Showing papers in "Journal of Applied Crystallography in 1991"
TL;DR: The MOLSCRIPT program as discussed by the authors produces plots of protein structures using several different kinds of representations, including simple wire models, ball-and-stick models, CPK models and text labels.
Abstract: The MOLSCRIPT program produces plots of protein structures using several different kinds of representations. Schematic drawings, simple wire models, ball-and-stick models, CPK models and text labels can be mixed freely. The schematic drawings are shaded to improve the illusion of three dimensionality. A number of parameters affecting various aspects of the objects drawn can be changed by the user. The output from the program is in PostScript format.
13,971 citations
TL;DR: A set of screening conditions for initial experiments in protein crystallization has been developed, tested, and is herein presented as discussed by the authors, which are empirically derived based on known or published crystallization conditions of various proteins in the past, so as to sample as large a range of buffer, pH, additive and precipitant variables as possible, using small amounts of proteins.
Abstract: A set of screening conditions for initial experiments in protein crystallization has been developed, tested, and is herein presented. These solution and precipitant conditions are empirically derived based on known or published crystallization conditions of various proteins in the past, so as to sample as large a range of buffer, pH, additive and precipitant variables as possible, using small amounts of proteins. The 50 crystallization conditions have been tested on 15 previously crystallized proteins, all of which were also crystallized in at least one form by this screen. This method is also shown to be highly successful in the crystallization of proteins which had not previously been crystallized.
2,139 citations
TL;DR: In this paper, a dichotomy method for indexing powder diffraction patterns for low-symmetry lattices is studied in terms of an optimization of bound relations used in the comparison of observed data with the calculated patterns generated at each level of the analysis.
Abstract: The dichotomy method for indexing powder diffraction patterns for low-symmetry lattices is studied in terms of an optimization of bound relations used in the comparison of observed data with the calculated patterns generated at each level of the analysis. A rigorous mathematical treatment is presented for monoclinic and triclinic cases. A new program, DICVOL91, has been written, working from the cubic end of the symmetry sequence to triclinic lattices. The search of unit cells is exhaustive within input parameter limits, although a few restrictions for the hkl indices of the first two diffraction lines have been introduced in the study of triclinic symmetry. The efficiency of the method has been checked by means of a large number of accurate powder data, with a very high success rate. Calculation times appeared to be quite reasonable for the majority of examples, down to monoclinic symmetry, but were less predictable for triclinic cases. Applications to all symmetries, including cases with a dominant zone, are discussed.
1,459 citations
[...]
TL;DR: RIBBONS 2.0 as mentioned in this paper allows real-time viewing of solid shaded ribbon models of macromolecules, including spheres, cylinders, dots, polygons and text.
Abstract: The program RIBBONS 2.0 allows real-time viewing of solid shaded ribbon models of macromolecules. The primary features of the software are the ability to create a wide variety of styles of ribbon drawings interactively and to toggle between various coloring schemes chosen to reflect assorted geometrical and biochemical properties. Spheres, cylinders, dots, polygons and text are also supported. The auxiliary programs included make RIBBONS 2.0 a powerful tool for visual structural analysis as well as for presentation graphics. The program is currently available only for the Silicon Graphics 4D series of workstations. A port to the Evans & Sutherland ESV workstation employing PEX is under development.
823 citations
TL;DR: The Fortran program package GNOM carries out SAS data treatment and interpretation for the disperse non-interacting dilute systems of different types using a general operator method based on Tikhonov's regularization technique.
Abstract: The Fortran program package GNOM carries out SAS data treatment and interpretation for the disperse non-interacting dilute systems of different types. A general operator method based on Tikhonov's regularization technique is used. Here a finite real-space distribution function [the correlation function p(r) for monodisperse systems and size distribution function D(R) for polydisperse ones] is searched for to fit the experimental data in accordance with the experimental conditions and the error bounds. It is possible to require non-negativity of the solution. The package GNOM is applicable for arbitrary experimental conditions (arbitrary weighting functions and form factors) and provides solutions stable to statistical noise and termination effects. The program makes it possible to treat together experimental curves from the same object measured under different experimental conditions and to choose the best model from the given set. Interactive and batch versions are available. The program is adapted to NORD, VAX and IBM-PC computers. It possesses convenient graphics possibilities.
