bluuebunny/crossref_metadata_2025 huggingface dataset:
Rotatory power measurements in the X-ray range with synchrotron radiation: Experimental set-up and preliminary results for NaBrO3 single crystals
Authors: Sauvage M. , Malgrange C. , Petroff J.-F.
Published: July 2002

The specific rotation of NaBrO3 single crystals has been measured with an X-ray beam extracted from the white synchrotron radiation spectrum by a specially designed silicon monochromator–polarizer. A detailed description of the monochromator principle and expected performance is given and the characteristics of the output beam are as follows: extremely narrow angular and wavelength spread (Δθ = 7 μrad, Δλ = 8 × 10–6 Å), nearly pure linear polarization (polarization ratio 2.5 × 108). The wavelength has been chosen very close to the bromine K edge (λ − λ

K
= 2 × 10–3 Å) in order to take advantage of possible resonant effects. The experimental set-up is of the crossed-polarizer type, the analyzer being a GaAs crystal adjusted for the 555 reflection with a Bragg angle θA
equal to 44.95°. The specific rotation measured in these conditions has been found to be 0.35 ± 0.31° cm−1. An explanation is proposed for this small value.
Dielectric and thermal properties, expansion, and high-temperature plastic deformation of Cd4Na(VO4)3
Authors: Abrahams S. C. , Nassau K. , Ravez J. , Simon A. , Olazcuaga R.
Published: July 2002

Cd4Na(VO4)3 has been reported as crystallizing in space group Pn21
a with all atoms displaced no more than 1/4 Å, from the corresponding positions in prototypic Pnma. Such an atomic arrangement is expected to be ferroelectric. Large optical-quality crystals, with no phase transition between 300 K and the melting point at 1165 K, have been grown. The piezoelectric d
22 coefficient is less than 0.1  pCN−1 and second-harmonic generation is undetectable at a sensitivity level capable of measuring a signal less than 0.001 that produced by quartz. The crystal is hence centrosymmetric at the 99% confidence level. The lattice constants at 294 K are a = 9.8189(3), b = 7.0298(2), c = 5.3610(1) Å, with linear thermal expansion coefficients of α(a) = 14.4, α(b) = 10.8, α(c) = 21.0 × 10−6 K −1 between 294 and 973 K. Plastic deformation sets in above 1060 K with an accompanying rapid rise both in dielectric permittivity and conductance.
An improved program for searching and matching of X-ray powder diffraction patterns
Authors: Lin Tian-Hui  , Zhang Sai-Zhu  , Chen Li-Jun  , Cai Xin-Xing
Published: July 2002

Two major improvements have been made in the present program: (1) a so-called `substance-type discrimination' has been introduced into the searching process, which can reduce missearching remarkably; (2) a new method for quantitative intensity matching has been proposed and can give reasonable results.
Crystal data for p-bromochlorobenzene/p-dibromobenzene mixed crystals at 293 K
Authors: Calvet T. , Cuevas M. A. , Tauler E. , Labrador M. , Haget Y. , Estop E.
Published: July 2002

The powder data for p-bromochlorobenzene/p-dibromobenzene mixed crystals [pBCB]
x
[pDBB]1 − x
at 293 K are reported; their thermal stability at 293 K is given. Vertical diffractometer, graphite monochromator, Cu Kα, λ = 1.54056 Å. They are all isomorphous, monoclinic, P21/a with Z = 2. a = 15.176(6), b = 5.847(3), c = 4.078(2) Å, β = 112.57(3)°, V = 334.2 Å3, Dx
= 1.947 Mg m−3 for [pBCB]0.90[pDBB]0.10; JCPDS No. 36-1975. a = 15.222 (7), b = 5.847 (3), c = 4.083(2) Å, β = 112.60(3)°, V = 335.5 Å3, Dx
= 1.984 Mg m−3 for [pBCB]0.80[pDBB]0.20; JCPDS No. 36-1974. a = 15.260(5), b = 5.839(2), c = 4.084(2) Å, β = 112.57(1)°, V = 336.1 Å3, Dx
= 2.024 Mg m−3 for [pBCB]0.70[pDBB]0.30; JCPDS No. 36-1973. a = 15.298(4), b = 5.845(3), c = 4.091(2) Å, β = 112.60(2), V = 337.7 Å3, Dx
= 2.058 Mg m−3 for [pBCB]0.60[pDBB]0.40; JCPDS No. 36-1972. a = 15.340(4), b = 5.844(2), c = 4.092(2) Å, β = 112.60(2)°, V = 338.7 Å3, Dx
= 2.096 Mg m−3 for [pBCB]0.50[pDBB]0.50; JCPDS No. 36-1971. a = 15.370(5), b = 5.843(2), c = 4.097(2) Å, β = 112.62(2)°, V = 339.6 Å3, Dx
= 2.134 Mg  m−3 for [pBCB]0.40[pDBB]0.60; JCPDS No. 36-1970. a = 15.404(6), b = 5.840(2), c = 4.100(2) Å, β = 112.67(2)°, V = 340.4 Å3, Dx
= 2.172 Mg m−3 for [pBCB]0.30[pDBB]0.70; JCPDS No. 36-1969. a = 15.437(5), b = 5.842(2), c = 4.103(2) Å, β = 112.66(2)°, V = 341.4 Å3, Dx
= 2.209 Mg m−3 for [pBCB]0.20[pDBB]0.80; JCPDS No. 36-1968. a = 15.468(4), b = 5.838(2), c = 4.104(2) Å, β = 112.70(2)°, V = 341.9 Å3, Dx
= 2.249 Mg m−3 for [pBCB]0.10[pDBB]0.90; JCPDS No. 36-1967.
X-ray absorption spectroscopy in the dispersive mode with synchrotron radiation: optical considerations
Authors: Tolentino H. , Dartyge E. , Fontaine A. , Tourillon G.
Published: July 2002

