bluuebunny/crossref_metadata_2025 huggingface dataset:
The determination of a stress-free lattice parameter within a stressed material using elastic anisotropy
Authors: Daymond M. R. , Johnson M. W.
Published: July 2002

A method for the calculation of a stress-free lattice parameter from the analysis of diffraction data from stressed material is discussed, utilizing the elastic anisotropy of the material. The technique is demonstrated using data obtained during a uniaxial tension test on untextured austenitic (face-centred cubic) steel. The uncertainty in the calculated lattice parameter for various choices of number of diffraction peaks and different number of stress levels available for the calculation is considered. It is shown that when all the data are within the elastic regime, an accurate evaluation of the reference lattice parameter can be made. When some data are in the plastic regime, a more limited evaluation is possible. The use of plots of lattice parameter against Γhkl[= (h2k2+h2l2+k2l2)/(h2+k2+l2)2] as a method for monitoring plasticity as well as freedom from deviatoric stress is demonstrated.
Orientation distribution analysis in deformed grains
Authors: Glez Jean Christophe , Driver Julian
Published: July 2002

Some improvements are proposed for the statistical analysis of orientation data within individual grains, in particular by allowing for crystallographic symmetries. A method based on quaternions is then presented to characterize orientation spreads including anisotropic effects. Based on this approach, some analyses of disorientation distributions (orientation distribution functions, disorientation noise and the description of sub-boundary disorientation) are reconsidered. The analysis is illustrated by a practical application to the microtextures of a hot deformed aluminium alloy crystal.
Computer simulation approach to reliability and accuracy in EXAFS structural determinations
Authors: Ghigna Paolo , Di Muri Melissa , Spinolo Giorgio
Published: July 2002

The frequency distribution of different parameters of an EXAFS (extended X-ray absorption fine structure) spectrum can be directly sampled by analysing a population of simulated spectra produced by adding computer-generated noise to a reference pattern. The procedure gives statistical estimators of the parameters obtained with different data processing strategies in order to test the performance of a strategy, to evaluate the bias introduced by random noise, and to clarify the amount of information actually contained in an experimental spectrum. Results are given for the two simple local structures formed by an Ag atom surrounded by two O atoms or by six I atoms, respectively.
Neutron imaging with bent perfect crystals. I. Imaging conditions
Authors: Stoica A. D. , Popovici M. , Hubbard C. R.
Published: July 2002

Neutron imaging with bent crystals is considered in the linear approximation of neutron optics. A matrix formalism analogous to that of conventional lens optics is developed. Various imaging conditions are discussed in relation to the crystal deformation type and to possible applications in neutron scattering. All focusing conditions known previously are included as particular cases. Arrangements combining high resolution in imaging with high resolution in scattering are examined. The concept of Bragg mirrors as devices for non-dispersive imaging is introduced. The most powerful application appears to be that of Bragg mirrors combined with the time-of-flight method. Imaging with a thick packet of silicon wafers at the spatial resolution of a single thin wafer is demonstrated.
Neutron Laue diffraction experiments on a large unit cell: concanavalin A complexed with methyl-α-D-glucopyranoside
Authors: Kalb (Gilboa) A. Joseph , Myles Dean A. A. , Habash Jarjis , Raftery James , Helliwell John R.
Published: July 2002

A neutron Laue diffraction experiment on a complex of concanavalin A with methyl-α-D-glucopyranoside extensively soaked in D2O (space groupI213,a= 167.8 Å), which resulted in 3.5 Å diffraction data, is described. In a programme of structural studies of crystalline saccharide complexes of concanavalin A, the unit cell of the cubicI213 complex of concanavalin A with methyl-α-D-glucopyranoside is one of the largest. With its cell edge of 167.8 Å and its asymmetric unit of molecular weight 50 kDa, it represents a challenge for current neutron diffraction technology. The size of the crystal used in this experiment, although large (4 × 3 × 2 mm), was not the largest ever produced for this complex. The degree of spatial overlapping observed in the Laue experiment, however, suggests that use of larger crystals would be a disadvantage. On the basis of these observations, several technical improvements for macromolecular neutron crystallography are suggested.
Twin structure in Yb:YAl3(BO3)4crystal
Authors: Zhao Shanrong , Wang Jiyang , Sun Daliang , Hu Xiaobo , Liu Hong
Published: July 2002

Twin structure is a defect in the self-frequency-doubling laser crystal material Yb:YAl3(BO3)4(YbYAB). Based on atomic force microscopy (AFM) observations, a check-like twin structure in YbYAB crystals is found. This kind of twin structure has three twin composition planes: (10\bar{1}1), (0\bar{1}11) and (\bar{1}101). These three twin composition planes result in a series of twin boundaries in two directions, forming a check-like pattern on each face. From the orientation of the chemical etching pits on each side of the twin boundary, it is found that the twin element is `twin plane ⊥yaxis', alternatively indicated as `twin plane {11\bar{2}0}'. The two single crystals composing the twin are a right form and a left form. This kind of twin is similar to the Brazil twin found in quartz.
A high-performance neutron diffractometer for biological crystallography (BIX-3)
Authors: Tanaka I. , Kurihara K. , Chatake T. , Niimura N.
Published: July 2002

