bluuebunny/crossref_metadata_2025 huggingface dataset:
Synchrotron radiation topographic observation of KTiOPO4crystals under an electric field
Authors: Liu W. J. , Jiang S. S. , Ding Y. , Wu X. S. , Wang J. Y. , Hu X. B. , Jiang J. H.
Published: July 2002

In situwhite-beam synchrotron radiation topographic observations under an electric field have been made on the KTiOPO4(KTP) family of crystals. The investigation shows a strong enhancement of diffracted intensity forhklreflections (l ≠ 0) and topographic contrast in the form of fine striations when the external electric field is parallel to the polar axis. Several kinds of doped and undoped samples with various grown-in defects, such as domain boundaries, growth striations, growth-sector boundaries, dislocations,etc., have been studied in detail. The results suggest that these defects have very little effect on the field-induced striations. It is believed that the movement of K^+ ions driven by the electric field leads to a local accumulation of charges and a lattice distortion. The strongly anisotropic conductivity is a governing factor in the explanation of the field-related phenomena in this kind of quasi-one-dimensional conductor.
A real-space approach to the solution of a disordered structure of the three-dimensional coordination polymer Fe(CN)6[Sn(C4H9)3]3
Authors: Wang Xin , Chen Yan-Song , Su Wu-Pei
Published: July 2002

As a test of a real-space approach to the X-ray phase problem, a disordered structure of a three-dimensional polymer, hexacyanotris(tri-n-butylstannyl)iron, Fe(CN)6[Sn(C4H9)3]3, has been solved by simulated annealing. The methodology as well as the structure are described.
Diffuse scattering and Monte Carlo simulations of cyclohexane–perhydrotriphenylene (PHTP) inclusion compounds, C6H12/C18H30
Authors: Mayo S. C. , Proffen Th. , Bown M. , Welberry T. R.
Published: July 2002

A recently developed method for fitting a Monte Carlo computer simulation model to observed single-crystal diffuse X-ray scattering has been applied to (hk0) scattering data obtained for the guest–host system (inclusion compound) cyclohexane–perhydrotriphenylene (cyclohexane–PHTP), C6H12/C18H30, at 295 K. A simple two-dimensional model representing the projection of the structure downcwas used, which allowed refinement of short-range order parameters describing the mutual orientation of guest molecules within neighbouring channels, together with a parameter describing the coupling of these orientations to distortions of the host framework. Since the guest site still appears very disordered in projection, only a crude model for the shape of the guest molecule was used. The resulting calculated diffraction patterns showed good qualitative agreement with the observed pattern, but quantitatively the achieved agreement factor (R≃ 38%) was considerably higher than that achieved in comparable studies of a different system. Calculations of the contribution to the diffraction pattern of the guest alone and the host alone clearly indicate that the host framework distortion is modelled well, but the guest is not so well described. The lattice distortion revealed by the study indicates a tendency for the hexagonal network to distort towards local monoclinicity. This is consistent with the observation that at ~273 K, the compound undergoes a reversible phase transition to a micro-twinned monoclinic phase.
Angle calculations for a `4S+2D' six-circle diffractometer
Authors: You H.
Published: July 2002

Angle calculations are derived for the operation of a `4S+2D' six-circle X-ray or neutron diffractometer. The six degrees of freedom [four sample-orienting (`4S') and two detector-orienting (`2D')] of the diffractometer are used to control the scattering vector and a reference vector in the laboratory frame of reference. Several modes of operation unavailable in other types of diffractometer are presented.
An elastically bent silicon monochromator for a neutron diffractometer
Authors: Tanaka Ichiro , Niimura Nobuo , Mikula Pavel
Published: July 2002

A new type of elastically bent perfect Si (EBP-Si) monochromator for a neutron diffractometer dedicated to protein crystallography has been developed and successfully applied to two such diffractometers. It was experimentally demonstrated that the monochromatic neutron intensity increased and the beam was focused at the sample position when the monochromator was suitably bent. A specially designed bender for the EBP-Si monochromator, already applied to several diffractometers, is described. The distinctive features of the EBP-Si monochromator are summarized.
Contribution to residual-stress evaluation in high-stress-gradient zones by X-ray diffraction
Authors: Hennion V. , Sprauel J. M. , Michaud H.
Published: July 2002

The non-destructive X-ray diffraction method is usually employed to evaluate stress states at the near surface of materials. In specific zones with high-stress gradients parallel to the surface, like welding joins, areas affected by lasers, or ball-bearing raceways, the classical measurement method is not particularly well suited, as the irradiation spot covers regions of varying stress according to its size. This leads to inaccurate stress evaluations. A new X-ray diffraction measurement and data treatment method is therefore proposed. It is based on longitudinal step-by-step scanning in a fixed direction of the surface. As the acquired data results from the convolution of local diffraction peaks with the incident-beam intensity, an accurate model of the true two-dimensional intensity distribution inside the spot has been developed. Firstly, the true shape and size of the irradiation area is defined. The distribution of the intensity received by the sample surface is then computed. The true local strains and stresses along the scanning direction are finally determined from the acquired peaks using an optimization through a least-squares fitting by a nonlinear function.
Preparation of silver behenate coatings to provide low- to mid-angle diffraction calibration
Authors: Blanton T. N. , Barnes C. L. , Lelental M.
Published: July 2002

A method for the preparation of a well aligned silver behenate sample is described. Multiple (0 0 l) diffraction peaks, (0 0 1) to (0 0 48), are easily observed using a conventional laboratory X-ray diffraction instrument.
Small-crystal X-ray diffractometry with a crystal ante-monochromator: erratum
Authors: McL Mathieson A.
Published: July 2002