Aspects of the optics of the energy-dispersive scheme for X-ray absorption spectroscopy are discussed. The idea of a set of monochromatic focus points related to a set of local Rowland circles is introduced to account for the source-size effect on the energy resolution. It is shown that there exists an optimized location of the position-sensitive detector where the energy resolution is no longer source-size dependent. In addition, the stability of the dispersive optical system has been estimated and a 10 meV energy-scale reliability is currently achieved.
A global approach to the n-dimensional traveling salesman problem: application to the optimization of crystallographic data collection
Authors: Weinrach J. B. , Bennett D. W.
Published: July 2002

An algorithm for the optimization of data collection time has been written and a subsequent computer program tested for diffractometer systems. The program, which utilizes a global statistical approach to the traveling salesman problem, yields reasonable solutions in a relatively short time. The algorithm has been successful in treating very large data sets (up to 4000 points) in three dimensions with subsequent time savings of ca 30%.
The distribution of carbon among interstitial sites of the f.c.c. iron-alloy lattice
Authors: Mazzone G.
Published: July 2002

The distribution of carbon among octahedral and tetrahedral interstices of the f.c.c. iron-alloy lattice has been measured by neutron diffraction on a poly crystalline rod of an FeMn alloy containing 5.4 at.% carbon, quenched from 1373 K. The integrated intensities of six reflections have been collected for several sample positions in order to reduce the effect of preferred orientation in the sample. The fraction of carbon atoms occupying octahedral interstices has been found to be 0.9 (1). In analogy with previous work on spinels a thermodynamic relation has been deduced which allows the difference of nonthermal internal energy between the two sites to be calculated from measured occupancy.
Discrete thin-film multilayer design for X-ray and neutron supermirrors
Authors: Hayter J. B. , Mook H. A.
Published: July 2002

Multilayer structures, analogous to broadband optical filters, may be used to reflect X-rays or neutrons at angles larger than the total external reflection angle intrinsic to the bulk mirror material. These so-called supermirrors have generally been designed using continuum theories, which predict that the thickness of the jth layer in the multilayer should vary smoothly as j
−1/4. A new approach is proposed, based on considering the contribution of each bilayer to the extinction in a given stack of bilayers, and the discrete equations governing the choice of layer thicknesses are derived and solved. In the limit of zero layer thickness, the continuum result is recovered. The design produces essentially perfect reflectivity over the entire supermirror range. An optimal technique for trading reflectivity to gain angular range also emerges naturally from the physics of the design.
Quantitative texture measurement on thin wires
Authors: Langouche F. , Aernoudt E. , Van Houtte P.
Published: July 2002

A method is proposed for measuring textures on the surfaces of thin wires. The method is based on a numerical absorption correction analysis of the sample. Analytical formulas are given for thicker or more strongly absorbing specimens. Experimental work is presented to illustrate the use of the method.
Small-angle neutron scattering method for in situ studies of the dense cores of biological cells and vesicles: application to isolated neurosecretory vesicles
Authors: Krueger S. , Lynn J. W. , Russell J. T. , Nossal R.
Published: July 2002

Small-angle neutron scattering was used to study the structure of the dense cores of intact neurosecretory vesicles (NSV). Contrast-variation techniques were used to minimize the scattering due to vesicle membranes and emphasize that due to the cores. By examination of a suspension of NSV membranes along with the intact NSV, residual membrane scattering was suppressed. The resultant scattering is inconsistent with model calculations which assume that the dense state of the vesicle cores is achieved by random dense packing or crystallization of small globular particles. Rather, the data suggest that the core constituents exist in a disordered state, forming aggregates with radii of gyration significantly larger than 100 Å.
A direct investigation of lattice relaxation at a crystal surface by optical transforms of surface profile images
Authors: Harada J. , Takata M. , Miyatake H. , Koyama H.
Published: July 2002

Rod-shaped scattering, referred to as crystal truncation rod (CTR) scattering in X-ray diffraction, can also be observed in optical diffraction patterns obtained from the surface profile image of high-resolution electron micrographs. The characteristics of the CTR scattering are shown to be in agreement with those observed by X-ray scattering. With this technique, information about the lattice relaxation of the image of surfaces or interface boundaries observed in the electron microscope (EM) can be easily obtained and the lattice spacing of a GaAs crystal is shown to be shrunk at the interface boundary between the (001) surface and the amorphous oxide layer. This is precisely opposite to the effect observed for an Si (001) wafer surface. Several effects of surface modulation on CTR scattering are demonstrated using an optical diffractometer and masks of the f.c.c. lattice.
PAP: a protein analysis package
Authors: Callahan T. , Gleason W. B. , Lybrand T. P.
Published: July 2002