A high-performance neutron diffractometer for biological crystallography (BIX-3) has been constructed at JRR-3M in the Japan Atomic Energy Research Institute (JAERI) in order to determine the hydrogen-atom positions in biological macromolecules. It uses several recent technical innovations, such as a neutron imaging plate and an elastically bent silicon monochromator developed by the authors. These have made it possible to realise a compact vertical arrangement of the diffractometer. Diffraction data have been collected from the proteins rubredoxin and myoglobin in about one month, to a resolution of 1.5 Å. The data were good enough to identify the hydrogen atoms with high accuracy. By adopting a crystal-step scan method for measuring Bragg diffraction intensities, the signal-to-noise ratio was much better than that of the Laue method. This shows that BIX-3 is one of the best-performing machines for neutron protein crystallography in the world today.
Least-squares structure refinement based on profile analysis of powder film intensity data measured on an automatic microdensitometer
Authors: Malmros G. , Thomas J. O.
Published: July 2002

The Rietveld profile-analysis refinement procedure [Rietveld, J. Appl. Cryst. (1969), 2, 65–71] is here applied to microdensitometer-measured Guinier–Hägg X-ray powder film data (strictly monochromated Cu Kα1 radiation). Various functions for describing the profile form of an individual reflection are tested; the most satisfactory fit is obtained for a modified and asymmetrized Lorentz function. The crystal structure of α-Bi2O3 has been refined and no significant deviations are observed from the results of an earlier single-crystal study [Malmros, Acta Chem. Scand. (1970), 24, 384–396]. An RF
value [roughly comparable with the conventional R(F) value of single-crystal work] of 0.068 was obtained, with standard deviations on the refined positional parameters on average three times larger than those obtained from the single-crystal study. Further examples of the refinement technique are also given which illustrate both its reproducibility and range of application.
Final Report of the International Project for the Calibration of Absolute Intensities in Small-Angle X-ray Scattering
Authors:   Commission on Crystallographic Apparatus
Published: July 2002

An international intercomparison project was performed to test the reproducibility and the comparative accuracy of the various absolute intensity calibration techniques in current use in small-angle X-ray scattering with the participation of fifteen investigators from eight different laboratories in six countries. In the project, the absolute differential X-ray scattering cross sections of standard samples of glassy carbon and polystyrene were calibrated using five different calibration techniques and two different X-ray wavelengths. The results have been intercompared with a variety of statistical techniques. It is concluded that angularly dependent errors associated with determining the zero of angle, dead-time corrections, collimation corrections, and insufficiently close data point spacing are more important in accounting for discrepancies between laboratories than are differences in the absolute intensity calibration methods themselves.
Determination of mean and anomalous absorption parameters in potassium feldspars
Authors: Carletti E. , Martinelli G. , Sacerdoti M.
Published: July 2002

Values of mean and anomalous absorption parameters in potassium feldspar, for operating diffraction vector g = 20{\bar 1} at an accelerating voltage of 100 kV, are reported. These values are ξg/ξ'0 = 0.35±0.05 and ξg/ξ'g = 0.27±0.07.
Crystal data for synthetic K2Mg(CO3)2
Authors: Simons B. , Sharma S. K.
Published: July 2002

Dipotassium magnesium carbonate, K2Mg(CO3)2, was crystallized at 573 K and 0.025 GPa in a cold-seal hydrothermal vessel with CO2 as pressure-transmitting medium. Cell parameters [a = 5.1543(5); c = 17.307(2) Å] were determined by the powder diffraction method. V = 398.2(2) Å3, Z = 3, Dx
= 2.784 g cm−3, Cu Kα. Crystalline K2Mg(CO3)2 is isostructural either to buetschliite, K2Ca(CO3)2, with space group R{\bar 3}2/m or eitelite, Na2Mg(CO3)2, with space group R{\bar 3}. K2Mg(CO3)2 may be present as a significant component in natural buetschliite as well as in eitelite, if not present as a natural mineral itself. The JCPDS Diffraction File No. for K2Mg(CO3)2 is 33-1495.
An interactive program for the interpretation and simulation of Laue patterns
Authors: Laughier J. , Filhol A.
Published: July 2002

An interactive Fortran IV program is described which allows the orientation matrix of a crystal to be determined and refined without ambiguity, from a transmission or reflection Laue photograph. The indexing method is that of Riquet & Bonnet [J. Appl. Cryst. (1979), 12, 39–41] applied to a small number of data (less than ten) and adapted for interactive use. The verification of calculated solutions or even the trial-and-error search for the right solution is made by means of the visual comparison between the experimental diagram and simulated patterns displayed on a graphic screen. In addition, the rotation angles for setting the crystal to any new orientation may be computed.
A synchrotron radiation study of modulated [(CH3)4N]2ZnCl4 crystals
Authors: El-Korashy A. , Roberts K. J. , Scheffen-Lauenroth T. , Dam B.
Published: July 2002

Synchrotron radiation Laue diffraction is applied to an investigation of the modulated phases of nearly perfect single crystals of tetramethylammonium tetrachlorozincate. Well resolved satellite reflections, consistent with the superspace group Pcmn (1s1), are observed in cooling from the paraelectric room temperature phase I to the incommensurate pseudoquintupled phase II. The transformation to the tripled phase IV is accompanied by the generation of much lattice disorder. The high lattice perfection of the samples is illustrated by the observation of anomalous-dispersion contrast in X-ray topographs.
Improved calibtration curve for the Sm2+:BaFCl pressure sensor
Authors: Comodi P. , Zanazzi P. F.
Published: July 2002

