bluuebunny/crossref_metadata_2025 huggingface dataset:
A method for checking X-ray diffractometer stability and its application
Authors: Fetisov G. V. , Markov V. T.
Published: July 2002

A statistical model of step-scanning of Bragg reflections on an X-ray diffractometer has been suggested [Dietrich (1976). J. Appl. Cryst.
9, 205–208]. A method and software for stability checking of X-ray automatic diffractometers at the user's laboratory have been developed on the basis of this model. The method is an extension of the technique proposed by Dietrich (1976) and reveals the operational instability of the instrument by processing the data of repeated measurements of a reflection intensity profile of a test crystal. The developed software package gives information about the error in initial positioning, the speed instability of the goniometer and the instability of the X-ray source and detector. The method was applied to a number of automatic four-circle X-ray CAD-4 (Enraf–Nonius, Holland) and Syntex (USA) diffractometers. The results obtained indicate that the method has high sensitivity and can be useful for practical crystallographers to reveal defects of their instruments.
Reflection electron microscopy
Authors: Yagi K.
Published: July 2002

Reflection electron microscopy (REM) in ultra-high vacuum (UHV) conditions is reviewed. UHV-REM can characterize surface structures of monolayer levels such as steps, domains of reconstructed surface structures and their boundaries and these capabilities are used to observe surface dynamic processes such as phase transitions of reconstructed surface structures and adsorbate structures and adsorption processes, oxidations, sublimations and ion-sputtering and annealing. The method is compared with other surface-imaging techniques.
Mass attenuation coefficients of the elements Ti, V, Fe, Co, Ni, Cu and Zn for the K emission lines between 4.51 and 10.98 keV
Authors: Unonius L. , Suortti P.
Published: July 2002

Mass attenuation coefficients of seven elements between atomic numbers 22 and 30 were measured at 20 X-ray energies between 4.5 and 11 keV. These energies correspond to fluorescence radiation excited by monochromatic Mo Kα radiation. The measurements were made on thin metal foils, which were spread flat and perpendicular to the beam scattered from the fluorescent sample. In this arrangement, the measured attenuation includes photoelectric absorption plus average elastic and inelastic scattering. The mass per unit area of a foil was determined by weighing and the impurity concentration by fluorescence analysis. The spectral lines corresponding to the components of the fluorescent radiation and those of the secondary radiation were resolved by the so-called singular value decomposition (SVD). The probable errors of the attenuation coefficients were typically 1%. The general agreement with previous experimental and theoretical values is good, but at low energies and at energies just above the absorption edges the present values are about 5% larger than theoretical values based on relativistic HFS (Hartree–Fock–Slater) calculations.
Calibration tests and use of a Nicolet/Xentronics imaging proportional chamber mounted on a conventional source for protein crystallography
Authors: Derewenda Z. , Helliwell J. R.
Published: July 2002

The results are presented of calibration tests and several single-crystal X-ray data collection experiments undertaken with a Nicolet/Xentronics Imaging Proportional Counter mounted on a conventional X-ray source. Considerable attention has been given to system optimization in collaboration with Nicolet, and this has led to a performance of high quality.
A measuring procedure for single-crystal diffraction using synchrotron radiation
Authors: Wendschuh-Josties M. , Wulf R.
Published: July 2002

The use of synchrotron radiation (SR) for single-crystal diffraction measurements requires special features of both the instrumentation and the measuring procedure. This paper describes a new algorithm for data collection developed for synchrotron radiation. It has been used successfully at the five-circle diffractometer at HASYLAB (DESY, Hamburg). The main features are: precise location of the reflection position, determination of the measuring parameters by pre-scanning, subsequent fine scanning, and real-time monitoring of several parameters of the beam conditions for data reduction. This allows fast and precise data collection.
SIR88 – a direct-methods program for the automatic solution of crystal structures
Authors: Burla M. C. , Camalli M. , Cascarano G. , Giacovazzo C. , Polidori G. , Spagna R. , Viterbo D.
Published: July 2002

SIR88 is an integrated package of computer programs for the solution of crystal structures. The package is based on the estimation of one- and two-phase structure seminvariants and three- and four-phase structure invariants according to the theory of representations [Giacovazzo (1977). Acta Cryst. A33, 933–944; (1980). Acta Cryst. A36, 362–372]. The program works in all the space groups and in most cases it is able to provide the correct solution without user intervention. Some prior information like the availability of a partial structure or of pseudotranslational symmetry is easily exploited to obtain the structure solution.
A cell for X-ray diffraction in liquids under moderate hydrostatic pressure
Authors: Geeves M. , Knight P.
Published: July 2002