613 citations
TL;DR: In this article, the effect of polydispersity on the small-angle X-ray and neutron scattering of fractals and disordered systems on a scale of lengths from about 10 to 2000 A is considered.
Abstract: Small-angle X-ray and neutron scattering are important techniques for studying the structure of fractals and other disordered systems on a scale of lengths from about 10 to 2000 A. This review begins with a brief outline of some properties of fractals. The small-angle scattering from fractal systems is then discussed and the effect of polydispersity is considered. The intensity of small-angle scattering from fractals and other disordered systems is often proportional to a negative power of the quantity q = 4πλ−1sin(θ/2), where θ is the scattering angle and λ is the X-ray or neutron wavelength. From the magnitude of the exponent that describes this type of scattering, which is often called power-law scattering, much important information can be obtained. Some situations in which power-law scattering can be expected are described. To illustrate the scattering from fractals and disordered systems, several experimental investigations of mass-fractal silicas and porous solids are reviewed and some calculations of the small-angle scattering from model fractal systems are outlined.
538 citations
TL;DR: In this article, a simple calculation which takes into account local correlations has been developed to furnish a reliable estimate of the values they would have if these correlations vanish, based on the Rietveld refinements.
Abstract: Estimated standard deviations which are obtained in Rietveld refinements are known to decrease towards zero with the recording step, but serial correlations appear at the same time. A simple calculation which takes into account local correlations has been developed to furnish a reliable estimate of the values they would have if these correlations vanish.
350 citations
TL;DR: In this paper, a program is developed to help design a geometric representation of the atomic structure of interfaces between two crystalline structures, such as sphalerite and bicrystal.
Abstract: A program has been developed to help design a geometric representation of the atomic structure of interfaces between two crystalline structures. The C-language program run on a graphics-oriented computer has proved to be a simple precise tool to use. It provides a clear picture of the atomic positions at the interface and, simultaneously, a stereographic projection and diffraction pattern for each crystal and bicrystal. Examples of multiple twinning in the sphalerite structure and of various metal–ceramic interfaces are presented.
233 citations
TL;DR: In this paper, a new method is proposed for the determination of positioning and mutual orientation of subunits in complex particles using the spherical harmonics technique, which is further developments of the indirect method based on the regularization technique.
Abstract: Applications of modern mathematical methods to the problems of small-angle scattering data treatment and interpretation are considered Special possibilities in data treatment, namely simultaneous treatment of data sets obtained with different experimental conditions and joint data processing for the anomalous-dispersion experiments, are presented The methods are further developments of the indirect method based on the regularization technique [Svergun, Semenyuk & Feigin (1988) Acta Cryst A44, 244–250] Recent improvements in the shape-determination technique based on the multipole expansion theory [Stuhrmann (1970) Z Phys Chem (Frankfurt am Main), 72, 177–184, 185–198] are described A new method is proposed for the determination of positioning and mutual orientation of subunits in complex particles using the spherical harmonics technique
195 citations
TL;DR: Methods, implemented into a computer program (LSABS), are described which calculate transmission factors and their first derivatives with respect to the positions of the faces of a convex polyhedral monocrystal.
Abstract: Methods, implemented into a computer program (LSABS), are described which calculate transmission factors and their first derivatives with respect to the positions of the faces of a convex polyhedral monocrystal. Both Gaussian and analytical integration procedures are considered in detail. LSABS has been implemented as a module of the Xtal system of programs. It functions independently of the instrument used to collect the intensity data. Test results obtained with this program are compared with standard literature values. Correlation coefficients of an absorption-corrected intensity data set of calcite, CaCO3 are reported. The need for transmission factor derivatives in the refinement of crystal shape and in the elimination of correlated absorption-corrected structure amplitudes is explained.
117 citations
TL;DR: In this article, the design and operation of a small-angle X-ray scattering instrument, optimized for high throughput at a synchrotron source, high angular and wavelength resolution, large sample cross-sectional area, accurate energy tuning, excellent signal-to-noise ratio and harmonic rejection are presented.