The pressure sensor proposed by Shen, Gregorian & Holzapfel [High Press. Res. (1991), 7, 73–75], based on the pressure-dependent shift of the luminescence line due to the 5
D
0−7
F
0 electronic transition of Sm2+ in a matrix of BaFCl, has been tested in a diamond-anvil cell and calibrated against the R
1−R
2 doublet shift of ruby and the known NaCl equation of state, in the pressure range between 0.0001 and 4.3 GPa. The parabolic dependence of the shift from the pressure can be approximated by the equation Δ(nm) = 1.46P − 0.047P
2, where the shift, Δ, is in nm and the pressure, P, in GPa. The estimated error in the pressure measurements is 5%. The Sm2+: BaFCl luminescence sensor can be advantageously used in the low to moderate pressure range (0.0001–5 GPa or more).
Refinement of hexamethylenetetramine based on diffractometer and imaging-plate data
Authors: Grochowski J. , Serda P. , Wilson K. S. , Dauter Z.
Published: July 2002

Two data sets were collected on single crystals of hexamethylenetetramine (urotropin) using a four-circle diffractometer with Cu Kα radiation and an imaging-plate two-dimensional detector with Mo Kα source using the rotation method. Both data sets extend to the same limit of sin θ/λ = 0.62 Å−1, corresponding to a resolution of 0.81 Å. Different processing protocols were employed for the two sets of data. Structure refinements carried out separately with each data set led to equivalent results of comparable accuracy. The imaging-plate scanner was able to provide X-ray data of high quality in a significantly shorter time than the diffractometer.
The Performance of the Australian Powder Diffractometer at the Photon Factory, Japan
Authors: Sabine T. M. , Kennedy B. J. , Garrett R. F. , Foran G. J. , Cookson D. J.
Published: July 2002

Australian scientists have built and installed an X-ray powder diffractometer of an unusual design on the Australian beamline at the Photon Factory synchrotron-radiation facility within the National Laboratory for High Energy Physics (KEK), Tsukuba, Japan. The diffractometer is a Debye–Scherrer camera of 0.573 m radius. The place of the cylindrical film in a conventional camera of this type is taken by image plates. To minimize scattering and absorption by air, the instrument can be evacuated. The instrument is now in operation and has been tested with a specimen of the rutile phase of TiO2. This material has been thoroughly studied previously and it has been demonstrated that time-of-flight neutron powder diffraction, conventional neutron powder diffraction, single-crystal neutron diffraction and single-crystal X-ray diffraction lead to a consistent set of values for the anisotropic thermal parameters and the one positional parameter. The powder specimen of rutile for use at KEK was diluted with gum tragacanth and inserted into a glass capillary of 0.5 mm diameter. The beam from the synchrotron is incident on a silicon (111) channel-cut monochromator. Data were collected to ±165°2θ at wavelengths of 0.62, 1.10, 1.54 and 1.90 Å. The exposure time for each data set was 10 min. The resolution of the instrument agrees with theoretical prediction and is such that the full width at half-maximum of a reflection varies from 0.04° at 20°2θ to 0.2° at 160°2θ for a wavelength of 1.54 Å. The intensity from a 10 min exposure is more than sufficient for Rietveld refinement (R
exp < 1%).
Improvements in the Quantitative Evaluation of Three-Dimensional Texture. I. The Nature of the Information Obtained from Pole Figures
Authors: Yu L.-G. , Guo H. , Hendrix B. C. , Xu K.-W. , He J.-W.
Published: July 2002

The sources of indefiniteness in the orientation-distribution-function (ODF) description of crystalline texture are shown to result from the integral nature of the pole-figure measurement. An equipartition-area theorem is proved and it is shown that current methods use too few pole figures, which are measured to an unnecessarily high angular resolution. The experimental resolution is considered and the number of pole figures needed for ODF analysis is calculated as a function of the required ODF resolution.
Quantitative Texture Analysis from X-ray Diffraction Spectra
Authors: Wang Y. D. , Zuo L. , Liang Z. D. , Laruelle C. , Vadon A. , Heizmann J. J.
Published: July 2002

A method to obtain the orientation distribution function (ODF) of a polycrystalline material directly from X-ray diffraction spectra is presented. It uses the maximum-texture-entropy assumption to reduce the diffraction data needed for the ODF analysis. The validity of this new method is illustrated through two model examples.
Accurate Site Occupancies for Light Atoms from Powder X-ray Data? Oxygen/Vacancy Ordering in 6H-BaFe0.67Ti0.33O3−δ(δ = 0.08 and 0.32)
Authors: Grey Ian E. , Cranswick Lachlan M. D. , Li Christina
Published: July 2002

The accuracy of site-occupancy results for light atoms obtained from powder X-ray data refinements was investigated for the case of the oxygen-deficient 6H-hexagonal ferrites, BaFe0.67Ti0.33O3−δ(δ = 0.08 and 0.32). Comparative Rietveld refinements were made for powder neutron diffraction (PND) data and for multiple powder X-ray diffraction (PXRD) data-sets collected on separate preparations using both fixed step counting time (FCT) and variable step counting time (VCT) strategies. The oxygen vacancies were found to be confined to the O1 sites which form the shared face ofM2O9binuclear octahedral units. The O1 site-occupation fractions (SOF) obtained from the averaged FCT refinements, the averaged VCT refinements and the PND refinements were all within three e.s.d.'s of the values calculated from chemical analyses. The deleterious effect of high peak overlap on the accuracy of the SOF results was demonstrated. The separate application of PND and PXRD refinements gave information on relative local displacements of titanium and iron atoms in theM2O9dimers due to oxygen removal. Anisotropic displacement refinements and Fourier maps were used to gain further information on local atomic displacements associated with anion vacancies. The results are consistent with the formation of Fe2O7corner-shared tetrahedral dimers, as well as (Fe, Ti)2O9and (Fe, Ti)2O8groups, as the oxygen vacancies undergo local ordering in response to the local distribution of the two types of metal atoms. The use of a low-angle truncation procedure in the PXRD refinements was shown to be effective in removing the influence of atomic valence states on the site-occupation refinement.
A neutron diffraction cell for studying lithium-insertion processes in electrode materials
Authors: Bergstöm Ö. , Andersson A. M. , Edström K. , Gustafsson T.
Published: July 2002

