Showing papers in "Journal of Applied Crystallography in 1998"
TL;DR: In this paper, two high-resolution, general-purpose, small-angle neutron scattering instruments have been constructed at the National Institute of Standards and Technology's Center for Neutron Research.
Abstract: Two high-resolution, general-purpose, small-angle neutron scattering instruments have been constructed at the National Institute of Standards and Technology's Center for Neutron Research. The instruments are 30 m long and utilize mechanical velocity selectors, pinhole collimation and high-data-rate two-dimensional position-sensitive neutron detectors. The incident wavelength, wavelength resolution and effective length of the instruments are independently variable, under computer control, and provide considerable flexibility in optimizing beam intensity and resolution. The measurement range of the instruments extends from 0.0015 to 0.6 A−1 in scattering wavevector, corresponding to structure in materials from 10 A to nearly 4000 A. The design and characteristics of the instruments, and their modes of operation, are described, and data are presented which demonstrate their performance.
643 citations
TL;DR: In this paper, the authors presented models compatible with the Rietveld method for the anisotropic broadening of the diffracted peaks produced by crystallite size and microstrain.
Abstract: Models are presented which are compatible with the Rietveld method for the anisotropic broadening of the diffracted peaks produced by crystallite size and microstrain. The models, derived from general crystallographic considerations, are different for different Laue groups.
560 citations
TL;DR: BEARTEX is a general PC-based Windows software package for quantitative texture analysis that provides corrections for experimental pole figures, orientation distribution calculations from complete or incomplete pole figures for all crystal and sample symmetries down to triclinic, graphical display of data, polycrystal tensor property determinations and other operations.
Abstract: BEARTEX is a general PC-based Windows software package for quantitative texture analysis. The 30 programs that it contains provide corrections for experimental pole figures, orientation distribution calculations from complete or incomplete pole figures for all crystal and sample symmetries down to triclinic, graphical display of data, polycrystal tensor property determinations and various other operations.
298 citations
TL;DR: In this article, a general model for the axial divergence aberration function has been developed for a conventional X-ray powder diffractometer equipped with either incident-beam and/or diffracted-beam Soller slits.
Abstract: A general model for the axial divergence aberration function has been developed for a conventional X-ray powder diffractometer equipped with either incident-beam and/or diffracted-beam Soller slits. In this model, the axial divergence function is generated from the geometrical dimensions of the diffractometer system, which include the axial lengths of the X-ray source, sample and receiving slit as well as the angular apertures of the incident-beam and diffracted-beam Soller slits. The paper describes how the diffractometer design and the extent of the axial divergence influence the shape of the axial divergence aberration function. The model is developed in three stages starting with analytical profile shapes for a single-ray incident X-ray beam. The single-ray results are then generalized to represent a line X-ray source and a specimen with axial length. In the third stage, the effects of the Soller slits in the beam path are incorporated by considering them as angular selective filters in the incident and diffracted beams. A general discussion is presented on the axial divergence aberration function with 2θ for a range of standard instrument configurations. This work forms the basis of an accurate fundamental parameters approach to profile analysis and Rietveld refinement.
184 citations
TL;DR: LAUEGEN version 6.0 as discussed by the authors is a significantly developed version of the original LAUEGENS program, which now carries out the stages of processing Laue diffraction data up to and including the integration of spot intensities.
Abstract: LAUEGEN version 6.0 is a significantly developed version of the original LAUEGEN program [Campbell (1995a). J. Appl. Cryst. 28, 228–236]. It is an X-Windows-based program which now carries out the stages of processing Laue diffraction data up to and including the integration of spot intensities. The integration routines have also been incorporated in a stand-alone integration program INTLDM. The programs are distributed as part of the Daresbury Laboratory Laue Software Suite.
170 citations
TL;DR: Altomare et al. as discussed by the authors used the positivity of the Patterson function as prior information for the decomposition of a powder diffraction pattern and proposed an automatic procedure which, integrated with the Le Bail method, is able to provide estimates of |F| values that are often remarkably better than those obtained by application of standard techniques.
