bluuebunny/crossref_metadata_2025 huggingface dataset:
A Small-Angle X-ray Scattering Study of the Binding of Cyclosporin A to Cyclophilin
Authors: Knott R. B. , Capel M. , Hansen S. , Handschumacher R. E.
Published: July 2002

The small-angle X-ray scattering (SAXS) technique was used to investigate structural characteristics of the protein cyclophilin in solution and to attempt to detect major changes induced by the binding of the immunosuppressant drug cyclosporin A. Maximum-entropy methods were used to analyse the experimental SAXS data. The measured radius of gyration, Rg
, for cyclophilin is 16.3 (5) Å. This is equivalent to a compact sphere of radius 21.0 Å. There is qualitative agreement between the experimental SAXS profiles and the derived distance-distribution function, p(r), for cyclophilin, and similar profiles calculated from the crystallographic structure. The notable discrepancy is the difference of approximately 1.5 Å in the estimated radius of the equivalent sphere. On binding cyclosporin A, the main structure-related change in cyclophilin observed under these experimental conditions is an increased propensity to form oligomers. Meaningful estimates of Rg
for the monomeric complex are not possible because of the presence of a significant population of aggregates. In a second series of experiments, both native cyclophilin and the cyclophilin/cyclosporin A complex readily formed aggregates under the prevailing experimental conditions.
Study of Biaxial Stress Induced by Cosputtered Thin Molybdenum Silicide Films in Silicon Single-Crystal Substrates
Authors: Lal K. , Mitra R. , Srinivas G. , Vankar V. D.
Published: July 2002

Bending of 25 mm-diameter (100) silicon wafers produced by 100 nm-thick cosputtered molybdenum–silicon films has been investigated just after deposition, after rapid thermal annealing and after etching of the deposits. Values of biaxial stress in the films were determined by the use of a widely used relation. The Mo:Si ratio in the as-deposited films was 89:11. Radii of curvature of substrates were measured by a double-crystal X-ray diffractometer. Blank wafers with three widely different radii of curvature (33, 62 and 250 m) were chosen as specimens and their curvatures were taken into account in the determination of the value of biaxial stress in wafers with deposits. All the wafers were convex when viewed from the polished surface side on which the films were deposited. Values of σ for wafers with as-deposited films were in the range 3 × 108 to 14 × 108 Nm−2 (tensile). Wafers with higher initial bending showed higher values of stress. Deposition led to degradation of perfection, as revealed by broadening of the diffraction curves and the contrast in the topographs. Rapid thermal annealing at 1273 and 1373 K (3–4 min) led to formation of the MoSi2 phase and to a notable relaxation of stress. The values of σ were in the range 1 × 107 to ~6 × 108 N m−2 (tensile). The value of the stress was lowest for the blank wafer with smallest bending. Annealing also improved the degree of crystalline perfection of the silicon. Experiments performed after etching of the annealed specimens showed no significant change in the radii of curvature.
Equipment for controlling nucleation and tailoring the size of solution-grown single crystals
Authors: König O. , Rechsteiner P. , Trusch B. , Andreoli C. , Hulliger J.
Published: July 2002

An improved version of the known temperature difference (ΔT) technique of crystallization from solution is presented. A Peltier device was used to control the local temperature of nucleation, dissolution and growth in order to reduce typically occurring delay times in the nucleation process within a given sample tube and to tailor the size of the single crystals to be obtained.
Small-Angle Neutron Scattering Investigations of Nanoscaled Microstructures
Authors: Wiedenmann A.
Published: July 2002

The formation and stability of nanoscaled microstructures upon annealing of two amorphous systems have been investigated by small-angle neutron scattering (SANS). The bulk amorphous alloy Zr41Be22.5Ti14Cu12.5Ni10 decomposes into two supercooled liquid phases upon annealing above the glass temperature. Dynamic scaling has been demonstrated for this phase-separation process. Crystallization is significantly slowed down by the two-phase nanoscaled microstructure formed upon decomposition. The temperature and magnetic field dependence of the anisotropic SANS scattering profiles in Fe73.5Si15.5B7
M
4 (M = Nb, W, Cu) alloys revealed superparamagnetic behaviour of Fe3Si nanocrystals which are decoupled from the amorphous ferromagnetic matrix. An Nb-enriched nonmagnetic layer formed around the Fe3Si crystals during diffusion-limited growth prevents magnetic exchange interactions and the coarsening of the nanocrystals.
Restoring Three-Dimensional Structure of Biopolymers from Solution Scattering
Authors: Svergun D. I.
Published: July 2002

The methods of small-angle scattering data analysis, developed to study the three-dimensional structure of biological macromolecules in solution, are reviewed. The direct methods include low-resolution shape determination in the homogeneous approximation and multiphase structure search by simultaneous fitting of the contrast-variation data sets from multicomponent particles. Approaches using crystallographic data involve evaluation of the solution scattering from the atomic coordinates, taking into account the solvation shell and relative positioning of subunits (domains) with known structure in complex particles. Practical applications of these methods to study biological macromolecules in solution are described.
Time-Resolved Small-Angle X-ray Scattering Combined with Wide-Angle X-ray Scattering
Authors: Bras W. , Ryan A. J.
Published: July 2002