A program package has been assembled for the analysis of protein coordinates which are in the Brookhaven Protein Data Bank (PDB) format. These programs can be used to make two types of φ–ψ plots: a Ramachandran-style scatter plot, and a plot of φ and ψ values as a function of the linear sequence. Programs are also available for the display of distance diagonal plots for proteins. Two protein structures can be compared and the resulting r.m.s. differences in the structures plotted as a function of sequence. Temperature factors can be analyzed and plotted as a function of the linear sequence. In addition, various utilities are supplied for splitting PDB files which contain multiple subunits into individual files and also for renumbering PDB files. A utility is also provided for converting Amber-style PDB files into standard PDB files. Priestle's program RIBBON [J. Appl. Cryst. (1988), 21, 572–576] has been converted to run in a stand-alone mode with interactive rotation of the three-dimensional ribbon picture. Programs are Silicon Graphics four-dimensional level and have been tested on 4D70/GT and personal Iris workstations, although programs which give Postscript output have been converted to run on Digital Equipment Corporation VAX computers and Sun workstations.
Bragg and diffuse components of X-ray reflection measured using acoustic excitation of an Si crystal with oxide precipitates
Authors: Entin I. R. , Khrupa V. I. , Datsenko L. I.
Published: July 2002

Bragg and diffuse scattering contributions to a reflection can be separated using the intensity dependence on the amplitude of transverse ultrasound excited in the crystal. The method is based on the fact that a weak ultrasound with wavelength equal to the extinction length suppresses almost completely the anomalous transmission of the coherent (Bragg) beam in a sufficiently thick absorbing crystal and does not affect diffuse scattering. Both parameters characterizing diffraction in a slightly imperfect crystal, the static Debye–Waller factor and the coefficient of additional absorption due to diffuse scattering, have been determined for a silicon crystal containing oxide precipitates after heat treatment.
The nature of the scattering tail in Cu–Ni–Fe and Invar alloys investigated by anomalous small-angle X-ray scattering
Authors: Simon J. P. , Lyon O.
Published: July 2002

A large rapidly decreasing intensity called the `scattering tail' is generally observed at the smallest recorded angles during small-angle measurements of metallic alloys. Since this tail was interpreted as caused by a bimodal phase separation in Cu–Ni–Fe alloys and by long-wavelength concentration fluctuations in Invar alloys, these two systems were re-examined with anomalous X-ray scattering. The variation of the alloying atomic contrasts allows a discrimination between the different types of particles or defects. In neither of the two systems can the tails be interpreted as caused by large-scale concentration fluctuations. In Cu–Ni–Fe alloys, the tail is due to some kind of superficial defect (surface roughness etc.). In Invar alloys, the tail is probably due to residual impurity particles.
Whole-pattern fitting in energy-dispersive powder diffraction
Authors: Honkimäki V. , Suortti P.
Published: July 2002

A new procedure of pattern decomposition in energy-dispersive powder diffraction is presented. The total observed pattern is taken as a sum of incoherent and coherent scattering. The incoherent part is calculated from theoretical cross sections for individual atoms, the coherent part is described as a sum of discrete Bragg peaks and acoustic and optic phonon thermal diffuse scattering (TDS) is calculated from the Debye and Einstein models, respectively. The total TDS is scaled to be the scattering missing from the Bragg reflections due to thermal motion. The model pattern is convoluted by the instrument function calculated from the diffraction geometry, and the pattern is fitted to the observed one by varying the integrated Bragg intensities and thermal motion parameters. The method is applied to patterns of Mg, Al and Ti powders, leading to an unambiguous and self-consistent division to the background and the pattern of Bragg reflections. As an application, the flux of continuous radiation from a W-anode X-ray tube is determined using theoretical integrated Bragg intensities.
Therman Expansion and Crystal Chemistry of (Sr1−x
, K2x
)Zr4(PO4)6 Ceramic
Authors: Liu D.-M. , Lin L.-J. , Chen C.-J.
Published: July 2002

Thermal expansion of (Sr1−x
, K2x
)Zr4(PO4)6 (SrKZP) (with x = 0–1) ceramic was investigated using both a dilatometer and a high-temperature X-ray diffractometer. The coefficients of thermal expansion (CTEs) of the SrKZP ceramic measured by the dilatometer demonstrate a similar trend as those from high-temperature X-ray diffraction. Both measurements show an ultra-low CTE at x = 0.5; nevertheless, this composition shows significant lattice thermal-expansion anisotropy (TEA), while the minimum TEA appears with composition x = 0.2. Although it possessed a sufficient degree of TEA, the x = 0.5 composition showed no visible microcracks or negligible microcracks over a grain size as large as 15 μm. A transition of space group from R{\bar 3} to R{\bar 3}c with composition between x = 0.3 and x = 0.5 has been observed. The crystal structure of the SrKZP ceramic with possible occupations of strontium and/or potassium within the lattice in relation to their influence on the CTEs is proposed.
Simulation of Powder Diffraction Diagrams from Disordered and Imperfect Graphitic Layers
Authors: Andreev Yu. G. , Lundström T.
Published: July 2002

Powder X-ray diffraction patterns from independent and piled honeycomb graphitic layers with finite lateral extent containing various types of distortions have been simulated through the use of the general equation of Debye. The list of the distortions included rotation of adjacent layers around the layer normal, displacement of atoms along the layer normal, in-plane strains and bowl-like bending of layers. Such an approach allows one to estimate the impact of both regular and random deteriorations of the graphite structure on the profile shape by taking into account the diffractional contribution from each pair of atoms comprising the structural unit under consideration.
Estimation of the Minimum Grain Number for the Orientation Distribution Function Calculation from Individual Orientation Measurements on Fe–3%Si and Ti–4Al–6V Alloys
Authors: Baudin T. , Jura J. , Penelle R. , Pospiech J.
Published: July 2002

