bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantitative Analysis of Time-Resolved Laue Diffraction Patterns
Authors: Ren Z. , Ng K. , Borgstahl G. E. O. , Getzoff E. D. , Moffat K.
Published: July 2002

Integration and quantification of time-resolved Laue images poses problems beyond those encountered with static Laue images. The flexible analytical profile-fitting technique [Ren & Moffat (1995). J. Appl. Cryst.
28, 461–481] has been extended to handle the integration of multiple-spot images with two or more exposures at different time points superimposed on a single detector flame but displaced by a small shift. Each Lane pattern on a multiple-spot image can be integrated separately; possible spatial overlaps between adjacent spots from either the same or different exposures can be resolved; streakiness and streakiness anisotropy are allowed to be different for each time point. Various strategies for time-resolved Lane diffraction data collection and processing are compared. Time-resolved Lane images obtained during the relaxation of photoactive yellow protein (PYP) from its photostationary state have been processed by the Laue data reduction package LaueView. Continuous laser illumination of PYP crystals establishes a photostationary state and termination of laser illumination starts a relaxation process. However, PYP crystals at the photostationary state are more anisotropically mosaic than those at the ground state, and the mosaicity and its anisotropy vary during the relaxation. Accurate integration of elongated and spatially overlapping spots therefore becomes more difficult. Two data processing strategies have been applied to calculate time-dependent difference Fourier maps of PYP. The first route takes advantage of both the wavelength normalization and the harmonic deconvolution [Ren & Moffat (I 995). J. Appl. Cryst.
28, 461–481, 482–493] algorithms. The second is the method of relative percentage changes of structure-factor amplitudes.
Use of R Factors in the Study of the Structural Defects in Phyllosilicates by X-ray Powder Diffraction
Authors: Taşer M. , Küçükçelebi H. , Armagˇan N. , Güler C.
Published: July 2002

The R factors employed in crystal structure analysis can also be used in connection with clay minerals to determine the amount of defects whose types are already known. For this purpose, using the unit-cell parameters of kaolinite with identical layers, (02,11) and (20,13) diffraction bands were calculated on the basis of a variable parameter model containing only ± b/3 translational stacking faults. The parameters of the model were varied to obtain the best fit between the calculated and observed patterns with respect to both intensity and profile. Three different R factors were used as a criterion for the best fit and the parameters yielding the smallest R value were accepted as describing the actual structure. In addition, a linear regression analysis is performed between observed and calculated intensities to determine the quality of the fit. Correlation coefficients, obtained from this analysis, are used as an indication of the effectiveness of corresponding R factors.
The Use of the Pseudo-Voigt Function in the Variance Method of X-ray Line-Broadening Analysis
Authors: Sánchez-Bajo F. , Cumbrera F. L.
Published: July 2002

A modified application of the variance method, using the pseudo-Voigt function as a good approximation to the X-ray diffraction profiles, is proposed in order to obtain microstructural quantities such as the mean crystallite size and root-mean-square (r.m.s.) strain. Whereas the variance method in its original form is applicable only to well separated reflections, this technique can be employed in the cases where there is line-profile overlap. Determination of the mean crystallite size and r.m.s. strain for several crystallographic directions in a nanocrystalline cubic sample of 9-YSZ (yttria-stabilized zirconia) has been performed by means of this procedure.
Two- and Three-Beam X-ray Diffraction Imaging of Domains in Magnetite
Authors: Medrano C. , Heyroth F. , Schlenker M. , Baruchel J. , Espeso J.
Published: July 2002

The X-ray diffraction topographic imaging process for ferrimagnetic domains in magnetite Fe3O4at room temperature is investigated, in two- and three-beam cases, for incident synchrotron radiation beams differing in angular divergence and energy spread. In the usual two-beam configuration, domain or wall contrast is obtained from the difference in Bragg conditions, or from the slight difference in the direction of the beams diffracted by neighbouring domains, revealed by the large crystal-to-film distance that can be used at a third-generation synchrotron radiation facility. A three-beamUmweganregungcase involving the weak 171 and the strong 131 reflections shows unusual domain contrast on the 171 topographs, even on images involving energy or angle integration; this contrast is particularly evident on white-beam topographs. The high angular sensitivity this implies is associated with the difference in dispersion relation between the two reflections.
Structural characterization of free-standing gallium arsenide coiled membranes produced by micromachining
Authors: Lal Krishan , Goswami S. Niranjana N. , Miao J. , Hartnagel H. L.
Published: July 2002

High-resolution X-ray diffraction techniques have been employed successfully to evaluate crystalline quality and long-term stability of coiled membranes. The process of fabrication involves photolithography, implantation by 2 MeV N^{2+} ions inn-type GaAs substrates, followed by selective etching. A five-crystal X-ray diffractometer was employed in (+, −, +) setting with an Mo Kα1exploring beam for high-resolution X-ray diffractometry and topography experiments. The exploring-beam width was reduced to illuminate different segments of the coiled membrane. Diffraction curves recorded from the bulk crystal surrounding the sensor had a half width of 26 arcseconds, whereas the half widths from sensor segments were in the range ∼58 to ∼166 arcseconds. Different segments (particularly vertical ones) were identified from the observed angular separations between different diffraction peaks as well as from the shape of the diffraction peaks. It was found that different segments of the sensor were tilted with respect to one another and the tilt angles were in the range 15–212 arcseconds. High-resolution X-ray diffraction topographs recorded from (5\bar 11) and (400) diffracting planes revealed that the sides of the trough below the membrane created by etching are not vertical, but tapered. Also, there is a thin strip of crystal freely hanging over the tapered regions as a result of underetching. The surface of the cavity is uneven. The structural perfection of different membrane segments could also be ascertained from the contrast in topographs.
Texture and residual-stress analysis using a kappa goniometer
Authors: Guillén R. , François M. , Bourniquel B. , Girard E.
Published: July 2002