The high X-ray intensity of synchrotron radiation (SR) beamlines makes it possible to perform time-resolved small-angle X-ray scattering (SAXS) experiments. The information that can be obtained by collecting the wide-angle diffraction pattern simultaneously not only increases the information content of an experiment but also increases the reliability of the time-correlations between SAXS and WAXS (wide-angle X-ray scattering) patterns. This is a great advantage for experiments with a time resolution below the level of 1 s per frame. With appropriate instrumentation, this is a time domain that is routinely accessible for a large group of research fields. This has had a considerable impact upon the understanding of fundamental aspects of phase transformations. Not only fundamental processes but also more applied fields have benefited from these developments. In polymer research this has led to a situation in which it has become possible to simulate materials processing techniques on-line. With the advent of third-generation synchrotron-radiation sources (e.g. ESRF, APS, Spring8), it has become possible to develop SAXS/WAXS beamlines that will open up new research opportunities by utilizing the higher intensity, the tuneability and the higher collimation offered by these SR sources. However, some of the instrumentation limits in detector and sample environments that have become apparent in research on second-generation synchrotron-radiation sources still have not been appropriately addressed, which means that in some fields it will not be possible to take full advantage of the superior X-ray beam quality that third-generation synchrotrons can offer. A way in which these instrumentation limits can be overcome is discussed, and the instrumentation for a new bending-magnet beamline at the ESRF is used as an example.
Small-Angle Neutron Scattering from Anisotropic Single-Crystalline Materials
Authors: Sequeira A. D. , Kostorz G. , Pedersen J. S.
Published: July 2002

`Isotropic' small-angle scattering (SAS), i.e. without azimuthal dependence around the primary beam, occurs only for specific configurations of the scattering objects if single-crystalline material is studied. For decomposing Ni-based alloy single crystals, SAS signals are generally highly anisotropic. From analysis of two-dimensional SAS patterns, important information on the evolution of the morphology and on the three-dimensional spatial arrangement of the precipitates from the early stages of decomposition can be extracted. The real-space information obtained from transmission electron microscopy is an excellent complement to the reciprocal-space information extracted from SAS data. The complementary use of these two techniques offers a valuable approach to the study of precipitation phenomena.
SANS Studies of Octyl-β-glucoside and Glycine Micellar Solutions
Authors: Giordano R. , Maisano G. , Teixeira J.
Published: July 2002

Small-angle neutron scattering (SANS) experiments were performed on micellar solutions of octyl-β-glucoside (OBG) in water and on solutions of OBG + glycine in water. A detailed analysis of the results obtained for different concentrations (from 0.026 to 1 M) of glycine and OBG shows that: (i) the structure of the micelles of OBG is always ellipsoidal with the long axis increasing as the concentration of OBG increases from the critical micellar concentration (c.m.c. = 0.022 M) to 1 M; (ii) there is a tendency to form aggregates at concentrations of OBG smaller than 0.5 M; (iii) steric repulsive interactions are clearly seen for concentrations above 0.6 M. The virial coefficient is evaluated as a function of the concentration in OBG. The negative values at low concentration reveal the presence of attractive interactions, while at high concentration the repulsive steric interaction dominates. At high concentration of OBG, the spectra with or without glycine are the same. The analysis shows that glycine remains in water and that its presence does not affect the structure of the micelles.
Small-Angle X-ray Scattering Study of Nanocrystalline and Amorphous States of the Fe73.5CuNb3Si13.5B9 Alloy
Authors: dos Santos D. R. , Torriani I. L. , Ramos A. Y. , Knobel M.
Published: July 2002

The structural evolution of the nanocrystalline material obtained from the amorphous compound Fe73.5CuNb3Si13.5B9 was investigated by small-angle X-ray scattering. Four sets of amorphous metallic ribbons were produced by melt spinning using different quenching rates. Samples of each set were annealed above the crystallization temperature; X-ray diffraction analysis showed the formation of an ordered Fe–Si solid solution with average grain sizes ranging from 9 to 11 nm. Small-angle scattering curves of the samples in the initial amorphous states revealed different intensities for different quenching rates, caused by the presence of heterogeneities with dimensions larger than those of the crystallites formed after thermal treatment. In addition, scattering measurements carried out in situ during isothermal annealing showed an intensity evolution as a function of time, attributed to electron-density contrast variation caused by the atomic diffusion process that occurs during crystallization.
Statistical Analysis and Modelling of Crystallization Outcomes
Authors: Sedzik J. , Norinder U.
Published: July 2002

This work describes a novel application of a recently developed statistical partial least-squares regression technique for the problem of establishing relationships between experimental variables in crystallization trials and the experimental results. To validate this method published sets of factorially designed crystallization trials were analyzed and it was discovered that these derived models show very good predictivity. These mathematical constructs cannot explain the detailed mechanism of crystallization, but are a pragmatic and powerful tool which enables the crystal growers to set up crystallization trials not only in a rational manner but also with confidence. This is a useful and a general methodological approach particularly when crystallizing proteins of limited supply.
Electric properties of KTiOPO4and NaTiOPO4from temperature-dependent X-ray diffraction
Authors: Dahaoui S. , Hansen N. K. , Protas J. , Krane H.-G. , Fischer K. , Marnier G
Published: July 2002

Single crystals of KTiOPO4(KTP) and NaTiOPO4(NaTP) show pronounced pyroelectric behaviour. In order to determine the origin of this property on an atomic scale, X-ray diffraction measurements have been carried out at several temperatures between 100 and 600 K. Modelling of the electron density and the evolution of the structure as a function of temperature has enabled the determination of values for the spontaneous polarization of the compounds and the pyroelectric coefficient of KTP, principally due to the alkaline-ion displacements with a value of 2.0 nC cm−2 K−1. Structure modifications, compared with NaTiOPO4, and the calculation of the electrostatic potential explain the anisotropic behaviour of ionic conductivity of KTP single crystals.
Rietveld refinement of site-occupancy parameters of Mg2−xMnxSiO4using a new weight function in least-squares fitting
Authors: Yamazaki S. , Toraya H.
Published: July 2002