In the paper by Mathieson [Acta Cryst. (1985), A41, 309- 316] the definition of k given in the Abstract should read k = (Δλ/λ).
Concerning phase relationships which are not structure invariants or seminvariants
Authors: Giacovazzo C.
Published: July 2002

Often some primitive random variables (e.g.the atomic coordinates) are non-uniformly distributed over the unit cell. Prior information about such distributions is often available but not always used in current direct-methods procedures. Examples are described according to which primitive random variables are restricted to some domains (attended domains) or, in an equivalent way, are prevented from frequenting some domains (forbidden domains). Emphasis has been put on the fact that non-uniform distributions generate phase relationships which are not structure invariants or seminvariants, but it is also anticipated that formulas for estimating phase invariants and seminvariants are deeply affected by such prior information.
Comments on the characteristics of incommensurate modulation in quartz: discussion about a neutron scattering experiment
Authors: Aslanyan T. A. , Shigenari T. , Abe K.
Published: July 2002

From analysis of the elastic neutron scattering data of Dolino et al. [J. Phys. (Paris) (1984), 45, 361–371], it is shown that, besides the well identified components u_x and u_y of the acoustic displacements in the incommensurate (IC) phase of quartz, there also exists a strong component of the u_z vector of the modulation. The existence of the large u_z is not consistent with the currently accepted model for the IC transition in quartz, since the long-period IC modulation observed in quartz cannot induce any noticeable acoustic component u_z. The need for a new model is keenly felt in order to understand the origin of the IC modulation in quartz.
The morphology of quasicrystals, incommensurate composite crystals and modulated crystals derived from the broken-bond model
Authors: van Smaalen Sander
Published: July 2002

The broken-bond model for the surface free energy of crystals is considered. The consequences are derived for the morphology of periodic crystals, quasicrystals, incommensurately modulated crystals and intergrowth compounds. It is found that low-index facets on periodic crystals and quasicrystals are the result of finding an optimized position along the vector normal to the surface of the surface plane. The same principle explains the normal facets on modulated crystals and intergrowth crystals. The so-called satellite facets are the result of surface pinning of the phase of the modulated wave. On intergrowth compounds, facets may be found that are stabilized by a combination of both mechanisms. It is shown that the most stable facets on intergrowth crystals are the normal facets that are common to the subsystems, independent of the details of the structure.
Three-wave grazing-incidence X-ray diffraction from thin crystal surface layers: determination of triplet phase invariants
Authors: Stetsko Yuri P. , Chang Shih-Lin
Published: July 2002

Numerical calculation of the angular and spectral distributions of the intensities of the specularly diffracted waves in the case of three-wave grazing-incidence X-ray diffraction is carried out using the dynamical theory. The angular and spectral distributions are shown to be uniquely and continuously dependent upon the value of the triplet phase invariant. A method of determining the value of the triplet phase invariant for thin crystal surface layers is developed, based on the comparison of experimentally measured three-wave peak profiles with the profiles calculated for different values of the triplet phase invariant. An analysis scheme of the phase sensitivity of the reflection coefficients is proposed taking into account the interference of the directly excited and the `Umweg'-excited specularly diffracted waves.
The role of projection operators in the theory of N-beam diffraction and the inversion of three-beam elastic scattering intensities
Authors: Hurley A. C. , Moodie A. F. , Johnson A. W. S. , Abbott P. C.
Published: July 2002

Commencing from a projection-operator description of N-beam diffraction, the mathematical basis for the recovery of phase and amplitude information from a three-beam convergent-beam electron diffraction pattern is given for both the centrosymmetric and noncentrosymmetric cases. The algebra is available in Mathematica Notebook form from the URL ftp://ftp.physics.uwa.edu.au/pub/EMC/3BeamAlgebra.nb.
The Darwin procedure in optics of layered media and the matrix theory
Authors: Dub P. , Litzman O.
Published: July 2002

The Darwin dynamical theory of diffraction for two beams yields a nonhomogeneous system of linear algebraic equations with a tridiagonal matrix. It is shown that different formulae of the two-beam Darwin theory can be obtained by a uniform view of the basic properties of tridiagonal matrices, their determinants (continuants) and their close relationship to continued fractions and difference equations. Some remarks concerning the relation of the Darwin theory in the three-beam case to tridiagonal block matrices are also presented.
Tensor distinction of domain pairs in ferroic crystals
Authors: Litvin Daniel B.
Published: July 2002

A classification of domain pairs in ferroic crystals is given in which all domain pairs in each class can be distinguished by the same set of physical property tensors. Tabulations are presented giving in which ferroic phase transitions such domain pairs arise. Whether a physical property tensor can or cannot distinguish between the domains of domain pairs of each class is given for a wide variety of physical properties.
Experimental determination of electric-field-induced differences in structure-factor phases of the order of 2%
Authors: Stahn J. , Pucher A. , Pietsch U. , Zellner J. , Weckert E.
Published: July 2002

In order to understand the induced electron-density response of covalently bonded materials to an externally applied electric field, the change of structure-factor phases of several weak reflections of GaAs has been measured by three-beam interferences. Using a modulation technique, phase variations of the order of 1° for a field strength of 1 kV mm−1 were determined. Although the statistics of these first experiments are rather poor, the experiment verifies qualitatively the predictions of a semi-empirical bond-charge model. The measured phase variation is the smallest value determined up to now.
A new stacking motif: complex alloy structures interpreted as modulated structures
Authors: Uchida Masaya , Matsui Yoshio
Published: July 2002

The structures of λ-Al4Mn, manganese aluminide, κ-Al177Cr49Ni, chromium nickel aluminide, and ∊-Al4Cr, chromium aluminide, which have been described in terms of the aggregation of clusters until now, are interpreted as modulated structures. They are basically composed of close-packed layers with ordered atomic vacancies. The observed stacking motif of the atoms can be found in many phases related to icosahedral and decagonal quasicrystals. This analysis will be helpful in the study of not only many complex alloys, but also quasicrystals.
High-pressure structures of α- and δ-ZrMo2O8
Authors: Krogh Andersen Anne Marie , Carlson Stefan
Published: July 2002