A kappa goniometer allowing the analysis of large samples (thickness up to 60 mm) has been constructed. It was designed to perform the analysis of crystalline texture and residual stress by X-ray diffraction with both χ- (or Ψ-) and Ω-type assembly configurations. Tilt-angle values up to 80° and 2θ values up to 165° can be reached. Validation has been achieved by checking the setting precision and the assembly reliability for the two types of configuration for which the system has been designed.
Flat-specimen effect as a convolution in powder diffractometry with Bragg–Brentano geometry
Authors: Ida T. , Kimura K.
Published: July 2002

The flat-specimen effect on the peak profile in powder diffractometry with Bragg–Brentano geometry can quantitatively be treated as a convolution with an asymmetric window function. The formula of the window function and a practical method for numerical calculation of the convolution are proposed. By applying the model function given by the convolution of the pseudo-Voigt function with asymmetric window functions based on vertical (axial) divergence and the flat-specimen effect, experimental profiles measured with a conventional diffractometer are fairly well reproduced with only a few parameters. When this function is applied to the analysis of experimental diffraction profiles of a lanthanum hexaboride (LaB6) standard sample measured with a conventional powder X-ray diffractometer, the lattice constant is estimated asa= 4.15685 (2)–4.15693 (2) Å at 299 (1) K, without the use of any other standard material or preceding correction of systematic instrumental errors, while the specified value isa= 4.15695 (6) Å.
Scanning system for high-energy electron diffractometry
Authors: Avilov A. S. , Kuligin A. K. , Pietsch U. , Spence J. C. H. , Tsirelson V. G. , Zuo J. M.
Published: July 2002

A new electron diffractometer with a diffraction-pattern scanning system in front of a fixed counter has been developed. Significant improvement was achieved in the measured diffraction intensities by using fast electronics and additional control of the stability of the electron beam. The measurement of and accounting for the gear-frequency characteristic of the registration system was performed, and the signal accumulation mode for intensity measurements together with advanced statistical data processing were employed. Good agreement between the experimental and Hartree–Fock structure factors for LiF, NaF and MgO was achieved (to avoid strong extinction effects, rather thin polycrystalline films were used as samples).
MIMS: a program for measuring four-dimensional Fourier maps of incommensurate modulated structures
Authors: Li Yang , Wan Zheng-Hua , Fan Hai-Fu
Published: July 2002

The programMIMS(measuring incommensurate modulated structures) has been written for the determination of structural parameters of incommensurate one-dimensionally modulated structures by an automatic search routine on four-dimensional Fourier maps. Test results show that the program works accurately and efficiently.
Solving crystal structures from powder data. V. Located molecular fragment and powder-pattern decomposition
Authors: Altomare Angela , Giacovazzo Carmelo , Guagliardi Antonietta , Moliterni Anna Grazia Giuseppine , Rizzi Rosanna
Published: July 2002

In direct procedures for crystal structure solution from powder data, information on the location and orientation of a molecular fragment may readily become available. Such information may be used retrospectively to improve the powder-pattern decomposition, with favourable effects on the phasing process. A method is described by which accurate estimation of a large number of structure-factor moduli is possible by exploiting the prior partial structural information.
Position-sensitive ionization chambers in the alignment of X-ray tubes and diffractometers
Authors: Arndt U. W. , Kyte M. P.
Published: July 2002

Split-plate air-filled ionization chambers have been used as X-ray beam position monitors in the alignment of microfocus X-ray tubes to X-ray diffractometers and goniometers.
Microstructural investigation of plastically deformed Pb(1−x)Snxalloys: an X-ray profile-fitting approach
Authors: Chatterjee Partha , Sen Gupta S. P.
Published: July 2002

A detailed X-ray line-profile analysis is presented on five compositions of Pb–Sn alloys in the face-centred cubic (f.c.c.) α phase. Analysis has been performed within the framework of the Warren–Averbach method and the Williamson–Hall method through a profile-fitting algorithm assuming a pseudo-Voigt (pV) profile shape function convoluted with an asymmetric function. The effect of cold-working on the alloys is analysed in terms of defect-related parameters and defect distribution. The alloys decompose beyond the composition Pb–13.2 at.% Sn. Cold-working produces very weak structural broadening in Pb–Sn alloys, along with negligible incidence of deformation-fault probability. However, the presence of the Sn-rich β phase enhances the fault probability. The plot of \ln \langle \varepsilon^2\rangle^{1/2}versus\ln L is a straight line and the slope is around −0.26 to −0.43, indicating that the strain-broadened profile is intermediate between a Gaussian and a Lorentzian profile. The origin of the `hook' effect in the size Fourier plot is attributed to non-Gaussian strain distribution or strain gradients in the diffracting columns. The effect of recovery is noticeable and is probably due to polygonization. Furthermore, the rate of recovery is greater in Pb–Sn alloys compared to Pb–Sb alloys for identical solute concentrations.
LiMn2O4low-temperature phase: synchrotron and neutron diffraction study
Authors: Massarotti V. , Capsoni D. , Bini M. , Scardi P. , Leoni M. , Baron V. , Berg H.
Published: July 2002

