new method for phase determination
Authors: Iwasaki H. , Yurugi T. , Yoshimura Y.
Published: July 2002

A new diffraction method has been developed in which the intensity of Bragg reflections is measured while changing continually the wavelength of the radiation over a range in the vicinity of the absorption edge of an atom contained in the crystal. It is shown that the intensity gradient with respect to the wavelength of the hkl reflection is in a simple relation to the real and imaginary parts of the structure factor of that reflection and, if the positions of the anomalously scattering atoms are known or properly assumed, the phase of F(hkl) can be derived by solving simultaneous linear equations. The procedure is particularly simple when the crystal is centrosymmetric. The method, called the wavelength-modulated diffraction (WMD) method, is free from the problem of intensity scaling encountered in other methods of phase determination. Synchrotron radiation is most suited to WMD measurements. Suggestions on how to measure the intensity gradient are given and the possible errors involved are discussed.
Reduced projection operators and algebraic expressions for the symmetry-adapted functions of the icosahedral group
Authors: Fan Peng-Dong , Chen Jin-Quan , Draayer J. P.
Published: July 2002

The algebraic expressions for the reduced projection operators \wp^{(\lambda)\bar{\mu}}_{\mu} = \sum_{i = 1}^4 u_i \hat{\beta_i} for the irreducible representation (irrep) \lambda of the icosahedral group I are found by using the double-induced technique and eigenfunction method, where \hat{\beta}_i are the double-coset generators of I with respect to the cyclic subgroup C_5. Simple algebraic expressions are derived for the symmetry-adapted functions (SAF's) by applying the reduced projection operators \wp^{(\lambda) \bar{\mu}}_\mu to Y_{l \bar m}. The SAF's are functions of the angular momentum l, the quantum numbers \lambda, \mu of the group chain I \supset C_5 and the multiplicity label \bar m. In this way, the SAF problem of the group I is solved once for all instead of for one angular momentum l each time.
Tunable X-ray polarization reflector with perfect crystals
Authors: Hasegawa Y. , Ueji Y. , Okitsu K. , Ablett J. M. , Siddons D. P. , Amemiya Y.
Published: July 2002

The possibility of a transmission-type X-ray linear polarizer is investigated using a thin Bragg reflector as a polarizing filter. In this device, the transmitted beam (rather than the Bragg-reflected beam) is the useful output of the device. Consequently, the position and the direction of the transmitted beam are unchanged as the energy is changed, or even when the polarization direction to be filtered out is changed. Theoretical considerations as well as preliminary transmissivity measurements are presented. The use of perfect crystals of silicon and diamond is examined. A polarization ratio, defined as I_H/I_V, higher than 105 was observed in experiments to measure the performance characteristics of the proposed X-ray polarizing reflector. The transmission-type X-ray linear polarizer is well suited for spectroscopic measurements with polarized X-rays.
Dynamical scattering and electron diffraction from thin polymer lamellar crystals – poly(tert-butylethylene sulfide). Erratum
Authors: Dorset Douglas L. , Dumas Philippe , Cartier Laurent , Lotz Bernard
Published: July 2002

In the paper by Dorset et al. [Acta Cryst. (1999), A55, 901–907], the last line of page 904 should read `…the S atoms. However, the autocorrelation function…'.
X-ray studies on crystalline complexes involving amino acids and peptides. XXXV. Invariance and variability in amino acid aggregation in the complexes of maleic acid with L-histidine and L-lysine
Authors: Pratap J. V. , Ravishankar R. , Vijayan M.
Published: July 2002

The crystal structures of complexes of maleic acid with L-histidine and L-lysine have been determined. The two crystallographically independent amino acid molecules in the L-histidine complex have different closed conformations, while the lysine molecule in its complex has the most favourable conformation sterically with an all-trans sidechain trans to the α-carboxylate group. The maleic acid molecules exist as semi-maleate ions of similar conformation and contain a symmetric O...H...O hydrogen bond. Amino acid cations and semi-maleate anions aggregate into alternate layers in both the structures. The arrangement of molecules in the histidine layer in L-histidine semi-maleate is closer to that in the crystals of the free amino acid than in other L-histidine complexes. On the other hand, the arrangement of lysine molecules in its semi-maleate complex is different from any observed so far. However, the well established characteristic interaction patterns involving amino and carboxylate groups still play a major role in holding the molecules together in the crystal of the complex.
Structure of the β form of calcium pyrophosphate tetrahydrate
Authors: Balić-Žunić Tonči , Christoffersen Margaret R. , Christoffersen Jørgen
Published: July 2002