Abstract: The positivity of the Patterson function is used as prior information for the decomposition of a powder diffraction pattern An automatic procedure is described which, integrated with the Le Bail method, is able to provide estimates of |F| values that are often remarkably better than those obtained by application of standard techniques The procedure, implemented in a computer program, has been applied to a set of test structures (experimental data); the results show that, particularly for structures with some heavy atoms, the new estimates of the structure-factor moduli are far better (on average) than those obtained via the default application of EXTRA [Altomare et al (1995) J Appl Cryst 28, 842–846] 1 Symbols and abbreviations The papers by Altomare, Carrozzini et al (1996), Altomare, Foadi et al (1996) and Carrozzini et al (1997) are referred to as papers I, II and III, respectively
131 citations
TL;DR: An accurate model for axial divergence aberration function has been implemented within a fundamental-parameters fitting procedure for diffraction profiles from a conventional X-ray powder diffractometer.
Abstract: An accurate model for the axial divergence aberration function has been implemented within a fundamental-parameters fitting procedure for diffraction profiles from a conventional X-ray powder diffractometer. The aberration function for axial divergence is derived from the geometrical dimensions of the diffractometer in the axial plane and the angular apertures of any Soller slits in the beam path. This function is then convoluted with other instrument and specimen aberration functions to form the final profile shape. As this process only requires a modest amount of computational effort, it has been incorporated into both a fundamental-parameters profile-analysis fitting program and a Rietveld refinement program. In these programs, the refinable parameters for the axial divergence contribution to the profile shape are the axial X-ray source length, the sample length, the receiving-slit length and the aperture angles of the primary and secondary Soller slits. The procedure developed has been evaluated by fitting diffraction patterns collected using two different X-ray diffractometers and a variety of Soller slits and sample configurations to introduce varying degrees of axial divergence in the incident and diffracted beams. The reference materials used for this work included Y2O3, LaB6 (SRM 660), Cr2O3 (SRM 674a) and CeO2 (SRM 674a). In all cases, the refined instrumental parameters defining the axial divergence were in good agreement with the directly measured values. Rietveld refinement using the present axial divergence model resulted in Rwp values that are significantly lower than those based on currently available models for this aberration function.
120 citations
TL;DR: In this paper, an algorithm is developed to determine the space group and the transformation to the standard setting of a given crystal structure, where no restrictions are placed on the original description of the structure.
Abstract: An important part of the crystallographic description of crystal structures, whether they belong to synthesized compounds or have been generated by computer, is the assignment of the correct space group. Since this task often proves to be highly nontrivial, we have developed an algorithm which determines the space group and the transformation to the standard setting of a given crystal structure, where no restrictions are placed on the original description of the structure.
112 citations
TL;DR: In this paper, high-resolution X-ray diffractometry of nanocrystalline Ni foils was used to study the effect of diffraction order on the grain size, dislocation densities and twin boundaries.
Abstract: Electrodeposited nanocrystalline Ni foils were studied by high-resolution X-ray diffractometry. The full width at half-maximum and Fourier coefficients were found to vary rather anisotropically as a function of diffraction order. The modified Williamson–Hall plot and the modified Warren–Averbach analysis, developed recently by taking into account the dislocation contrast in peak broadening, have been applied to interpret this anisotropic behaviour in terms of grain size, dislocation densities and twin boundaries. The average grain size has been found to range between 50 and 12 nm, in good agreement with transmission electron microscopy observations. The average dislocation density has been found to be 4.9 (5) × 1015 m−2 and the dislocations are of the screw character.
110 citations
TL;DR: In this paper, simple methods for handling, mounting, storage and transportation of crystals at cryogenic temperatures are presented, which are easy to learn and have a number of technical and operational advantages over currently popular methods.
Abstract: Simple methods are presented for handling, mounting, storage and transportation of crystals at cryogenic temperatures. They are easy to learn and have a number of technical and operational advantages over currently popular methods. In particular, the temperature of the crystal throughout all manipulations is known; it is shown never to rise above that of the warmest component of the cryogenic system, typically the cold gas stream of the low-temperature apparatus. Crystals can be mounted and inspected in the home laboratory prior to transportation to a synchrotron, giving dramatic savings in experimental time and effort. Provided appropriate care is taken, crystals remain frost free throughout any number of mount–dismount cycles.
90 citations
TL;DR: In this paper, a method is developed that allows the computation of the single-crystal elastic constants for crystals of cubic symmetry from the diffraction elastic constants, which can be obtained by measuring the hkl-dependent lattice strain response to an applied stress.