Abstract: The design and operation of a new small-angle X-ray scattering instrument, optimized for high throughput at a synchrotron source, high angular and wavelength resolution, large sample cross-sectional area, accurate energy tuning, excellent signal-to-noise ratio and harmonic rejection are presented. The principles of design and implementation are given, as are the details of primary calibration of absolute intensity and experimental desmearing. The instrument has been tested for application to anomalous-scattering measurements near the chromium K edge. Preliminary results on samples of a heat-treated steel are presented as a demonstration of the capability of this experiment to separate the microstructure evolution as a function of temperature of a chromium-rich precipitate from the thermal behavior of other precipitates in the steel.
TL;DR: In this paper, small-angle neutron scattering (SANS) is used to characterize scale-invariant (fractal) disordered materials such as cements, clays and porous rocks.
Abstract: Small-angle neutron scattering (SANS) provides a powerful tool in the non-destructive characterization of statistically representative microstructures in technologically important disordered materials. While qualitative microstructural models must be provided by other methods such as transmission electron microscopy (TEM), SANS quantifies the microstructural parameters and can characterize scale-invariant (fractal) disordered materials such as cements, clays and porous rocks. H2O/D2O contrast-variation methods have proved particularly useful for investigating the accessibility of different parts of the pore structure and in differentiating between pore-volume access (measured by other methods) and pore-surface accessibility (likely to control sorption and leaching processes). This paper describes SANS studies of disordered porous materials, particularly the hydration of cement and the real-time permeation of water in clay minerals. Other microstructural effects are also considered.
TL;DR: In this paper, the crystal structure of cimetidine (C10H16N6S) has been solved and refined from high-resolution synchrotron X-ray powder data collected on station 8.3 at Daresbury Laboratory.
Abstract: The crystal structure of cimetidine (C10H16N6S) has been solved and refined from high-resolution synchrotron X-ray powder data collected on station 8.3 at Daresbury Laboratory. The structure is monoclinic, P21/n, Z = 4, a = 10.7029 (3), b = 18.8262 (1), c = 6.8266 (2) A, β = 111.306 (2)°. The data were auto-indexed and the integrated intensities were obtained by pattern decomposition. The structure was solved by direct methods followed by iterative cycles of least-squares refinement and Fourier syntheses. All 17 non-H atoms were located. The structure was then refined using the method of Rietveld [J. Appl. Cryst. (1969), 2, 65–71]. A difference Fourier synthesis after the convergence of the non-H-atom model showed positive electron density at the expected positions of all 16 H atoms, ten of which were significantly above the background. The H atoms were allowed to refine unconstrained and the final agreement factors were RI = 1.9, Rwp = 8.5 and Rex = 6.9%; the equivalent R-factors without the H atoms were RI = 10.3 and Rwp = 16.2%, demonstrating the significant contribution of the H atoms to the profile.
TL;DR: In this article, the Brindley particle absorption contrast effect was used to improve the accuracy of X-ray powder diffraction analysis. But the analysis is grossly in error. And it was shown that the analysis was not correct to within 1 percentage point.
Abstract: It is shown that quantitative X-ray powder diffraction analyses using full profile multiphase refinement should include corrections for the Brindley particle absorption contrast effect. This is demonstrated with synthetic mixtures of the highly contrasting phases LiF and Pb(NO3)2 (μ/ρ = 20 and 231 cm−1 for Co Kα). With contrast corrections, the only parameter which needs to be input is an effective particle radius R for each phase. For mixtures of LiF and Pb(NO3)2 over the whole composition range, quantification is achieved, under the present specimen preparation method, with an effective particle radius of 51 μm for both phases, giving analyses correct to within 1 percentage point. Without Brindley corrections, the analysis is grossly in error.
TL;DR: In this article, the authors measured the instrumental resolution function for the Daresbury 9.1 high-resolution powder diffractometer as a function of wavelength, slit width and specimen geometry.
Abstract: The instrumental resolution function for the Daresbury 9.1 high-resolution powder diffractometer has been measured as a function of wavelength, slit width and specimen geometry. It has been demonstrated that the effects of size and strain from the sample of annealed barium fluoride used as a reference material were negligible, and that it is a suitable standard for studying instrumental broadening. The results of considering the angular dependence of the Lorentzian and Gaussian components of the integral breadths of instrumental line profiles are in excellent agreement with those predicted by the receiving-slit width and the known wavelength spread in the incident beam. The measured instrumental resolution function was found to be dependent on the slit dimensions at low and intermediate angles, and to be dominated by the effects of angular dispersion at higher angles. The line shapes are predominantly Gaussian at low angles and for a flat sample tend to a pure Lorentzian shape at 180°. The line profiles for a capillary sample are similar at low and intermediate angles and tend to an intermediate Gaussian/Lorentzian form at high angles.