The crystal structures of olivine-type Mg_{2-x}MnxSiO4(x = 0, 1, 1.4 and 2) were refined using high-resolution synchrotron radiation data and the Rietveld method, adopting the new weight function with the form 1/Y_o^e\;(e\simeq 2) (Yois the observed profile intensity) [Toraya (1998).J. Appl. Cryst.31, 333–343]. Positional parameters were accurately determined by optimizingein 1/Y_o^e. Derived structures were in accordance with observations on olivine structures, having a constant tetrahedral Si–O distance and octahedral metal–oxygen distances which increase linearly on substitution of octahedral atoms with those having larger atomic radii. Site-occupancy parameters of octahedral atoms were refined under various refinement conditions and could be determined within ±1.2% under the constraint of chemical composition. Deviations ofxfrom assumed chemical compositions, when the site-occupancy parameters were refined without the constraint, were 1.0–3.5% using the new weight function; they are comparable with values obtained in single-crystal studies on Mg–Fe olivine (0.5–2.7%). The use of atomic scattering factors for fully ionized atoms gave better site-occupancy parameters, by 0.1–2.5%, than those for neutral atoms in the present analysis.
LSCALE– the new normalization, scaling and absorption correction program in the DaresburyLauesoftware suite
Authors: Arzt Steffi , Campbell John W. , Harding Marjorie M. , Hao Q. , Helliwell John R.
Published: July 2002

A new program in the DaresburyLauesoftware suite has been developed for the scaling and normalization of Laue intensity data, to yield fully corrected structure amplitudes. Previously available routines have been improved, and additional options for refinement, control and statistical diagnostic output provided. A new feature, namely a wavelength- and position-dependent absorption correction that models a two-dimensional surface derived from the Laue data alone, is discussed in detail; it is tested on simulated and real data, and the improvement in data quality is demonstrated. The wavelength normalization function is now able, when sufficiently redundant experimental data are available, to model fine details such as the features arising from the modification of the incident intensity spectrum by a platinum mirror in the beamline optics. A full data set for tetragonal lysozyme is processed with the new program, and extensive statistical output is given.
MESO– a program to convert X-ray diffraction data from angular to reciprocal space
Authors: Locherer K. R. , Buckley A. , Salje E. K. H.
Published: July 2002

A computer code has been developed to convert X-ray diffraction data generated by Siemens X-1000 and Siemens HISTAR area detectors and INEL CPS 120 linear detectors from angular to reciprocal space. Programs have been written to visualize and analyse the resulting data using AVS (Advanced Visual Systems Inc., MA, USA). Two examples of the application ofMESOto problems in X-ray diffuse scattering are presented. Firstly, the `butterfly' associated with tweed microstructure in the high-temperature superconductor YBa2(Cu1–xMx)3O7–δ. Secondly, the `dog bone' generated by scattering off domain walls in the perovskite-like WO3.
Modeling hydroxyl and water H atoms
Authors: Nardelli Mario
Published: July 2002

The coordinates of hydroxyl (alcohol, phenol,etc.) and water H atoms can be easily calculated, within acceptable approximation, by combined geometric and force-field calculations on the basis of hydrogen-bonding interactions. To perform this kind of calculation the computer programHYDROGENhas been developed.
Difference structure-factor normalization for heavy-atom or anomalous-scattering substructure determinations
Authors: Blessing Robert H. , Smith G. David
Published: July 2002

Procedures are described for normalizing structure-factor difference magnitudes, |Δ|F||SIR= ||FDer| − |FNat|| ≤ |FHeavy| or |Δ|F||SAS= ||F+h| − |F−h|| ≤ 2|F′′|, to prepare data for probabilistic direct methods phasing to determine heavy-atom or anomalous-scattering substructures in SIR (single-derivative isomorphous replacement) or SAS (single-wavelength anomalous scattering) cases. Applications of the procedures in several recent determinations of multi-selenium substructures in selenomethionyl proteinsviaSnBdirect-methods phasing are briefly summarized.
Assessment of detector calibration materials for SANS experiments
Authors: Ghosh R. E. , Rennie Adrian R.
Published: July 2002

The properties of incoherent-scattering samples that are used as calibrants for small-angle neutron scattering (SANS) experiments have been investigated using time-of-flight analysis of the wavelengths of the scattered neutrons. Such samples are employed both as secondary intensity standards and to calibrate the relative efficiency of detectors. The data show that care must be taken to allow for wavelength dependence of the detector efficiency if the measured results are to be understood or compared with respect to different instruments. At ambient temperatures, glassy polymer sheets of poly(methyl methacrylate) or polycarbonate offer better performance as secondary standards than water. The results reveal the limits for the use of transmission measurements to estimate incoherent scattering cross sections. Use of uniform scattering as a basis for a measure of the effective cross section is, in practice, instrument dependent, owing to extensive inelastic multiple scattering.
X-ray scattering by crystals with local lattice rotation fields
Authors: Barabash R. I. , Klimanek P.
Published: July 2002

X-ray (or neutron) scattering by crystals with local rotation fields arising from dislocations is treated on the basis of the formalism of the kinematical diffraction theory. Such fields mostly change the intensity distribution of reflections in the azimuthal plane. Scattering intensity in the azimuthal plane for crystals with one or two sets of different-type dislocation walls, causing local rotations in the lattice, is analysed. In this case the intensity distribution is close to Lorentzian in the radial direction and to Gaussian in the azimuthal direction. The expressions for the scattering intensity are valid when averaging over a large statistical ensemble of defects. If this condition is not fulfilled, the intensity distribution in the azimuthal plane will split into several spikes. The mean distance between these spikes in the reciprocal-lattice space is connected with the disorientation between the walls. The conditions necessary for such splitting of the reflection into spikes are considered. The values of the limiting disorientation angle for some common scattering volumes and distances between dislocation walls are evaluated.
The small-angle scattering correlation function of the cuboid
Authors: Gille Wilfried
Published: July 2002