The structural evolution of LiMn2O4spinel was followed from 320 K down to 10 K. The structural transformation, recently studied down to 230 K [Rodriguez-Carvajal, Rousse, Masquelier & Hervieu (1998).Phys. Rev. Lett.81, 4660–4663], takes place near room temperature with a significant hysteresis: the high-temperature cubic phase transforms to a superstructure orthorhombic cell. The present study indicates that the nuclear structure is stable down to 10 K, while neutron diffraction patterns below 80 K show the rise of a magnetic ordering in the spinel phase. From Mn—O bond-length analysis of the MnO6octahedra, a temperature-independent charge ordering in the structure can be deduced.
Form factors of block copolymer micelles with spherical, ellipsoidal and cylindrical cores
Authors: Pedersen Jan Skov
Published: July 2002

The form factor of a micelle model with a spherical core and Gaussian polymer chains attached to the surface has previously been calculated analytically by Pedersen and Gerstenberg [(1996). Macromolecules 29, 1363–1665.]. Non-penetration of the chains into the core region was mimicked in the analytical calculations by moving the center of mass of the chains R

g
away from the surface of the core, where R

g
is the radius of gyration of the chains. In the present work, the calculations have been extended to micelles with ellipsoidal and cylindrical cores. Non-penetration was also for these taken into account by moving the center of mass of the chains R

g
away from the core surface. In addition results for worm-like micelles, disk-shape micelles and micelles with a vesicle shape are given.
Self-organization of OPV-PEG diblock copolymers in THF/water
Authors: Urban V. , Wang H.H. , Thiyagarajan P. , Littrell K.C. , Wang H.B. , Yu L.
Published: July 2002

Oligo(phenylenevinylene)-poly(ethyleneglycol) (OPV-PEG) diblock copolymers in tetrahydrofuran (THF) solution at concentrations of 5 to 25 g/l self-assemble into rod-like structures with a radius of about 80 Å, for an OPV-PEG diblock copolymer comprising 13 PV and 45 EG monomers. These aggregates consist of a liquid crystalline OPV core and a PEG shell. Addition of about 10% water to the solution induces the formation of a phase of packed rods, as revealed by a sudden and dramatic transition of the scattering pattern. Further addition of water leads to swelling and at about 30% ultimately to disruption of the packed-rod phase.
Non-self-similar aggregation of methylcellulose
Authors: Guillot S. , Lairez D. , Axelos M. A. V.
Published: July 2002

Aggregation of methylcellulose in aqueous dilute solution is studied by light and small angle neutron scattering. For temperature T<293.15K, the free chains adopt a swollen conformation. For T>313.15K, a slow aggregation kinetics occurs. At qRg<1, the association number is found to vary as Rg
3. However at qRg>1, the scattered intensity decreases as q−(1<d≤1.3) depending on the temperature. At even higher q, one observes a local compaction as aggregation proceeds, as revealed by a more and more pronounced Porod's behaviour. Results are discussed in terms of non self-similar growing aggregates, having neither a rod-like nor a dense structure.
General formula for determination of cross-section from measured SANS intensities
Authors: Strunz P. , Šaroun J. , Keiderling U. , Wiedenmann A. , Przenioslo R.
Published: July 2002

A detailed derivation of the formula connecting measured SANS intensity with the smeared cross-section (raw-data treatment) is presented. The derivation is carried out considering also a possible high probability of the coherent small-angle scattering. The information content of the so-called transmission measurement is analyzed. The individual effects which contribute to the primary-beam attenuation as well as the different modes of the transmission measurement are described. Multiple scattering contribution to the excess of the forward scattering of the water calibration standard is discussed.
The new high resolution ultra small-angle neutron scattering instrument at the High Flux Reactor in Grenoble
Authors: Hainbuchner M. , Villa M. , Kroupa G. , Bruckner G. , Baron M. , Amenitsch H. , Seidl E. , Rauch H.
Published: July 2002

In 1998, the combined Interferometer and Ultra Small Angle Neutron Scattering (USANS) instrument S18 at the 58 MW High Flux Reactor at the Institute Laue-Langevin in Grenoble (France) started operation. The instrument has been upgraded to allow more advanced neutron optics experiments for fundamental, nuclear and condensed matter physics. The new supermirror guide along with the two channel cut silicon perfect crystals, configured as an advanced high resolution Bonse-Hart camera, provides optimal intensity conditions. A large range of wavelengths is accessible by variations of the Bragg angle and by using different reflecting planes of a properly cut silicon monochromator block in combination with different channel-cut analyzer crystals. The fine adjustment analyzer system is achieved by an advanced piezo drive, which has an accuracy of 0.036 seconds of arc. The basic features of the USANS camera, which takes advantage of a new tail suppression method, and the results of various test measurements concerning intensities, long term stability and the accessible wavelength range will be presented. A high resolution position sensitive detector can either be used to analyze beam profiles or to perform neutron radiography experiments.
Small-angle neutron scattering investigations of magnetic nanostructures using polarized neutrons
Authors: Wiedenmann A.
Published: July 2002

Crystalline and magnetic nanostructures have been studied by means of small angle neutron scattering. Using polarised neutrons, the relative contrasts are strongly modified which allowed magnetisation, density and composition profiles at surfaces and interfaces to be evaluated. In nanocrystalline Fe3O4 embedded in a glass ceramics matrix the magnetic order is strongly disturbed at the surface of the particles, leading to a magnetically inactive layer. In partially crystallised metallic Fe-Si-B-(Nb,Cu) alloys the presence of a non-magnetic interface between Fe3Si nanocrystals and the amorphous matrix has been demonstrated which breaks the direct ferromagnetic exchange interactions. In Co-ferrofluids the superparamagnetic core is encapsulated by a shell of surfactant molecules which was found to be impenetrable for the solvent.
The fractal microstructure of ancient sedimentary rocks
Authors: Radlinski A.P. , Radlinska E.Z. , Agamalian M. , Wignall G.D. , Lindner P. , Randl O.G.
Published: July 2002