An electrochemical cell has been constructed forin situneutron 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 LiMn2O4powder, mixed with electrolyte, occupies the space between separator and current collector.
SIR97: a new tool for crystal structure determination and refinement
Authors: Altomare Angela , Burla Maria Cristina , Camalli Mercedes , Cascarano Giovanni Luca , Giacovazzo Carmelo , Guagliardi Antonietta , Moliterni Anna Grazia Giuseppina , Polidori Giampiero , Spagna Riccardo
Published: July 2002

SIR97 is the integration of two programs,SIR92 andCAOS, the first devoted to the solution of crystal structures by direct methods, the second to refinementvialeast-squares–Fourier procedures. Several new features have been introduced inSIR97 with respect to the previous version,SIR92: greater automatization, increased efficiency of the direct methods section, and a powerful graphics interface. The program also provides publication tables and CIF files.
A hinge goniometer head
Authors: Katrusiak Andrzej
Published: July 2002

A stable diffractometer goniometer head for mounting crystals enclosed in high-pressure cells, cryostats furnaces, or other heavy crystal-environment attachments, has been designed. Its construction is based on a hinge, thus it is called a hinge goniometer head, and allows adjustments in three directions. Under a load of 9.8 N perpendicular to the φ axis, the goniometer head yields less than 5 µm.
UMWEG-98: a program for calculation and graphical representation of multiple diffraction patterns
Authors: Rossmanith Elisabeth
Published: July 2002

UMWEG-XX was designed as a software tool for taking into account multiple diffraction effects in many areas of crystal structure analysis. TheUMWEG-98 version of the program makes possible the calculation and graphical representation of multiple diffraction patterns (Renninger scans, multiple diffraction peak location plots) of forbidden and weak reflections and can be applied to synchrotron as well asKα1,2radiation. The program is written in standard Fortran90 and may be ported to Unix platforms, such as HP, Sun, Silicon Graphics and IBM, as well as to DOS and Windows platforms. The program runs without any commercial subroutine libraries. Consequently it can be used without licence problems. A current version is distributedviahttp://www.rrz.uni-hamburg.de/mpi/rossmanith. Executable (binary) files are available free of charge for academic use only.
Ab initiostructure determination of sulfathiazole polymorph V from synchrotron X-ray powder diffraction data
Authors: Chan Fung Choy , Anwar Jamshed , Cernik Robert , Barnes Paul , Wilson Rory McHardy
Published: July 2002

The crystal structure of sulfathiazole [4-amino-N-(2,3-dihydro-2-thiazolylidene)benzenesulfonamide, C9H9N3O2S] polymorphic form V has been determined from high-resolution synchrotron X-ray powder diffraction data. The structure is monoclinic, space groupP21/n,Z= 8, with two molecules in the asymmetric unit. The unit-cell dimensions area= 14.3296 (3),b= 15.2733 (2),c= 10.4428 (2) Å and β = 91.052 (1)° with cell volumeV= 2285.13 (8) Å3. The structure has been solved by direct methods without recourse to any computational modelling techniques for generating possible structures. The unrefined structure obtained from direct methods gave anRwpvalue of 36.5%. Refinement of atomic and displacement parameters yielded a finalRwpof 12.54% (Rp= 9.37%). The conformations of the two molecules in the asymmetric unit are nearly identical and very similar to that found in other forms of sulfathiazole. The molecular packing is characterized by molecular sheets lying perpendicular to theaaxis. Each sheet is two molecules thick, being integrated by hydrogen bonding. With 16 non-H atoms in the molecule and two molecules in the asymmetric unit, this structure represents a further advance in terms of the complexity of an organic structure solved from X-ray powder diffraction data.
Direct analysis of small-angle smeared intensity tails. II. Applications
Authors: Ciccariello Salvino , Sobry Roger
Published: July 2002

The tails of the smeared small-angle X-ray intensities, scattered by two triblock copolymers of poly(methyl methacrylate-b-butadiene-b-methyl methacrylate) are analysed by using the smeared leading terms of the intensity asymptotic expansion. In this way, it is possible to determine the particle shape and size as well as the area and the Kirste–Porod curvosity of the sample interfaces.
XAct: a program for construction, automated setup and bookkeeping of crystallization experiments
Authors: Brodersen D. E. , Jenner L. B. , Andersen G. R. , Nyborg J.
Published: July 2002

XActis a stand-alone application for Microsoft Windows™ 95/98 that, through a spreadsheet-like interface, allows the user to construct crystallization experiments in a highly flexible way. The object-oriented data structure helps to maintain an overview of many experiments and provides reusable stock and tray objects. The program features a hardware interface that enables automatic setup of experiments by use of a Gilson 222 autosampler (Gilson Inc., Wisconsin, USA). Additionally, experimental information can be imported from external spreadsheets and World Wide Web applications.
Analysis of weighting schemes
Authors: Spagna Riccardo , Camalli Mercedes
Published: July 2002