The calculation of characteristics describing texture as well as relations between orientations and morphological features of microstructure are based on single orientation measurements. For such experimental data, it is essential to estimate the number of necessary measurements of single orientations for a statistically significant representation of the investigated quantity, which, in the present paper, is the orientation distribution function (ODF). In a previous article [Pospiech, Jura & Gottstein (1993) Mater Sci. Forum, 157–162, 407–412], this number has been estimated by a criterion that is used here for a cubic and a hexagonal material. This approach is very useful since it allows one to estimate the minimum orientation number with or without referring to an ODF calculated from pole figures measured by X-ray or neutron diffraction.
Remarks on the geometric parameters in describing diffraction optics of asymmetric Bragg crystals
Authors: Hung H.-H.
Published: July 2002

This communication gives the asymmetric factor b and deviation parameter η describing the diffraction geometry in a more general way including very asymmetric scattering arrangements. Darwin's parameter Y defined on the dispersion surface and the associated deviation parameter ηY
given by the Takagi–Taupin equations are proposed to characterize the angular deviation from the exact resonance of Bragg diffraction. With respect to ηY
of a high accuracy, the validity of conventional forms of the deviation parameter is further discussed.
Small-Angle Scattering of Heterogeneous Catalysts
Authors: Benedetti A.
Published: July 2002

A heterogeneously catalyzed reaction involves the adsorption of the reactants on the surface of the catalyst. Consequently, catalytic activity is usually strongly related to the microstructural features of the catalyst. Small-angle scattering (SAS) has been extensively used to study heterogeneous catalysts and it has been shown to provide useful information about the specific surface of the active phases. In particular, the majority of the research has dealt, and still deals, with supported metal catalysts, which are three-phase systems (support, voids and metal). Several attempts have been made experimentally and theoretically to overcome this problem and to obtain values for the three surface areas. In the first part of the paper, earlier results and recent approaches will be reviewed in order to give a general picture of the problem. In the second part, some recent results on other kinds of heterogeneous catalysts, such as sulfated zirconia, with strongly acidic properties, and functionalized silicas, will be discussed. Knowledge of the degree of smoothness of the particles (angularity) and thickness of the organic films obtained by SAS experiments can be very useful in clarifying morphological aspects of these porous systems.
Investigation of Polymer Brushes and Adsorbed Layers under Shear
Authors: Nguyen D. , Clarke C. J. , Eisenberg A. , Rafailovich M. H. , Sokolov J. , Smith G. S.
Published: July 2002

Neutron reflectometry was used to investigate the density profiles of polymer brushes and an adsorbed polymer layer under shear. The brushes consisted of end-functionalized deuterated polystyrene. The adsorbed polymer was a random copolymer of deuterated styrene and 4-vinylpyridine. The brush and the adsorbed layer were sheared by the flowing of solvents over the polymer, with a maximum shear rate of 104s−1. Density profiles of the brush and the layer were determined before, during and after shear. The shear had no observable effect on the polymer profiles in any of the experiments. We discuss these results in the context of other experiments and a recent theory.
Position-Resolved Small-Angle X-ray Scattering of Complex Biological Materials
Authors: Fratzl P. , Jakob H. F. , Rinnerthaler S. , Roschger P. , Klaushofer K.
Published: July 2002

Many biological materials, like bone or wood, are hierarchically organized and optimized at all levels for their specific mechanical function. At the lowest level, these materials are fiber composites, where the fiber direction as well as the size of the individual components varies considerably with position inside a given specimen. For bone or wood, some of these parameters can be readily obtained by small-angle X-ray scattering (SAXS) in a position-resolved way. A scanning-SAXS system based on a pinhole camera with rotating anode and area detector is presented, and first applications to the study of bone and wood are reported.
The Morphology of Block Copolymer Micelles in Supercritical Carbon Dioxide by Small-Angle Neutron and X-ray Scattering
Authors: Londono J. D. , Dharmapurikar R. , Cochran H. D. , Wignall G. D. , McClain J. B. , Betts D. E. , Canelas D. A. , DeSimone J. M. , Samulski E. T. , Chillura-Martino D. , Triolo R.
Published: July 2002

Above its critical point, carbon dioxide forms a super-critical fluid, which promises to be an environmentally responsible replacement for the organic solvents traditionally used in polymerizations. Many lipophilic polymers such as polystyrene (PS) are insoluble in CO2, though polymerizations may be accomplished via the use of PS-fluoropolymer stabilizers, which act as emulsifying agents. Small-angle neutron and X-ray scattering have been used to show that these molecules form micelles with a CO2-phobic PS core and a CO2-philic fluoropolymer corona. When the PS block was fixed in length and the fluorinated corona block was varied, the number of block copolymer molecules per micelle (six to seven) remained constant. Thus, the coronal block molecular weight exerts negligible influence on the aggregation number, in accordance with the theoretical predictions of Halperin, Tirrell & Lodge [Adv. Polym. Sci. (1992), 100, 31–46]. These observations are relevant to understanding the mechanisms of micellization and solubilization in supercritical fluids.
An open-flow helium cryostat for single-crystal X-ray diffraction experiments
Authors: Hardie M. J. , Kirschbaum K. , Martin A. , Pinkerton A. A.
Published: July 2002