The analytical expression of the correlation function γ(r) for the cuboid with edgesa,bandcis established. The calculation is based on the chord-length distribution. Details of these structure functions at essentialrpositions are analysed, including higher derivatives of the correlation function at the maximum chord length. The result was checked on closer analysis of the corresponding scattering intensityI(h) and its asymptotic behaviourI∞(h).
Structural properties ofp+-type porous silicon layersversusthe substrate orientation: an X-ray diffraction comparative study
Authors: Faivre C. , Bellet D.
Published: July 2002

The structural properties of (001)- and (111)-orientedp+-type porous silicon samples have been investigated using X-ray techniques. X-ray reflectivity applied to thin layers of both orientations allows the estimation of the layer thickness, the porosity and the interface roughness. High-resolution X-ray diffraction was used to obtain symmetrical and asymmetrical rocking curves, as well as maps of the reciprocal space. The lattice-mismatch parameter was measured and some indications about the pore shape, orientation and size were deduced. The obtained X-ray curves as well as differential scanning calorimetry data are compared to discuss the influence of the substrate orientation on the structural properties ofp+-type porous silicon material.
X-ray diffraction study of polycrystallinep-chloronitrobenzene
Authors: Meriles C. A. , Schneider J. F. , Mascarenhas Y. P. , Brunetti A. H.
Published: July 2002

The X-ray diffraction pattern of powdered 1-Cl-4-NO2-benzene was measured at several temperatures in order to characterize the structure of two different phases. ForT< 282 K, the crystal array is ordered and exhibitsP21symmetry with two molecules in the unit cell; the observed lattice parameters atT= 190 K area= 5.838 (4),b= 5.218 (3),c= 10.716 (5) Å and β = 96.43 (5)°. A molecular arrangement inside the unit cell which minimizes the crystalline packing energy was calculated and, in combination with the Rietveld method, was used to reproduce the observed diffraction pattern. The refined structure yields an excellent agreement with the experimental results and confirms a qualitative model previously suggested to explain the measured low-frequency Raman spectrum. The high-temperature phase is monoclinic,Z= 2 witha= 3.84 (1),b= 6.80 (1),c= 13.37 (1) Å and β = 97.4 (1)° atT= 290 K. As confirmed by a Rietveld refinement, this phase exhibits an orientationally disordered arrangement in which dipoles of equivalent molecules point along opposite directions. This leads to a statistically centrosymmetrical molecule and enablesP21/csymmetry, in agreement with previous studies. The X-ray diffraction pattern also reveals an important increase of the background radiation which, in turn, exhibits a marked modulation of its intensity. On this basis, a simple analytical model has been developed to predict the angular dependence of diffuse scattering due to orientational disorder. The scope and limitations of this model are exhaustively discussed through a detailed comparison with the experimental results.
Distance distribution function of interacting uniform spheres
Authors: Vass Sz.
Published: July 2002

The scattering intensity equals the Fourier-transform of the distance distribution function p(t) = t
2Δ\tilde{\rho}2(t) of the sample, where Δ\tilde{\rho}2(t) is the autocorrelation function of the scattering contrast Δρ(r). Δ\tilde{\rho}2(t) carries information both on the structure of the elementary scatterers and on their spatial distribution. In the present paper a method is described how to separate the two types of information in a system of interacting hard spheres.
A new method of generating plane groups of simple and multiple antisymmetry
Authors: Jablan S. V.
Published: July 2002

A new method of generating groups of simple and multiple antisymmetry of Mm
type, based on presentations of groups of symmetry and on a newly defined term of antisymmetric characteristics, is suggested and applied to generate plane groups of simple and multiple antisymmetry of Mm
type from 17 plane groups of symmetry.
Statistical inference and crystallite size distributions
Authors: Guérin D. M. A. , Alvarez A. G. , Rebollo Neira L. E. , Plastino A. , Bonetto R. D.
Published: July 2002

An information theory approach is devised in order to obtain crystallite size distributions from X-ray line broadening. The method is shown to be superior to those based on Fourier expansions, as illustrated by numerical examples and a realistic situation. The powder model of Warren and Averbach is considered, in which the sample is thought of as a 'column-like' structure of unit cells perpendicular to the diffraction plane. Errors in excess of 100% arise as a result of truncating the diffraction peak. It is shown that, with the present approach, the corresponding figure is reduced to 5%, which confirms the power of information theory, and makes this method especially convenient in those cases in which there are large overlaps between the tails of two diffraction peaks.
Second-order piezomagnetism in polychromatic crystals
Authors: Rama Mohana Rao K.
Published: July 2002

The group-theoretical method established for obtaining the non-vanishing independent number of constants required to describe a magnetic/physical property in respect of the 18 polychromatic crystal classes [Rama Mohana Rao (1987). J. Phys. A, 20, 47-57] has been explored to enumerate the second- order piezomagnetic coefficients (n

i
′) for the same classes. The advantage of Jahn's method [Jahn (1949). Acta Cryst.
2, 30-33] is appreciated in obtaining these n

i
′ through the reduction of a representation. The different group-theoretical methods are illustrated with the help of the point group 4. The results obtained for all 18 classes are tabulated and briefly discussed.
The probability distribution of structure factors with non-integral indices. II. The P1¯ case
Authors: Giacovazzo Carmelo , Siliqi Dritan , Carrozzini Benedetta , Guagliardi Antonietta , Grazia Moliterni Anna
Published: July 2002