Eighteen weighting schemes were implemented, together with two procedures of weighting modification to lessen the effects of outliers. Thirteen known structures were refined using the available weighting functions, obtaining for each structure many different minima. The solutions having the lowestRindex do not give the best distributions of 〈wΔ2〉 against |F| and resolution. Possibly the accepted model could be one which gives the `best' estimates of the standard uncertainties rather than the lowestR.
Variation of phase lengths in wet and dry nylon 6 using SAXS data
Authors: Annadurai V. , Gopalakrishna urs R. , Somashekar R.
Published: July 2002

Hosemann's formula for X-ray scattering by linear paracrystal is put into a form suitable for numerical analysis using exponential distribution function for the lengths of two phases namely amorphous and crystalline. A stepping refinement procedure was used to obtain the paracrystalline parameters so that the discrepancy between the calculated and the experimental intensity was minimised. It is observed that there are significant changes in distribution of phases lengths in wet and dry nylon 6 samples.
Micro-focus X-ray scanning on layers of smectic superstructures
Authors: Gurke I. , Wutz C. , Gieseler D. , Janssens B. , Heidelbach F. , Riekel C. , Kricheldorf H.R.
Published: July 2002

The investigated main chain poly(ester imide)s (PEI) form smectic layers with different superstructures due to a nanophase separation of the rigid mesogens and flexible spacers, which give rise to X-ray reflections in the small angle region (2θ =1-6°; d=2-8 nm). The novel micro-focus X-ray scanning technique at the beamline ID13 (ESRF, France) enables a visualisation of the local orientation of smectic layers in different morphologies such as bâtonnets and spherulites. Previous investigations of the spherulites formed from PEI by optical and electron microscopy, and SAXS indicated a negative birefringence and a lamellar internal structure. The particular feature of these spherulites is their internal smectic layer structure. The present X-ray scanning experiments reveal a radial arrangement of the smectic layers within the spherulite. Furthermore, the micro-beam scanning experiments provide maps of the molecular orientation in different smectic LC-textures. The internal SC-order of a single bâtonnet (10 * 90 µm) could be examined.
Structure of weakly bonded PPG-silica nanocomposites
Authors: Chaker J.A. , Dahmouche K. , Craievich A.F. , Santilli C.V. , Pulcinelli S.H.
Published: July 2002

The structure of silica-polypropyleneglycol (PPG) nanocomposites with weak physical bonds between the organic (PPG) and inorganic (silica) phase, prepared by the sol-gel process, was investigated by small angle X-ray scattering (SAXS). These nanocomposite materials are transparent, flexible, have good chemical stability and exhibit high ionic conductivity when doped with lithium salt. Their structure was studied as a function of silica weight fraction x (0.06 ≤ x ≤ 0.29) and [O]/[Li] ratio (oxygens being of ether-type). The shape of the experimental SAXS curves agrees with that expected for scattering intensity produced by fractal aggregates sized between 30 and 90Å. This result suggests that the structure of the studied hybrids consists of silica fractal aggregates embedded in a matrix of PPG. The correlation length of the fractal aggregates decreases and the fractal dimension increases for increasing silica content. The variations in structural parameters for increasing Li+ doping indicate that lithium ions favor the growth of fractal silica aggregates without modifying their internal structure and promote the densification of the oligomeric PPG matrix.
Synchrotron studies of polymers at DND-CAT
Authors: Londono J.D. , Davidson R.V. , Brill D. , Roach D.H. , Leach R.A. , Quintana J.P.
Published: July 2002

Two examples are presented that illustrate the capabilities of DND-CAT instrumentation for the study of the effects of processing on polymers. Firstly, a thermoplastic elastomer, Hytrel®, was stretched while 2-D data were collected simultaneously. The Hytrel® data show that the yield point of the stress-strain curve is associated with the sudden appearance of a four-point pattern. At higher deformations, strain-induced crystallization and the destruction of the hard segment domains lead to a substantial decrease of the contrast as monitored by the SAXS invariant. Prior to breakage, the extent and intensity of an equatorial streak develops as the material fibrillates. Secondly, SAXS and WAXS data were collected from quenched and annealed Poly(ethylene terephthalate) (PET) samples mounted on a DSC cell, to characterize the pre-melting shoulder in this material. Results show that substantial melting and re-crystallization occurs within the range of this shoulder in the quenched sample.
Double monochromator with three pairs of crystals for a SANS machine: Monte Carlo simulation studies
Authors: Aswal V. K. , Joshi J. V. , Goyal P. S. , Pimpale Ashok V.
Published: July 2002

Small-angle neutron scattering (SANS) diffraction experiments involve scattering of a monochromatic beam of neutrons and measuring the angular distribution of the scattered neutrons. Unlike conventional diffraction experiments, the scattering angles involved in a SANS experiment are small. In view of the small angles involved, the uncertainties in the angles are usually quite large (∼10%) and thus one uses a comparable wavelength resolution (Δλ/λ) for the incident neutrons to optimize the intensity and the resolution. That is, while conventional diffractometers use Δλ/λ ≃ 2%, SANS machines require Δλ/λ ≃ 10%. This paper presents the results of Monte Carlo simulations of a double-crystal monochromator system in which the wavelength resolution is spoiled by replacing the individual crystals of the double monochromator by three slightly misaligned pairs of crystals. It is seen that for a misalignment of 1° and a mosaic spread of 0.8° for all the crystals, three wavelengths selected by monochromatization are well resolved, with a width of 0.05 Å around each wavelength, thus giving a total wavelength spread of about 5%. Based on the above, a monochromator system has been built and tested. The measured wavelength distribution is in reasonable agreement with the calculated one.
Fractal approach in petrology: combining ultra small angle, small angle and intermediate angle neutron scattering
Authors: Triolo F. , Triolo A. , Agamalian M.M. , Lin J.-S. , Heenan R.K. , Lucido G. , Triolo R.
Published: July 2002

