bluuebunny/crossref_metadata_2025 huggingface dataset:
Phase analysis of bulk samples using sample-tilting X-ray diffractometry
Authors: Cong Q.
Published: July 2002

In the Bragg–Brentano X-ray powder diffractometer, the sample-tilting X-ray diffraction (STD) technique probes a fixed depth of penetration from the sample surface. In this way, phase analysis can be carried out from the surface layers to the depth probed by the CBD (conventional Bragg–Brentano geometry X-ray diffraction) method. In the present paper, after derivation of the diffracted intensity and the observed crystal-plane azimuthal equations, attention is focused on investigations of the geometrical optics of X-ray surface reflection by comparing the STD and CBD methods. Some examples are given to illustrate the applications of the STD technique for solving phase analyses and related problems.
Background correction of the SAXS intensities scattered by semicrystalline materials
Authors: Meng C.
Published: July 2002

A new method of background correction of small-angle X-ray scattering (SAXS) intensities scattered by semicrystalline materials is proposed. The intensities scattered by the sample when in the amorphous state were taken as the background scattering. This method was applied to background correction of the SAXS intensities scattered by aged nylon 1010 and the width of the transition layer between crystalline and amorphous phase was determined to be 2.0 nm.
The detective quantum efficiency of CCD and vidicon-based detectors for X-ray crystallographic applications
Authors: Stanton M. , Phillips W. C. , Li Y. , Kalata K.
Published: July 2002

The detective quantum efficiency (DQE) characterizes the measurement uncertainty introduced by a detector. A formulation of the DQE for CCD and vidicon-based X-ray integrating detectors is developed and illustrated with five model detectors. A comparison of the calculated and measured DQEs for two of these detectors is presented. From an examination of the DQEs for the model detectors, generalizations are made regarding detector designs.
Corrections to Equations for Estimating the Counting-Statistical Errors in Diffraction Stress Measurements
Authors: Winholtz R. A.
Published: July 2002

Two corrections are made to the equations for estimating the counting statistical errors in diffraction stress measurements. It is shown that the previous equations provide a conservative estimate of the counting-statistical component of the random errors in stress measurements. The results from the corrected equations are compared to a Monte Carlo model and to replicated measurements. A procedure to handle other sources of random error is also suggested.
Space-Group Determination of the Orthorhombic Form of NaV6O11
Authors: Kanke Y. , Shigematsu H. , Ohshima K. , Kato K.
Published: July 2002

The unit cell and space group of orthorhombic NaV6O11 (low-temperature form) are found by an X-ray single-crystal diffraction study at 300 K (hexagonal, P63/mmc), 100 K (hexagonal, P63
mc) and 20 K (orthorhombic). The orthorhombic form (o) shows no superstructure and its unit cell is related to the hexagonal one (h): a

o
≃ a

h
+ b

h
, a

o
≃ −a

h
+ b

h
and c

o
≃ c

h
. Bijvoet-pair examination confirms that it crystallizes in the noncentrosymmetric space group Cmc21. The hexagonal (P63
mc)-orthorhombic (Cmc21) structural phase transition is proved to be of second order.
A High-Energy Monochromatic Laue (MonoLaue) X-ray Diffuse Scattering Study of KMnF3 using an Image Plate
Authors: Gibaud A. , Harlow D. , Hastings J. B. , Hill J. P. , Chapman D.
Published: July 2002

The technique of high-energy monochromatic Laue X-ray scattering using image plates to record the diffraction patterns is presented. A tunable wiggler beamline is used as an X-ray source. It is shown that such experimental conditions present many advantages over conventional tube sources and photographic films. A study of diffuse scattering in the perovskite compound KMnF3 is presented to illustrate this in a qualitative way.
The Use of Quartz as an Internal Pressure Standard in High-Pressure Crystallography
Authors: Angel R. J. , Allan D. R. , Miletich R. , Finger L. W.
Published: July 2002

The unit-cell parameters of quartz, SiO2, have been determined by single-crystal diffraction at 22 pressures to a maximum pressure of 8.9 GPa (at room temperature) with an average precision of 1 part in 9000. Pressure was determined by the measurement of the unit-cell volume of CaF2 fluorite included in the diamond-anvil pressure cell. The variation of quartz unit-cell parameters with pressure is described by: a −4.91300 (11) = −0.0468 (2) P + 0.00256 (7) P
2 − 0.000094 (6) P
3, c − 5.40482 (17) = − 0.03851 (2) P + 0.00305 (7) P
2 − 0.000121 (6) P
3, where P is in GPa and the cell parameters are in ångstroms. The volume–pressure data of quartz are described by a Birch–Murnaghan third-order equation of state with parameters V
0 = 112.981 (2) å3, K
T0 = 37.12 (9) GPa and K′ = 5.99 (4). Refinement of K′′ in a fourth-order equation of state yielded a value not significantly different from the value implied by the third-order equation. The use of oriented quartz single crystals is proposed as an improved internal pressure standard for high-pressure single-crystal diffraction experiments in diamond-anvil cells. A measurement precision of 1 part in 10 000 in the volume of quartz leads to a precision in pressure measurement of 0.009 GPa at 9 GPa.
Implementation of a six-dimensional search using theAMoRetranslation function for difficult molecular-replacement problems
Authors: Sheriff Steven , Klei Herbert E. , Davis Malcolm E.
Published: July 2002