A portable cell is described which allows X-ray diffraction data to be collected from suspensions or mounted specimens in liquid under hydrostatic pressure. Pressure is applied via either an oil pump or a high-performance liquid chromatography pump. The temperature of the cell is controlled by a thermostatted baseplate. X-rays pass through the stainless steel block via windows of Kapton V plastic sheet which will withstand a pressure difference of at least 15 MPa. Distortion of the plastic under pressure increases the liquid path of the X-rays. This distortion can be reduced by stretching over dome-shaped supports. Under these conditions 10 MPa causes a reduction of intensity of between 10 and 50%, depending on window geometry. Specimens can be mounted on a removeable frame, and the solution in the sample chamber can be changed without disassembling the cell. The apparatus has been used successfully to study the low-angle diffraction from muscle.
Linear least-squares adjustment of UB matrix elements and the prediction of reflection positions
Authors: Wilkinson C.
Published: July 2002

A method is presented for the linear least-squares refinement of small changes in the UB matrix elements and diffractometer offsets in single-crystal diffractometer experiments. It thus affords a rapid method for calculating the consequent changes in the positions of the centres of Bragg reflections.
A new method of highly accurate orientation of single crystals on a standard powder diffractometer
Authors: Michalski E.
Published: July 2002

A new method of obtaining high accuracy in a diffractometer refinement of a single-crystal orientation is presented in this paper. The primary errors in goniometer orientations are caused by the fact that, whereas the measurement required is the angle between the atomic plane and the plate face, goniometers in general measure the angle between the atomic plane and a plane defined by the goniometer, which is not necessarily parallel to the plate face. The methods of correcting for these inaccuracies associated with the goniometer orientations that have been used and cited so far are based on the laser-assisted system described by Vig [Proc. 29th Annual Symposium on Frequency Control (1975), pp. 240–247]. The method described in this paper allows one to eliminate these errors by a specially invented method of measurements and calculations without a laser-assisted system or any other additional instruments.
The φ Extent of the Reflection Range in the Oscillation Method According to the Mosaicity-Cap Model
Authors: Bolotovsky R. , Coppens P.
Published: July 2002

New expressions for the φ extent of the reflection range in the oscillation method are derived by the combination of analytical geometry with the reciprocal-space and Ewald-sphere formalisms. The effect of crystal mosaicity is described by the extension of a reciprocal-lattice point into a cap. The effective cylindrical coordinates of the reciprocal-lattice point, corresponding to the cap points lying in the plane in which the cap has its maximum φ extent, are introduced. The effect of beam divergence and wavelength spread are accounted for by consideration of a nest of the Ewald spheres. The expressions for intersection of the mosaicity cap with the nest of the Ewald spheres are used to check whether a reflection is fully or partially registered during the crystal oscillations. The presented formulas have been incorporated into software for processing area-detector data.
X-ray Scattering Studies of Metalloproteins in Solution: a Quantitative Approach for Studying Molecular Conformations, a Quantitative Approach for Studying Molecular Conformations.
Authors: Günter Grossmann J. , Hasnain S. S.
Published: July 2002

The model-independent approach based on the multipole expansion method using spherical harmonics [Stuhrmann (1970a). Acta Cryst. A26, 297–306] has been applied to obtain structural information on a variety of metalloproteins studied by synchrotron X-ray solution scattering. The method is applied to examples (nitrite reductase, transferrin and nitrogenase), not only with the view of comparing protein conformations in solution with those in the crystalline state, but also defining conformational changes and protein-protein interactions which are of functional importance. The shape restoration is found to be straightforward at low resolution (L≤ 3). For correct treatment using higher harmonics, overall molecular symmetry, if present, must be included in the multipole expansion.
K2(NbO)2Si4O12: a new ferroelectric
Authors: Foster M. C. , Arbogast D. J. , Photinos P. , Nielson R. M. , Abrahams S. C.
Published: July 2002

The atomic coordinates of K2(NbO)2Si4O12reported in space groupP4bmsatisfy the structural criteria for ferroelectricity. The estimated phase-transition temperature,Tc= 2230 (250) K, is substantially in excess ofTmelting= 1476 (5) K; the hypothetical paraelectric phase in space groupP4/mbmis hence experimentally inaccessible. A strong indication that the phase transition does not occur atT<Tmeltingis provided by the absence of both calorimetric and dielectric permittivity anomalies belowTmelting, the linear thermal dependence of unit-cell parameters with expansion coefficients α11= –0.53 (12) × 10–6and α33= 24.66 (11) × 10–6 K–1between 5 and 843 K, and the generation of second harmonics between 300 and 1353 K. Demonstration of dielectric hysteresis under both direct and alternating current, with spontaneous polarizationPs≃ 0.2 C m–2, provides unambiguous verification of the ferroelectric property.
On the number of independent reflections in a powder diffraction pattern
Authors: David W. I. F.
Published: July 2002

Peak overlap in powder diffraction patterns necessarily implies that the number of visible peaks is less than the number of Bragg peaks. While the number of Bragg reflections is easily enumerated, the number of statistically independent Bragg peaks is not. In this paper, it is shown that, although there is no single or simple answer to this problem, Bragg peak intensity correlation is a well behaved statistical quantity that can be used to express a measure of the number of independent peaks.
Small-angle scattering from collections of interacting hard ellipsoids of revolution studied by Monte Carlo simulations and other methods of statistical thermodynamics
Authors: Sjöberg Bo
Published: July 2002