TL;DR: In this article, the authors investigated the structure self-assembling in the spinodal decomposition of polymer blends in its late stage by a time-resolved light scattering technique, with a particular emphasis on the time evolution of the interface structure.
Abstract: Structure self-assembling in the spinodal decomposition (SD) of polymer blends in its late stage has been explored for a near-critical mixture of polybutadiene and polyisoprene by a time-resolved light scattering technique, with a particular emphasis on the time evolution of the interface structure. By analysis of a scaled structure factor F(x, t) ≡ I(q, t)qm(t)3 over wide ranges of a reduced scattering vector x ≡ q/qm(t) and time, it was found relevant to divide the late stage of SD into two stages, I and II. Here, I(q, t) denotes the scattered intensity as a function of the scattering vector q and time t. In the intermediate stage preceding the late one, F(x, t) became sharper with its peak at x = 1 increasing with t. However, as time elapsed, F(x, t) turned out to be universal for t, first in the range of x smaller than about 2 and then over the entire range of x accessible by the present experiment. The time interval in which the former occurred is defined as late stage I and the one in which the latter was realised is called late stage II. In late stage I, the average thickness of phase-phase interfaces decreases towards an equilibrium value and the time evolution of the interfacial area density Σ(t) does not scale with qm(t), i.e. the exponents γ and α in the power laws Σ(t) ~ t−γand qm(t) ~ t−α do not coincide (actually, α < γ). Late stage II corresponds to the process in which these exponents become equal and the interface thickness reaches equilibrium. Such conditions probably ensure the establishment of a complete dynamical scaling law in the SD process.
TL;DR: In this paper, two methods for performing indirect Fourier transformation of small-angle scattering data have been introduced by Glatter [J. Appl. Co'st. this paper.
Abstract: Two methods for performing indirect Fourier transformation of small-angle scattering data have been introduced by Glatter [J. Appl. Co'st. (1977), 10, 415-421] and Moore [J. Appl. Co'st. (1980), 13, 168-175]. These two methods are here compared to one using maximum entropy. The methods have been applied to simulated as well as experimental scattering data. The comparison of the three methods for the simulated data shows that the maximum-entropy method and the method of Glatter give similar results, which agree well with the original distributions. The method of Moore occasionally gives results which have artificial oscillations.
TL;DR: In this article, the Fourier cosine coefficients are calculated for a model with paracrystalline disorder and an assumed distribution of crystal sizes and the parameters describing this model are refined to minimize the discrepancy between the calculated and experimental values of the coefficients.
Abstract: The intensity profile of the X-ray reflection from a crystalline material is related to the lattice disorder and the distribution of crystal sizes through its Fourier cosine coefficients. However, existing methods of obtaining these structural parameters from the coefficients require more than one order of reflection and this is seldom available with polymer fibres. They also rely heavily on the low-order harmonics which are those determined with least accuracy. The development and testing of a method which overcomes this weakness and which is suitable for use with a single order is described. The coefficients are calculated for a model with paracrystalline disorder and an assumed distribution of crystal sizes and the parameters describing this model are refined to minimize the discrepancy between the calculated and experimental values of the coefficients. Provided the distribution of lengths is asymmetric this discrepancy is no greater than would be expected from experimental error and so the assumed model cannot be rejected on the evidence available. Since a range of model parameters all gave equally good agreement with experiment, it was not possible with a single order to obtain a well defined set of values. Diffraction patterns displaying two orders had been chosen and results from the second order were consistent with the first, only a narrow range satisfying both simultaneously. The method was further developed by calculating the intensity profile from the harmonics and using this in the refinement. There was no advantage over using harmonics; indeed, on occasions the refinement algorithm was unstable producing unreliable results.
TL;DR: The use of X-ray and neutron small-angle scattering in the study of microstructural changes in inorganic materials is reviewed in this article, with emphasis on recent developments.