A six-dimensional molecular-replacement procedure has been implemented using Perl scripts and theCCP4 version of the translation function of the molecular-replacement programAMoRe. These tools have allowed the structure determination of CTLA-4 monomer from NMR-derived coordinates, and of a potential complex of MurD with a substrate. In both cases other molecular-replacement programs,X-PLORandAMoRe, when used in a conventional manner, either failed or did not yield an obvious solution.
The influence of cylindrical geometry on X-ray stress tensor analysis. II. Applications
Authors: Dionnet B. , François M. , Sprauel J. M. , Nardou F.
Published: July 2002

The usual formulation used for determining stresses by X-ray diffraction with a flat specimen is not valid when the sample has a curved surface. In the first paper of this series [Françoiset al.(1995).J. Appl. Cryst.28, 761–767], a general formulation was presented, showing that two effects arise simultaneously: a translation and a rotation effect. In the present paper, analytical formulae, derived from the general expression and valid under certain assumptions, are given. They exhibit a linear relation between the measured strain and sin2ψ and they can be used in many practical cases. Experimental and numerical examples of the use of the simplified and the general formulation on thick and thin wires, thin layers on wires, convex and concave specimens, and isotropic and textured samples are given. The range in which the classical and the present formulation are valid is also studied.
A high-temperature furnace for X-ray diffraction with directly machined α-Al2O3ceramic parts
Authors: Estermann Michael , Reifler Hans , Steurer Walter , Filser Frank , Kocher Peter , Gauckler Ludwig J.
Published: July 2002

A complex high-temperature furnace has been produced from presintered α-Al2O3precompacts. The furnace is designed for the Mar345 imaging-plate detector system. Up to now, the design complexity of ceramic parts has been limited by the difficulty of machining hard ceramic materials or by the moulds used in hot-pressing and casting. However, the machining of soft presintered ceramic materials enables the realisation of much more demanding designs, provided that the final sintering shrinkage of the ceramic is homogeneous and predictable. The design of the furnace and its realisation and application in an X-ray diffraction study with synchrotron radiation are presented.
Localized destructive interference in X-ray specular reflectivity
Authors: Eymery J.
Published: July 2002

Localized destructive interference can be obtained for specific values of the angle of incidence when studying semiconductor superlattices with X-ray reflectivity at fixed wavelength. Kinematical calculations show that the difference between the real parts of the refractive index of the layers must be small, and that the number of superlattice periods must be optimized to enhance the destructive interference. An experimental example, involving a CdTe/CdZnTe superlattice, and optical matrix simulations for SiGe/Si superlattices are presented to illustrate this effect. Finally, it is shown that such structures can be tailored to act as energy filters at fixed angles of incidence.
Cryocrystallography with oil – an old idea revived
Authors: Riboldi-Tunnicliffe A. , Hilgenfeld R.
Published: July 2002

A method is described that permits the cooling of protein crystals without the use of conventional cryoprotective agents. Covering the crystal completely with highly liquid dry paraffin oil (Merck) prevents ice formation. It is suggested that this method, because of its simplicity, be tried as a first choice in cryocrystallography, since it does not require the growth or soaking of crystals in solvents which could disturb the packing by diffusing into the crystal.
Ultra-small-angle X-ray and neutron scattering studies on colloidal crystals
Authors: Matsuoka H. , Harada T. , Ikeda T. , Yamaoka H.
Published: July 2002

The formation of colloidal crystals by charged colloidal particles in dispersion at a low ionic strength has been interpreted by the DLVO theory that assumes the electrostatic repulsion between particles to be a dominant factor. However, several recent observations can not be interpreted by the DLVO theory, one of which is our finding obtained by the ultra-small-angle X-ray scattering (USAXS) technique, i.e., the effect of added salt concentration on the interparticle distance in colloidal crystals. We obtained interesting and significant information by the ultra-smallangle scattering techniques on the interparticle interaction, which support the existence of a novel factor.
Accuracy of molecular structures determined from high-resolution powder diffraction. The example ofm-fluorobenzoic acid
Authors: Pattison Philip , Knudsen Kenneth D. , Fitch Andrew N.
Published: July 2002

The accuracy with which it is possible to determine atomic coordinates of light atoms at arbitrary positions using X-ray powder data alone is demonstrated. For this purpose use is made of synchrotron powder diffraction data obtained for an organic molecule,m-fluorobenzoic acid, consisting of ten non-hydrogen atoms (C7H5FO2). The results are compared with those of a single-crystal study on the same compound, and in particular the differences in bond lengths and bond angles are studied. The structure solution produced by direct methods is compared with the final result after the Rietveld refinement. The effect of performing the data collection at room temperature and at 100 K is also investigated. The bond lengths extracted from the powder data were found to differ from the single-crystal values by an average of 0.025 Å when data collected at room temperature were employed, and by 0.021 Å when using 100 K data. For the bond angles the corresponding values were 3.5 and 1.5°, respectively. With the low-temperature data it was possible to refine all atoms, including the hydrogen atoms, without restraints, whereas for the data collected at room temperature, strong restraints had to be used for the hydrogen atoms.
Crystal structure refinement of α-Si3N4using synchrotron radiation powder diffraction data: unbiased refinement strategy
Authors: Toraya H.
Published: July 2002

The crystal structure of α-silicon nitride (Si3N4) was refined by the Rietveld method using synchrotron radiation powder diffraction data (wavelength = 1.2 Å) collected at station BL-4B2 in the Photon Factory. A refinement procedure that adopted a new weight function,w= 1/Y_o^e (Yois the observed profile intensity ande≃ 2), for the least-squares fitting [Toraya (1998).J. Appl. Cryst.31, 333–343] was studied. The most reasonable structural parameters were obtained withe= 1.7. Crystal data of α-Si3N4: trigonal,P31c,a= 7.75193 (3),c= 5.61949 (4) Å,V= 292.447 (3) Å3,Z= 4;Rp= 5.08,Rwp= 6.50,RB= 3.36,RF= 2.26%. The following five factors are considered equally important for deriving accurate structural parameters from powder diffraction data: (i) sufficiently large sin θ/λ range of >0.8 Å−1; (ii) adequate counting statistics; (iii) correct profile model; (iv) proper weighting on observations to give a uniform distribution of the mean weighted squared residuals; (v) high-angular-resolution powder diffraction data.
Diffraction by a randomly distorted crystal. I. The case of short-range order
Authors: Becker P. , Al Haddad M.
Published: July 2002