Computer simulations using Monte Carlo methods are used to investigate the effects of interparticle correlations on small-angle X-ray and neutron scattering from moderate or highly concentrated systems of ellipsoids of revolution. Both oblate and prolate ellipsoids, of varying eccentricities and concentrations, are considered. The advantage with Monte Carlo simulation is that completely general models, both regarding particle shapes and interaction potentials, can be considered. Equations are also given that relate the nonideal part of the chemical potential, βμni, with the scattering at zero angle,I(0), and the compressibility factor,z. The quantity βμnican be obtained during the Monte Carlo simulations by using Widom's test-particle method. For spherical particles, the simulations are compared with approximation formulas based on the Percus–Yevick equation. A method is also suggested for the calculation of both βμniandzfrom experimental values ofI(0) recorded as a function of concentration.
A model-independent maximum-entropy method for the inversion of small-angle X-ray diffraction patterns
Authors: Elliott J. A. , Hanna S.
Published: July 2002

A model-independent maximum-entropy method is presented which will produce a structural model from small-angle X-ray diffraction data of disordered systems using no other prior information. In this respect, it differs from conventional maximum-entropy methods which assume the form of scattering entitiesa priori. The method is demonstrated using a number of different simulated diffraction patterns, and applied to real data obtained from perfluorinated ionomer membranes, in particular Nafion™, and a liquid crystalline copolymer of 1,4-oxybenzoate and 2,6-oxynaphthoate (B–N).
Ternary mixture of a homopolymer blend and diblock copolymer studied near the Lifshitz composition by small-angle neutron scattering
Authors: Mortensen K. , Schwahn D. , Frielinghaus H. , Almdal K.
Published: July 2002

A ternary mixture of two homopolymers and the corresponding diblock copolymer represents a good model system to study critical phenomena near the mean field predicted Lifshitz point. The structure factor of concentration fluctuations is in this regime of the phase diagram described by a formula which independently accounts for the peak-position q*, the scattering intensity at q*, S(q*), and the forward scattering, S(0). The susceptibility, which is given by the maximum value of either S(0) or S(q*), shows markedly renormalized critical behavior with critical exponents significant larger than the 3d-Ising value relevant for simple binary blends.
Evaluation of double-crystal SANS data influenced by multiple scattering
Authors: Šaroun J.
Published: July 2002

Evaluation of small-angle neutron scattering (SANS) data is often complicated by multiple scattering effects if large particles of relatively high volume fraction have to be studied and dilution or contrast reduction is impossible. The use of pin-hole SANS instruments is often limited due to the contradictory requirements of high resolution and short wavelength needed to keep scattering contrast as low as possible. Double crystal (DC) SANS diffractometers of Bonse-Hart and bent-crystal type are useful alternatives in such cases, as they permit reaching very high resolution with thermal neutrons. A method for SANS data evaluation suited to DC instruments is presented. It includes the common scheme of the indirect Fourier transformation method, but takes multiple scattering into account. The scattering medium is described by the frequency function g(x) defined as the cosine Fourier transform of slit-smeared data. Although a simplistic model of polydisperse spheres is used to represent g(x), resulting g(x) function and some integral parameters are independent of this model. Tests on simulated data show, that the method reproduce well true values of microstructural parameters, though systematic errors are observed in the cases when the unscattered part of incident beam completely disappears. If the scattering power  is known and kept fixed during fitting, then other parameters are reproduced well also in the regime of strong multiple scattering. The evaluation procedure permits simultaneously fitting to several sets of data measured for different Q-regions, resolutions and sample thicknesses. It has proved to provide reliable results for particle sizes ranging from about 100 Å to several microns and  < 10.
SAXS studies on Kynar-based membranes for fuel cells
Authors: Jokela K. , Serimaa R. , Torkkeli M. , Elomaa M. , Sundholm F. , Walsby N. , Kallio T. , Sundholm G.
Published: July 2002

The structure of the styrene grafted and sulfonated poly(vinylidenefluoride-co-hexafluoropropylene) was studied by small angle x-ray scattering, SAXS. The starting material is lamellar and the grafting and sulfonation processes and the hexafluoropropylene, HFP, contents affected the lamellar period. The SAXS intensity curve of the sulfonated membranes also included an ionic peak which arises from the ionic aggregates. The effects of the humidy and metal ion neutralization on the ionic aggregates were studied.
Agglomeration of nano-MgAl2O4 spinel from microemulsion and CVD techniques
Authors: Winter R. , Quinten M. , Dierstein A. , Hempelmann R. , Altherr A. , Veith M.
Published: July 2002