In situ high-pressure synchrotron X-ray powder diffraction studies of trigonal α-ZrMo2O8, zirconium molybdate, have been performed from ambient conditions to 1.9 GPa, over the α–δ phase transition at 1.06–1.11 GPa. The monoclinic structure of δ-ZrMo2O8, stable between 1.1 and 2.5 GPa at 298 K, has been solved by direct methods and refined using the Rietveld method. Significant distortions of the ZrO6 and MoO4 polyhedral elements are observed for δ-ZrMo2O8, as compared to the ambient conditions of the α-phase, while the packing of anions becomes more symmetric at high pressure.
Kinetic behaviour investigations and crystal structure of nitric acid dihydrate
Authors: Lebrun N. , Mahe F. , Lamiot J. , Foulon M. , Petit J. C. , Prevost D.
Published: July 2002

X-ray powder diffraction experiments are performed to prove the possible crystallization of nitric acid dihydrate (HNO3·2H2O, further denoted NAD) and to determine the best thermal conditions for growing a single crystal. It is shown that the kinetic behaviour of NAD strongly depends on the preliminary thermal treatment. One good single crystal obtained by an in situ adapted Bridgman method procedure enabled determination of the crystal structure. The intensities of diffracted lines with h odd are all very weak. The H atom of nitric acid is delocalized to one water molecule leading to an association of equimolar nitrate (NO3
−) and an H5O2
+ ionic group. The asymmetric unit contains two such molecules. These two molecules are related by a pseudo a/2 translation (with a 0.3 Å mean atomic distance difference), except for one H atom of the water molecules (0.86 Å) because of their different orientations in the two molecules. The two molecules, linked by very strong hydrogen bonds, are arranged in layers. Two layers which are linked by weaker hydrogen bonds are approximately oriented along the c axis. The structure may be described by translations of this set of two layers along the c axis without hydrogen bonds leading to a two-dimensional hydrogen-bond network. The structures of the monohydrate (NAM) and trihydrate (NAT) are re-determined for comparisons. These structures may be described by one- and three-dimensional hydrogen-bond networks, respectively.
Topochemically controlled solid-state methyl rearrangement in thiocyanurates
Authors: Greenberg Mark , Shteiman Vitaly , Kaftory Menahem
Published: July 2002

4,6-Dimethoxy-3-methyl-1,3,5-triazine-2(3H)-thione crystallizes in two polymorphic forms, needles and plates. In the needle-shaped crystals (9a) the molecules occupy the crystallographic mirror plane, thus the layers are stacked along the b axis. The molecules of the other polymorph [plate-shape crystals, (9b)] are packed in a herringbone packing mode. Upon heating, (9b) undergoes a phase transition to form (9a). At 378 K the needles undergo O → S topochemically controlled methyl transfer in the solid state to produce 1-methyl-4-methoxy-6-methylthio-1,3,5-triazine-2(1H)-one in 75% yield. The enthalpy of the rearrangement is estimated to be −39.1 kJ mol−1. 1-Methyl-6-methoxy-4-methylthio-1,3,5-triazine-2(1H)-thione crystallizes in space group P21 with two crystallographically independent molecules in the asymmetric unit. Compound (9b) undergoes O → S methyl transfer in the solid state at 373 K. The rearrangement is topochemically assisted and the product, 1-methyl-2,4-bismethylthio-1,3,5-triazine-6(1H)-one, is obtained in quantitative yield. The enthalpy of the rearrangement is estimated to be −58.8 kJ mol−1. The crystal structures of the compounds as well as their DSC thermographs are described and discussed. Energy calculation by ab initio methods shows that the driving force for the reactions is the difference between the molecular energies of the pre-rearranged compounds and their products, 54.2 and 59.3 kJ mol−1 in the two cases, respectively.
Five-dimensional structure refinement of natural melilite, (Ca1.89Sr0.01Na0.08K0.02)(Mg0.92Al0.08)(Si1.98Al0.02)O7
Authors: Bindi Luca , Bonazzi Paola , Dušek Michal , Petříček Václav , Chapuis Gervais
Published: July 2002

The structure of a crystal of natural melilite from San Venanzo, Umbria (Italy) of the general formula X
2
T1(T2)2O7, where X = Ca0.945Sr0.005Na0.04K0.01, T1 = Mg0.92Al0.08 and T2 = Si0.99Al0.01, has been solved and refined as an incommensurate structure in five-dimensional superspace. The structure is tetragonal, superspace group P\bar 421
m:p4mg, cell parameters a = 7.860 (1), c = 5.024 (1) Å, modulation vectors q
1 = 0.2815 (3)(a* + b*), q
2 = 0.2815 (3)(−a* + b*). The data collection was performed on a KumaCCD diffractometer. The structure was refined from 7606 reflections to final R = 0.0481. A special modification of the refinement program Jana2000 was necessary to take into account overlapping of satellite reflections m × n = ±1, which could not be properly separated in the integration procedure. The final model includes modulations of the atomic positions as well as modulations of the thermal parameters. The latter are induced by strong differences in the neighbourhood of the actual modulated positions. The occupational modulation was neither significant for X nor for T1 sites and the sites were supposed to be occupied only by Ca and Mg, respectively. As a consequence of the Ca and O positional modulations six-, seven- and eightfold Ca coordination occur throughout the structure and the thermal ellipsoid changes its shape correspondingly. The positional modulation of the atoms causes variations in the interatomic distances which, however, do not affect bond-valence sums considerably, but induce flattening and rotation in T1 and T2 tetrahedra, respectively.
Observation of 4f electron transfer from Ce to B6 in the Kondo crystal CeB6 and its mechanism by multi-temperature X-ray diffraction
Authors: Tanaka Kiyoaki , Ōnuki Yoshichika
Published: July 2002