Kato's statistical theory of diffraction [Kato (1980). Acta Cryst. A36, 763-769, 770-778] is reformulated in a self-consistent manner. The local displacement field u(r) occurs through the phase factor φ(r) = exp [2πi
h . u(r)]. The present paper is concerned with the limiting case where (φ~(r)) = E = 0: this corresponds to the situation where only secondary extinction is present. There are two correlation lengths in the problem, the first one τ for the phase factor φ, the second one Γ for the wave-field amplitudes. Kato assumed Γ » τ. It is shown in the present paper that Γ ≃ τ, a property which has important consequences for the general theory, where E ≠ 0, to be discussed in the second paper of this series.
A `three-beam' analysis of resonance scattering in reflection high-energy electron diffraction
Authors: Anstis G. R.
Published: July 2002

Enhanced reflection of fast electrons from a crystal surface and a decrease in the depth of penetration of the primary beam occurs when diffraction conditions are such as to set up a wave travelling just beneath the crystal surface. This is the surface resonance condition for reflection high-energy electron diffraction (RHEED). Quantitative prediction of these effects can be achieved by assuming that only the primary and two diffracted beams are significant. Expressions for the coefficient of reflection and the depth of penetration in terms of a few Fourier coefficients of an effective potential are derived. These coefficients depend sensitively on incident-beam direction and are significantly different from the values for the bulk crystal. In particular, the mean potential experienced by the electrons in the resonance state is increased. It can be estimated using Bethe's perturbation approach. Predictions of the position, height and width of the peak in reflectivity resulting from resonance scattering from the (111) surface of platinum are in reasonable agreement with the values obtained from many-beam computations. The three-beam approach gives insight into resonance scattering using the standard formalism of diffraction theory.
Enumeration of four-connected three-dimensional nets. I. Conversion of all edges of simple three-connected two-dimensional nets into crankshaft chains
Authors: Han Shaoxu , Smith Joseph V.
Published: July 2002

Various topological approaches to mathematical analysis, classification and enumeration of four-connected three-dimensional (3D) nets are listed. Four-connected 3D nets are being generated systematically by conversion of edges of a vertical stack of congruent three-connected two-dimensional (2D) nets into chains. This paper describes 57 3D nets obtained by converting into crankshaft chains all edges of the simpler 2D nets in the catalog of the Consortium of Theoretical Frameworks. Atomic coordinates are given for distance-least-squares modeling in the highest space group. Nine nets occur in known structures: tridymite, Zn2P2O8 \cdot organic, aluminophosphates AlPO-5 (International Zeolite Association Structure Commission code AFI), AlPO-8 (AEI), AlPO-11 (AEL), AlPO-25 (ATV), AlPO-41 (AFO), AlPO-54 (VFI) and AlPO-H2 (AHT).
Reverse Monte Carlo modelling of neutron powder diffraction data
Authors: Mellergård A. , McGreevy R. L.
Published: July 2002

A new reverse Monte Carlo (RMC) method for modelling both lattice and magnetic disorder in powder crystalline materials by direct calculation of the structure factor has been developed. The method, the program and the basic theory are described in some detail. Initial results from modelling the lattice and magnetic structure of MnO around the Néel temperature are also presented.
Electron-density-based calculations of intermolecular energy: case of urea
Authors: Suponitsky Kyrill Yu. , Tsirelson Vladimir G. , Feil Dirk
Published: July 2002

The intermolecular interaction energy in crystalline urea has been calculated both from diffraction data and from the Hartree–Fock crystalline electron-density distribution, using a modified atom–atom approximation scheme. The electrostatic part of this energy has been calculated from the atomic multipole moments, obtained by adjustment of the multipole model to experimental X-ray and to theoretical Hartree–Fock structure amplitudes. To obtain the induction energy, multipole moments were calculated from structure amplitudes for the crystalline electron density and from those that refer to the electron density of a superposition of isolated molecules. This worked well for the calculation of the interaction energy from Hartree–Fock data (6% difference from the sublimation-energy value), but not for the interaction energy from experimental data, where the moments of the superposition have to come from Hartree–Fock calculations: the two sets of multipole moments are far too different. The uncertainty of the phases of the structure amplitudes, combined with systematic errors in the theoretical data and noise in the experimental values, may account for the discrepancies. The nature of the different contributions to intermolecular interactions for urea is examined.
On the errors in molecular dipole moments derived from accurate diffraction data
Authors: Coppens Philip , Volkov Anatoliy , Abramov Yuriy , Koritsanszky Tibor
Published: July 2002

The error in the molecular dipole moment as derived from accurate X-ray diffraction data is shown to be origin dependent in the general case. It is independent of the choice of origin if an electroneutrality constraint is introduced, even when additional constraints are applied to the monopole populations. If a constraint is not applied to individual moieties, as is appropriate for multicomponent crystals or crystals containing molecular ions, the geometric center of the entity considered is a suitable choice of origin for the error treatment.
Experimental and theoretical charge density of DL-alanyl-methionine
Authors: Guillot Régis , Muzet Nicolas , Dahaoui Slimane , Lecomte Claude , Jelsch Christian
Published: July 2002