MgAl2O4 nanoparticles have been prepared using a microemulsion synthesis technique (ME) and a chemical vapour deposition (CVD) approach. In the wet-chemical route, the chain length of the surfactant molecules used to stabilise the microemulsion as well as the annealing conditions (temperature and time) after preparation were varied. For CVD samples, the influence of the target temperature and the consecutive annealing temperature were investigated. A pinhole small-angle X-ray scattering camera was employed to determine the size and size distribution of primary particles as well as agglomerates. ME material is primarily obtained in the form of amorphous non-agglomerated particles; CVD raw material is crystalline and agglomerated. Upon annealing, agglomerates of ME material form and then grow faster than the primary particles do; in CVD material the agglomerates do not grow. Agglomeration is suppressed if the CVD process is carried out at elevated target temperature.
Neutron diffraction investigation of the nuclear and magnetic extinction in MnP
Authors: Baruchel J. , Patterson C. , Guigay J. P.
Published: July 2002

The absolute values of the reflecting powers ρ are measured for the 200 and 2 ± τ, 0, 0 set of magnetic and nuclear reflections in the helimagnetic phase of a good-quality crystal of MnP as a function of its thickness. Severe and very different extinction effects are observed for the magnetic and nuclear reflections (y
magnetic ~ 0.4, y
nuclear ~ 0.02 for the largest thickness). This corresponds to the spectacular result that the magnetic reflecting powers ρ
± are twice as big as the nuclear one ρN
, in spite of the fact that the scattering cross sections |F
±|2 are about ten times smaller than the nuclear |FN
|2. The nuclear results appear consistent with dynamical theory while the magnetic ones are not. They can be explained by Zachariasen's type II secondary extinction model based on the chirality domain pattern. The same measurements were performed in the ferromagnetic phase, yielding y
ferro ≃ 0.03. A model using the relative sizes of the ferromagnetic and chirality domains is presented.
Correction for the dynamical electron diffraction effect in crystal structure analysis
Authors: Sha B. D. , Fan H. F. , Li F. H.
Published: July 2002

A method is proposed to correct for the dynamical electron diffraction effect in crystal structure analysis. A rough structure model is first obtained by conventional structure-analysis methods neglecting the dynamical diffraction effect. From the rough structure model, multislice calculations are used to estimate the crystal thickness through the observed dynamical diffraction wave amplitudes. With this estimated thickness, the observed diffraction wave amplitudes are calibrated to give a set of fictitious observed kinematic structure-factor magnitudes. Based on such a set of magnitudes, a traditional least-squares procedure is used to refine structural parameters. The reliability of the result is checked by the consistency between the observed dynamical diffraction wave amplitudes and those found from the multislice calculation. The process can be made iterative. Tests were performed with two known structures, Bi-2212 and Pb-doped Bi-2223 high-Tc
superconductors, and satisfactory results were obtained.
Influence of intermolecular interactions on multipole-refined electron densities
Authors: Spackman Mark A. , Byrom Patrick G. , Alfredsson Maria , Hermansson Kersti
Published: July 2002

This work examines the effect of intermolecular interactions on molecular properties derived from simulated X-ray diffraction data. Model X-ray data are computed from a superposition of ab initio molecular electron densities in the crystal, as well as from periodic crystal Hartree–Fock electron densities, for the hydrogen-bonded systems ice VIII, formamide and urea, as well as the weakly bound acetylene. The effects of intermolecular interactions on the electron density are illustrated at both infinite and finite data resolution, and it is concluded that multipole models are capable of quantitative retrieval of the interaction density, despite the known shortcomings of the radial functions in the model. Multipole refinement reveals considerable enhancement of the molecular dipole moment for hydrogen-bonded crystals, and negligible change in molecular second moments. Electric field gradients at H nuclei are significantly reduced in magnitude upon hydrogen bonding, and this change is also faithfully represented by the rigid pseudoatom model.
Probability distribution of the four-phase structure invariants of isomorphously related structure factors and its applications
Authors: Liu Yong-Sheng , Hu Ning-Hai , Wang Shu-Yun
Published: July 2002

The probability distribution of the four-phase structure invariants (4PSIs) involving four pairs of structure factors is derived by integrating the direct methods with isomorphous replacement (IR). A simple expression of the reliability parameter for 16 types of invariant is given in the case of a native protein and a heavy-atom derivative. Test calculations on a protein and its heavy-atom derivative using experimental diffraction data show that the reliability for 4PSI estimates is comparable with that for the three-phase structure invariants (3PSIs), and that a large-modulus invariants method can be used to improve the accuracy.
Positive-definite conditions of elastic constants of two-dimensional quasicrystals with noncrystallographic symmetries
Authors: Wang Jianbo , Zhang Ruikang , Ding Di-Hua , Wang Renhui
Published: July 2002

In the present work, the positive-definite conditions of the elastic constants of two-dimensional (2D) quasicrystals with non-crystallographic symmetries have been derived. These results are necessary for studying elastic and mechanical properties of two-dimensional quasicrystals both in theory and in experiment.
Progress on the direct-methods solution of macromolecular structures using single-wavelength anomalous-dispersion (SAS) data
Authors: Langs David A. , Blessing Robert H. , Guo Dongyao
Published: July 2002