Electron density distributions (EDD) in CeB6 were measured by X-ray diffraction at 100, 165, 230 and 298 K. Analysis with a weak-field model, in which the spin-orbit interaction dominates the energy splitting of the 4f levels, revealed that more 4f electrons were donated from Ce to B6 at the lower temperature. Donated electrons localize around the B—B bonds connecting B6 octahedra. The localized electrons and an expansion of the outermost 5p orbitals change the effective atomic potentials and enhance the anharmonic vibration (AHV) of constituent atoms at lower temperature. Enhanced AHV increases the entropy and makes the electron donation inevitable. Changes in crystal structure, EDD, electron configuration and AHV are found to be closely correlated with one another and the mechanism of the electron transfer in the Kondo crystal CeB6 in the studied temperature range was elucidated. This is, to the authors' knowledge, the first multi-temperature measurement of EDD that elucidates a mechanism of change from the temperature dependence of the EDD. Parameters change consistently at all the temperatures except 298 K, at which the excited states Γ7 of the Ce 4f states have significant electron population. The thermal excitation to Γ7 levels expands the B6 octahedra, since Γ7 has main lobes along 〈111〉 or from Ce to the centre of B6 octahedra. The energy gap between the ground state Γ8 and Γ7 was calculated to be 470 K from the ratio of electron populations of both states. The present experiment opens the door to accurate X-ray EDD analyses of rare earth complexes.
Structurally ferroelectric SrMgF4. Erratum
Authors: Abrahams S. C.
Published: July 2002

In the paper by Abrahams (2002) Acta Cryst. (2002), B58, 34–37 two errors were printed. First, in the abstract the Curie temperature was incorrectly given as ∼1450 K. The correct value is ∼450 K. The second error occurs in section (a) of p. 36. The polar displacement expression is currently given as Δzl
≃ 1 Å. The correct expression should be Δzi
≃ 1 Å.
X-ray diffraction study of Nd2CuO4 single crystals at 20 K
Authors: Makarova I. P. , Simonov V. I. , Blomberg M. K. , Merisalo M. J.
Published: July 2002

Single crystals of Nd2CuO4, a parent compound for the electron-doped high-Tc
superconductors, have been studied using X-ray diffraction data collected at 20 K. Structure refinements indicated that the specimen was nonstoichiometric, with the approximate formula Nd1.89CuO3.85. Difference Fourier maps showed significant residual peaks around the Cu and Nd atoms. The residual density around the Cu atom had features characteristic of the valence state +2. Part of the nonspherical effects observed around the Nd atom could be described using the Gram–Charlier series expansion of the Gaussian density function, indicating either static or dynamic displacements of this atom, while part of the residual density was attributed to bonding effects. The bonding of the O(1) atom, located in the plane of the Cu atoms, was found to be different from the bonding of the O(2) atom, situated between the Cu—O(1) layers.
Effects of H/D substitution on thermal vibrations in piperazinium hexanoate-h11,d11
Authors: Luo J. , Ruble J. R. , Craven B. M. , McMullan R. K.
Published: July 2002

The crystal structures of piperazinium hexanoate-h11, {{1}\over{2}}C4H12N2+
2.C6H11O−
2, and piperazinium hexanoate-d11, {{1}\over{2}}C4H12N2+
2.C6D11O−
2, have been determined from neutron diffraction data collected at 15 K. Nuclear anisotropic displacement parameters have been analyzed to obtain the internal molecular displacements of the H and D nuclei, given by 〈
u

2
obs〉 − 〈
u

2
ext〉 where 〈
u

2
ext〉 is the contribution assuming all H/D to be carried rigidly on the vibrating molecular framework consisting of the heavier nuclei. In both crystal structures the cation ring is well fitted by the rigid-body model and the anion chain by a model with two rigid segments. In the piperazinium cations the corresponding protons in the two structures have about the same internal vibrational directions and magnitudes except for the two N—H protons, perhaps owing to differences in N—H...O hydrogen bonding. The internal vibrations of corresponding H/D in the h11 and d11 anions have approximately the same vibrational directions. The internal mean-square displacements of the H nuclei are systematically greater than the values of the corresponding D nuclei by an average factor 1.7 (3). For both anions, normal-mode analyses have been carried out using the force fields derived from ab initio quantum-mechanical calculations with HF/3-21 G and HF/6-31G** basis sets. The values of the resultant H/D internal displacements for C—H(D) bond stretching and methylene out-of-plane vibrations are in good agreement with experiment. However, with either basis set, theory predicts methylene in-plane mean-square displacements significantly greater than the experimental values.
Distance limits of OH...Y hydrogen bonds (Y = Cl, Br, I) in solid hydroxides, structure refinement of laurionite-type Ba(OD)I, Sr(OD)I and Sr(OH)I by neutron and synchrotron X-ray powder diffraction
Authors: Peter S. , Cockcroft J. K. , Roisnel T. , Lutz H. D.
Published: July 2002