Abstract: The use of X-ray and neutron small-angle scattering in the study of microstructural changes in inorganic materials is reviewed, with emphasis on recent developments. In the kinetics of diffusion-controlled phase separation, reliable data on the evolution of size and shape of the decomposition products may be obtained especially from in situ ageing experiments. Magnetic neutron scattering is discussed for inhomogeneous ferromagnets. While for the structural characterization of binary systems single-wavelength experiments are sufficient, variable X-ray wavelengths now available at synchrotron-radiation facilities open possibilities for the study of partial structure functions in systems containing several components. The combined use of small-angle scattering and complementary techniques is of particular value in more complex cases. These points are illustrated by a survey of recent results on phase separation and precipitates in binary and ternary crystalline alloys and on radiation-induced microstructural changes in metallic and semiconducting materials.
TL;DR: In this article, a combination of multiple-crystal diffraction profiles and topography has been used to characterize semiconductor materials using high-resolution diffractometers and X-ray topography.
Abstract: Multiple-crystal diffraction profiles and multiple-crystal topography have been combined to characterize semiconductor materials. X-ray topographs have been measured on the same region of the sample as the diffraction profiles and diffraction space maps, by insetting a film cassette in the diffracted beam path of a high-resolution diffractometer. This has proved to be a very powerful combination for providing a deeper understanding of the structural features that give rise to the high-resolution diffraction profiles. The data collection mode is discussed and the problems associated with interpreting `rocking curves' of imperfect materials. The misorientation and nature of the mosaic blocks in semi-insulating GaAs have been revealed by this method.
TL;DR: In this paper, small-angle neutron scattering has been used to study both water condensation and freezing/defreezing processes in porous Vycor glass, and the overall form of the scattering is modelled in terms of a percolating system of empty pores connected to the surface during desorption and of a random distribution of clusters of filled pores on adsorption.
Abstract: Small-angle neutron scattering has been used to study both water condensation and freezing/ defreezing processes in porous Vycor glass. When the system is partially saturated with a matched D2O/H2O water mixture, an interesting new feature appears in the scattering which gives a straight line on a log-log plot of scattering intensity versus scattering vector, Q. This feature extends over about one and half orders of magnitude in Q, but with a substantially different gradient for desorption and adsorption, the gradients being 1.75 ± 0.1 and 0.5 ± 0.1 respectively. The overall form of the scattering is modelled in terms of a percolating system of empty pores connected to the surface during desorption and of a random distribution of clusters of filled pores on adsorption. Further, when water in porous Vycor undergoes a freezing/defreezing cycle, the density of the water changes gradually over a large temperature range and there is also ~ 10 K hysteresis between the freezing and defreezing processes.
TL;DR: In this paper, Cahn's scheme for simulating the morphology of the late-stage spinodal decomposition of a phase-separating two-component alloy is extended to the case of bicontinuous microemulsions with an equal volume fraction of oil and water.
Abstract: The ubiquitous scattering peak found in all disordered bicontinuous microemulsions, when scattering measurements are made with an oil-water contrast, is attributed to the existence of two length scales in the system. The two lengths, d and ξ, appear explicitly in the Debye correlation function for the microemulsion in a phenomenological model proposed by Teubner & Strey [J. Chem. Phys. (1987), 87, 3195–3200] (T–S model). The precise physical meaning of these two lengths, however, was not clear in the original paper. Cahn's scheme for simulating the morphology of the late-stage spinodal decomposition of a phase-separating two-component alloy is extended to the case of bicontinuous microemulsions with an equal volume fraction of oil and water. In the simulation, a length scale {\bar d}=2π/{\bar k}, representing the average interdomain distance (proportional to the average domain size), and another parameter z, relating to the dispersion of the domain size by Δk/{\bar k} = (z + 1)−1/2, are imposed. It is shown that the ratio ξ/d in the T–S model is a unique function of the parameter 1/z. The extended Cahn model gives both the real-space structure of a disordered bicontinuous microemulsion and the exact Debye correlation function for the calculation of the corresponding scattering intensity. A criterion is given for the realisation of the disordered bicontinuous structure in terms of a universal range for the dispersion (i.e. ξ/d). The existence of the two lengths, having a universal ratio, also implies that the scattering function I(Q) satisfies a certain scaling relation. Our SANS data are used to support the validity of such a scaling relation.