Small-angle neutron scattering (SANS), ultra-small-angle neutron scattering and scanning electron microscopy (SEM) were used to analyze the pore-fabric interface in a 1400 million years old shale. The pore-rock fabric interface is a surface fractal (of fractal dimension 2.82) over three orders of magnitude of the length scale and ten orders of magnitude in intensity (macroscopic differential cross section range 10−1 – 109 cm−1). Both SANS and SEM indicate similar fractal dimension for scales less than 4 µm and a breakdown of fractal properties for scales larger than 4 µm.
Propagation of phase errors during phase refinement and extension
Authors: Makowski L.
Published: July 2002

A formalism is developed for estimating phase errors during refinement using non-crystallographic symmetry, solvent flattening or density modification. This formalism, based on a separation of all structure factors into known (best estimate) and unknown (random variable) parts, leads directly to simple expressions for the propagation of phase errors during a refinement process. Phase extension and treatment of unmeasured reflections are readily incorporated into this scheme. The formulation provides a direct method for evaluating the success of a refinement process. This may be useful in cases where examination of the resulting electron density map does not provide a quantitative evaluation of the calculations, such as at low to moderate resolution or when phase extension has been used. It may also provide a basis for designing optimal refinement strategies.
Extension of molecular replacement: a new search strategy based on Patterson correlation refinement
Authors: Brünger A. T.
Published: July 2002

A new search strategy is presented to obtain initial phases for single-crystal diffraction data by molecular replacement. It consists of carrying out 'Patterson refinements' of a large number of the highest peaks of a rotation function. The target function for Patterson refinement is proportional to the negative correlation coefficient between the squared amplitudes of the observed and the calculated normalized structure factors. If the root-mean-square difference between the search model and the crystal structure is within the radius of convergence of the minimization procedure employed, the correct orientation can be identified by having the lowest value of the target function after refinement. Similar to conventional crystallographic R-factor refinement, the target function for Patterson refinement may be combined with an empirical energy function describing geometric and non-bonded interactions. Patterson refinement of individual atomic coordinates or of rigid-group parameters may be carried out. Search models of crambin and of myoglobin with 1.6-2.0 Å backbone atomic r.m.s, differences from the target crystal structures show that the Patterson refinement strategy can solve crystal structures that cannot be solved by conventional molecular replacement or even by full six-dimensional searches.
A method calculating bond valences in crystals
Authors: O'Keeffe M.
Published: July 2002

A method of calculating the expected bond valences from the connectivity matrix of complex crystals is described. The method is exact (does not require iteration) and is suitable for implementation on a microcomputer.
Peculiarities of the low-symmetry phase structure near the phase-transition point
Authors: Chechin G. M. , Ipatova E. A. , Sakhnenko V. P.
Published: July 2002

The structure of the low-symmetry phase near the transition point depends on the number of arbitrary parameters, which is smaller than that determined by the space group GD
of this phase. Such a feature gives rise to some nontrivial structural effects in the vicinity of the transition point: there are numerical relationships among the displacements of the atoms belonging to different orbits of the GD
group. In some cases, atoms may be displaced in a direction not singled out by symmetry in any of the 230 space groups. These effects are revealed by group-theoretical methods and the Landau concept of one irreducible representation.
X-ray multislice computation using the Moodie–Wagenfeld equations: divergent-beam pattern simulation in three-beam and six-beam Laue cases
Authors: Goodman P. , Liu Linda
Published: July 2002

Pattern simulations for three-beam and six-beam X-ray diffraction are presented using multislice calculations based on Moodie & Wagenfeld's formulation of the X-ray equations, which factorize Maxwell's equations into Dirac format, using circular-polarization bases. The results are presented in three forms: one-dimensional rocking curves, Pendellösung thickness fringes, and convergent/divergent-beam patterns of single-diffraction orders, using experience gained from CBED (convergent-beam electron diffraction) and LACBED (large-angle CBED) techniques developed for high-voltage electron diffraction transmission patterns. This latter and quite new technique displays the results in the most compact form. The acronym DBXRAD (divergent-beam X-ray diffraction) is used for these patterns. The optics required for these patterns has only recently become available for radiations up to Mo Kα1 in energy and for limited angular divergences, but with capillary focusing currently undergoing rapid development these limits are likely to be extended. However, these simulations define critical angular ranges within reach of current designs. Simulations for light- and heavy-atom structures belonging to the enantiomorphic space-group pair P3121 and P3221 provide clear evidence of chiral interaction between radiation and structure, highlighting divergences from the use in structure analysis of the well studied CBED pattern symmetries. Mo Kα1 and Ta Kα1 wavelengths were used to minimize absorption for the two structures studied, an important factor owing to the large thicknesses (up to 20 mm) required.
A two-phase charge-density real-space-pairing model of high-Tc
superconductivity
Authors: Humphreys Colin J.
Published: July 2002