Synchrotron X-ray and neutron powder diffraction experiments on Sr(OH)I, Sr(OD)I and Ba(OD)I have been performed at ambient temperature. The structure data obtained by Rietveld refinement confirm the laurionite-type structure {Pnma, Z = 4; RI
= 5.9, 7.9 and 7.8%; a = 7.7294 (1), b = 4.24697 (7) and c = 10.7374 (1) Å [Sr(OH)I]; a = 7.7303 (8), b = 4.2465 (4) and c = 10.7399 (10) Å [Sr(OD)I]; a = 8.0291 (9), b = 4.4961 (5) and c = 11.0196 (16) Å [Ba(OH)I]; a = 8.0203 (13), b = 4.4912 (7) and c = 11.0193 (14) Å [Ba(OD)I]} proposed from IR and Raman spectra. In contrast to the respective lead compound, only very weak hydrogen bonds are formed. The strength of the hydrogen bonds of OH− ions in condensed material is discussed with respect to intermolecular H...Y distances, furcation of the bonds and M—O bond lengths (synergetic effect). The longest H...Y distances (Y′ = Cl, Br, I), for which hydrogen bonds due to OH− have been established, are 2.80, 2.90 and 3.25 Å, respectively.
Structural chemistry of NaCoPO4
Authors: Hammond R. , Barbier J.
Published: July 2002

Sodium cobalt phosphate, NaCoPO4, occurs as two different polymorphs which transform reversibly at 998 K. The crystal structures of both polymorphs have been determined by single-crystal X-ray diffraction. The low-temperature form α-NaCoPO4 crystallizes in the space group Pnma with cell parameters: a = 8.871 (3), b = 6.780 (3), c = 5.023 (1) Å, and Z = 4 [wR(F
2) = 0.0653 for all 945 independent reflections]. The α-phase contains octahedrally coordinated Co and Na atoms and tetrahedrally coordinated P atoms, and is isostructural with maracite, NaMnPO4. The structure of high-temperature β-NaCoPO4 is hexagonal with space group P65 and cell parameters: a = 10.166 (1), c = 23.881 (5) Å, and Z = 24 [wR(F
2) = 0.0867 for 4343 unique reflections]. The β-phase belongs to the large family of stuffed tridymites, with the P and Co atoms occupying tetrahedral sites and the Na atoms located in the cavities of the tetrahedral framework. The long c axis corresponds to a 3 × superstructure of the basic tridymite framework (c ≃ 8 Å) and is caused by the displacement of the Na atoms, tetrahedral tilts and strong distortions of the CoO4 tetrahedra. A bond-valence analysis of these phases reveals that the polymorphism in NaCoPO4 is due in part to over-/underbonding of the Na atom in the low-/high-temperature structures, respectively.
Neutron diffraction of a complex of 1,8-bis(dimethylamino)naphthalene with 1,2-dichloromaleic acid
Authors: Wozniak K. , Wilson C. C. , Knight K. S. , Jones W. , Grech E.
Published: July 2002

A neutron study of the crystalline complex of 1,8-bis(dimethylamino)naphthalene (DMAN) with 1,2-dichloromaleic acid (ClMH2) has been carried out at 100 K using the Laue time-of-flight technique. The moieties are planar. The neutron data indicate that both [N—H...N]+ and [O—H...O]− hydrogen bonds in the complex are asymmetric. There are significant differences between the neutron and X-ray temperature factors, C—H, N—H and O—H bond lengths. There is a strong correlation between the neutron and X-ray temperature factors for non-H atoms and no correlation for H-atom temperature factors. According to the neutron data the involvement of a given H atom in a weak C—H...O hydrogen bond can be correlated with the ratio of equivalent temperature factors of the H and non-H atoms to which they are attached.
Inclusion compounds of thiourea and peralkylated ammonium salts. III. Hydrogen-bonded host lattices built of thiourea and nitrate ions
Authors: Li Q. , Mak T. C. W.
Published: July 2002

The new inclusion complexes tetraethylammonium nitrate–thiourea (1:3), (C2H5)4N+.NO3
−. 3(NH2)2CS (1), tetra-n-propylammonium nitrate–thiourea–water (1:3:1), (n-C3H7)4N+.NO3
−.3(NH2)2CS.H2O (2), tetramethylammonium nitrate–thiourea (1:1), (CH3)4N+.NO3
−.(NH2)2CS (3), tetra-n-propylammonium nitrate–thiourea (1:1), (n-C3H7)4N+.NO3
−. (NH2)2CS (4), and tetra-n-butylammonium nitrate–thiourea (1:1), (n-C4H9)4N+.NO3
−.(NH2)2CS (5) have been prepared and characterized by X-ray crystallography. Crystal data, Mo Kα: (1), space group P{\bar 1}, a = 10.300 (2), b = 14.704 (3), c = 15.784 (4) Å, α = 75.30 (3), β = 86.98 (3), γ = 72.25 (3)°, Z = 4 and RF
= 0.039 for 5034 observed data; (2), space group P21/n, a = 8.433 (2), b = 9.369 (2), c = 34.361 (7) Å, β = 91.01 (3)°, Z = 4 and RF
= 0.050 for 2475 observed data; (3), space group Pnma, a = 15.720 (3), b = 8.218 (2), c = 8.709 (2) Å, Z = 4 and RF
= 0.073 for 579 observed data; (4), space group P21/n, a = 8.784 (2), b = 14.421 (3), c = 15.078 (3) Å, β = 92.31 (3)°, Z = 4 and RF
= 0.046 for 2507 observed data; (5), space group Pna21, a = 19.934 (3), b = 12.680 (2), c = 9.092 (3) Å, Z = 4 and RF
= 0.047 for 1646 observed data. In the crystal structure of (1) infinite chains each composed of an alternate arrangement of a twisted thiourea trimer and a nitrate ion are cross-linked to form a puckered layer and further hydrogen bonding between such layers leads to a channel host framework for accommodation of the cationic guests. In the crystal structure of (2) two independent thiourea molecules are used to construct a hydrogen-bonded puckered layer normal to the c axis, whereas the remaining thiourea molecule, together with the nitrate ion and water molecule, generate another puckered layer that is parallel to the first. Hydrogen bonding between these two types of layers gives rise to a channel system running parallel to the [100] direction and the cations are stacked regularly within each column. Compounds (3), (4) and (5) have closely related crystal structures in which the cations are separated by one-dimensional, infinitely extended thiourea–nitrate composite ribbons in a sandwich-like packing arrangement.
An extension of the Voronoi analysis of crystal structures
Authors: Thomas N. W.
Published: July 2002