Ultra small angle neutron scattering (USANS) instruments have recently covered the gap between the size resolution available with conventional intermediate angle neutron scattering and small angle neutron scattering (SANS) instruments on one side and optical microscopy on the other side. New fields of investigations are now open and important areas of material science (ceramics, glass fibers, natural materials) and fundamental physics (phase transition, phase separation and critical phenomena) can be studied in bulk samples with an accuracy previously unobtainable owing to a combination of favourable features of the neutron-matter interaction: high penetrability of neutrons, even cold neutrons, ability to easily manipulate local scattering amplitudes by means of isotopic substitution methods, small absorption for most nuclei and hardly any radiation damage. In particular, neutrons see rocks as two-phase systems, and therefore the data analysis is enormously simplified. Rocks showing fractal behavior in over two decades of momentum transfer and seven orders of magnitude of intensity are examined and fractal parameters are extracted from the combined USANS, SANS and intermediate angle neutron scattering curves.
Schemes to determine the crystal potential under dynamical conditions using voltage variation
Authors: Rez Peter
Published: July 2002

Charge densities and crystal structures can be determined routinely from X-ray diffraction as X-ray scattering is relatively weak and single scattering can be assumed. The strong dynamical diffraction of high-energy electrons has prevented electron diffraction from being used in the same way. Dynamical diffraction describes both the propagation of the Bragg diffracted wave in the crystal and the scattering by the crystal potential. The balance between these two processes changes as a function of voltage due to relativistic effects. The difference in diffracted intensities recorded at two voltages is shown to be directly proportional to the crystal potential. This is confirmed by calculations using first-order perturbation theory which show negligible differences compared to exact calculation. It should therefore be possible to use differences in intensity measured as a function of voltage to determine the crystal potential directly. If the full complex wave function is available, then there is a particularly simple procedure to recover the potential, even under dynamical conditions.
Room-temperature and low-temperature structure of Nd1−x
Ca
x
MnO3 (0.3 ≤ x ≤ 0.5)
Authors: Richard O. , Schuddinck W. , Van Tendeloo G. , Millange F. , Hervieu M. , Caignaert V. , Raveau B.
Published: July 2002

The structure of the insulating manganites Nd1−x
Ca
x
MnO3 (0.3 ≤ x ≤ 0.5) has been studied against temperature using electron diffraction and neutron diffraction. At room temperature, the three compounds with x = 0.3, 0.4 and 0.5 exhibit the orthorhombic GdFeO3-type structure (a ≈ 21/2
ap
, b ≈ 2ap
, c ≈ 21/2
ap
). Below the charge-ordering temperature of ∼250 K, a doubling of the lattice is observed along the a axis which could be interpreted as an ordering of the d

z
2
Mn3+ orbitals. The low-temperature polymorph has orthorhombic symmetry for x = 0.5 and monoclinic symmetry for x = 0.4 and 0.3. The low-temperature structure of the x = 0.4 sample was refined using neutron diffraction data. The HREM study at room temperature confirms the average GdFeO3-type structure and indicates the existence of defective areas with a lower symmetry.
The joint probability distribution function of structure factors with rational indices. V. The estimates
Authors: Giacovazzo Carmelo , Siliqi Dritan , Fernández-Castaño Cristina , Comunale Giuliana
Published: July 2002

The probabilistic formulae [Giacovazzo, Siliqi & Fernández-Castaño (1999). Acta Cryst. A55, 512–524] relating standard and half-integral index reflections are modified for practical applications. The experimental tests prove the reliability of the probabilistic relationships. The approach is further developed to explore whether the moduli of the half-integral index reflections can be evaluated in the absence of phase information; i.e. by exploiting the moduli of the standard reflections only. The final formulae indicate that estimates can be obtained, even though the reliability factor is a constant.
Three-wave X-ray diffraction: an analysis scheme of the sensitivity in determining triplet phase invariants
Authors: Stetsko Yuri P. , Chang Shih-Lin
Published: July 2002

A model for analysing the phase sensitivity of the reflection coefficients of a diffracted wave in the case of three-wave X-ray diffraction is proposed. This model considers three-wave diffraction as the interference of the directly excited and the Umweg-excited diffracted waves and seems to account properly for the phase sensitivity as well as the behaviour of an involved diffracted wave as a function of the triplet phase invariant, the polarization state of the incident wave and the diffraction geometry. The practical issues for phase determination are also considered.
Faster calculation of the full matrix for least-squares refinement
Authors: Templeton David H.
Published: July 2002

Equations derived from a statistical model and valid for all space groups give estimates of the elements of the matrix for least-squares refinement of atomic coordinates and isotropic thermal parameters for a large crystal structure with many diffraction data. The elements are functions of the lengths and directions of Patterson vectors and the distribution of weights as a function of Bragg angle. For a large data set, this matrix, or a portion of it, can be calculated in a time that is approximately proportional to the number of elements calculated and independent of the number of reflections.
Molecular fragment electric moments derived from the fit of the experimental electrostatic potential. Application to the water molecule
Authors: Bouhmaida Nouzha , Ghermani Nour Eddine , Lecomte Claude , Thalal Abdelmalek
Published: July 2002