The probability distribution of the structure factors with non-integral indices is derived in P1¯. For integral values of at least one of the indices, the intensity distribution coincides with that provided by Wilson's statistics, but may strongly differ when the indices are (or are close to) half-integers. The deviations are stronger when the integral part of the indices is small, and increase with the size of the structure. In favourable circumstances, moduli and phases of the reflections may be accurately estimated.
Ab initio structure determination and Rietveld refinement of a high-temperature phase of zirconium hydrogen phosphate and a new polymorph of zirconium pyrophosphate from in situ temperature-resolved powder diffraction data
Authors: Krogh Andersen Anne Marie , Norby Poul
Published: July 2002

The collected in situ temperature-resolved synchrotron powder data revealed that the transformation of the recently reported three-dimensional τ-Zr(HPO4)2 to cubic ZrP2O7 goes through two intermediate phases. The first intermediate phase, ρ-Zr(HPO4)2, is formed in a reversible phase transition at 598 K, which involves both rearrangement and disordering of the hydrogen phosphate groups of τ-Zr(HPO4)2. At 688 K condensation of the hydrogen phosphate groups leads to the formation of the second intermediate, a new polymorph of zirconium pyrophosphate (β-ZrP2O7). Heating above 973 K results in the gradual transformation of β-ZrP2O7 to cubic zirconium pyrophosphate (α-ZrP2O7). The crystal structures of the two intermediate phases were solved from the in situ powder diffraction data using direct methods and refined using the Rietveld method. Both phases are orthorhombic, space group Pnnm and Z = 2. The lattice parameters for the two phases are: ρ-Zr(HPO4)2: a = 8.1935 (2), b = 7.7090 (2), c = 5.4080 (1) Å; β-ZrP2O7: a = 8.3127 (5), b = 6.6389 (4), c = 5.3407 (3) Å. The formation mechanism for the new zirconium pyrophosphate polymorph, β-ZrP2O7, is discussed in relation to structurally restricted soft chemistry.
Structure and thermal behavior of the new superprotonic conductor Cs2(HSO4)(H2PO4). Erratum
Authors: Chisholm Calum R. I. , Haile Sossina M.
Published: July 2002

Table 2 in the paper by Chisholm & Haile [(1999), Acta Cryst. B55, 937–946] was printed incorrectly. The following is the correct version, including H-atom coordinates.
Structures and phase transitions of B-Ta2O5 and Z-Ta2O5: two high-pressure forms of Ta2O5
Authors: Zibrov I. P. , Filonenko V. P. , Sundberg M. , Werner P.-E.
Published: July 2002

A sample of Ta2O5, ditantalum pentaoxide, heat-treated in a `toroid'-type high-pressure chamber at P = 8 GPa and T = 1470 K, was studied by X-ray powder diffraction and high-resolution transmission electron microscopy (HRTEM). Two high-pressure modifications of Ta2O5, isostructural with B-Nb2O5 and Z-Nb2O5, were identified from the X-ray powder pattern. Both structures were refined by the Rietveld method from the X-ray diffraction data: B-Ta2O5, a = 12.7853 (4), b = 4.8537 (1), c = 5.5276 (2) Å, β = 104.264 (2)°, V = 332.45 Å3, Z = 4, space group C2/c; Z-Ta2O5, a = 5.2252 (1), b = 4.6991 (1), c = 5.8534 (1) Å, β = 108.200 (2)°, V = 136.53 Å3, Z = 2, space group C2. The Z-Ta2O5 modification is new. The Ta atoms are six-coordinated in B-Ta2O5 and seven-coordinated in Z-Ta2O5. The two structures are closely related, which makes an intergrowth and a transformation between them possible. An idealized model of the intergrowth structure has been given. The HRTEM study showed defect-rich B-Ta2O5 crystals, which could be interpreted as an intergrowth between the B-Ta2O5 and Z-Ta2O5 phases.
Characterization of the room-temperature phase of Tl2MoO4: crystal structure, symmetry mode analysis and second-harmonic generation measurements
Authors: Friese Karen , Aroyo Mois I. , Folcia Cesar L. , Madariaga Gotzon , Breczewski Tomasz
Published: July 2002

The crystal structure of the glaserite-related compound dithallium(I)–molybdate(VI), which at 293 K crystallizes monoclinic, space group C121 with lattice parameters a = 10.565 (3), b = 6.418 (1), c = 8.039 (2) Å, β = 91.05 (4)°, has been determined. The structure was refined as an inversion twin to a final R(F
all) value of 0.0611 for 1006 unique reflections [R(F
obs) = 0.0285 for 644 observed reflections]. Second-harmonic generation measurements led to a value of d
eff = 5.5 ± 0.5 pm V−1 as an estimation of the second-harmonic conversion efficiency at phase matching. Symmetry mode analysis shows that, in general, primary modes have the highest amplitudes, yet surprisingly some of the secondary modes assume amplitudes of comparable magnitude. A comparison of the phase at 293 K with that at 350 K (space group P\overline{3}m1) shows that the main change can be described as a rotation of the molybdate tetrahedra around the trigonal a(b) axis. The molybdate tetrahedra as well as the octahedra around one of the symmetry-independent Tl atoms are more strongly distorted in the monoclinic phase. The coordination number for the other two Tl atoms is decreased from 12 and 10 in the high-symmetry phase to 10 and 9 in the monoclinic phase. Furthermore, the number of common edges between the Tl and Mo coordination polyhedra is reduced and the common face which is observed between them in the high-temperature phase is changed to a common edge in the low-temperature phase. The contribution of the primary symmetry modes leads exactly to this change in the coordination spheres of the atoms.
Direct observation of deuterium migration in crystalline-state reaction by single-crystal neutron diffraction. III. Photoracemization of 1-cyanoethyl cobaloxime complexes
Authors: Ohhara Takashi , Uekusa Hidehiro , Ohashi Yuji , Tanaka Ichiro , Kumazawa Shintaro , Niimura Nobuo
Published: July 2002