The Voronoi method of constructing polyhedra about all atoms in a given crystal structure is extended to provide a general, quantitative and concise means of discriminating crystal structures from one another. Although applicable to all crystal structural types, the methodology is developed here by reference to binary non-molecular inorganic systems, AmBn
, corresponding mainly to ionic crystals and intermetallic compounds. Consideration is given to the geometrical forms of the Voronoi polyhedra, with AB
2 compounds singled out for illustrative purposes. Two methods are proposed for summarizing structural geometry: (i) the use of representative points for elements of Voronoi polyhedra (i.e. corners, edges and faces) and relating these to space-group symmetry; (ii) calculation of the ratio of the volumes of Voronoi polyhedra, VV
A
/VV
B
. Topological structural properties (i.e. numbers of A...A, A...B and B...B pairwise interactions), by comparison, may be quantified by the indices IAA
, IAB
, IBA
and IBB
, whose meanings are defined and elucidated. By adopting statistically disordered structures as reference points, indices I′AA
, I′AB
,I′BA
and I′BB
are defined, which lead to the generation of two-dimensional topological structure diagrams with I′AA
and I′BB
as axes. Alternatively, geometrical-topological diagrams may be generated, with VV
A
/VV
B
and I′AA
— I′BB
as axes. The straightforward modifications required for a chemical–geometrical rather than a topological–geometrical analysis are also described. It is anticipated that the technique will find widespread application in solid-state chemistry and materials science.
Experimental and Theoretical Studies of Charge Density Distribution in E-Tetraethyl-1,4-diammoniumbut-2-ene.2PF6
Authors: McCormack K. L. , Mallinson P. R. , Webster B. C. , Yufit D. S. , Slater L. A. , Robins D. J.
Published: July 2002

The [PF6]− salt of E-tetraethyl-l,4-diaminobut-2-ene has been synthesized and the crystal structure determined. The experimental electronic charge distribution has been obtained by fitting a multipole model to low-temperature, high-resolution, single-crystal X-ray diffraction data. A high-level ab initio molecular orbital calculation has also been performed for the cation, which is used as an antifungal agent. Both experimental and theoretical Laplacian distributions display maxima only at the bond critical points. This correlates with a calculated electrostatic potential which is positive at all points.
Molecular and Crystal Structure of the Complex Composed of 2,3,5-Triphenyltetrazolium Cation and Dichloro-(1,3,5-triphenylformazanato)cobaltate(II) Anion
Authors: Kawamura Y. , Yamauchi J. , Azuma N.
Published: July 2002

The molecular and crystal structure of, cobalt(II) complex prepared by the reaction of 1,3,5-triphenylformazan with cobalt dichloride, 2,3,5-triphenyltetrazolium dichloro(1,3,5-triphenylformazanato)cobaltate(II), has been determined by means of X-ray diffraction. The complex crystallized in a triclinic space group P\overline 1, with M

r
= 728.55, a = 10.214 (3), b = 18.208 (7), c = 9.985 (2) Å, \alpha = 96.44 (3), \beta = 107.92 (2), \gamma = 81.79 (3)°, Z = 2, D

x
= 1.387 Mg m−3, \mu = 0.683 mm−1, F(000) = 750, R = 0.041, wR = 0.040 and S = 1.40. The complex is composed of a pair of ions, the 2,3,5-triphenyltetrazolium cation and the dichloro(1,3,5-triphenylformazanato)cobaltate(II) anion. The cobalt(II) ion is tetrahedrally coordinated by two N atoms and two chloride ions. One of the chloride ions of the anion has electrostatic intermolecular contacts with the tetrazolium ring of the cation. The conductivity of the complex in nitromethane solution showed complete dissociation of a pair of ions to separate ions.
Structure of Magnesium Hydroxide Sulfate [2MgSO4.Mg(OH)2] and Solid Solution in Magnesium Hydroxide Sulfate Hydrate and Caminite
Authors: Fleet M. E. , Knipe S. W.
Published: July 2002

Magnesium hydroxide sulfate [2MgSO4.Mg(OH)2; MHS] is tetragonal with a = 7.454 (1), c = 12.885 (2) Å, V = 716.0 Å3, space group P43212, Z = 4 and D

x
2.774 g cm−3. The structure (single-crystal X-ray, R = 0.025, wR = 0.023) comprises spiral (43) single chains of corner-shared Mg(2) octahedra cross-linked by SO4 tetrahedra and face-shared Mg(1) octahedra. A linear ternary group of face-shared Mg octahedra [Mg(2)—Mg(1)—Mg(2)] alternates with an unoccupied octahedral position in rows along [\overline 110]. A crystal of MHS was grown hydrothermally (0.15 GPa, 673 K) from gold-bearing Mg—S—O—H fluid. The MHS structure, with Mg(1) octahedra sharing two octahedral faces with Mg(2) octahedra, revises the structure assumed for the complex solid solution magnesium hydroxide sulfate hydrate and the related ocean-floor mineral caminite [2MgSO4.xMg(OH)2.(2 − 2x)H2O; 0.5 < x < 1.0]. The substitution reaction appears to be Mg(1)2+ ⇋ 2H+. The H-substituted MHS structure is distinguished from that of kieserite (MgSO4.H2O), which has straight single chains of corner-shared Mg octahedra.
Distribution of Molecular Centres in Crystallographic Unit Cells
Authors: Motherwell W. D. S.
Published: July 2002