It is usually assumed that high-Tc
superconductors have a periodic band structure and a periodic charge density, although amorphous low-Tc
superconductors are known. In this paper, it is suggested that the CuO2 conduction planes of cuprate superconductors consist of regions of two different charge densities which do not normally repeat periodically. It is suggested that the pairing of holes occurs in real space in cuprate superconductors. It is proposed that the hole-pairing mechanism is magnetic exchange coupling and the pairing force is strong, the pairing energy being greater than kT at room temperature. The bound hole pair is essentially a bipolaron. A real-space model is very tentatively suggested in which the CuO2 planes of YBa2Cu3O7 contain nanodomains of a 3 × 3 hole lattice surrounded by interfaces one unit cell wide in which the holes are paired. In the superconducting state in this model, the existing hole pairs condense and move coherently and collectively around the insulating nanodomains, like trams running around blocks of houses, with one hole on each tramline. The hole pairs move in an elegant manner with hole pairs hopping from oxygen to oxygen via adjacent copper sites. The model explains the superconducting current being in the ab plane and it also explains the very short coherence lengths. Because the pairing force is strong, the model suggests that room-temperature superconductivity might be possible in carefully designed new oxide materials.
Study of NH4
+ in the zeolite phillipsite by combined synchrotron powder diffraction and IR spectroscopy
Authors: Gualtieri A. F.
Published: July 2002

The Rietveld refinement on synchrotron data of three natural hydrated NH4
+-exchanged phillipsites with different Si/Al ratios allowed acceptable assignment of the NH4
+ sites. These phillipsites display the typical monoclinic P21/m cell, whose volume increases with increasing numbers of NH4
+ ions, which are refined in the two sites labelled I and II. Some residual Na+ is retained in the site labelled II′. Distortion of the Si/Al tetrahedra, which affects some absorption bands in the IR spectra, seems to be mainly due to the distribution of Al in the framework, while the interaction of NH4
+ with the O atoms of the framework seems to play a minor role. NH4
+ in both sites acts as a donor in hydrogen bonding with O atoms of the framework and water molecules. NH4
+ in site I has three H atoms interacting with O atoms of the framework and a number of interactions with water O atoms. In all three samples, monocentric and dicentric connections are formed. NH4
+ in site II has one H atom interacting with O atoms of the framework and a number of interactions with water O atoms. Two monocentric connections and a quadricentric one were found in sample (1), three dicentric connections and a monocentric one in sample (2), and three monocentric connections and a dicentric one in sample (3).
Adducts of meso and racemic 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane with trigonally trisubstituted benzene carboxylic acids: supramolecular structures in one and two dimensions
Authors: Burchell Colin J. , Ferguson George , Lough Alan J. , Glidewell Christopher
Published: July 2002

The meso and racemic forms of 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, C16H36N4 (tet-a and tet-b, respectively), form adducts with trigonally trisubstituted benzene carboxylic acids; tet-a–3,5-dinitrobenzoic acid (1/2) (1), tet-a–5-hydroxyisophthalic acid–water (1/1/1) (3) and tet-b–5-hydroxyisophthalic acid–water (1/1/1) (4) are all salts, [C16H38N4]2+·2[C7H3N2O6]− (1) and [C16H38N4]2+·[C8H4O5]2−·H2O (3) and (4). The conformations of the [(tet-a)H2]2+ and [(tet-b)H2]2+ cations are entirely different: [(tet-a)H2]2+ is precisely centrosymmetric in (1) and approximately so in (3), while [(tet-b)H2]2+ has approximate C
2 symmetry in (4). In each salt the cation forms two intramolecular N—H...N and four intermolecular N—H...O hydrogen bonds. In (1) the supramolecular structure is one-dimensional, a C
2
2(13)[R
2
4(16)] chain of rings. Compounds (3) and (4) crystallize in space groups P212121 and P21/c, respectively, but the supramolecular structures are very similar: in each, the anions and the water molecules form a C(7)[R
3
3(13)] chain of rings, generated in (3) by a 21 axis and in (4) by a glide plane. These chains are linked, in both (3) and (4), by cations to form sheets. Adjacent meso cations in (3) are related by a 21 axis and adjacent chiral cations in (4) are related by a glide plane.
Structure predictions allowing more than one molecule in the asymmetric unit. Erratum
Authors: van Eijck Bouke P. , Kroon Jan
Published: July 2002

In the paper by van Eijck & Kroon (2000) the fifth and sixth sentences in the third paragraph of §2.1 are incorrect. The correct paragraph is here reprinted in full. Table 6 also contains errors and thus is also reprinted below.
X-ray investigations of sulfur-containing fungicides. II. Intramolecular forces governing the conformation of four novel α-phenylazo- and α-phenylhydrazono-β-ketosulfones
Authors: Wolf W. M.
Published: July 2002

The crystal and molecular structures of four novel β-ketosulfones, phenyl benzoyl(phenylhydrazono)methyl sulfone (I), phenyl benzoyl(4-nitrophenylhydrazono)methyl sulfone (II), (benzoyl)(phenyl)(phenylazo)methyl phenyl sulfone (III) and (benzoyl)(phenyl)(3-chlorophenylazo)methyl 4-tolyl sulfone (IV), have been investigated using X-ray analysis and density functional theory supplemented by ab initio Hartree–Fock calculations. The conformations of (I) and (II) are stabilized by strong intramolecular resonance-assisted hydrogen bonds formed between the sulfonyl and the α-hydrazono moieties. The following increase of a positive charge on the S atoms is compensated via π-conjugation and stereoelectronic back-donation from the nearby β-carbonyl group. Compounds (III) and (IV) adopt a strongly distorted propeller shape with pivotal tetrahedral C1 atoms. Distortion mostly follows from the approximate coplanarity of the α-azophenyl and α-phenyl moieties opposite each other. The main stereoelectronic interactions involve the donation of electron density from the α-azo N2 towards the sulfonyl S and the β-carbonyl C2 atoms.
Ethane-1,2-diphosphonic acid as a building block in supramolecular chemistry; a pillared-layer framework and framework-encapsulated cations
Authors: Wheatley Paul S. , Lough Alan J. , Ferguson George , Burchell Colin J. , Glidewell Christopher
Published: July 2002