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. The effectiveness of the system has been demonstrated using test data for oxalic acid, terbium vanadate and dysprosium vanadate.
Twinning in Natural Diamond. II. Interpenetrant Cubes
Authors: Machado W. G. , Moore Moreton , Yacoot A.
Published: July 2002

Single crystals of natural diamond which exhibit an approximately cubic morphology are known to have grown either (a) by nonfaceted cuboid growth or (b) by fibrous growth branching in a multiplicity of 〈111〉 columns, to give a mean surface orientation of {100}. Interpenetrant twins of diamond are uncommon; but when they occur they are usually coloured and appear (by X-ray topography) to have been formed by fibrous growth upon a twin origin, with the 14 growth sectors adequately filled by fibres, without the necessity of branching.
VALMAP2.0: contour maps using the bond-valence-sum method
Authors: González-Platas Javier , González-Silgo Cristina , Ruiz-Pérez Catalina
Published: July 2002

VALMAP2.0 is a Microsoft-Windows-based program designed to assist material scientists in accurate structural investigations. The aim ofVALMAPis to calculate the sum of bond valences that a particular atom would have if it were placed at any arbitrary point in the crystal. By movement of this atom through all possible points, its valence-sum contour map can be displayed. Parameters of the bond-valence model are available and may be modified. The program was tested in a number of cases and two examples of applications are reported: (i) finding probable atom sites in crystal structures; (ii) displacive and order–disorder phase transition mechanisms, analysing steric effects.
X-ray design constraints forin situelectrochemical cells: importance of window material, electrolyte and X-ray wavelength
Authors: Nahlé A. H. , Walsh F. C. , Brennan C. , Roberts K. J.
Published: July 2002

The need to calculate X-ray absorption losses through window materials and electrolytes in electrochemical cells used forin situX-ray studies is highlighted. Calculations of the loss of beam intensity through Mylar windows of various thicknesses and at different wavelengths are carried out. Other window materials, such as polyethylene, polyimide (Kapton), polymethylmethacrylate (Perspex), polycarbonate (Lexan) and polyamide (Kevlar), are considered. Linear absorption coefficients and loss of intensity through different thicknesses of these materials at the CuKα wavelength are considered. Calculations of loss of intensity for a typical experimental case involving a Mylar cell window and a 1.029 MH2SO4electrolyte are presented.
Small-angle scattering of interacting particles. II. Generalized indirect Fourier transformation under consideration of the effective structure factor for polydisperse systems
Authors: Weyerich B. , Brunner-Popela J. , Glatter O.
Published: July 2002

The indirect Fourier transformation (IFT) is the method of choice for the model-free evaluation of small-angle scattering data. Unfortunately, this technique is only useful for dilute solutions because, for higher concentrations, particle interactions can no longer be neglected. Thus an advanced technique was developed as a generalized version, the so-called generalized indirect Fourier transformation (GIFT). It is based on the simultaneous determination of the form factor, representing the intraparticle contributions, and the structure factor, describing the interparticle contributions. The former can be determined absolutely free from model assumptions, whereas the latter has to be calculated according to an adequate model. In this paper, various models for the structure factor are compared,e.g.the effective structure factor for polydisperse hard spheres, the averaged structure factor, the local monodisperse approximation and the decoupling approximation. Furthermore, the structure factor for polydisperse rod-like particles is presented. As the model-free evaluation of small-angle scattering data is an essential point of the GIFT technique, the use of a structure factor without any influence of the form amplitude is advisable, at least during the first evaluation procedure. Therefore, a series of simulations are performed to check the possibility of the representation of various structure factors (such as the effective structure factor for hard spheres or the structure factor for rod-like particles) by the less exact but much simpler averaged structure factor. In all the observed cases, it was possible to recover the exact form factor with a free determined parameter set for the structure factor. The resulting parameters of the averaged structure factor have to be understood as apparent model parameters and therefore have only limited physical relevance. Thus the GIFT represents a technique for the model independent evaluation of scattering data with a minimum ofa prioriinformation.
Grazing-incidence X-ray diffraction in the study of metallic clusters buried in glass obtained by ion implantation
Authors: d'Acapito Francesco , Zontone Federico
Published: July 2002

A novel X-ray technique is described for carrying out structural investigations of metallic nanoparticles buried in glass obtained by ion implantation. The method consists of maximizing the scattering contribution of the cluster-rich layer by working at the critical angle for total external reflection at the implanted-layer–substrate interface. By using the refracted beam as a probe, the diffraction profile of the metallic clusters in very dilute samples can be extracted by a simple subtraction procedure. The new procedure is applied to SiO2glasses implanted with Ag ions. The study was performed at the European Synchrotron Radiation Facility. The results indicate that a complete structural investigation can be performed by using highly collimated, very intense synchrotron radiation beams, a grazing-incidence geometry and two-dimensional detectors. The procedure is shown to be very useful for complementing the information from electron techniques (transmission electron microscopy, micro-beam electron diffraction) and X-ray spectroscopic (EXAFS) methods. In particular, the accuracy of the lattice-parameter determination is shown to be a factor of nine better than the accuracy of EXAFS results.
Natural partitioning of orientation elements and determination of the ODF from individual orientations according to the maximum-entropy principle
Authors: Wang Y. D. , Vadon A. , Bessières J. , Heizmann J. J.
Published: July 2002