A survey has been made of the position of molecular centres in unit cells for the seven most populated space groups in the Cambridge Structural Database. Visual inspection of histograms and scattergrams reveals very clear peaks and clusters, showing preferred locations for molecules. These preferred positions are midway between centres of symmetry or screw axes. Some simple calculations of interaction energies of molecules with neighbours in a molecular coordination sphere suggest reasons for these preferred positions. These diagrams show that simplification of molecules to spheres is helpful in visualizing the prevalence of layers of molecules and tendencies to the ideal close packing of spheres. global minimum, but not always. This observation has been noted both by workers who have exhaustively explored packing parameter space (Williams, 1969; van Eijck, Mooij & Kroon, 1995) or have used Monte Carlo methods plus simulated annealing to take statistical samples of the total space and proceed to a local minimum (Karfunkel & Gdanitz, 1992). What is certain is the importance of the principle of close packing. The present study takes the approach of looking at the total available data on packing of molecular crystals to identify certain tendencies in packing and certain commonly occurring patterns. If such patterns exist then these will have implications as regards the methods of prediction, if only on the basis of probability of occurrence.
High-Pressure Study of h.c.p.-Argon
Authors: Wittlinger J. , Fischer R. , Werner S. , Schneider J. , Schulz H.
Published: July 2002

Argon, used as pressure medium in high-pressure experiments, crystallizes in a hexagonal close-packed structure when the argon is loaded cryogenically in the diamond anvil cell. The behaviour under pressure reveals elastic properties comparable to that of f.c.c.- argon (f.c.c. = face-centred cubic). The bulk modulus is 6.5 (1.3) GPa. The initially good quality of the individual crystals suffers strongly under pressure increase. Heating the cell to 393 K at 8.5 GPa recreates the crystal structure, accompanied by a pressure decrease of more than 1 GPa.
Neutron Diffraction Study of Aromatic Hydrogen Bonds: 5-Ethynyl-5H-dibenzo[a,d]cyclohepten-5-ol at 20K
Authors: Steiner T. , Mason S. A. , Tamm M.
Published: July 2002

The neutron diffraction crystal structure of the title compound, C17H12O, was determined at 20 K. One of the benzyl groups accepts intermolecular hydrogen bonds from a hydroxyl and an ethynyl group, one to each face of the ring. The bond donated by the hydroxyl group points almost linearly to an aromatic C atom with a H...C separation of 2.339 (6) Å. The bond donated by the ethynyl group points to the aromatic midpoint M with a H...M separation of 2.587 (5) Å. The average acetylenic C—H bond length determined by low- temperature neutron diffraction is only 1.062 (6) Å, appreciably shorter than observed for chemically different C—H bonds.
Monoclinic and Hexagonal Nepheline Structures of (Na3/4/K1/4)AlGeO4
Authors: Hammond R. P. , Barbier J.
Published: July 2002

Hexagonal (Na3/4K1/4)AlGeO4 crystallizes in the space group P63 with unit-cell parameters a = 10.164 (2), c = 8.540 (2) Å and Z = 8 [wR(F
2) = 0.066 for all 3060 independent reflections]. Monoclinic (Na3/4K1/4)AlGeO4 crystallizes in the space group P21 with unit-cell parameters a = 10.0477 (4), b = 8.5764 (4), c = 10.2118 (4) Å, β = 119.035 (1)° and Z = 8 [wR(F
2) = 0.120 for all 3194 independent reflections measured on a twinned crystal]. Both structures belong to the large family of stuffed tridymites, with the Al and Ge atoms occupying tetrahedral sites, and the alkali atoms occupying the cavities of the tetrahedral framework. Hexagonal (Na3/4K1/4)AlGeO4 is isostructural with the silicate mineral nepheline (Na3/4K1/4)AlSiO4, while monoclinic (Na3/4K1/4)AlGeO4 corresponds to a minor distortion of the nepheline structure. Chemical analysis by electron microprobe and structure determination of flux-grown single crystals indicate that the hexagonal form with the chemical formula (Na0.78K0.19)Al0.97Ge1.03O4 may be stabilized by an alkali deficiency similar to that found in hexagonal natural nephelines. In contrast, all alkali sites are fully occupied in the monoclinic form of composition (Na0.75K0.25)AlGeO4 and the lower symmetry eliminates the oxygen disorder present in the hexagonal form.
Refinement of Twinned Structures with SHELXL97
Authors: Herbst-Irmer R. , Sheldrick G. M.
Published: July 2002

Four examples of the refinement of twinned structures are discussed. These structures illustrate three different types of twins: twinning by merohedry, pseudo-merohedry and non-merohedral twins. How the twinning was detected, how the structures were solved and how they were refined are shown in this paper. The difference between a refinement as a disordered model and a twinned model is illustrated. Sometimes the twin law was necessary to solve the structure, while in the other examples the twinning was first recognized during refinement of the structures. Finally, a list of `characteristic warning signs' is presented which are indicative of possible twinning.
Two crystal structures towards the discotic columnar mesophase of (1,4,8,11,15,18,22,25-octahexylphthalocyaninato)nickel. Erratum
Authors: Helliwell M. , Deacon A. , Moon K. J. , Powell A. K. , Cook M. J.
Published: July 2002

The low-temperature apparatus used in this work [Helliwell, Deacon, Moon, Powell & Cook, Acta Cryst. (1997), B53, 231–240] was the Oxford Cryosystems Cryostream, and not as stated a cryo-cooling system manufactured by Oxford Instruments. Also, on p. 235, lines 8–10, right-hand column, should read '...for molecule (2), whilst two are approximately perpendicular to it. .. For molecule (1)...'. © 1997 International Union of Crystallography Printed in Great Britain - all rights reserved Acta Crystallographica Section B ISSN 0108–7681 © 1997
Structure of the Tannin Geraniin Based on Conventional X-ray Data at 295 K and on Synchrotron Data at 293 and 120 K
Authors: Luger P. , Weber M. , Kashino S. , Amakura Y. , Yoshida T. , Okuda T. , Beurskens G. , Dauter Z.
Published: July 2002