The 1:1 adduct of piperazine and ethane-1,2-diphosphonic acid is a salt [C4H12N2]2+·[C2H6O6P2]2−, in which both ions lie across centres of inversion in space group P21/c. The anions are linked by a single type of O—H...O hydrogen bond [O...O, 2.562 (3) Å; H...O, 1.73 Å, O—H...O, 169°] into (6, 3) nets built from a single type of R^4_4(22) ring. The cations lie between these nets, linked to them by two types of N—H...O hydrogen bond [N...O, 2.635 (3) and 2.735 (3) Å; H...O 1.72 and 1.82 Å, N—H...O, 175 and 177°] such that the cations link adjacent sheets, thus forming a pillared-layer framework. The aquated adduct formed between trimethylenedipiperidine and ethane-1,2-diphosphonic acid is also a salt [C13H28N2]2+·[C2H6O6P2]2−·2.8[H2O], in which there are 12 different types of hydrogen bond, eight O—H...O and four N—H...O. The anions are linked into chains by pairs of O—H...O hydrogen bonds and these chains are linked by the water molecules into a continuous three-dimensional framework. Within the anion/water framework are large voids which contain pairs of cations, linked to the framework by N—H...O hydrogen bonds.
Patterns of soft C—H...O hydrogen bonding in diaryl sulfones
Authors: Glidewell Christopher , Harrison William T. A. , Low John N. , Sime Jamie G. , Wardell James L.
Published: July 2002

In bis(4-tolyl) sulfone, C14H14O2S (1), 2,5,4′-trimethyldiphenyl sulfone, C15H16O2S (2), and 4-chlorodiphenyl sulfone, C12H9ClO2S (3), the molecules are linked by soft C—H...O hydrogen bonds into three different types of one-dimensional aggregate: simple chains in (1), molecular ladders in (2) and chains of fused rings in (3). In each of 3,4-dimethyl-4′-chlorodiphenyl sulfone, C14H13ClO2S (4), and 2,5-dimethyldiphenyl sulfone, C14H14O2S (5), the C—H...O hydrogen bonds link the molecules into two different types of two-dimensional sheet, based on a (4,4) net in (4) and a (3,6) net in (5). The patterns of soft C—H...O hydrogen bonds in (1)—(5) are compared with those in other diaryl sulfones, mainly retrieved from the Cambridge Structural Database, whose substitution patterns preclude the formation of hard hydrogen bonds. Observed aggregation modes range from the formation of no C—H...O hydrogen bonds at all, via finite (zero-dimensional) arrays through one-, two- and three-dimensional systems.
Structure determination of two intercalated compounds VOPO4·(CH2)4O and VOPO4·OH—(CH2)2—O—(CH2)2—OH; synchrotron powder diffraction and molecular modelling
Authors: Goubitz Kees , Čapková Pavla , Melánová Klára , Molleman Wim , Schenk Henk
Published: July 2002

The crystal structures of two intercalated compounds have been determined using a combination of synchrotron powder diffraction and molecular mechanics simulations: (1) vanadyl phosphate intercalated with tetrahydrofuran, VOPO4·(CH2)4O, and (2) vanadyl phosphate intercalated with diethylene glycol, VOPO4·HO(CH2)2O(CH2)2OH. Both intercalates preserve the tetragonal space group P4/n, as found in the host structure VOPO4·2H2O. (1): a = 6.208, c = 8.930 Å, Z = 2, Dx
= 2.51 g cm−3; (2): a = 6.223, c = 11.417 Å, Z = 2, Dx
 = 2.66 g cm−3. Both intercalates exhibit the same type of orientational disorder in the arrangement of guest molecules, as observed in the same host compound intercalated with water. These two intercalates also exhibit, rather surprisingly, perfect ordering in layer stacking without the displacement disorder, characteristic of many intercalated layered structures. Thanks to this regularity in the arrangement of guests and layers, synchrotron powder diffraction could be used in the present structure determination. The present results also enabled the analysis of the effect of geometrical parameters characterizing the mutual host–guest complementarity and the effect of host–guest and guest–guest interaction on the crystal packing of intercalates.
Space-group changes: a revision to a revision
Authors: Marsh Richard E. , Sparks Robert A.
Published: July 2002

The space group for the entry under the reference codes FEBMUU and FEBMUU01 in the Cambridge Structural Database (1992) should be further corrected to space group C2/c rather than P1 (FEBMUU) or C2 (FEBMUU01).
Isostructuralism in a series of methylester/methylamide derivatives of (R,R)-O,O′-dibenzoyl tartaric acid; inclusion properties and guest-dependent homeotypism of the crystals of (R,R)-O,O′-dibenzoyltartaric acid diamide
Authors: Rychlewska Urszula , Warżajtis Beata
Published: July 2002