In the past few years, a number of strategies have been outlined to resolve the SAS phase ambiguity given that unique estimates \omega(h, k) of the triple invariants are available. A new least-squares method is described that can in principle resolve the phase ambiguity to determine macromolecular phases provided that \omega(h, k) estimates are unbiased. Limitations of the method in practical applications are discussed. An example is given where the correct solution can be identified by use of the SAS tangent formula in the instance that traditional SAS phasing methods have lead to an incorrect heavy-atom substructure.
A new structure type in the hexagonal perovskite family; structure determination of the modulated misfit compound Sr9/8TiS3
Authors: Gourdon O. , Petricek V. , Evain M.
Published: July 2002

Sr9/8TiS3, strontium titanium sulfide, a new phase in the hexagonal perovskite-like Sr
x
TiS3 system, has been prepared and its structure solved from single-crystal X-ray data within the (3 + 1)-dimensional [(3 + 1)D] formalism. Sr9/8TiS3 crystallizes with trigonal symmetry [R3¯m(00γ)0s superspace group], with the following lattice parameters: a

s
= 11.482 (3), c

s
= 2.9843 (8) Å, q = 0.56247 (7)c* and V

s
= 340.7 (3) Å3. The structure was considered as commensurate [R3¯c three-dimensional (3D) space group], but refined within the (3 + 1)D formalism to a residual factor R = 2.79% for 64 parameters and 1084 independent reflections. Original crenel functions were used for the sulfur and strontium description. The structure is different from that of the hexagonal perovskite-like oxide counterparts. The main difference is related to the presence of a new type of polyhedron in the [MS3] transition metal chains, intermediate between the octahedra classically found in such chains and the trigonal prismatic sites encountered in the oxides.
Modulated structure of the pseudohexagonal InFe1−x−4δTi
x+3δO3+x/2 (x = 0.61) composite crystal
Authors: Michiue Yuichi , Onoda Mitsuko , Watanabe Mamoru , Brown Francisco , Kimizuka Noboru
Published: July 2002

The structure of pseudohexagonal-type InFe1−x−4δTi
x+3δO3+x/2 (x = 0.61, δ = 0.04), indium iron titanium oxide, was refined on the basis of a four-dimensional superspace group. The crystal has a compositely modulated structure consisting of two orthorhombic subsystems mutually incommensurate in b. The first subsystem InFe1−x−4
δTi
x+3
δO2 has a delafossite structure with lattice parameters a = 5.835 (3), b
1 = 3.349 (1) and c = 12.082 (7) Å. The second subsystem with b
2 = 2.568 (6) Å consists of O atoms. The superspace group of the overall structure is Ccmm(1, 1.305, 0)s00, which can be converted to Amam(0, 0, 0.305)0s0 (No. 63.8). Refinement on 1105 unique reflections converged to R = 0.0303 and wR = 0.0325 with 63 structural parameters. The structure of the first subsystem is the alternate stacking of an edge-shared InO6 octahedral layer and an Fe/Ti triangle-lattice plane along c. A sheet of O atoms in the second subsystem is also extending on the Fe/Ti plane, where displacive modulation of atoms is prominent.
Modified photoreactivity due to mixed crystal formation. I. Three mixed crystals between isostructural cobaloxime complexes
Authors: Vithana Champika , Uekusa Hidehiro , Sekine Akiko , Ohashi Yuji
Published: July 2002

Three crystals of 2-cyanoethyl cobaloxime complexes with 3-chloropyridine, 3-bromopyridine and 3-methylpyridine as axial base ligands are isostructural to one another. Three mixed crystals were formed between the pairs: (3-bromopyridine)(2-cyanoethyl)cobaloxime–(2-cyanoethyl)(3-methylpyridine)cobaloxime(0.45/0.55);(3-chloropyridine)(2-cyanoethyl)cobaloxime–(2-cyanoethyl)(3bromopyridine)cobaloxime (0.61/0.39); (3-chloropyridine)(2-cyanoethyl)cobaloxime–(2-cyanoethyl)(3-methylpyridine)cobaloxime (0.44/0.56). The X-ray crystal structure analysis revealed that the mixed compounds are also isostructural to the component crystals. It was found from the change in IR spectra that the 2-cyanoethyl groups in the three mixed crystals isomerized to 1-cyanoethyl groups on exposure to a xenon lamp, as observed for the 2-cyanoethyl groups in the component crystals. Rate constants for the three mixed and three component crystals, measured under the same conditions, are approximately the same, as the reaction cavities for the 2-cyanoethyl groups in the six crystals have almost the same size. For the mixed crystal between 3-chloropyridine and 3-methylpyridine complexes, the isomerization proceeded with retention of the single-crystal form. The conformation and configuration of the 1-cyanoethyl group that was produced were well explained by the shape of the reaction cavity before irradiation.
Twinning and intergrowth of rare earth boride carbides
Authors: Oeckler Oliver , Bauer Josef , Duppel Viola , Mattausch Hansjürgen , Simon Arndt
Published: July 2002