The orientation distribution function (ODF) of a polycrystalline material is usually constructed from individual orientations by the harmonic method on the assumption of a certain function distribution in the Euler space around each orientation. In the present paper, a new method is developed to determine the ODF from individual orientations. A natural partitioning of the orientation elements in the Euler space around some clustered orientations is proposed. Thus, the preliminary values of orientation density in the elements are directly estimated by the volumes of the orientation elements and the number of grains (or measured points) in each orientation element. Then, the texture vector is further refined using the maximum-entropy method with the preliminary orientation densities as constraints. The validity of this method is exemplified by the texture analysis of a cubic material from individual orientations modelled by Gaussian distribution.
The structure of approximant μ-Al4Mn interpreted as a modulated crystal
Authors: Uchida Masaya , Horiuchi Shigeo
Published: July 2002

The structure of the μ-Al4Mn phase, which is an approximant of icosahedral and decagonal quasicrystals, is interpreted as a modulated crystal structure. It basically comprises modulated layers, in which the ordering of atomic vacancies takes place in the close-packed layers. The modulation is considered to be due to the rearrangement of atomic vacancies as a response to the occurrence of charge-density waves.
Stabilization of cubic Na-modified ZrO2: a neutron diffraction study
Authors: Canton P. , Fagherazzi G. , Frattini R. , Riello P.
Published: July 2002

Neutron diffraction patterns, collected at room temperature and, within situthermal treatment, from 981 to 1173 K, on a zirconia sample containing 3 wt% of sodium, show the presence and the stability of the cubic form of zirconia from 298 to 1073 K. The analysis has also demonstrated the presence of a significant amount of the tetragonal form of zirconia, which cannot be clearly detected using conventional X-ray diffraction. We report and discuss the Rietveld refinements performed on the samples, analysed at 298 and at 981 K. The cubic form appears to be stabilized by the incorporation of sodium ions, which randomly replace the zirconium ions, thus forming a substitutional solid solution.
PDFFIT, a program for full profile structural refinement of the atomic pair distribution function
Authors: Proffen Th. , Billinge S. J. L.
Published: July 2002

The programPDFFITis designed for the full profile structural refinement of the atomic pair distribution function (PDF). In contrast to conventional structure refinement based on Bragg intensities, the PDF probes thelocalstructure of the studied material. The program presented here allows the refinement of atomic positions, anisotropic thermal parameters and site occupancies as well as lattice parameters and experimental factors. By selecting individual atom types one can calculate partial and differential PDFs in addition to the total PDF. Furthermore one can refine multiple data sets and/or multiple structural phases. The program is controlled by a command language which includes a Fortran style interpreter supporting loops and conditional statements. This command language is also used to define the relation between refinement parameters and structural or experimental information, allowing virtually any constraint to be implemented in the model.PDFFITis written in Fortran-77, and the source code and documentation are availableviathe World Wide Web.
Structure of sodiumpara-hydroxybenzoate, NaO2C–C6H4OH by powder diffraction: application of a phenomenological model of anisotropic peak width
Authors: Dinnebier R. E. , Von Dreele R. , Stephens P. W. , Jelonek S. , Sieler J.
Published: July 2002

Theab initiostructure solution of sodiumpara-hydroxybenzoate from high-resolution X-ray powder diffraction data is reported. The compound is of interest with respect to understanding the mechanism of Kolbe–Schmitt type reactions. It crystallizes in space groupP21,Z= 2, with unit-cell parametersa= 16.0608 (3),b=  5.38291 (9),c=  3.63834 (6) Å, β =  92.8692 (5)° andV=  314.153 Å3. The compound consists of layers of distorted NaO6prisms perpendicular to theaaxis and phenol rings perpendicular to these layers pointing up and down. The molecular structure is held together by van der Waals forces between the phenyl groups of different layers and additional hydrogen-bridge bonding between the phenolate oxygen atoms. The sample showed powder peak widths which are not a smooth function of diffraction angle; a recently implemented phenomenological model was able to describe this effect sufficiently well to obtain excellent Rietveld fits to the data. The accuracy of modeling the data makes this one of the rare cases where the position of a hydrogen atom could be unambiguously determined by powder techniques.
Estimation of radial distribution functions in electron diffraction experiments: physical, mathematical and numerical aspects
Authors: Nörenberg H. , Säverin R. , Hoppe U. , Holzhüter G.
Published: July 2002

Radial distribution functions (RDFs) can be obtained from transmission electron diffraction experiments. Polycrystalline gold specimens have been used to study how different mathematical methods extract the RDF information from electron diffraction data. Fourier transform (FT) and a maximum-entropy (ME) algorithm have been used in these calculations. Results obtained by the two methods are very similar and reproduce the interatomic distances accurately. Between the two methods, FT and ME, no significant difference could be found. ME calculations are very sensitive towards input parameters whereas the FT is a very robust algorithm. Calculations on reduced sets of experimental data with little error margin showed no improvement of resolution in the RDF calculated by ME.
Subpicosecond X-ray diffraction study of laser-induced disorder dynamics above the damage threshold of organic solids
Authors: Rousse A. , Rischel C. , Uschmann I. , Förster E. , Albouy P. A. , Geindre J. P. , Audebert P. , Gauthier J. C. , Antonetti A.
Published: July 2002