The compounds studied are methyl ester, amide and methylamide derivatives of (R,R)-O,O′-dibenzoyl tartaric acid. The molecules adopt the planar T conformation of the four-carbon chain with the terminal C=O bonds situated antiperiplanar with respect to the nearest C*—O bond. All investigated molecules occupy a twofold symmetry site in the crystal, including the mono(N-methylamide) monomethylester which lacks the C
2 molecular symmetry. Connected with this is the static disorder in which the methylester and the N-methylamide groups replace each other and isostructuralism within the methylester/methylamide series. (R,R)-O,O′-Dibenzoyltartaric acid diamides [(R,R)-O,O′-dibenzoyl-2,3-dihydroxybutanediamides], both primary and secondary, form hydrogen-bond aggregation patterns typical for amides, despite the presence of other hydrogen-bond acceptors in the molecule. However, in primary amides such packing leads to the creation of homeotypic crystal structures in which structural voids are filled by cyclic solvent molecules (pyridine, 1,4-dioxane). The presence of polyamide ladders, consisting of `fused' hydrogen-bond rings, seems to be responsible for the low solubility and high melting point of these substances.
Klockmannite, CuSe: structure, properties and phase stability from ab initio modeling
Authors: Milman Victor
Published: July 2002

The details of the electronic and crystal structure, the nature of the interatomic bonding and the phase stability of three modifications of klockmannite, CuSe, are analysed using first principles modeling. The hexagonal modification of CuSe is predicted to be less stable than the orthorhombic phase under pressure. The stabilizing force for the orthorhombic phase is identified as the Cu—Cu bond formation between the Cu atoms in the flat hexagonal CuSe layer and in the buckled Cu2Se2 layer. Furthermore, klockmannite is shown to be unstable under compression with respect to the decomposition into umangite, Cu3Se2, and krutaite, CuSe2 II.
Polymorphism of 7-dimethylaminocyclopenta[c]coumarin: packing analysis and generation of trial crystal structures
Authors: Gavezzotti A.
Published: July 2002

The crystal structures of two polymorphs of the title compound [Pbca and Pna21; Jasinski & Woudenberg (1995). Acta Cryst. C51, 107–109] were analysed. Packing energies and indices were compared. Molecules in the two forms show a slight conformational difference; both conformers were packed in some of the most frequent space groups for organic molecules (P21, P21/c, P212121 and Pna21) using a computer program for crystal structure generation and prediction (PROMET3). The results of such calculations are used to provide tentative explanations for the preference of the two conformers for centrosymmetric and non-centrosymmetric space groups. Several comments on general problems encountered in crystal structure prediction are also presented, concerning in particular the multi-minima structure of the potential energy hypersurface.
New ferroelectric inorganic materials predicted in point group 4mm
Authors: Abrahams S. C.
Published: July 2002

The Inorganic Crystal Structure Database containing a total of 209 entries for 128 different materials reported in point group 4 mm, of which eight are for previously known ferroelectrics. Analysis of the remaining entries, assuming the structure determinations reported are correct, leads to the prediction of an additional 43 new ferroelectric materials. 15 were determined in space group P4mm, 11 in P4bm, one in P42
nm, one in P4cc, two in P42
mc, one in P42
bc, seven in I4mm, two in I4cm, one in I41
md and two in I41
cd. Numerous other structures are shown to have been assigned, most likely, to an incorrect space group. All but one of the materials reported in space group P4bm are predicted to be ferroelectric. New ferroelectrics predicted in point group 4 mm include K2[Pt(CN)4]Br0.3.3H2O, Nd0.33TaO3, YBaCuFeO5+δ, Ba2(TiO)(Si2O7), K2(NbO)2(SiO3)4, Ba6CoNb9O30, Sr2SbMnO6, Hg2AlF5.(H2O)2, Rb5Nb3OF18 and SrNi2(VO4)2.
β-Al4.5FeSi: A Combined Synchrotron Powder Diffraction, Electron Diffraction, High-Resolution Electron Microscopy and Single-Crystal X-ray Diffraction Study of a Faulted Structure
Authors: Hansen V. , Hauback B. , Sundberg M. , Rømming Chr. , Gjønnes J.
Published: July 2002

A previous single-crystal X-ray and electron diffraction structure study [Rømming et al. (1994). Acta Cryst. B50, 307–312] of the heavily faulted alloy phase β-Al4.5FeSi has been extended by synchrotron powder data and further electron microscopy and diffraction observations. Reflections that were omitted in the single-crystal work could be included in the powder refinement, which resulted in some adjustment of cell parameters and atom coordinates. The double c axis reported by some authors is explained by periodic faults in the structure, which is described in terms of a tetragonal sub-unit. Apparent discrepancies between refinement from single-crystal and powder data are discussed briefly.
Temperature-dependent neutron powder diffraction evidence for splitting of the cationic sites in ferroelectric PbHf0.4Ti0.6O3
Authors: Muller C. , Baudour J.-L. , Madigou V. , Bouree F. , Kiat J.-M. , Favotto C. , Roubin M.
Published: July 2002

Temperature-dependent neutron powder diffraction experiments (diffractometer 3T2-LLB, Saclay, France, λ = 1.227 Å) have been performed on the perovskite-like lead hafnate titanate PbHf0.4Ti0.6O3. This compound belongs to the solid solution denoted PHT, which derives from the well known ferroelectric PZT series. It exhibits a ferroelectric-to-paraelectric phase transition around 620 K, between the low-temperature tetragonal phase and the high-temperature cubic phase. The tetragonal structure of the ferroelectric phase has been refined at 10 and 300 K using a Rietveld-type method: space group P4mm with Z = 1; at
= 3.999 (1), ct
= 4.120 (1) Å, c/a = 1.030, V = 65.89 Å3 at 10 K; at
= 4.012 (1) and ct
= 4.100 (1) Å, c/a = 1.022, V = 65.99 Å3 at 300 K. The cubic structure of the paraelectric phase has also been refined at 720 K: space group Pm3¯m, Z = 1, ac
= 4.046 (1) Å, V = 66.23 Å3. Cation displacements and oxygen-octahedra elongations have been observed as a function of temperature. Evidence for peculiar behaviour associated with the relative shifts of the Hf and Ti atoms (thought until now to be on the same crystallographic site) was found through an anomaly of the mean-square atomic displacements of the Hf/Ti pseudo-nucleus. The PDF Nos for PbHf0.4Ti0.6O3 are 48-49-9 and 48-49-10.
Disorder determined by high-resolution powder diffraction: structure of pentamethylcyclopentadienyllithium
Authors: Dinnebier Robert E. , Schneider Martin , van Smaalen Sander , Olbrich Falk , Behrens Ulrich
Published: July 2002