The H atoms bonded to the chiral C atoms (stereogenic center) of the 1-cyanoethyl groups in two cobalt complexes, [(R)-1-cyanoethyl]bis(dimethylglyoximato)(pyridine)cobalt(III) (2) and [(R,S)-1-cyanoethyl]bis(dimethylglyoximato)(piperidine)cobalt(III) (3), were replaced with D atoms, such as Co—C*D(CH3)CN. The crystals of the two cobalt complexes were irradiated with a xenon lamp for 72 h and 27 d, respectively. The unit-cell dimensions were gradually changed with retention of the single-crystal form. The crystal structures after irradiation were determined by neutron diffraction. In each crystal the chiral 1-cyanoethyl group of one of the two crystallographically independent molecules was partly inverted to the opposite configuration, whereas that of the other molecule kept the original configuration. The C*—D bond in the inverted group was completely conserved in the process of the inversion of the chiral alkyl group. This suggests that the inversion of the chiral 1-cyanoethyl group proceeds with the rotation of the cyanoethyl radical after the Co—C bond cleavage by photo-irradiation so that the opposite side of the radical faces the Co atom. This is followed by recombination of the Co—C bond to form the inverted 1-cyanoethyl group.
Parametrization of perovskite structures: an ab initio study
Authors: Magyari-Köpe B. , Vitos L. , Johansson B. , Kollár J.
Published: July 2002

An overview of the pressure, temperature and chemical composition dependence of the lattice distortion in orthorhombic ABO3 perovskite structures is presented. Within the framework of the so-called global parametrization method (GPM) [Thomas (1998). Acta Cryst. B54, 585–599] an improved description for the position of the A cation in terms of the AO_{12} and BO_6 polyhedral volume ratio is proposed. The relationship is derived from an extensive ab initio study based on the density functional theory. The applicability of the improved GPM in combination with ab initio total energy calculations in the prediction of changes in the structural distortion under increasing hydrostatic pressure is investigated. Test calculations are performed for the geophysically important magnesium silicate perovskite and the results are compared with the available theoretical and experimental data.
Disordered crystal structure of pentamethylcyclopentadienylsodium as seen by high-resolution X-ray powder diffraction
Authors: Tedesco Consiglia , Dinnebier Robert E. , Olbrich Falk , van Smaalen Sander
Published: July 2002

The crystal structure of pentamethylcyclopentadienylsodium, [NaC10H15] (NaCp*), has been determined from high-resolution X-ray powder diffraction. The compound crystallizes in space group Cmcm with lattice parameters a = 4.61030 (3), b = 16.4621 (3), c = 14.6751 (2) Å, V = 1113.77 (4) Å3 (Z = 4). NaCp* forms polymeric multidecker chains along the a axis. The Rietveld refinement (Rp
= 0.050 and RF
= 0.163) shows that the Cp* moieties occupy, with disorder, two different orientations rotated away from the eclipsed conformation by ±13.8°.
Structure of compounds E(SnMe3)4 (E = Si, Ge) as seen by high-resolution X-ray powder diffraction and solid-state NMR
Authors: Dinnebier Robert E. , Bernatowicz Piotr , Helluy Xavier , Sebald Angelika , Wunschel Markus , Fitch Andy , van Smaalen Sander
Published: July 2002

The compounds tetrakis(trimethylstannyl)germane, Ge(SnMe3)4 (1), and tetrakis(trimethylstannyl)silane, Si(SnMe3)4 (2), have crystal structures with the quasispherical molecules in a closed-packed stacking. At room temperature both structures have the space group P\bar 1 (Z = 2) with a = 9.94457 (5), b = 14.52927 (8), c = 9.16021 (5) Å, α = 90.53390 (30), β = 111.73080 (30), γ = 90.0049 (4)°, and V = 1229.414 (12) Å3 for (1) and a = 9.92009 (7), b = 14.51029 (11), c = 9.13585 (7) Å, α = 90.4769 (4), β = 111.6724 (4), γ = 89.9877 (6)°, and V = 1222.037 (16) Å3 for (2). The molecules are found to be ordered as a result of steric interactions between neighboring molecules, as shown by analyzing the distances between the atoms. Upon heating, both compounds undergo a first-order phase transition at temperatures Tc
= 348 ± 5 K, as characterized by a relative jump of the lattice parameter of ∼16%. At 353 K, both structures have the space group P\bar 1 (Z = 4), with a = 14.2037 (2) Å, and V = 2865.52 (7) Å3 for (1) and a = 14.1346 (2) Å, and V = 2823.90 (7) Å3 for (2). Rietveld refinements were performed for the low-temperature phases measured at T = 295 K [R
wp = 0.0844 for (1), R
wp = 0.0940 for (2)] and for the high-temperature phases measured at T = 353 K [R
wp = 0.0891 for (1), R
wp = 0.0542 for (2)]. The combination of high-resolution X-ray powder diffraction measurements and variable-temperature magic-angle-spinning 13C, 29Si and 119Sn NMR experiments demonstrates low crystallographic and molecular (C
1) symmetries for the low-temperature phases of (1) and (2) at temperatures T < 348 ± 5 K and high crystallographic symmetry due to rotational disorder for the high-temperature phases at temperatures T > 348 ± 5 K.
Structure prediction as a tool for solution of the crystal structures of metallo-organic complexes using powder X-ray diffraction data
Authors: Bond Andrew D. , Jones William
Published: July 2002