The electrostatic potential is a multicenter property that can be expressed as a sum of the contributions of electric moments located at each atomic site of a molecule. Independently of the model used to generate the electrostatic potential around the system, these atomic moments can be accurately obtained by the fit of this physical property outside the van der Waals envelop. However, the larger the system, the greater the number of parameters. In this study a way is proposed to reduce the number of centers in the representation of the electrostatic potential which becomes a sum of fragment contributions rather than atomic ones. A sample of six water molecules in different crystal environments was chosen to discuss the derived values of the electric moments referred to the molecular center of mass.
Systematic ab initio study of the compressibility of silicate garnets
Authors: Milman V. , Akhmatskaya E. V. , Nobes R. H. , Winkler B. , Pickard C. J. , White J. A.
Published: July 2002

The structural properties of the silicate garnets andradite, Ca3Fe2Si3O12, uvarovite, Ca3Cr2Si3O12, knorringite, Mg3Cr2Si3O12, goldmanite, Ca3V2Si3O12, blythite, Mn^{2+}_3Mn^{3+}_2Si3O12, skiagite, Fe^{2+}_3Fe^{3+}_2Si3O12, calderite, Mn^{2+}_3Fe^{3+}_2Si3O12, and khoharite, Mg3Fe^{3+}_2Si3O12, have been investigated with a quantum-mechanical model as a function of applied pressure. The study has been performed with the density functional theory code CASTEP, which uses pseudopotentials and a plane-wave basis set. All structural parameters have been optimized. The calculated static geometries (cell parameters, internal coordinates of atoms and bond lengths), bulk moduli and their pressure derivatives are in good agreement with the experimental data available. Predictions are made for those cases where no experimental data have been reported. The data clearly indicate that the elastic properties of all silicate garnets are dominated by the compressibility of the dodecahedral site. The compression mechanism is found to be based on a bending of the angle between the centers of the SiO4 tetrahedra and the adjacent octahedra, as in the aluminosilicate garnets. An analysis of the relationship between ionic radii of the cations and the compressibility of silicate garnets is presented.
Anisotropic thermal expansion of potassium dinitramide: a variable-temperature crystallographic study
Authors: Hardie Michaele J. , Martin Anthony , Pinkerton A. Alan , Zhurova Elizabeth A.
Published: July 2002

The structure of potassium dinitramide (KDN), KN3O4, has been refined in the temperature range 85–298 K from single-crystal X-ray diffraction data. The unit-cell axial lengths and the cell volume decrease linearly on cooling with the b axis being the most sensitive to the change of temperature. The β cell angle increases with decreasing temperature. The thermal expansion of KDN is significantly anisotropic, expanding along the b axis [010] more than three times the amount parallel to any other crystallographic direction. Other eigenvectors of the thermal expansion tensor lie approximately parallel to the diagonals of the ac plane. A rigid-body analysis of the dinitramide ion using the TLS formalism was performed and shows that the thermal motion of the anion is well represented by the rigid-body model. The eigenvalues of the libration tensor show significant anisotropy, whereas the translation tensor is close to isotropic. The variation of all descriptions of the thermal motion with respect to temperature indicates an anharmonic contribution to the mean field potential. The direction of greatest unit-cell expansion coincides with the largest components of the displacement tensor of the potassium ions and the direction of the largest atomic amplitudes due to the libration of the dinitramide anions.
A novel perovskite-like Ta-bronze KTa1+z
O3: preparation, stoichiometry, conductivity and crystal structure studies
Authors: Arakcheeva Alla , Chapuis Gervais , Grinevitch Vladimir , Shamray Vladimir
Published: July 2002

A new cubic Ta-bronze (1) KTa_{1+z}^{+(5 -\delta)}O3 [z ≃ 0.107 (3)] was obtained on a cathode by molten salt electrolysis of the system K2TaOF5–K3TaO2F4–(KF + NaF + LiF)eutectic. Black, metallic cubic crystals of (1) are formed together with tetragonal β-Ta. The perovskite-like crystal structure of (1) [a = 4.005 (1) Å, space group Pm\bar3m] was refined with anharmonic displacement parameters for Ta and K atoms and anisotropic displacement parameters for a split O-atom position [KM4CCD diffractometer; λ(Mo Kα); 3320 measured reflections with I > 3σ(I); R = 0.0095, wR = 0.0065, Δρmin = −0.91 e Å−3, Δρmax = 0.65 e Å−3]. Defects in the O and K atomic positions were found. (1) is a semiconductor in the temperature range 4–300 K, whereas the well studied and closely related colourless transparent crystals KTa+5O3 (2) are dielectric. Differences in the properties of (1) and (2) are assumed to be connected with the existence of Ta dumb-bells statistically distributed into the KTaO3 matrix.
Experimental electron density of urea–phosphoric acid (1/1) at 100 K
Authors: Rodrigues Bernardo Lages , Tellgren Roland , Fernandes Nelson G.
Published: July 2002

The deformation electron density of the urea–phosphoric acid adduct has been studied from 100 K X-ray and neutron diffraction experiments. Data were interpreted according to the Hirshfeld model. The long hydrogen bonds show characteristics of electrostatic interaction. Deformation density maps on the short hydrogen bond shows hydrogen more strongly bonded to urea than to phosphoric acid, and peak maxima at almost midway between the two O—H bonds.
Structure determination of the [Fe(teec)6](BF4)2 metal complex from laboratory and synchrotron X-ray powder diffraction data with grid-search techniques
Authors: Dova Eva , Stassen Arno F. , Driessen René A. J. , Sonneveld Ed , Goubitz Kees , Peschar René , Haasnoot Jaap G. , Reedijk Jan , Schenk Henk
Published: July 2002