The crystal structure of pentamethylcyclopentadienyllithium, [Li(C10H15)] (LiCp*), has been determined from a high-resolution powder pattern by modelling and the maximum entropy method (MEM). The compound crystallizes in space group R3m with lattice parameters a = b = 14.7711 (5), c = 3.82206 (6) Å and V = 722.19 (4) Å3 (Z = 3). LiCp* forms polymeric `multidecker' chains along the c axis. The pentamethylcyclopentadienyl anions are coplanar with each other and show threefold rotational disorder. The MEM calculations did not only confirm the structural model and the type of disorder, but also discovered additional symmetry compared with the Rietveld analysis. This is the first solid-state structure of a Lewis-base-free alkali metal Cp* compound.
X-ray charge densities and chemical bonding by P. Coppens
Authors: Bryan R. F.
Published: July 2002

A review of this book, by Tibor Koritsansky, has been published in the September 1998 issue of Acta Crystallographica Section A, page 686.
Solution and refinement of the crystal structure of Bi7Ta3O18
Authors: Ling Christopher D. , Schmid Siegbert , Withers Ray L. , Thompson John G. , Ishizawa Nobuo , Kishimoto Shunji
Published: July 2002

The structure of heptabismuth tritantalum octadecaoxide, Bi7Ta3O18, has been solved and refined using single-crystal X-ray diffraction data collected at a synchrotron source in conjunction with unit-cell and symmetry information derived from electron diffraction. The space-group symmetry is triclinic C1 but is very close to monoclinic C2/m. A twin component observed during data collection was successfully modelled in the refinement. The C2/m prototype fitted all the Rietveld-refinable features of a medium-resolution neutron powder diffraction pattern. The metal-atom array is approximately face-centred cubic (fluorite type), punctuated by regularly spaced displacement faults perpendicular to the [111]fluorite direction every 2.5 fluorite unit cells. The metal-atom populations and O-atom positions are fully ordered. The Ta5+ cations are octahedrally coordinated, with TaO6 octahedra forming columns. The remaining O atoms occupy distorted fluorite positions. The Bi3+ cations occupy octahedral, square pyramidal or trigonal prismatic sites within the O-atom array; strain in the latter coordination environment appears to be responsible for the lowering of symmetry from monoclinic to triclinic.
A relation between structure factor, triple products and a single Patterson vector, and its application to sign determination
Authors: Krabbendam H. , Kroon J.
Published: July 2002

The Fourier coefficients D
h

of a hypersection of the double Patterson function at a fixed value of U can be written as D
h

= (1/V) Σ
h′
F

h

F

h′
F_{\overline {\bf h + h^\prime}} cos 2π
h′. U/cos 2π
h. (U/2). It is shown that also D
h

= F

h

G
h, where G
h is the Fourier coefficient of a reduced structure derived from the original structure by positioning an atom with form factor Σ
h′
f

i
(h′)fj
({\overline {\bf h + h^\prime}}) halfway in between those atoms i and j, which are a vector U apart. If U is a single vector between two atoms with form factor f(h), then the reduced structure contains only one atom, at the origin, and G

h
= (1/V) Σ
h′
f(h′)f({\overline {\bf h + h^\prime}}). We obtain the relation Σ
h′
F

h

F

h′
F_{\overline {\bf h + h^\prime}} cos 2 π
h′. U/[cos 2π
h. (U/2) Σ
h′
f(h′)f{\bf h + h^\prime;}] = F

h
. Using this formula with probabilities for the signs of the triple products, the single vectors U are found by a scanning process, as we demonstrate on a heavy atom structure. The signs of the structure factors have thus been calculated directly from the triple products.
Parallel beams of neutrons or X-rays by multiple Bragg reflection
Authors: Kottwitz D. A.
Published: July 2002

A method of producing extremely parallel (and monochromatic) beams of neutrons or X-rays by multiple Bragg reflection in perfect crystals is proposed. The scheme is based on the simulation of a forbidden reflection by the cooperative action of three other reflections. The conditions necessary for obtaining the desired effect are discussed, and a short list of several reflections satisfying some of the conditions is given.
On the application of phase relationships to complex structures. XIV. The additional use of statistical information in tangent-formula refinement
Authors: Hull S. E. , Irwin M. J.
Published: July 2002

Suitable weighting schemes for use in tangent-formula phase development and refinement are discussed, and a statistically based weighting scheme, which can easily be incorporated in existing computer programs, is proposed. Examples of the use of this method in structure solution and completion are presented for a previously unknown structure and for several structures which had been difficult to solve by other methods. In addition a method is given for subtracting the contribution of heavy atoms from observed |E| values on a statistical basis which is very useful when the presence of such atoms leads to problems in defining the enantiomorph. These methods add greatly to the power of the MUL TAN computer program.