X-ray diffraction data up to d = 0.50 Å resolution have been collected at 100 K for a DL-alanyl-methionine single crystal using a CCD area detector. Multipolar crystallographic refinement was carried out and the electron density of the molecule has been analyzed. The deformation electron density around the S atom reveals two lone pairs with an sp
3 hybridization and agrees with the results of density functional theory calculations. The topological properties of the covalent bonds and of the hydrogen bonds have been investigated. Two weak polar intramolecular interactions of the type C5 (pentagonal cyclic structure) have unfavorable geometrical parameters for hydrogen bonds and are devoid of critical points. The two electron lone pairs of the carbonyl oxygen appear asymmetric in the experimental deformation density. This could be attributed to the different strength of the hydrogen bond and intramolecular polar interaction involving the carbonyl oxygen. In the ab-initio-derived deformation maps, the asymmetry of the electron doublets is reproduced only very partially.
Structure of N,4-dinitroaniline and its complex with sulfolane at 85 K; on the proton donor–acceptor affinity of the primary nitramine (HNNO2) group
Authors: Zaleski J. , Daszkiewicz Z. , Kyzioł J. B.
Published: July 2002

The NNO2 group of the title compound is significantly less twisted with respect to the aromatic ring in comparison to a typical secondary nitramine. The amide nitrogen is trigonally hybridized. The nitramino group is almost planar. The C—C—N—N torsion angles vary between ca 13 and 42°, whereas the twist along the N—N bond is much smaller and amounts to between ca 1 and 15°. Those twist angles are governed by a crystal packing and are much larger in the case of crystals of pure N,4-dinitroaniline in comparison to that of its complex with sulfolane. The deviations of the internal angles of the aromatic ring from 120° do not exceed 3°. The presence of the nitro group increases the C—C—C valence angle of ca 2.0–2.6°, whereas an analogous effect associated with the nitramino group is much smaller (ca 0.3–1.3°), pointing to its weak electron-withdrawing properties. The nitramino group displays no tendency to conjugate with an electron-demanding substituent across the ring. It participates in hydrogen bonding only as a hydrogen-bonding donor. It does not act as a proton acceptor, despite the fact that nitramine rearrangement is catalysed by acids.
The NIST Surface Structure Database – SSD version 4
Authors: Van Hove M. A. , Hermann K. , Watson P. R.
Published: July 2002

SSD-4, the fourth version of the NIST Surface Structure Database, has appeared in early 2002 as a significant upgrade to both the database and graphics software; it now also includes nearly 1300 structures published through the end of 2000. SSD is an interactive PC-based database of critically selected surface structures determined with a variety of experimental techniques. The data include not only atomic coordinates, bond lengths and bond angles, but also information about experimental preparation, measurement and analysis methods. The software provides advanced search and display facilities, as well as interactive three-dimensional color visualization and analysis tools of great flexibility. In addition, the program can generate publication-quality color or grayscale prints of any structure, with a host of user-selectable options (such as view angle, perspective, colors, ball and bond styles, and labels).
Charge density around a Jahn-Teller-distorted site: (ND4)2Cu(SO4)2.6D2O at 85 K
Authors: Figgis B. N. , Khor L. , Kucharski E. S. , Reynolds P. A.
Published: July 2002

Diammonium hexaaquacopper(II) disulfate-d
20, [ND4]2[Cu(D2O)6](SO4)2, Mr
= 419.7, monoclinic, P21/a, a = 9.399 (2), b = 12.673 (2), c = 6.071 (1) Å, β= 107.13 (1)°, V= 691.1 (4) Å3, Z = 2, Dx
= 2.02 Mg m−3, Mo Kα radiation, λ = 0.71069 Å, μ = 2.008 mm−1, F(000) = 415.3, T= 85 (2) K, R(I) = 0.020,R(F) = 0.014 for 7287 reflections. The CuO6 octahedron has a large Jahn–Teller distortion; Cu—O(8) 2.301 (1), Cu—O(7) 2.010 (1), Cu—O(9) 1.960 (1) Å. It was necessary to use quartic anharmonic thermal parameters at the CuII site. These modelled potential softening associated with the Jahn–Teller distortions, important even at this temperature. At the ammonium, the sulfate and the hexaaqua-ion sites the charge densities, at this experimental accuracy, can be described by simple valence functions which reflect only local symmetry. Large lower-symmetry densities are not observed; any small such effects are comparable with uncertainties in the treatment of the thermal motion. The valence refinement gave Cu 3d populations of 3d^{1.58 (6)}_{xy}3d^{2.12 (3)}_{xz, yz}3d^{2.18(6)}_{z^{2}}3d^{0.82 (7)}_{x^{2} - y^{2}}. This shows the hole expected in 3d_{x^{2}_y^{2}}, modified, taking overlap into account, by a covalent or acceptance of 0.52 (1) and π back donation of 0.18 (10) e. The rhombic distortion causes 3d_{z^2}/3d_{x^{2} - y^{2}} mixing corresponding to a 3d_{z^2} coefficient of −0.21 (7) in a spin-hole wavefunction. Ionic charges from the model are Cu(OD2)^{17+}_{6}, ND^{6+}_{\xB0} and SO^{1.5-}_{4}. These are more ionic than for the isomorphous CrII salt, where σ and π charge flows are both to the metal, not opposed as here. Qualitatively these results agree with the conclusions from ESR, NMR and optical spectra.
Electron density study of a new non-linear optical material: L-arginine phosphate monohydrate (LAP). Comparison between X–X and X–(X + N) refinements
Authors: Espinosa E. , Lecomte C. , Molins E. , Veintemillas S. , Cousson A. , Paulus W.
Published: July 2002