The structure of the coordination compound [Fe(teec)6](BF4)2, hexa[1-(2-chloroethyl)tetrazole]iron(II) di(borotetrafluoride), has been determined using the grid-search techniques of the program suite MRIA. A Guinier-camera data set was used to determine the unit cell, the space group and to position the initial model. A high-resolution synchrotron powder data set was used to position a more detailed model using torsion-angle variation and to refine the structure leading to Rp = 0.0689, Rw = 0.0805 and GoF = 1.38. The crystal structure at room temperature shows the existence of two symmetry-equivalent iron(II) ions in the high-spin state.
Phosphate tungsten bronze series: crystallographic and structural properties of low-dimensional conductors
Authors: Roussel P. , Pérez O. , Labbé Ph.
Published: July 2002

Phosphate tungsten bronzes have been shown to be conductors of low dimensionality. A review of the crystallographic and structural properties of this huge series of compounds is given here, corresponding to the present knowledge of the different X-ray studies and electron microscopy investigations. Three main families are described, monophosphate tungsten bronzes, Ax
(PO2)4(WO3)2m
, either with pentagonal tunnels (MPTBp) or with hexagonal tunnels (MPTBh), and diphosphate tungsten bronzes, Ax
(P2O4)2(WO3)2m
, mainly with hexagonal tunnels (DPTBh). The general aspect of these crystal structures may be described as a building of polyhedra sharing oxygen corners made of regular stacking of WO3-type slabs with a thickness function of m, joined by slices of tetrahedral PO4 phosphate or P2O7 diphosphate groups. The relations of the different slabs with respect to the basic perovskite structure are mentioned. The structural description is focused on the tilt phenomenon of the WO6 octahedra inside a slab of WO3-type. In this respect, a comparison with the different phases of the WO3 crystal structures is established. The various modes of tilting and the different possible connections between two adjacent WO3-type slabs involve a great variety of structures with different symmetries, as well as the existence of numerous twins in MPTBp's. Several phase transitions, with the appearance of diffuse scattering and modulation phenomena, were analysed by X-ray scattering measurements and through the temperature dependence of various physical properties for the MPTBp's. The role of the W displacements within the WO3-type slabs, in two modulated structures (m = 4 and m = 10), already solved, is discussed. Finally, the complexity of the structural aspects of DPTBh's is explained on the basis of the average structures which are the only ones solved.
A pentagonal cluster in certain approximants to decagonal quasicrystals
Authors: Boström Magnus , Hovmöller Sven
Published: July 2002

A certain pentagonal cluster occurring in several approximants to the decagonal quasicrystal is discussed. The term `cluster' is used here to denote a structure motif which is a certain assemblage of coordination polyhedra. The cluster resembles a wheel with an `axis' and a `tyre'. It is built up of seven intergrown icosahedra. The `wheel cluster' builds up structures of infinite strands or nets perpendicular to the pentagonal wheel cluster axis. The wheel cluster is the main constituent of the decagonal approximant structure types Al3Mn, Al60Mn11Ni4 and Ga137Mn123.
Intersubsystem chemical bonds in the misfit layer compounds (LaS)1.13TaS2 and (LaS)1.14NbS2
Authors: Jobst Andreas , van Smaalen Sander
Published: July 2002

The modulated structures of incommensurate composite crystals (La0.912S)1.13TaS2 at room temperature and of (La0.949S)1.14NbS2 at T = 115 K are refined against high-resolution X-ray data. The compounds are isostructural with superspace group F′m2m(α,0,0)00s. For (LaS)1.13TaS2, lattice parameters of the first subsystem TaS2 were obtained as a = 3.2922 (1), b = 5.7776 (2) and c = 23.013 (2) Å. For the second subsystem LaS, the same b and c parameters were found, but a = 5.8090 (8) Å. Refinements led to a final structure model with R = 0.036 for 4767 observed unique reflections (R = 0.023 for 2147 main reflections, R = 0.099 for 1554 first-order satellites and R = 0.112 for 1042 second-order satellites). The final model includes modulation parameters up to the second-order harmonics for the displacements of the atoms, for the occupational parameters and for the temperature parameters. A clear correlation is found between the relative positions of the subsystems, the displacement modulation, the occupational modulation and the modulation of the temperature parameters. The analysis shows that the variations in environments are resolved by correlated variations in the temperature factors. For (LaS)1.14NbS2, lattice parameters at T = 115 K of the NbS2 subsystem were obtained as a = 3.3065 (4), b = 5.7960 (5) and c = 22.956 (3) Å. For the LaS subsystem, the same values for b and c were obtained, but a = 5.7983 (7) Å. Refinements led to a final structure model with R = 0.048 for 5909 observed unique reflections (R = 0.034 for 2528 main reflections, R = 0.092 for 2171 first-order satellites and R = 0.113 for 1103 second-order satellites). The final structure model is similar to that of (LaS)1.13TaS2. In particular, it is found that the values of the modulation parameters are almost equal and it is concluded that the modulations are independent of the temperature and the replacement of Ta with Nb, and thus represent a general mechanism of resolving the strain between the mutually incommensurate layers.