TL;DR: In this article, high-resolution powder diffraction coupled with the total pattern-fitting technique revealed unusual and hitherto unreported diffraction effects for samples of copper(II) oxide.
Abstract: High-resolution powder diffraction coupled with the total-pattern-fitting technique revealed unusual and hitherto unreported diffraction effects for samples of copper(II) oxide. Data were obtained with a conventional diffractometer and strictly monochromatic Cu Kα1 radiation and also with the Daresbury Laboratory synchrotron source (9.1 HRPD). It was found that most lines are skewed, by varying amounts, towards higher or lower angles and there is a small amount of line broadening, which is mainly order dependent. It is shown that the line profile asymmetry varies systematically throughout reciprocal space and that the magnitude of the effect is sample dependent. No significant peak displacements were detected, but the skewing of lines is consistent with a distribution in the dimensions of the unit cell and hence Cu—O bond lengths.
TL;DR: In this paper, the authors compared the performance of pulsed-source and reactor-based small-angle neutron scattering (SANS) detectors at pulsed sources and compared their performance at very low and moderate intensity versus momentum transfer (Q), respectively.
Abstract: The importance of small-angle neutron scattering (SANS) in biological, chemical, physical and engineering research mandates that all intense neutron sources be equipped with SANS instruments. Four existing instruments at pulsed sources are described and the general differences between pulsed-source and reactor-based instrument designs are discussed. The basic geometries are identical, but dynamic range is generally achieved by using a broad band of wavelengths (with time-of-flight analysis) rather than by moving the detector. This allows optimization for maximum beam intensity at a given beam size over the full dynamic range with fixed collimation. Data-acquisition requirements at a pulsed source are more severe, requiring large fast histograming memories. Data reduction is also more complex, as all wavelength-dependent and angle-dependent backgrounds and nonlinearities must be accounted for before data can be transformed to intensity versus momentum transfer (Q). A comparison is shown between the Los Alamos pulsed instrument and DII (Institut Laue–Langevin) and examples from the four major topics of the conference are shown. The general conclusion is that reactor-based instruments remain superior at very low Q or if only a narrow range of Q is required, but that the current generation of pulsed-source instruments is competitive at moderate Q and may be faster when a wide range of Q is required.
TL;DR: In this paper, the authors present a pure enantiomer β-(2S,3S,4S,5R)-Fructofuranosyl-α-(1R,2R,3s,4s,5r)-glucopyranoside or sucrose, with formula C12H22O11 and crystallizing with two molecules per cell in the monoclinic space group P21, a = 10.8631 (9), b = 8.7044 (6), c = 7.7624
Abstract: β-(2S,3S,4S,5R)-Fructofuranosyl-α-(1R,2R,3S,4S,5R)-glucopyranoside or sucrose, with formula C12H22O11 and crystallizing with two molecules per cell in the monoclinic space group P21, a = 10.8631 (9), b = 8.7044 (6), c = 7.7624 (7) A, β = 102.938 (7)° at 300 (1) K, is readily available as a pure enantiomer. Its small cell volume, stability, purity, low thermal motion and the absence of its enantiomorph in a material available from grocery stores commend it as a test compound for experimental set-ups aimed at the determination of absolute structures. Practical details of recrystallization, crystal selection and mounting, experimental method as well as data processing are given together with lists of indices for Bijvoet differences sensitive to the hand of the molecule.
TL;DR: In this article, small-angle neutron scattering was used to investigate the microstructure of the high-temperature Lα phase in the AOT/water/decane system [AOT = sodium bis(2-ethylhexyl) sulfosuccinate].
Abstract: Small-angle neutron scattering was used to investigate the microstructure of the high-temperature Lα phase in the AOT/water/decane system [AOT = sodium bis(2-ethylhexyl) sulfosuccinate]. The system was modeled as randomly oriented lamellar stacks with a one-dimensional paracrystalline distortion. For samples injected into cells at high temperature, the two-dimensional Q maps show anisotropic scattering consistent with partial alignment of the phase, while samples injected near room temperature do not exhibit this feature. From the latter samples, it was found that a stack typically contains about 15 lamellae, about 80 A apart. Each layer is coated by a diffuse surfactant interface with a characteristic thickness of approximately 9 A. The fact that the scattering spectra do not exhibit maxima beyond the first order is explained by the observation that the Hosemann g factor gives about a 20% variation in the interlamellar spacing. The mean spacing is inversely proportional to the surfactant concentration, as expected for a lamellar phase. Over-estimation of the zero-order scattering predicted by the paracrystal model is attributed primarily to improper treatment of the contrast between each layer and the space-filling lamellar matrix.