A simulated-annealing direct-space approach has been applied to predict the crystal structures of a series of metallo-organic complexes containing Zn, Cu and Ni. The prediction methodology generates a set of energetically reasonable crystal structures among which the actual structure is present, but it is not always possible to specify unambiguously the known crystal structure solely on the basis of energy. In each case, however, the ambiguity may be resolved by recourse to laboratory powder X-ray diffraction (PXRD) data. In this manner, structure prediction is shown to be a powerful tool for structure solution using PXRD data, with the additional advantage that indexing of the PXRD profile is not required at the outset.
Structures of trans-[PtCl2(PBz3)2], trans-[PtI2(PBz3)2], trans-[Pt(NCS)2(PBz3)2]·0.5C6H6 and trans-[PdI2(PBz3)2]
Authors: Johansson Maria H. , Otto Stefanus , Oskarsson Åke
Published: July 2002

A series of structures of trans-[MX
2(PBz3)2] [M = Pt, X = Cl−; PBz3 = tribenzylphosphine (1), I−, trans-diiodobis(tribenzylphosphine)platinum(II) (2), and NCS−, trans-di(thiocyanate)bis(tribenzylphosphine)platinum(II) (3); M = Pd, X = I−, trans-diiodobis(tribenzylphosphine)palladium(II) (4)] have been characterized by X-ray crystallography. In all compounds each tribenzylphosphine has one benzylcarbon close to the coordination plane. In (1), (2) and (4) those (in-plane) C atoms, from the two different PBz3, exhibit an anti conformation along the P—P axis, while (3) has the gauche conformation. Root mean square (RMS) calculations and half-normal probability plots show that the complexes in (2) and (4) are very similar and the only significant differences between them are the M—P bonds, 2.354 (4) and 2.330 (5) Å, and the M—I bond distances, 2.604 (1) and 2.611 (2) Å, for Pd and Pt, respectively. Calculations of the steric demand of the PBz3 ligands based on the Tolman model gave values ranging from 155 to 178° for the effective and 156 to 179° for the Tolman angles, respectively.
Crystal and electronic structure of a new metallic modification of (ET)2[KHg(SCN)4]
Authors: Kazheva Olga N. , Canadell Enric , Aleksandrov Grigorii G. , Kushch Nataliya D. , Dyachenko Oleg A.
Published: July 2002

A new room-temperature metallic modification of the well known radical cation salt (ET)2[KHg(SCN)4] has been prepared by electrochemical oxidation of ET. Its crystal and electronic structure have been examined at 110 K. The salt has a layered structure in which the conducting layers are characterized by the δ-type packing.
Nucleophilic addition reaction in coordinated non-linear pseudohalides: experimental charge density analysis in trans-bis(cyanamidonitrato-N:O)bis-(imidazole-N
3)copper(II) complex
Authors: Kožíšek Jozef , Hansen Niels K. , Fuess Hartmut
Published: July 2002

The experimental crystal charge density of the complex [Cu(iz)2(can)2] (I) (where iz = imidazole, can = cyanamidonitrate anion), [Cu(C3H4N2)2(CN3O2)2], crystallizing in centrosymmetric space group P21/c, has been determined at 120.0 (2) K, using Hansen and Coppens' aspherical atom multipole model. The central Cu atom is pseudo-octahedrally coordinated by four N atoms in an equatorial plane [two from iz molecules, 1.9660 (4) Å, two from can anions, 2.0273 (5) Å] and by two O atoms (from two adjacent molecules) in axial positions [2.5566 (5) Å]. The d-orbital population analysis shows that the oxidation state for the copper is +2 with the d
9 configuration and that there is a hole in the d_{x^2 - y^2} orbital, located in the plane of the four nitrogen-ligating atoms. Electron lone pairs of these four N atoms are pointing into these regions of depleted electron density. In accordance with the medium-length Cu—O axial bonds, the d
z2
orbital is almost fully occupied [1.79 (3) electrons]. The non-bonding d

xy
orbital is fully occupied [2.05 (3) electrons]. Analysis of the atomic charges does indicate a deficit of +0.35 (4) e− on the nitrile carbon, which provides evidence of β-carbon activation for a possible nucleophilic addition reaction. This reaction has been indirectly confirmed experimentally.
Neutron and X-ray diffuse scattering of calcium-stabilized zirconia at temperatures up to 1500 K
Authors: Proffen Th. , Keilholz M. , Neder R. B. , Frey F. , Keen D. A.
Published: July 2002

The temperature dependence of disorder diffuse scattering of calcium-stabilized zirconia was measured with neutron and X-ray scattering methods up to 1500 K. A quantitative interpretation of the diffuse scattering is given in terms of a correlated distribution of two types of microdomains. A pronounced decrease of the integrated intensity of the diffuse maxima was found in the temperature range 1250–1300 K in both neutron and X-ray cases. However, the half-width of the diffuse maxima and the corresponding correlation length shows a different temperature behaviour. The neutron data indicate an almost constant correlation length as a function of temperature, whereas the X-ray data show a maximum at 1170 K. A tentative interpretation is given.
Structure of 1,3,5,7-tetranitro-3,7-diazabicyclo-[3.3.0]octane. Structure solution from molecular packing analysis
Authors: Ammon H. L. , Du Z. , Gilardi R. D. , Dave P. R. , Forohar F. , Axenrod T.
Published: July 2002