The crystal structure, thermal vibrations and electron density of L-arginine phosphate monohydrate (formally C6H15N4O2
+.H2PO4
−.H2O) have been analysed using 130 K single-crystal X-ray diffraction data to a resolution of (sin θ/λ)max = 1.20 Å−1. A multipolar pseudo-atom density model was fitted against the 6805 observed data with I > 3σ(I), [R(F) = 0.016,Rw
(F) = 0.014, S = 1.39] in order to map the static valence-electron density distribution. Positional and thermal vibration parameters for H atoms were taken from neutron diffraction results. A comparison between the electron density ρ(r), ∇2
ρ(r) and the electrostatic potential calculated from X−X and X−(X + N) refinements shows that reliable results may be obtained from X−X data only.
Static disorder in the incommensurate structure of the high T

c
superconductor Bi2Sr2CaCu2O8+δ
Authors: Grebille D. , Leligny H. , Ruyter A. , Labbé Ph. , Raveau B.
Published: July 2002

The incommensurate modulated structure of the high Tc
superconductor of nominal composition Bi2Sr2CaCu2O8 has been refined using an optimized data set of 1530 reflections (496 main reflections, 753 first-order and 281 second-order satellites) in superspace group Bbmb(Oβ1) using a Fourier expansion of the modulation functions up to second order. A large improvement of the R (wR) factor [global: 0.056 (0.051); main reflections: 0.049 (0.043); first-order satellites: 0.056 (0.058); second-order satellites: 0.078 (0.079)] is obtained by the introduction of a static modulated disorder on the Bi site without any thermal factor modulation and by considering occupational modulation functions. A detailed comparison with previous studies in modulated structure models or in supercell models is given, in particular for the description of the BiO layers.
Importance of True Satellite Reflections in the Analysis of Modulated, Composite Crystal Structures. II. The Structure of [M'2Cu2O3]7[CuO2]10, M' = Bi0.04Sr0.96
Authors: Jensen A. F. , Petříček V. , Larsen F. K. , McCarron E. M.
Published: July 2002

The structure of the modulated, composite crystal bismuth strontium cuprate [M′2Cu2O3]7[CuO2]10, with M′ = Bi0.04Sr0.96, has been determined and refined using a model which allows modulation of the atomic positional parameters for all atoms, as well as modulation of thermal parameters for the M′ site. The structure is orthorhombic, with c being the misfit direction and a = 11.4712 (6), b = 13.3765 (13) Å. Sublattice 1, of composition (CuO2)4, has c
l = 2.7509 (10) Å, while sublattice 2, of composition (M′2Cu2O3)4, has c
2 = 3.9316 (3) Å. Superspace group Amma(001 + γ)ss\overline 1; q = γc
l* with γ = 0.6997 (3); λ = 0.6565 Å, μ = 25.4 mm−1. The final residual is R
all = 0.0474 for all 1174 reflections, R
main = 0.0451 for the 549 main reflections, R
1.sat = 0.0562 for the 442 first-order satellite reflections and R
2.sat = 0.1044 for the 183 second-order satellite reflections. The satellite reflections have been collected with X-ray synchrotron radiation. It is shown that these reflections help to unambiguously determine the superspace group. The orthorhombic compound has a layered structure, stacked along the b axis. The first sublattice consists of layers of CuO2 ribbons, with CuO4 squares sharing edges along the c axis. The second sublattice consists of CuO4 squares, which are sharing edges in a zigzag pattern. In-between these layers the disordered layers of Bi and Sr atoms are sandwiched. A comparison with structurally and chemically related compounds is made.
Revision of Ferroelastic Structures of n-Heptyl- and n-Octylammonium Dihydrogen Phosphate Crystals
Authors: Fábry J. , Petrícek V. , Císarová I. , Kroupa J.
Published: July 2002

This study deals with the structure determination of C7H15NH3
+.H2PO4
− (C7ADP) and C8H17NH3
+.H2PO4
− (C8ADP). The samples used in this study were not subjected to a phase transition after they had been crystallized. Unlike a previous structure determination, weak reflections, now with indices h = 2n + 1, were included. This means that both structures are described in unit cells with the lattice parameters a twice as long as given previously. Both structures are quite similar; two double layers of dihydrogen phosphates, which are interconnected by hydrogen bonds (2.52–2.62 Å), pass through each unit cell. Alkylammonium groups interact with these dihydrogen phosphates via longer hydrogen bonds (>2.75 Å), while the rest of the aliphatic chains interact via van der Waals contacts. All H atoms were localized and no disorder of the H atoms was detected. Both structures described in the space group P121/n1 exhibit a reproducible ferroelastic switching. The hypothetical prototypic phase is orthorhombic with the space group number 60 P2/b21/n21/a. All atoms except two hydrogen species exist in pairs linked by the lost symmetry operations derived from the prototypic space group and are brought close to each other – up to 0.25 Å – under the action of them. Each of these two different H atoms is involved in an asymmetric hydrogen bond between an oxygen pair. Under the action of a lost symmetry operation each of these H atoms is displaced from one oxygen towards the other. Therefore, it is assumed that during the ferroelastic switching the jumps of these two hydrogen species take place between the pertinent hydrogen-bond acceptor and donor O atoms. Hence, these O atoms reverse their role as hydrogen-bond donors and acceptors during the ferroelastic switching.
Inclusion Compounds of Thiourea and Peralkylated Ammonium Salts. VI. Hydrogen-Bonded Host Lattices Constructed From Thiourea and Anions of Oxalic Acid and Fumaric Acid
Authors: Li Q. , Mak T. C. W.
Published: July 2002