Abstract: In this work a method is developed that allows the computation of the single-crystal elastic constants for crystals of cubic symmetry from the diffraction elastic constants. The diffraction elastic constants can be obtained by measuring the hkl-dependent lattice strain response to an applied stress. Because of their hkl dependence they represent, partially, the anisotropic nature of the single-crystal elastic constants. The computation of the single-crystal elastic constants is carried out by a least-squares refinement which fits the calculated diffraction elastic constants to the measured ones.
TL;DR: Wu et al. as mentioned in this paper applied the Rietveld refinement of the neutron powder diffraction profiles of deuterium-cycled LaNi5 and β-PdD0.66.
Abstract: In paper I [Wu et al. (1998). J. Appl. Cryst. 31, 356–362] an approach was developed to the problem of modelling dislocation-induced X-ray or neutron-diffraction-line broadening. This paper applies those findings to the Rietveld refinement of the neutron powder diffraction profiles of deuterium-cycled LaNi5 and β-PdD0.66. These interstitially modified materials exhibit, respectively, strong and weak anisotropic strain broadening. The broadening in LaNi5 is consistent with a dislocation slip system a/3 (\bar 2110) {0\bar 110}, in agreement with transmission electron microscopy studies. In PdD0.66 the model predicts a regular distribution of screw dislocations, which remains to be confirmed by other techniques.
TL;DR: In this paper, it was shown that the addition of an azimuthal detector rotation to a (2 + 2)-type diffractometer makes it fully equivalent to a z-axis type.
Abstract: It is shown that the addition of an azimuthal detector rotation to a (2 + 2)-type diffractometer makes it fully equivalent to a z-axis type. Using this equivalence, the geometric correction factors for this (2 + 3)-type diffractometer, which are needed to obtain structure factors from measured integrated intensities, are derived. The (2 + 3)-circle diffractometer combines the ideal resolution behaviour of a z-axis diffractometer with the mechanical simplicity and range of a (2 + 2)-circle diffractometer. For sample alignment, two additional degrees of freedom are needed, leading to a total of seven circles.
TL;DR: In this article, the transmission factor and peak shift for absorbing cylindrical specimens or crystals are given, which can be used over a range of μR useful in X-ray scattering experiments, typically μR < 10.
Abstract: Analytical expressions for the transmission factor and peak shift for absorbing cylindrical specimens or crystals are given. These expressions can be used over a range of μR useful in X-ray scattering experiments, typically μR < 10. Comparison with tabulated values of the transmission factor obtained by numerical calculation shows that the maximum difference occurs at θ = 45° and ranges from 1% at μR = 1 to 20% at μR= 20. The analytical expressions were tested on synchrotron data from rutile at four wavelengths (0.628, 1.100, 1.536 and 1.895 A) using the Rietveld method. Separate μR values were obtained at each wavelength, as well as a single μR value from a combined refinement of all wavelengths. The results confirmed that the absorption can be modelled using these expressions.
TL;DR: SYMMOL as discussed by the authors is a stand-alone program to find the maximum symmetry compatible with a given tolerance, where the input requirements are cell parameters, atomic coordinates and the tolerance, and the output is a set of symmetrized coordinates, the point-group label and the pointgroup elements.
Abstract: SYMMOL is a new stand-alone program to find the maximum symmetry compatible with a given tolerance. The input requirements are cell parameters, atomic coordinates and the tolerance. The output is a set of symmetrized coordinates, the point-group label and the point-group elements. Some parameters quantifying the deviation from the exact symmetry are also calculated.
TL;DR: In this paper, the authors investigated flash cooling of macromolecular crystals by measurement of the cooling rates achieved by different cryoprotectant solutions (glycerol, sucrose and polyethylene glycol) with different sample volumes (0.2-0.8mm3) and by different cooling agents (cold nitrogen or helium gas, liquid nitrogen and liquid propane).