The structure was solved with the molecular packing program MOLPAK, starting with a molecular mechanics-geometry optimized model of an isolated molecule. The best predicted crystal structures from the MOLPAK procedure were subjected to lattice energy refinement with the WMIN program. The MOLPAK/WMIN-predicted crystal structure, whose cell parameters were closest to the experimental values, gave an initial R of 0.48 for the 173 data to 0 = 25°. Four cycles of least-squares refinement of x, y, z and U gave an R of 0.27 for the 277 data to 0 = 30°. Final R = 0.038, wR = 0.044. There are no unusual intermolecular distances and crystal packing is normal.
Hydrogen-bond patterns in 1,4-dihydro-2,3-quinoxalinediones: ligands for the glycine modulatory site on the NMDA receptor
Authors: Kubicki M. , Kindopp T. W. , Capparelli M. V. , Codding P. W.
Published: July 2002

The crystal structures of five 1,4-dihydro-2,3-quinoxalinediones, antagonists of the NMDA modulatory glycine binding site on the excitary amino acid (EAA) receptor complex, have been determined: (I) 6,7-dinitro-1,4-dihydro-2,3-quinoxalinedione (DNQX); (II) 5,7-dinitro-1,4-dihydro-2,3-quinoxalinedione (MNQX); (III) 6-nitro-1,4-dihydro-2,3-quinoxalinedione hydrate; (IV) 6,7-dichloro-1,4-dihydro-2,3-quinoxalinedione; (V) 5,7-dichloro-1,4-dihydro-2,3-quinoxalinedione dimethylformamide. The crystal structure of the most active compound (II) contains a unique intramolecular N—H...O(NO2) hydrogen bond, which may be important for activity, as semiempirical calculations show that this bond is stable over a wide range of dihedral angles between the planes of the molecule and of the nitro group. In the other compounds the intermolecular hydrogen bonds connect molecules into three-dimensional networks. In compounds (I), (III) and (IV) head-to-tail: π-stacking is found between molecules connected by a center of symmetry. The geometries of the hydrogen-bonded —NH—C=O fragments show evidence of π-cooperativity or resonance-assisted hydrogen bonding. Graph-set analysis of the hydrogen-bond patterns of quinoxalinedione derivatives shows a tendency to form two types of hydrogen-bonding motifs: a centrosymmetric dimeric ring and an infinite chain. Even though this pattern may be modified by the presence of additional hydrogen-bond acceptors and/or donors, as well as by solvent molecules, general similarities have been found. Comparison of all quinoxalinedione structures suggests that the hydrogen-bonding pattern necessary for the biological activity at the glycine binding site contains one donor and two acceptors.
Structure and conformation of the hydrate of 8,11-dihydroxypentacyclo[5.4.0.02,6.03,10.05,9]undecane-8,11-carbolactam
Authors: Kruger H. G. , Martins F. J. C. , Viljoen A. M. , Boeyens J. C. A. , Cook L. M. , Levendis D. C.
Published: July 2002

The crystalline modification of the title compound from aqueous medium depends critically on factors such as concentration and pH. Crystallographic analysis of a hydrated crystal showed that this effect related to the formation of an extended network of hydrogen bonds that requires the water molecule, as H2O, to be hydrogen bonded three ways. The crystals are monoclinic, C2/c, a = 10.030 (1), b = 9.840 (2), c = 21.625 (2) Å, β = 90.87 (1)°, Z = 8. The hydrogen-bond network between water of hydration and the functional groups on the cage compound has identical geometries for both chiralities and during crystallization there is hence no discrimination between enantiomers. Both forms are thus accommodated at the same sites in random distribution, causing disorder of the molecular fragment remote from the functional region. The final arrangement is similar to a hydrated solid solution of the two enantiomers with an IR spectrum sufficiently different from the anhydrous form to suggest a different molecular arrangement.
Structure of KLiSi2O5 and the hygroscopicity of glassy mixed alkali disilicates
Authors: de Jong B. H. W. S. , Supèr H. T. J. , Spek A. L. , Veldman N. , van Wezel W. , van der Mee V.
Published: July 2002

KLiSi2O5, Mr
= 182.21, λ(Mo Kα) = 0.71073 Å, 150 K, monoclinic, P21, a = 5.9803 (6), b = 4.7996 (6), c = 8.1599 (11) Å, β = 93.477 (10)°, V = 233.78 (5) Å3, μ(MoKα) = 15.7 cm−1, Z = 2, F(000) = 180, Dx
= 2.5885 (6) Mg m−3, R
1 = 0.0331 for 1023 reflections with I > 2.0σ(I), wR
2 = 0.0864 for all 1064 reflections. The crystal structure of potassium lithium phyllosilicate has been determined and the hygroscopicity of glassy single and mixed alkali disilicates has been measured. The potassium lithium phyllosilicate sheet topology is the same as that of lithium phyllosilicate, consisting of six-membered silica rings in a chair configuration. Li is fourfold oxygen coordinated by three non-bridging O and one bridging O, while K is sixfold oxygen coordinated by five non-bridging O and one bridging O. The K atoms are sandwiched between the LiSi2O5 layer. Crystalline and glassy lithium disilicate are not hygroscopic. Crystalline and glassy potassium disilicate are the most hygroscopic alkali disilicates known to date, whereas the mixed system, crystalline and glassy lithium potassium disilicate, is again not hygroscopic. Calculated valences for lithium potassium phyllosilicate do not provide a rationale for these observations.