New inclusion complexes tetra-n-butylammonium hydrogen oxalate–thiourea (1/2), (n-C4H9)4N+.HC2O4
−.2[(NH2)2CS] (1), tetramethylammonium hydrogen fumarate–thiourea (1/1), (CH3)4N+.HC4H2O4
−.(NH2)2CS (2), di(tetraethylammonium) fumarate–thiourea (1/2), [(C2H5)4N+]2.C4H2O4
2−.2[(NH2)2CS] (3) and tetra-n-propylammonium hydrogen fumarate–thiourea–water (1/1/2), (n-C3H7)4N+.HC4H2 O4
−.(NH2)2CS.2H2O (4) have been prepared and characterized by X-ray crystallography. Crystal data, Mo Kα radiation: (1), space group P21/n, a = 8.854 (6), b = 9.992 (3), c = 32.04 (2) Å, β = 97.34 (3), Z = 4, R

F
= 0.055 for 2261 observed data; (2), space group P\overline 1, a = 6.269 (2), b = 8.118 (4), c = 14.562 (8) Å, α = 104.79 (4), β= 91.72 (4), γ = 101.30 (4)°, Z = 2, R

F
= 0.078 for 1543 observed data; (3), space group P21/n, a = 11.340 (2), b = 9.293 (6), c = 14.619 (2) Å, β = 102.41 (2)°, Z = 2, R

F
= 0.050 for 1856 observed data; (4), space group P2/n, a = 16.866 (4), b = 8.311  (1), c = 17.603 (2) Å, β = 104.94 (1)°, Z = 4, R

F
= 0.048 for 2785 observed data. In the crystal structure of (1) the tetra-n-butylammonium ions are sandwiched between puckered layers, which are constructed from thiourea-hydrogen oxalate ribbons. In the crystal structure of (2), zigzag O--H...O and C--H...O hydrogen-bonded hydrogen fumarate ribbons are linked by thiourea dimers to form a wide puckered ribbon and the crystal structure is built of a packing of these thiourea–anion composite ribbons and the cationic columns. In the layer-type crystal structure of (3) a series of thiourea–fumarate layers match the (002) planes and the (C2H5)4N+ cations occupy the intervening space. In the crystal structure of (4) the thiourea, hydrogen fumarate ions and water molecules are connected by hydrogen bonds to form wide puckered ribbons, which are crosslinked to generate a three-dimensional host framework containing open channels aligned parallel to the a axis, with the tetra-n-propylammonium cations accommodated in a single column within each channel.
Two Crystal Structures Towards the Discotic Columnar Mesophase of (1,4,8,11,15,18,22,25-Octahexylphthalocyaninato)nickel
Authors: Helliwell M. , Deacon A. , Moon K. J. , Powell A. K. , Cook M. J.
Published: July 2002

(1,4,8,11,15,18,22,25-Octahexylphthalocyaninato)nickel is a thermotropic liquid crystal, undergoing a transition to a discotic columnar mesophase at 418 K. Its crystals are weakly diffracting needles. Single-crystal X-ray diffraction studies at 103 K, using intense synchrotron radiation of wavelength 0.504 Å at Station F2, CHESS and the `Princeton 1k' CCD area detector, and at 295 K using a Cu Kα rotating anode diffractometer, have respectively shown that the symmetry of the crystals is increased from primitive monoclinic (space group P21/n) to C-centred monoclinic (space group C2/c) when the temperature is raised. Solution of the structure at each temperature shows that the increase in symmetry at 295 K arises from slight movement of the molecules, as well as increased thermal motion of the hexyl groups, and small conformational changes in these groups, indicating the first stage of the transition to the discotic columnar mesophase, when the hexyl groups are thought to become completely mobile. The quality of the refinement of the structure is indicative of what can now be realized by employing multipole wiggler synchrotron radiation and a CCD area detector, as well as cryocooling, for difficult small molecule samples. The temperature of the space group transition was determined at the Daresbury SRS, on Station 7.2, using a CCD detector to monitor an area of the diffraction pattern, whilst varying the temperature systematically. A gradual appearance of spots for h + k = 2n + l occurred between 205 and 195 K. Tests were also made using an image plate system at 195 and 293 K, which confirmed the transition from primitive to C-centred lattice symmetry with temperature. No other changes were apparent on the CCD or IP detectors.
Crystal Engineering Using Bisphenols. Chains, Ladders and Sheets Formed by the Adducts of 1,4-Diazabicyclo[2.2.2]octane with Bisphenols: Structures of Adducts with 4,4'-Isopropylidenediphenol (1/1), 4,4'-Oxodiphenol (1/1) and 4,4'-Thiodiphenol (1/1 and 2/1)
Authors: Ferguson G. , Coupar P. I. , Glidewell C.
Published: July 2002