Geraniin, C41H28O27.7H2O, is the main tannin from Geranium thunbergii Sieb. et Zucc., which is one of the most popular folk medicines and also an official antidiarrheic drug in Japan. Conventional X-ray diffractometer data at 295 K and synchrotron data at 293 and 120 K (at EMBL/DESY Hamburg with a MARRESEARCH imaging plate) were measured. The structure could not be determined with any of the currently distributed direct-methods programs, but was easily solved with DIRDIF using several conformers of the hexahydroxydiphenoyl group as input fragments into a vector-search procedure. A molecular structure was established where a cyclohexenetrione moiety attached to O4 of a central glucose unit was in a hydrated six-membered hemiacetal ring structure, as expected from the NMR study of crystalline geraniin. Owing to the high oligocyclic substitution the glucopyranosyl ring is in the unusual 1
C
4 conformation. Refinement of the low-temperature synchrotron data allowed identification of almost all the H atoms, even within the seven water molecules, so that a rather complex system of approximately 30 different hydrogen bonds can be studied in some detail.
Supramolecular Alcohol–Amine Crystals and Their Hydrogen-Bond Patterns
Authors: Loehlin J. H. , Franz K. J. , Gist L. , Moore R. H.
Published: July 2002

An analysis of the structures of five new cocrystals with saturated hydrogen bonding between amines and alcohols is presented. These five are all cocrystals with p-phenylenediamine (PDA) and a mono- or diol. The cocrystals are p-phenylenediamine–phenol (1/2) [PDA·2 phenol, (I)], p-phenylenediamine–p-phenylphenol (1/2) [PDA·2 p-phenylphenol (II)], p-phenylenediamine–2,6-dihydroxynaphthalene (1/1) [PDA·2,6-dihydroxynaphthalene (III)], p-phenylenediamine–1,6-hexanediol (1/1) [PDA·1,6-hexanediol (IV)] and p-phenylenediamine–p-chlorophenol (1/2) [PDA·2 p-chlorophenol (V)]. These crystals have two distinctly different supramolecular hydrogen-bond patterns, when considering only the hydroxyl and amine groups. There are, in addition, variations in the ways covalently bonded crosslinks connect these hydrogen-bond networks. The graph sets of the hydrogen-bond networks of these and other published saturated amine–alcohol crystals are compared and suggestions made on how to present the graph-set descriptions, especially those involving infinite two- and three-dimensional networks of intersecting chains which are characteristic of most of these materials.
Residual short-range order in the heavy fermion compound CeInCu2
Authors: Takahashi Isao , Kagayama Akiko , Oomi Gendo , Onuki Yoshichika , Komatsubara Takemi
Published: July 2002

By using the X-ray diffraction technique, residual short-range order was detected in CeInCu2, cerium dicopper indium, which is known to be a heavy fermion compound. In spite of the long-range order of this substance, diffuse scattering exhibiting short-range order was observed at room temperature. The correlation parameters obtained showed that an incorrectly occupied lattice site has a tendency to gather atoms of different species at the neighboring sites along the 〈111〉 directions. Thus, the disordered region would form a cluster composed of several disordered atoms. Furthermore, a chain-type correlation which has a period of 20–23 Å along the same directions was indicated. The superstructure-like feature of the disordered atoms hardly increases the residual resistivity. It is consistent with the behavior of the residual resistivity under high pressure.
Analysis of the environment of beryllium, magnesium and alkaline earth atoms in oxygen-containing compounds
Authors: Blatov V. A. , Pogildyakova L. V. , Serezhkin V. N.
Published: July 2002

About 2100 inorganic and organometallic compounds containing beryllium, magnesium and alkaline earth atoms (M) were investigated with Voronoi–Dirichlet polyhedra (VDPs). It is shown that the coordination numbers (CNs) of the M atoms in MO
n
coordination polyhedra can be determined by means of VDPs without crystal-chemical radii. The distributions of the M—O distances in the coordination spheres of the M atoms are bimodal for M = Be or Mg and monomodal for the other alkaline earth metals. Beryllium and magnesium coordination polyhedra containing weak M—O contacts were classified by variants of their distortions. It is found that the volume of the domains of the Mg, Ca, Sr and Ba atoms is independent of their CNs at CN \ge 6 (up to 16 for barium). The possibility of using the model of deformable spheres to describe the crystal structure of the compounds investigated is suggested.
Structures of Bi14WO24 and Bi14MoO24 from neutron powder diffraction data
Authors: Ling Christopher D. , Withers Ray L. , Thompson John G. , Schmid Siegbert
Published: July 2002

The (isomorphous) structures of Bi14WO24, tetradecabismuth tungsten tetracosaoxide, and Bi14MoO24, tetradecabismuth molybdenum tetracosaoxide, have been solved and refined using neutron powder diffraction data in the space group I4/m. The metal-atom array is fully ordered in terms of composition, and in terms of atomic positions deviates only slightly from a fluorite-type δ-Bi2O3-related parent structure. Three independent O-atom sites (accounting for 70 out of 78 O atoms in the unit cell) are also very close to fluorite-type parent positions. The remaining two O-atom sites, which coordinate W, exhibit partial occupancies and displacive disorder, neither of which could be better modelled by lowering of symmetry. The W site is coordinated by four O atoms in highly distorted tetrahedral coordination, the tetrahedron necessarily being orientationally disordered on that site. Nonetheless, the structure appears to be chemically reasonable.