Abstract: Flash cooling of macromolecular crystals was investigated by measurement of the cooling rates achieved by different cryoprotectant solutions (glycerol, sucrose and polyethylene glycol) with different sample volumes (0.2–0.8 mm3) and by different cooling agents (cold nitrogen or helium gas, liquid nitrogen and liquid propane). Samples were not cooled instantaneously; it usually took 0.5 to 1 s to reach the glass transition temperature and 1 to 2 s to reach the final temperature. The cooling rate varied from ~50 to 700 K s−1 depending on the volume of the sample, the cooling agent used and the temperature at the time of measurement. The cooling rate may affect the distribution of unit-cell dimensions, structural states and the average structure throughout the volume of the crystal, and hence its mosaic spread.
TL;DR: In this article, the radial scattering length density distribution of particles in strongly polydisperse samples and a parameter that quantifies the degree of polydispersity present in the system is defined.
Abstract: The internal structure of colloidal particles of spherical, rod-like or lamellar symmetry can be determined from small-angle scattering data by means of the well established `indirect Fourier transformation' technique followed by the numerical deconvolution of the pair distance distribution function The method works without any preliminary knowledge of the particles under investigation except a rough estimate of their maximal size However, its use has hitherto been restricted to monodisperse or very weakly polydisperse systems In this paper a new method is presented that is capable of determining the radial scattering length density distribution of particles in strongly polydisperse samples and provides a parameter that quantifies the degree of polydispersity present in the system The only additional information necessary is the function type which describes the size distribution of the particles under investigation in the best way possible However, the method is limited to polydisperse systems which may be characterized by only one linear size parameter In addition, it is now possible to represent the density profile by smooth spline functions, ie the technique is no longer restricted to step functions as basis functions The mathematical procedure is outlined in detail and numerical results of simulations as well as of experimental data from small-angle neutron scattering and small-angle X-ray scattering measurements of surfactant aggregates in water and water/oil systems are given
TL;DR: Lists of software presented and/or reviewed in the Journal of Applied Crystallography are available on the World Wide Web at the above address, together with information about the availability of the software where this is known.
Abstract: The category Computer Program Abstracts provides a rapid means of communicating up-to-date information concerning both new programs or systems and significant updates to existing ones. Following normal submission, a Computer Program Abstract will be reviewed by one or two members of the IUCr Commission on Crystallographic Computing. It should not exceed 500 words in length and should follow the standard format given on page 189 of the June 1985 issue of the Journal [J. AppL Cryst. (1985). 18, 189190] and on the World Wide Web at http://www.iucr. org/journals/jac/software/. Lists of software presented and/or reviewed in the Journal of Applied Crystallography are available on the World Wide Web at the above address, together with information about the availability of the software where this is known.
TL;DR: An open-flow helium cryostat for single-crystal X-ray diffraction experiments capable of reaching 14'K has been developed using off-the-shelf components as discussed by the authors.
Abstract: An open-flow helium cryostat for single-crystal X-ray diffraction experiments capable of reaching 14 K has been developed using off-the-shelf components. Solid/liquid air build-up is prevented using the transfer-line helium back-flow and a heated nozzle. The system has run for over 30 h with no frost build-up.In a recent experiment the system has run for over 60 h with no frost build-up. The effectiveness of the system has been demonstrated using test data for oxalic acid, terbium vanadate and dysprosium vanadate.
TL;DR: In this article, a combination of the early approaches of the Debye model for specific heat and the Gruneisen theory of the thermal expansion of solids is used to interpolate the thermal variation of lattice parameters.
Abstract: A combination of the early approaches of the Debye model for specific heat and the Gruneisen theory of the thermal expansion of solids is used to interpolate the thermal variation of lattice parameters. From this analysis, relevant parameters such as the Debye temperature and the value of lattice parameters extrapolated at 0 K are deduced. This method and its limitations are reported. A program has been developed which can be readily used with any computer. Practical applications are given as an illustration. The method is first applied to an ideal cubic compound Na2Ca3Al2F14, which we propose as a standard for thermal expansion calibrations. Then, magnetoelastic phenomena, which occur at very low temperature, are displayed in cubic rare earth garnets by comparison of experimental and extrapolated data. The model is extended to an anisotropic hexagonal system, where the values of lattice parameters extrapolated at 0 K are determined within 10−5. Debye temperatures are estimated within a relative error of 10% and are in good agreement with those calculated from specific heat data; they prove the relevance of this easy method.