β-Ca2P2O7·4H2O is monoclinic, P21/c, a = 12.287 (6), b = 7.511 (3), c = 10.775 (5) Å, and β = 112.54 (1)°. Five of the terminal O atoms from a pyrophosphate group bind to Ca atoms, together with O atoms from three of the water molecules. The fourth H2O forms only hydrogen bonds. Both Ca atoms have coordination number 7 and show characteristics between those of a capped octahedron and a pentagonal bipyramid. The analysis of coordination distortions suggests that regularity and volume efficiency of a Ca coordination polyhedron increases with the number of bound water O atoms. The structure is layered after {100}, reflected also in the morphology of crystals which are formed as extremely thin plates. The central parts of the layers are formed by chains of Ca coordination polyhedra which run along the b axis and are interconnected by pyrophosphate groups. Water molecules form the surfaces of the layers. A peculiar auto-inhibition of growth from supersaturated solutions is supposed to be caused by a direct attachment of CaP2O7
2− and P2O7
4− to the water molecules on the surfaces of layers. Ca2P2O7·4H2O is known in two polymorphs. The unit-cell volume of the β form compared with that of α suggests that the former is a low-temperature modification.
Modulated structure of La2Co1.7 from neutron and X-ray diffraction data
Authors: Dusek Michal , Chapuis Gervais , Schobinger-Papamantellos Penelope , Wilkinson Clive , Petricek Vaclav , Tung L. D. , Buschow Karl Heinz Juergen
Published: July 2002

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.
Crystal engineering in the gem-alkynol family; synthon repetitivity and topological similarity in diphenylethynylmethanols: structures that lack O—H...O hydrogen bonds
Authors: Bilton Clair , Howard Judith A. K. , Madhavi N. N. L. , Nangia Ashwini , Desiraju Gautam R. , Allen Frank H. , Wilson Chick C.
Published: July 2002

The structures of four para-substituted derivatives of diphenylethynylmethanol have been determined [ditolylethynylmethanol, di(4-chlorophenyl)ethynylmethanol, di(4-bromophenyl)ethynylmethanol and bis(4,4′-biphenylyl)ethynylmethanol]. The dimethyl, dichloro, dibromo and diphenyl compounds have been analysed using X-ray diffraction at 150 K, and the dichloro compound has also been studied using neutron diffraction at 150 K. In common with the parent diphenylethynylmethanol [Garcia, Ramos, Rodriguez & Fronczek (1995). Acta Cryst. C51, 2674–2676], all four derivatives fail to form the expected strong O—H...O hydrogen bonds due to steric hindrance. Instead, the supramolecular structural organization in this family of gem-alkynols is mediated by a variety of weaker interactions. The two most acidic protons, O—H and C[triple-bond]C—H, participate in weak hydrogen bonds to π-acceptors, forming synthons that stabilize all five structures. These primary interactions are reinforced by a variety of other weak hydrogen bonds involving C—H donors and the hydroxy-O as an acceptor, and by halogen...halogen interactions in the dichloro and dibromo compounds.
Quantifying distortion from ideal closest-packing in a crystal structure with analysis and application
Authors: Thompson Richard M. , Downs Robert T.
Published: July 2002

A parameter, U
cp, that quantifies the distortion of the anion skeleton in a crystal from ideal closest-packing has been developed. It is a measure of the average isotropic displacement of the observed anions from their ideal equivalents. An ideal closest-packed structure can be fit to an observed structure by varying the radius of the ideal spheres, orientation and translation, such that U
cp is minimized. Ideal structures were fit to the M1M2TO4 polymorphs, pyroxenes and kyanite. The distortions of these crystals were analyzed in terms of the two parameters, U
cp and the ideal radius. Changes in structures due to temperature, pressure and compositional effects were characterized in terms of these parameters.
Ferroelectric–paraelectric phase transitions with no group–supergroup relation between the space groups of both phases?
Authors: Kroumova E. , Aroyo M. I. , Pérez-Mato J. M. , Hundt R.
Published: July 2002