Ultrafast disordering in Langmuir–Blodgett (LB) films and TlAP (thallium acid phthalate) crystals has been investigated by optical-pump–X-ray-probe experiments using intense femtosecond laser pulses. A laser-produced plasma X-ray source at 7.12 Å wavelength is used to study atomic dynamics by subpicosecond time-resolved X-ray diffraction. It is found that a drop of the X-ray diffracted intensity appears with time constants from below 600 fs up to a few tens of picoseconds in LB films optically excited at laser fluxes from 1.8 to 27 J cm−2. This loss in the diffracted intensity is understood as a displacement of the diffracting atoms by ∼8 Å from their equilibrium position. The response of the TlAP crystal was significantly slower. The relation to the atomic structure of the materials is discussed.
Small-angle X-ray scattering from semi-dilute solutions of poly(N-isopropylacrylamide) in aqueous acetonitrile solutions
Authors: Shimizu S. , Sekine R. , Kurita K.
Published: July 2002

Interaction parameters of poly(N-isopropylacrylamide) in acetonitrile–water mixed solvent were determined by using small-angle X-ray scattering. Small-angle X-ray scattering measurements were carried out in conditions of the semi-dilute poor solvent regime and just below the lower critical solution temperature. From concentration dependence of the correlation length, the binary and ternary cluster integrals were determined. The contribution of the segment–segment interactions to the entropy S
int and the enthalpy H
int has been calculated from the temperature dependence of these cluster integrals. It has been found that both values are positive. On addition of a small amount of acetonitrile into aqueous solution of poly(N-isopropylacrylamide), S
int and H
int increase with increasing acetonitrile content. Upon further addition of acetonitrile, these values decreased.
Polymer bristles: a SANS study
Authors: King S. , Washington C. , Attwood D. , Booth C. , Mai S. , Yang Y.-W. , Cosgrove T.
Published: July 2002

We report the results of a SANS study into the structure of the adsorbed layers formed at the interface in a dilute perfluorodecalinin-water emulsion by five short chain diblock copolymers of oxyethylene (OE) and oxybutylene (OB). The results are discussed in the context of the Marques-Joanny-Leibler scaling description of block copolymer adsorption from selective solvents. The volume fraction profiles are best described by parabolic and, to a lesser extent, Gaussian functional forms. On increasing the temperature the buoy segments of one copolymer were found to contract toward the surface, giving rise to profiles that were more block-like in accordance with the mean-field predictions of Wijmans and Zhulina.
Small and wide-angle X-ray scattering study of metallocene isotactic poly(propylene)
Authors: Dai P.S. , Cebe P. , Capel M. , Alamo R.G. , Mandelkern L.
Published: July 2002

We report a study of metallocene isotactic poly(propylene) (m-iPP) which crystallizes into α and γ crystal modifications. Simultaneous in-situ small- and wide-angle X-ray scattering (SAXS and WAXS) were used to study kinetics during crystallization. Both techniques provide information about time development of crystallinity, while WAXS gives also kinetics of formation of α and γ crystals. During the earliest stages of crystal formation, the SAXS Bragg peak occurs simultaneously, or slightly lags, the appearance of crystalline WAXS reflections. We conclude crystallization occurs by a nucleation and growth process in this m-iPP.
Analysis of the structure of dendrimers in solution by small-angle neutron scattering using contrast variation
Authors: Pötschke D. , Ballauff M. , Lindner P. , Fischer M. , Vögtle F.
Published: July 2002

The analysis of dendrimers of fourth (G4) and fifth generation (G5) by small-angle Neutron scattering (SANS) in solution is presented. The contrast of the solute towards the solvent is changed systematically in order to elucidate the radial scattering length density (contrast variation). Contrast variation allows to determine the molecular weight of the dendrimers without additional assumptions. The data obtained here demonstrate that the dendrimer of fourth generation is a perfect structure whereas G5 has defects. Moreover, the analysis demonstrates that both dendrimer under consideration here have a compact structure where the density has its maximum at the center of the molecule.
MarqX: a new program for whole-powder-pattern fitting
Authors: Dong Y. H. , Scardi P.
Published: July 2002

MarqXis a computer program for the modelling of powder diffraction data. It can be used for an unconstrained profile fitting (pattern decomposition, PD) or constrained modelling of the whole powder pattern (Pawley method, PM), for single- as well as multiple-phase samples. The program output includes: lattice parameters or peak positions (for PM and PD, respectively), width and shape of the diffraction peak (in terms of half width at half-maximum and mixing parameter of a pseudo-Voigt function), corrected for the instrumental broadening component, intensity, peak area and profile asymmetry. In addition, errors on the goniometer zero and shift in sample position with respect to the goniometric axis can also be modelled, together with distance and relative intensity of the spectral components of the X-ray beam (e.g.Kα1andKα2). Specific output files are provided for line-profile analysis, including the Williamson–Hall plot and Warren–Averbach method.
The diffraction pattern of projected structures
Authors: Elser V.
Published: July 2002

A method for calculating the properties of structures obtained by projection is developed and applied to a three-dimensional generalization of the Penrose tiling. The diffraction pattern is shown in general to consist of a dense set of delta-function peaks. For the Penrose model the pattern in addition has the symmetry of the icosahedron.
Bloch-wave solution in the Bragg case
Authors: Ma Y. , Marks L. D.
Published: July 2002