TL;DR: In this paper, an electrochemical cell was constructed for in situ neutron diffraction studies of lithium-insertion/extraction processes in electrode materials for Li-ion batteries, using a Pyrex tube, gold plated on its inside, which functions as a current collector, and a central lithium rod, which serves as the negative electrode.
Abstract: An electrochemical cell has been constructed for in situ neutron diffraction studies of lithium-insertion/extraction processes in electrode materials for Li-ion batteries. Its key components are a Pyrex tube, gold plated on its inside, which functions as a current collector, and a central lithium rod, which serves as the negative electrode. The device is demonstrated here for a neutron diffraction study of lithium extraction from LiMn2O4: a mechanical Celgard© separator soaked in the electrolyte surrounds the lithium electrode. The LiMn2O4 powder, mixed with electrolyte, occupies the space between separator and current collector.
TL;DR: The distribution of substituting elements Mn, Al and Co on the two latter sites was determined from synchrotron powder diffraction experiments by using the diffraction contrast produced by anomalous dispersion effects as discussed by the authors.
Abstract: The battery electrode material LaNi 3.55 Mn 0.4 Al 0.3 Co 0.75 is a substitutional derivative of LaNi 5 in which La occupies site 1(a), and Ni, Mn, Al and Co sites 2(c) and 3(g) of space group P6/mmm. The distribution of the substituting elements Mn, Al and Co on the two latter sites was determined from synchrotron powder diffraction experiments by using the diffraction contrast produced by anomalous dispersion effects. The results of joint Rietveld refinement on three patterns collected close to the K edges of Ni, Co and Mn and on one pattern collected away from these edges showed that Al and Mn occupy almost exclusively site 3(g), while Co is distributed over sites 2(c) and 3(g) with a slight preference for site 3(g).
TL;DR: In this paper, a new procedure to perform quantitative analysis without any internal standard has been developed within Rietveld analysis, which permits solution of the problem due to the presence of an amorphous phase when its chemical composition or the global sample composition is known.
Abstract: A new procedure to perform quantitative analysis without any internal standard has been developed within Rietveld analysis. This new approach permits solution of the problem due to the presence of an amorphous phase when its chemical composition or the global sample composition is known. The equations developed, based on a study of the global intensities diffracted by each phase in reciprocal space, reduce to the well known formula of Hill & Howard [J. Appl. Cryst. (1987), 20, 467–474] when applied to completely crystalline materials.
TL;DR: Several different types of X-ray mirrors suitable for use with a microfocus Xray tube and intended principally for single-crystal diffractometry are discussed and compared in this article.
Abstract: Several different types of X-ray mirrors suitable for use with a microfocus X-ray tube and intended principally for single-crystal diffractometry are discussed and compared. The construction and performance of ellipsoidal mirrors made by an electroforming method are described in detail. The intensity at the mirror focus is comparable with the flux obtained with conventional arrangements with X-ray tubes dissipating more than 100 times the power of the microfocus tube.
TL;DR: In this article, the anisotropy of the intermolecular interactions in the low-temperature ordered phases of three chemically and structurally related compounds [neopentyl glycol, (CH3)2C(CH2OH)2, pivalic acid, CH3)3C(COOH), and neopentylon alcohol, all of which display an orientationally disordered high temperature phase, has been shown by means of the isobaric thermalexpansion tensor.
Abstract: The anisotropy of the intermolecular interactions in the low-temperature ordered phases of three chemically and structurally related compounds [neopentylglycol, (CH3)2C(CH2OH)2, pivalic acid, (CH3)3C(COOH), and neopentylalcohol, (CH3)3C(CH2OH)], all of which display an orientationally disordered high-temperature phase, has been shown by means of the isobaric thermal-expansion tensor. The variation of the directions of the principal components of the thermal-expansion tensor as a function of temperature, as well as the variation of its principal coefficients, is evidence of the large differences in the intermolecular interactions for each compound; or, more precisely, between the strong intermolecular hydrogen bonds and the weak van der Waals interactions. In addition, the differences in the hydrogen-bonding schemes expected a priori from the molecular structures of the studied compounds have been enhanced. Finally, the volume expansivity as well as the packing coefficient have been analysed in the orientationally disordered high-temperature phase of each of the three compounds.Part of this work was presented at the EPDIC 1997 meeting (5th European Powder Diffraction Conference).