Twins and intergrown crystals of tetragonal rare earth boride carbides, especially those with the La5B2C6 structure type, have been investigated by high-resolution electron microscopy and X-ray diffraction. The structure of the twin interface has been determined. It provides an explanation for coherently intergrown domains of different structure. The Sc3C4 structure type is remarkable because it is frequently intergrown with La5B2C6-type phases. It provides, for instance, a model for the intergrowth of other types, e.g. Gd4B3C4 and Gd5B2C5. The presence of metal-atom square nets in different orientations in the structures accounts for a number of intergrowth phenomena. The possibilities and limitations of X-ray structure determinations are discussed with respect to actual examples.
Use of the Inorganic Crystal Structure Database as a problem solving tool
Authors: Kaduk James A.
Published: July 2002

The information in the Inorganic Crystal Structure Database (ICSD) can be used to determine the structures of new compounds by analogy; this database is thus a useful tool for practical problem solving. A model for the structure of (NH4)Fe(CO3)(OH)2 [Cmcm, a = 6.6154 (6), b = 12.0639 (10), c = 6.0263 (5) Å, Z = 4] was found by searching the ICSD for compounds with C-centered orthorhombic cells and containing C, H, N and O. [Al(H2O)6]2[Al(H2O)5(SO4)]2-(H3O)2(SO4)5 [P\bar{1}, a = 12.5881 (5), b = 13.0726 (5), c = 7.3354 (2) Å, α = 108.162 (2), β = 70.536 (2), γ = 112.658 (2)°, Z = 1] is isostructural to a Cr compound of the same stoichiometry, which was located by searching on the formula type ANX = A4B7X52. Suitable models for MgCl2(H2O)4 [P21/c, a = 5.9001 (9), b = 7.2709 (7), c = 8.4199 (7) Å, β = 111.007 (15)°, Z = 2], MgCl2(H2O)2 [C2/m, a = 7.3886 (29), b = 8.5498 (24), c = 3.6488 (17) Å, β = 98.96 (3)°, Z = 2], and MgCl2(H2O) [Pnma, a = 8.899 (7), b = 3.6339 (20), c = 11.398 (8)  Å, Z = 4] were located by combining searches on chemistry and formula type. Quantum-chemical calculations were performed on all five structures to locate the H-atom positions and to obtain quantitative understanding of the hydrogen bonding important in all the structures.
Structural studies on monofluorinated derivatives of the phytohormone indole-3-acetic acid (auxin)
Authors: Antolić A. , Kojić-Prodić B. , Tomić S. , Nigović B. , Magnus V. , Cohen J. D.
Published: July 2002

As part of the molecular recognition studies on the phytohormone indole-3-acetic acid (IAA) a series of fluorinated IAA's has been examined. The phenyl ring substitution at positions 4, 5, 6 and 7 resulted in four compounds, which were analyzed. Structure–activity correlation includes the analysis of their molecular conformations, based on the X-ray diffraction and computational chemistry results, and bioactivity determinations in the Avena coleoptile and the Pisum
sativum stem straight-growth tests, lipophilicity and UV absorbance. The conformations of monofluorinated IAA's and a free hormone are defined by rotations about two bonds: one describes the relative orientation of a side chain towards the indole plane and the second the orientation of the carboxylic group. The results of X-ray structure analysis revealed the folded shape of the molecules in all compounds studied. Molecular mechanics and dynamics located the folded conformation as the local minimum, but failed to detect the planar conformation as one of the local minima, which according to ab initio results on IAA and 4-CI-IAA could also be possible. Crystal data at 295 K for 4-F-IAA and at 297 K for 5-F-IAA, and at 100 K for 6-F-IAA and 7-F-IAA using Mo Kα radiation (λ = 0.71073 Å) and Cu Kα (λ = 1.5418 Å, for 7-F-IAA), are as follows: 4-F-IAA, C10H8NO2F, Mr
= 193.18, monoclinic, C2/c, a = 17.294 (5), b = 13.875 (4), c = 7.442 (4) Å, β = 103.88 (6)°, V = 1734 (1) Å3, Z = 8, Dx
= 1.480 g cm−3, μ = 1.1 cm−1, F(000) = 800, R = 0.043, wR = 0.044 for 823 symmetry-independent [I ≥ 3σ(I)] reflections; 5-F-IAA, C10H8NO2F, monoclinic, P21/c, a = 19.284 (5), b = 5.083 (4), c = 9.939 (4) Å, β = 117.28 (6)°, V = 865.9 (1) Å3, Z = 4, Dx
= 1.482 g cm−3, μ = 1.1  cm−1, F(000) = 400, R = 0.062, wR = 0.057 for 729 symmetry-independent [I ≥ 3σ(I)] reflections; 6-F-IAA, C10H8NO2F, monoclinic, P21/a, a = 9.360 (1), b = 5.167 (4), c = 17.751 (4) Å, β = 93.75 (1)°, V = 856.7 (8) Å3, Z = 4, Dx
= 1.498 g cm−3, μ = 1.1 cm−1, F(000) = 400, R = 0.048, wR = 0.048 for 1032 symmetry-independent [I ≥ 2σ(I)] reflections; 7-F-IAA, C10H8NO2F, monoclinic, P21/a, a = 9.935 (5), b = 5.0059 (4), c = 17.610 (1) Å, β = 102.13 (1)°, V = 856.3 (1) Å3, Z = 4, Dx
= 1.498 g cm−3, μ = 9.8 cm−1 (Cu Kα, F(000) = 400, R = 0.035, wR = 0.040 for 1504 symmetry-independent [I ≥ 2σ(I)] reflections.
A revision of the structure of (bipyridyl-N,N')-dicyanoplatinum(II)
Authors: Connick W. B. , Henling L. M. , Marsh R. E.
Published: July 2002