The structures of Sr3(FeF6)2, β-NbO2, TlBO2 and CrOF3, previously reported as possible ferroelectrics with no group–supergroup relation between the ferroelectric and the paraelectric symmetries, have been carefully studied. We could not confirm any structural pseudosymmetry with respect to a space group which is not a supergroup of their room-temperature polar space group. In all cases, pseudosymmetry was indeed detected, but only for non-polar supergroups of the actual space groups of the structures. In this sense, the four compounds are possible ferroelectrics, but fulfilling the usual group–supergroup relation between the phase symmetries.
Ab initio determination of the valence electron distribution in the average structure of the incommensurately modulated calaverite AuTe2
Authors: Caracas Razvan , Gonze Xavier
Published: July 2002

The valence-electron density distribution of the average structure of incommensurately modulated calaverite, AuTe2, has been computed using density-functional theory. High-density regions, centered around the Au and Te atoms, are not spheric, but present charge concentrations along the Au—Te and Te—Te bonds. The electronic band structure and its corresponding density of states reveal the presence of three electronic band groups, constituted mainly by Te 5s, Au 5d and hybrids of Te 6p + Au 6s + Au 5d orbitals. The electrons belonging to the last block are responsible for the chemical bonds.
Bulk modulus and non-uniform compression of Nb3Te4 and In
x
Nb3Te4 (x < 1) channel compounds
Authors: Wunschel Markus , Dinnebier Robert E. , Carlson Stefan , van Smaalen Sander
Published: July 2002

The crystal structures of Nb3Te4 and In
x
Nb3Te4 [x = 0.539 (4)] are reported for a series of pressures between 0 and 40 GPa. Both compounds crystallize in space group P63/m with a = b = 10.671 and c = 3.6468 Å for Nb3Te4, and a = b = 10.677 and c = 3.6566 Å for In
x
Nb3Te4 at ambient conditions. Phase transitions were not observed. High-pressure X-ray powder diffraction was measured using a diamond anvil cell and synchrotron radiation. Full Rietveld refinements provided the values of the lattice parameters and the values of the atomic coordinates at each pressure. The bulk modulus is found as K
0 = 70 (5) GPa for Nb3Te4 and as K
0 = 73 (4) GPa for In
x
Nb3Te4. The analysis of the pressure dependences of the detailed crystal structures shows that the compression along c involves the folding up of the quasi-one-dimensional zigzag chains of Nb. The compression perpendicular to c is entirely due to the reduction of the diameter of the channels. The presence of intercalated In atoms is found to have hardly any influence on the compression behaviour up to 40 GPa.
Some 60 new space-group corrections
Authors: Marsh Richard E. , Kapon Moshe , Hu Shengzhi , Herbstein Frank H.
Published: July 2002

Some 60 examples of crystal structures are presented which can be better described in space groups of higher symmetry than used in the original publications. These are divided into three categories: (A) incorrect Laue group (33 examples), (B) omission of a center of symmetry (22 examples), (C) omission of a center of symmetry coupled with a failure to recognize systematic absences (nine examples). Category A errors do not lead to significant errors in molecular geometry, but these do accompany the two other types of error. There are 19 of the current set of examples which have publication dates of 1996 or later. Critical scrutiny on the part of authors, editors and referees is needed to eliminate such errors in order not to impair the role of crystal structure analysis as the chemical court of last resort.
New Powder Diffraction File (PDF-4) in relational database format: advantages and data-mining capabilities
Authors: Kabekkodu Soorya N. , Faber John , Fawcett Tim
Published: July 2002

The International Centre for Diffraction Data (ICDD) is responding to the changing needs in powder diffraction and materials analysis by developing the Powder Diffraction File (PDF) in a very flexible relational database (RDB) format. The PDF now contains 136 895 powder diffraction patterns. In this paper, an attempt is made to give an overview of the PDF-4, search/match methods and the advantages of having the PDF-4 in RDB format. Some case studies have been carried out to search for crystallization trends, properties, frequencies of space groups and prototype structures. These studies give a good understanding of the basic structural aspects of classes of compounds present in the database. The present paper also reports data-mining techniques and demonstrates the power of a relational database over the traditional (flat-file) database structures.