TL;DR: In this paper, two types of multilayers have been installed in the standard monochromator tank of beamline 4-2 at the Stanford Synchrotron Radiation Laboratory; the multilayer beam has been characterized for studies of small-angle X-ray scattering/diffraction from biological materials.
Abstract: Many biological applications of small-angle X-ray scattering, in particular time-resolved studies, are often limited by the flux incident on the sample due to the smaller scattering cross section of biological specimens. The wider-energy bandpass of a monochromator that consists of a pair of synthetic multilayer microstructures can, in principle, provide a flux two orders of magnitude higher than that of an Si(111) double-crystal monochromator. Two types of multilayers have been installed in the standard monochromator tank of beamline 4-2 at the Stanford Synchrotron Radiation Laboratory; the multilayer beam has been characterized for studies of small-angle X-ray scattering/diffraction from biological materials. Reflectivity and topography measurements indicate that the multilayers are quite adequate for these applications and a pair of Mo/B4C multilayers provided a 10–30 times increase in flux, compared with the flux level obtained with an Si(111) double-crystal monochromator. The increased flux level is very useful in time-resolved scattering studies as well as for recording weak scattering at higher angles. Having carried out many solution scattering and fiber diffraction experiments, we conclude that the use of multilayer does not result in significant broadening of diffraction peaks nor does it have appreciable effects on small-angle resolution. No significant increase in background is observed.
TL;DR: In this paper, an X-ray diffraction peak-decomposition procedure using a Pearson type VII function is described and implemented in a Microsoft Excel spreadsheet to aid a beginning student of X ray diffraction.
Abstract: An X-ray diffraction peak-decomposition procedure using a Pearson type VII function is described and implemented in a Microsoft Excel spreadsheet to aid a beginning student of X-ray diffraction.
TL;DR: In this article, a theory is presented to describe the anomalous peak shift encountered in neutron diffraction residual stress measurements as the specimen is translated into and out of the sampling volume, defined by a pair of masking slits inserted before and after the specimen.
Abstract: A theory is presented to describe the anomalous peak shift encountered in neutron diffraction residual stress measurements as the specimen is translated into and out of the sampling volume, which is defined by a pair of masking slits inserted before and after the specimen. Analytical formulae for the anomalous peak shift were obtained for both position-sensitive-detector-based diffractometers and conventional scanning diffractometers. The results indicate that the observed peak shift is a complex function of many variables, including the in-pile collimation, slit widths, slit-to-axis distances, mosaic spread of the monochromating crystal, and mismatch in lattice spacing between the sample and the monochromator. Calculations based on the derived analytical formulae are in good agreement with experimental observations. It is shown that by the choice of appropriate experimental conditions, this peak shift anomaly can be suppressed or, in some cases, eliminated altogether.
TL;DR: The theory of dislocation-induced X-ray or neutron diffraction line broadening developed by Krivoglaz et al. and Wilkens has been adapted for Rietveld refinement of the powder diffraction profile by fitting a Voigt function to each peak as mentioned in this paper.
Abstract: The theory of dislocation-induced X-ray or neutron diffraction line broadening developed by Krivoglaz et al. and Wilkens has been adapted for Rietveld refinement of the powder diffraction profile by fitting a Voigt function to each peak. Information on both the type of slip system and the density of dislocations in the crystallites may then be found by evaluating the shape parameter and the index-dependent breadth of the Voigt function.
TL;DR: The BLANC program suite is a set of programs which can be used for macromolecular structure determination by X-ray crystallography to provide experienced crystallographers and students with a number of simple tools, at the same time allowing the building and testing of new algorithms.
Abstract: The BLANC program suite is a set of programs which can be used for macromolecular structure determination by X-ray crystallography. The suite is designed to provide experienced crystallographers and students with a number of simple tools, at the same time allowing the building and testing of new algorithms. Besides a set of small programs, the BLANC system introduces so-called superprograms which represent larger programs composed of several smaller ones. They utilize the so-called black-box principle, requiring minimum preparation or intervention from a user. The programs are written in standard Fortran77 and communicate with each other by the standard BLANC data files. The package has been ported to all the major platforms such as UNIX, VMS and DOS. A current version of the suite is distributed by anonymous ftp.