In a previous X-ray crystallographic study of crystals of Pt(bpy)(CN)2 (bpy = 2,2′-bipyridine) the planar molecules were reported to be exactly eclipsed, stacked directly on top of one another with a spacing of 3.33 Å so as to form a linear Pt...Pt...Pt chain. A reinvestigation shows this structure to be incorrect. The presence of weak intermediate layer lines indicates that the repeat distance along the stacking direction is 6.66 Å rather than 3.33 Å. Successive molecules within the stack are rotated by 180° and the resulting Pt-atom chain is slightly zigzag with a Pt...Pt...Pt angle of 168.6 (1)°. The implications are discussed of the determination and refinement of an apparently satisfactory, although grossly wrong, structure that was based on an incorrect unit cell and an incorrect space group.
The sigma phase revisited: an alternative pathway to a trial structure
Authors: Bergman G.
Published: July 2002

A new derivation of a trial structure of the sigma phase is described. It is based on the stochastic method in a form which is particularly useful in the derivation of trial structures of high symmetry, provided that the radii of the component metals are of the appropriate relative magnitudes. In this form the initial assumption is made that the structure is tetrahedrally close-packed and that the coordination polyhedra are normal with coordination numbers 12, 14, 15 and 16, respectively. These restrictions are first invoked in the search for the correct structural arrangements along the symmetry axes. Only those arrangements are considered which have small packing strains along these axes when calculated using the metallic radii for coordination number 12.
Hydrogen-Bond Acceptor and Donor Properties of Divalent Sulfur (Y-S-Z and R-S-H)
Authors: Allen F. H. , Bird C. M. , Rowland R. S. , Raithby P. R.
Published: July 2002

The hydrogen-bond acceptor ability of divalent sulfur in Y—S—Z systems, Y, Z= C, N, O or S, and the donor ability of thiol S—H have been studied using crystallographic data retrieved from the Cambridge Structural Database. Of 1811 Y—S—Z substructures that co-occur with N—H or O—H donors, only 86 (4.75%) form S...H—N,O bonds within S...H < 2.9 Å. In dialkylthioethers, the frequency of S...H bond formation is 6.24%, but drops below 3% when the alkyl groups are successively replaced by Csp
2 centres. This parallels an increasing \delta-positivity of S as calculated using ab initio methods. A similar frequency trend is observed for O...H—N,O bond formation by analogous oxyethers. Mean intermolecular >S...H distances for O—H [2.67 (3) Å] and N—H [2.75 (2) Å] donors (with H positions normalized to neutron values) are ca 0.25 Å longer than in C=S...H—N,O systems, indicative of very weak hydrogen bonding to >S. Intramolecular >S...H are slightly more frequent (8.56%), with S...H slightly shorter than for the intermolecular case. In contrast, 26 (70.3%) out of 37 S—H donors that co-occur with suitable acceptors form X...H—S bonds. The C=O...H—S system is predominant with a mean O...H distance of 2.34 (4) Å, considerably longer (weaker) than in C=O...H—O systems.
Aminoguanidinium(2+) Hexafluorozirconate Monohydrate: A Co-Product of Preparing the Ferroelectric Anhydrous Salt
Authors: Ross C. R. , Paulsen B. L. , Nielson R. M. , Abrahams S. C.
Published: July 2002

Preparation of anhydrous aminoguanidinium(2+) hexafluorozirconate, CN4H8ZrF6, shown previously to satisfy the structural criteria for ferroelectricity [Abrahams et al. (1996). Acta Cryst. B52, 806–809], generally results in the co-formation of a series of related fluorozirconates. The structure of the monohydrate salt, one of the co-products, has been redetermined to improve understanding of the preparation pathway, locate the H atoms and compare corresponding atom positions [Gerasimenko et al. (1989). Koord. Khim.
15, 130–135]. The positions of the H atoms were not established in the latter study. All 16 H atoms in the two symmetry-independent CN4H8(2+) ions are now located and refined, with R
1 = 0.0299 and S = 1.119. Both independent water molecules are disordered. Normal probability analysis reveals uncompensated error and/or underestimated uncertainty associated with ten non-H-atom position coordinates. The relative concentrations of HF, CN4H7Cl and H2ZrF6 are among the major variables controlling the formation of the related fluorozirconates.
New Measure of Distortion for Coordination Polyhedra
Authors: Makovicky E. , Balić-Žunić T.
Published: July 2002