TL;DR: In this article, profile fitting of energy-dispersive (ED) X-ray diffraction spectra of GeO2 polymorphs, hexagonal (P3221, Z = 3) and tetragonal (T42/mmm), was examined under high-pressure and high-temperature conditions.
Abstract: Profile fitting of energy-dispersive (ED) X-ray diffraction spectra of GeO2 polymorphs, hexagonal (P3221, Z = 3) and tetragonal (P42/mmm, Z = 2), was examined under high-pressure and high-temperature conditions These ED spectra were observed with a diamond-anvil cell on a synchrotron-radiation source Structure refinement of hexagonal GeO2 at several pressures was made, in which diffraction intensities were corrected for the energy spectrum of the incident radiation, escape efficiency of the detector, background scattering and absorption It was confirmed that the Ge—O distances in hexagonal GeO2 vary little with pressure, whereas the mean O—O distance and volume of the GeO4 tetrahedron decrease linearly with pressure The decrease in the Ge—O—Ge linking angle between tetrahedra causes the deformation of the framework composed of corner-linked tetrahedra, which is mainly responsible for the bulk volume compression The bulk modulus of the two polymorphs has been calculated on the basis of the volumetric compressibility up to 10 GPa
TL;DR: In this paper, the crystal structure of magnesium boron nitride in the low pressure phase, Mg3BN3(L), has been solved ab initio from X-ray powder data.
Abstract: The crystal structure of magnesium boron nitride in the low-pressure phase, Mg3BN3(L), has been solved ab initio from X-ray powder data. The cell is hexagonal (space group P63/mmc, Z = 2) with a = 3.54453 (4), c = 16-03536 (30)/k. Initial positional parameters for the Mg atoms were obtained from Patterson functions generated by 50 integrated intensities derived from a whole-powder pattern decomposition. The remaining atoms were located by trial-and-error model building, followed by Rietveld refinements (R,p = 8.5%). The structure can be described as consisting of ABB'BACC'CA... layers perpendicular to the c axis with linear N-~-B~N molecular anions at position A, Mg 2+ at positions B and C and Mg 2+ with three coordinating N atoms at positions B' and C', although Mg3BN3(L) is not a layer compound. A very similar structure has also been obtained by applying standard direct methods to the same intensity data. A high-quality electrondensity map has been calculated from the structurefactor data using the maximum-entropy method.
TL;DR: In this paper, the crystal structures of Zr2CO11 and HfCo7 intermetallic compounds were examined by transmission electron microscopy using both selected-area and convergent-beam electron diffraction.
Abstract: The crystal structures of Zr2CO11 and HfCo7 intermetallic compounds were examined by transmission electron microscopy using both selected-area and convergent-beam electron diffraction. Results show that both have an orthorhombic crystal structure, space group Pcna. The unit cells of both compounds appear to be comprised of two long-period superlattices in antiphase relation to one another along [001].
TL;DR: In an effort to evaluate the reliability of pole-figure measurements, a sample of experimentally deformed polycrystalline calcite was circulated among nine neutron diffraction facilities as discussed by the authors.
Abstract: In an effort to evaluate the reliability of pole-figure measurements, a sample of experimentally deformed polycrystalline calcite was circulated among nine neutron diffraction facilities This report compares results of the project both in terms of experimental pole figures and three-dimensional orientation distributions In general textures agree very closely For pole figures with strong diffraction intensities, standard deviations from the mean are 004–006 mrd (multiples of a random distribution) with a spread of maxima values of 018 mrd The spread is considerably larger for pole figures with weak diffraction intensities (037 mrd for 110) and so are standard deviations (020 mrd) For weak diffraction peaks one-dimensional position-sensitive detectors have an advantage over single-tube detectors Two-dimensional position-sensitive detectors combined with time of flight (TOF) offer new possibilities but data processing needs to be improved Greater care must be devoted to angle conventions: four of nine pole figures were inverted This study can provide a basis for quantitative texture analysis and estimation of experimental uncertainties