4,4′-Isopropylidenediphenol-1,4-diazabicyclo[2.2.2]octane (1/1), (1), C15H16O2.C6H12N2, monoclinic, P2/a, a = 11.385 (2), b = 6.5565 (12), c = 13.076 (2) Å, \beta = 96.240 (11)°, with Z = 2; the two components of the adduct, which each lie across twofold axes, are joined into simple chains via O—H...N hydrogen bonds in a motif with graph set C_{2}^2(17). 4,4′-Oxodiphenol-1,4-diazabicyclo[2.2.2]octane (1/1), (2), C12H10O3.C6H12N2, orthorhombic, P212121, a = 9.4222 (11), b = 11.1886 (15), c = 15.694 (2), with Z = 4; the diamine component is disordered by rotation about the N...N vector, having two orientations [populations 0.76 (1) and 0.24 (1)] rotated by 48 (3)° from coincidence: the components are joined into chains via O—H...N hydrogen bonds in a motif with graph set C_{2}^2(17); pairs of these chains are joined into ladders by C—H...O hydrogen bonds in a motif of graph set R_{2}^2(22). 4,4′-Thiodiphenol-l,4-diazabicyclo[2.2.2]octane (1/1), (3), C12H10O2S.C6H12N2, isomorphous, a = 9.5785 (11), b = 11.4525 (13), c = 15.759 (2) Å (and ipso
facto isostructural), with (2); the diamine disorder is characterized by two equally populated orientations related by a rotation about the N...N vector of 37.1 (2)° and pairs of chains are now joined into ladders by C—H...S hydrogen bonds. 4,4′-Thiodiphenol-1,4-diazabicyclo[2.2.2]octane (2/1), (5), (C12H10O2S)2.C6H12N2, monoclinic, P21/n, a = 8.3198 (9), b = 11.4006 (13), c = 15.056 (2) Å, \beta = 104.955 (8)°, with Z = 2; the diamine component of the adduct is disordered across a centre of inversion, and the bisphenol components are linked into chains by O—H...O hydrogen bonds in a motif with graph set C(12). These chains form cross-links via the diamine component by means of O—H...N hydrogen bonds in a C_{3}^3(19) motif to yield sheets within which are large hydrogen-bonded rings described by the unusual graph set R_{8}^8(62).
Rigid-Body Disorder Models for the High-Temperature Phase of Ferrocene
Authors: Brock C. P. , Fu Y.
Published: July 2002

Ferrocene, [Fe(C5H5)2], which crystallizes at room temperature in space group P21/a with Z = 2, is described in many textbooks as having D
5d symmetry. Previous work has shown, however, that the librational amplitude associated with motion about the fivefold axis does not decrease with temperature and that the crystals are probably disordered. Ferrocene molecules in triclinic crystals grown below 169 K have approximate D
5 symmetry and an almost eclipsed conformation; the low- and high-temperature phases may be related by an order–disorder transition, during which the number of independent atoms changes by a factor of 4. The structure of the high-temperature phase has been reinvestigated with rigid-body refinements of the neutron diffraction data collected at 173 and 298 K by Takusagawa & Koetzle [Acta Cryst. (1979), B35, 1074–1081]. The C5H5 ring was treated as a rigid group of C
5 symmetry; C—C and C—H distances were allowed to vary, as was the displacement of the H atoms from the C5 plane. The rigid-body motion of the C5H5 ligand was described by the TLS model. All the rigid-body disorder models fit better than conventional independent-atom models. A disorder model that includes three sites for each C5H5 ring is the best of the models that were investigated, which indicates that the structure of the high-temperature phase cannot be described by the superposition of the two independent ferrocene molecules in the low-temperature phase. The phase transition between the high- and low-temperature phases is not a simple order–disorder transition.
Hydrogen-Bond Acceptor Properties of Nitro-O Atoms: A Combined Crystallographic Database and Ab Initio Molecular Orbital Study
Authors: Allen F. H. , Baalham C. A. , Lommerse J. P. M. , Raithby P. R. , Sparr E.
Published: July 2002

Crystallographic data for 620 C—nitro-O...H—N,O hydrogen bonds, involving 560 unique H atoms, have been investigated to the van der Waals limit of 2.62 Å. The overall mean nitro-O...H bond length is 2.30 (1) Å, which is much longer (weaker) than comparable hydrogen bonds involving >C=O acceptors in ketones, carboxylic acids and amides. The donor hydrogen prefers to approach the nitro-O atoms in the C—NO2 plane and there is an approximate 3:2 preference for hydrogen approach between the two nitro-O atoms, rather than between the C and O substituents. However, hydrogen approach between the two O acceptors is usually strongly asymmetric, the H atom being more closely associated with one of the O atoms: only 60 H atoms have both O...H distances \leq 2.62 Å. The approach of hydrogen along putative O-atom lone-pair directions is clearly observed. Ab-initio-based molecular orbital calculations (6-31G** basis set level), using intermolecular perturbation theory (IMPT) applied to the nitromethane–methanol model dimer, agree with the experimental observations. IMPT calculations yield an attractive hydrogen-bond energy of ca −15 kJ mol−1, about half as strong as the >C=O...H bonds noted above.
Temperature Dependence of the Rigid-Body Motion of Anthraquinone
Authors: Fu Y. , Brock C. P.
Published: July 2002

Anthraquinone has been studied at five temperatures between 296 and 162 K. The unsatisfactory agreement factors obtained in earlier studies probably resulted from the use of data collected from twinned crystals; needle-like crystals with approximately equidimensional cross sections are usually contact twins related by reflection across (102¯). The temperature dependence of the T and L tensors is normal. The out-of-plane displacement parameters of the O atom indicate a large-amplitude internal vibration.
Determination of the Bonding and Valence Distribution in Inorganic Solids by the Maximum Entropy Method
Authors: Rao G. H. , Brown I. D.
Published: July 2002