The Bloch-wave method for reflection diffraction problems, primarily electron diffraction as in reflection high-energy electron diffraction (RHEED) and reflection electron microscopy (REM), is developed. The basic Bloch-wave approach for surfaces is reviewed, introducing the current flow concept which plays a major role both in understanding reflection diffraction and determining the allowed Bloch waves. This is followed by a brief description of the numerical methods for obtaining the results including specific results for GaAs near to the [010] zone axis. A number of other Bloch-wave phenomena are also discussed, namely resonance diffraction and its relationship to internal and external reflection and variations in the boundary conditions and Bloch-wave character, splitting of diffraction spots due to stepped surfaces, which can be completely explained, and the reflection equivalent of thickness fringes.
Orthogonal bases of radial functions for charge density refinements
Authors: Restori R.
Published: July 2002

Charge density determination from X-ray measurements necessitates the evaluation of the Fourier-Bessel transforms of the radial functions used to expand the charge density. Analytical expressions are given here for four sets of orthogonal functions which can substitute for the 'traditional exponential functions' set in least-squares refinements.
Estimation of triplets with interatomic vectors
Authors: Kronenburg M. J.
Published: July 2002

This paper investigates the exponential triplet probability distribution and the corresponding expectation values that incorporate structural information of an a priori known part of the electron density. The expectation values improve from the Cochran expectation values (no structural information) to the true values (all information known). Combination of the expectation values gives an estimation formula for triplets with structural information. The validity of the resulting estimated triplets is tested with mean absolute errors and correlation coefficients against the true values of the triplets. The single and double Patterson functions may serve to find the structural information ab initio. Some problems are mentioned that mean application of the formulae to standard structure-solution methods is still difficult. The development of more general formulae for other multiplets and higher Patterson functions is expected.
Nonsymmetrical X-ray diffraction in a perfect rectangular t × l crystal. Extinction and absorption
Authors: Thorkildsen Gunnar , Larsen Helge B.
Published: July 2002

A theoretical investigation of diffraction in a rectangular t\times l crystal for nonsymmetrical coplanar scattering has been undertaken. The asymmetry in the scattering geometry, measured by an angle \gamma, causes different weights for the mixed Laue–Bragg contributions to the integrated power. Primary extinction and ordinary absorption are only moderately affected when the value of the geometrical parameter \zeta = (t/l)\tan\theta_{oh} \,\lt\, 1, \theta_{oh} being the Bragg angle. For increasing values of \zeta and \gamma the surface integration set-up for the Laue and Bragg regions is extensively changed, leading to pronounced effects. Analytical results for the normal absorption factor are presented.
A likelihood-based search for the macromolecular position in the crystalline unit cell
Authors: Petrova T. E. , Lunin V. Y. , Podjarny A. D.
Published: July 2002

The goal of this work is to analyse whether the generalized likelihood criterion can be used to find the best spherical envelope of a macromolecule in a unit cell. A family of spherical envelopes is ranged in accordance with their likelihood values calculated by means of a Monte Carlo-type computer procedure. Two kinds of envelope families were tested. The first one was composed of spherical envelopes of fixed radius but different positions in the unit cell. In the second case, the sphere radii were linked to their centre position so that the total volume occupied by all symmetry-related spheres was roughly equal to the total volume occupied by the real molecule. The experiments showed that when using the first type of envelope the level of the signal for the right solution is higher than the one obtained with the straightforward R-factor-based single-Gaussian-atom search, but spurious maxima (usually placed on the symmetry axes) may still exist. The use of the second type of envelope family reduces the level of the spurious maxima.
The tetragonal phase of Na0.5Bi0.5TiO3 – a new variant of the perovskite structure
Authors: Jones G. O. , Thomas P. A.
Published: July 2002

The structure of the tetragonal phase of the A-site-substituted perovskite sodium bismuth titanate, Na0.5Bi0.5TiO3, has been determined by neutron powder diffraction at 698 K. The structure was refined in space group P4bm with a (= b) = 5.5191 (1), c = 3.9085 (1) Å, V = 119.055 (5) Å3, Z = 2 and D

x
= 5.91 Mg m−3. The structure exhibits an unusual combination of in-phase (a
0
a
0
c
+) tilts and antiparallel cation displacements along the polar c axis, which results in a new variant of the perovskite structure.
X-ray structure determination of the monoclinic (121 K) and orthorhombic (85 K) phases of langbeinite-type dithallium dicadmium sulfate
Authors: Guelylah A. , Madariaga G. , Morgenroth W. , Aroyo M. I. , Breczewski T. , Bocanegra E. H.
Published: July 2002

The structures of the monoclinic and the orthorhombic phases of type I langbeinite Tl2Cd2(SO4)3 have been determined at 121 and 85 K, respectively, by X-ray diffraction. A precise analysis of these structures shows the existence of some differences compared to langbeinites of type II. The monoclinic structure differs very little from the high-temperature cubic structure and the distortion relating the monoclinic structure to the cubic one is very small. SO4 tetrahedra seem to rotate under orthorhombic symmetry in the monoclinic phase. A symmetry distortion analysis of the ferroelectric monoclinic distortion discloses the importance of the secondary modes with orthorhombic symmetry, especially for the O atoms of the SO4 groups.