The distribution of valence among the bonds in the bond graph of an inorganic compound is used to calculate an `entropy'. We show that the distribution of valence that maximizes this entropy (ME) is similar, but not identical, to that obtained using the equal-valence rule (EVR) proposed by Brown [Acta Cryst. (1977), B33, 1305–1310]. Since the ME solutions are maximally non-committal with regard to missing information, they give better predictions of the observed valence distributions than the EVR solutions when lattice constraints or electronic anisotropies are present, but worse predictions when these effects are absent. Since valences calculated using ME are necessarily positive, they give significantly better predictions in cases where EVR predicts a negative bond valence. In the absence of electronic distortions the observed bond graph is either the graph with the highest maximum entropy or it has an entropy within 1% of this value. Since the entropy depends on the oxidation states of the atoms, compounds with the same stoichiometry and cation coordination numbers but different atomic valences may adopt different bond graphs and hence different structures.
Structural Determination of the Hydrofullerene C60D36 by Neutron Diffraction
Authors: Hall L. E. , McKenzie D. R. , Davis R. L. , Attalla M. I. , Vassallo A. M.
Published: July 2002

A mixture of C60D36 with 24.5 \pm 4.5% C60 by weight has been analysed by neutron diffraction techniques. The diffraction data was converted to a reduced density function G(r) by Fourier transformation. The C60 component of the G(r) was subtracted out. This enabled a comparison for five molecular models of C60D36, with symmetries T, Th
, S
6 and two D
3

d
isomers, with the experimental G(r). This specimen of C60D36 was found to be best described by a T symmetry isomer, in agreement with 13C NMR and IR data for C60H36 [Attalla et al. (1993). J. Phys. Chem. pp. 6329–6331].
Intramolecular and intermolecular geometry of thiophenes with oxygen-containing substituents
Authors: Blake Alexander J. , Clark Bernard A. J. , Gierens Hedi , Gould Robert O. , Hunter Gordon A. , McNab Hamish , Morrow Mark , Sommerville Craig C.
Published: July 2002

The intramolecular and intermolecular geometries of six thiophenes carrying oxygen-containing substituents have been determined. Crystals of 2-methoxythiophene and 3-methoxythiophene were grown in situ on a diffractometer from liquid samples. The 2-methoxy group introduces significant distortions to the thiophene nucleus and each molecule participates in four S...O contacts leading to an infinite bilayer. The extended structure of 3-methoxythiophene comprises zigzag chains of molecules linked by S...O contacts. Molecules of 2-acetyl-3-methoxythiophene are arranged in pairs about inversion centres, with ring centroids 3.835 Å apart. 5-Cyano-3-hydroxythiophene adopts the hydroxythiophene tautomeric form which allows conjugation between the S atom and the nitrile group: O—H...N hydrogen bonding leads to chains which are cross-linked by S...O contacts to give infinite two-dimensional layers. 5-(Methylthio)thiophen-3(2H)-one exists exclusively as the thiophen-3(2H)-one form in the solid state, allowing maximum conjugative interaction of both the ring heteroatom and the substituent with the carbonyl group: each molecule is linked to two of its neighbours through pairwise C—H...O interactions, forming ribbons. Spiro[cyclohexane-1,2′-2′,3′-dihydrothiophen]-3′-one crystallizes with four independent molecules in the asymmetric unit with only minor differences between these: the five- and six-membered rings in each molecule are approximately orthogonal and C—H...O hydrogen bonding generates chains. The last two structures differ from the others in that they lack a fully aromatic thiophene system.
Determining the crystal structure of twinned 2-methylpyrazine
Authors: Boese Roland , Gehrke Annette , Kapon Moshe , Herbstein Frank H.
Published: July 2002

Although systematic absences and symmetry relations among reflections pointed to space group I4122 (one molecule in the asymmetric unit), a direct methods solution could only be obtained in I4¯ (two molecules in the asymmetric unit). Refinement in I4¯ was unsatisfactory until merohedral twinning was taken into account. The resulting molecular dimensions are in excellent agreement with analogous values in the literature. The molecular arrangement is described.
Phase determination and structure refinement by density and shift functions
Authors: Jagodzinski H. , Philipp D.
Published: July 2002

Any crystal structure may be described in terms of a sublattice of points, each of which represents a certain fraction of the electron density. Multiplying this sublattice by a density function f(x) and applying a shift function s(x), which brings the atoms into the right positions, the correct crystal structure can be given in many different ways. It is shown that the shift function s(x) yields phase relations between the structure factors F(h), which may be evaluated directly, if the coefficients of the Fourier representation of s(x) converge rapidly. This behaviour is demonstrated for the case of a one-dimensional acentric model structure consisting of 50 atoms. Complete information on the structure may be obtained by routine methods with the aid of 5 given phases of the structure factor. This procedure may also be applied to three-dimensional structures, if the corresponding computer programs are available.
Die Messung von Schallgeschwindigkeiten in optisch anisotropen Medien mit dem Schaefer–Bergmann-Verfahren
Authors: Küppers H.
Published: July 2002

If the optical wave front is not normal to one of the principal axes of the indicatrix, diffraction of light in crystals, caused by ultrasound, is observed with maximum intensity when the incident beam is off the Bragg angle by an amount depending on double refraction. In a crystal plate of finite dimensions a standing ultrasonic wave also generates waves with propagation directions which are inclined to the normal of the plate, and interfere with the measurement of sound velocities by the improved Schaefer–Bergmann method. These difficulties are overcome by an appropriate choice of the angle of the incident light beam. Experiments with triclinic and trigonal crystals are reported. Formulae are derived for calculating the angles of incidence necessary for any measurements of sound velocities in crystals.
Aspects of symmetry in electron diffraction patterns and optical transforms of very small homo-atomic aggregates using computer simulation
Authors: Larroque P. , Brieu M.
Published: July 2002

The study investigates the reasons why it may happen that the first simulation of an electron diffraction pattern is not always centrosymmetrical whereas the second one is bound to be so. The formal link between the two simulations is established. A number of examples are given.