A new global measure of distortion for coordination polyhedra is proposed, based on a comparison of the ratios Vs
(circumscribed sphere)/Vp
(polyhedron) calculated, respectively, for the real and ideal polyhedra of the same number of coordinated atoms which have the same circumscribed sphere. This formula can be simplified to υ (%) = 100[Vi
(ideal) − Vr
(real)]/Vi
, where Vi
and Vr
are the volumes of the above-defined polyhedra. The global distortion can be combined with other polyhedral characteristics, e.g. with the eccentricity of the central atom in the polyhedron or with the degree of sphericity of the coordination sphere [Balić Zõunić & Makovicky (1996). Acta Cryst. B52, 78–81].Vs
/Vp
ratios are given for a number of ideal polyhedra, including several types of trigonal coordination prisms, with the aim of facilitating the distortion calculations. The application examples included in the paper are: complex sulfides based on PbS and SnS archetypes, coordination polyhedra of large cations in feldspars, a phase transformation in a monoclinic amphibole and the subdivision of structures isopointal to ilmenite.
Anhydrous ammonioguanidinium(2+) and dihydrated bis[aminoguanidinium(1+)] hexafluorosilicates: new co-products of preparing ferroelectric ammonioguanidinium(2+) hexafluorozirconate
Authors: Ross C. R. , Bauer M. R. , Nielson R. M. , Abrahams S. C.
Published: July 2002

Ammonioguanidinium hexafluorosilicate, CH8N_4^{2+}·SiF_6^{2-}, and bis(aminoguanidinium) hexafluorosilicate dihydrate, 2CH7N_4^+·SiF_6^{2-}·2H2O, are new materials formed as by-products in course of preparing ferroelectric CH8N4ZrF6 in the presence of glassware. Their structures were determined for comparison with the corresponding hexafluorozirconates. All atoms including the eight H atoms in the CH8N_4^{2+} cation and the seven H atoms in the CH7N_4^+ cation have been located and refined with wR(F
2) = 0.0653, R = 0.0255, S = 1.146 and wR(F
2) = 0.0745, R = 0.0301, S = 1.065, respectively. The N2C—N—N backbone of the 2+ cation is close to planarity, while that of the 1+ cation does not differ significantly from planarity. The SiF_6^{2-} octahedron is nearly ideally regular in both materials, with  = 1.684 (unbiassed estimator of standard uncertainty = 0.016) Å in the anhydrous hexafluorosilicate and 1.6801 (unbiassed estimator of standard uncertainty = 0.0006) Å in the dihydrate. The combination of coulombic and NH...F interactions in CH8N4SiF6 results in a relatively dense variant of the NaCl structure. In addition to similar forces, the dihydrate is also characterized by the role of the water molecule with its strong NH...O interactions; its packing efficiency is, however, appreciably less than that of the anhydrous hexafluorosilicate with an ∼8% increase in void space. Cleaved crystals of the dihydrate are frequently twinned across the (001) composition plane, with a twofold rotation about the b axis as the twin operation.
Polynuclear chloromercurate(II) systems in their chloropyridinium salts
Authors: Linden Anthony , James Bruce D. , Liesegang John , Gonis Nick
Published: July 2002

The chloromercurate(II) salts of 2-, 3- and 4-chloropyridine display a variety of anion stoichiometries and structures, including the rare [Hg3Cl10]4− stoichiometry. 2-Chloropyridinium trichloromercurate(II), (I), (C5H5ClN)[HgCl3], monoclinic, P21/n, a = 9.094 (8), b = 18.143 (4), c = 12.902 (3) Å, β = 106.13 (4)° with Z = 8, has the [HgCl3]− stoichiometry, but the anions are infinite chains composed of [HgCl3]−, HgCl2 and Cl− moieties linked by longer Hg...Cl contacts. Hydrogen bonds link the cations to the formal Cl− ions. Tetrakis(3-chloropyridinium) decachlorotrimercurate(II), (II), (C5H5ClN)4[Hg3Cl10], monoclinic, P21/n, a = 7.522 (2), b = 28.046 (3), c = 9.165 (2) Å, β = 105.78 (2)° with Z = 2, has the rare [Hg3Cl10]4− stoichiometry and contains infinite one-dimensional double-stranded {([HgCl4]2−)2 [HgCl2]}
n
anionic chains made up of linear HgCl2 and distorted [HgCl4]2− entities linked together by longer Hg...Cl contacts. The HgCl2 moieties are joined by double [HgCl4]2− bridges. Hydrogen bonds link the cations to the sides of the anionic columns. Tetrakis(4-chloropyridinium) decachlorotrimercurate(II), (III), (C5H5ClN)4[Hg3Cl10], triclinic, P1¯, a = 9.907 (3), b = 13.226 (2), c = 7.282 (2) Å, α = 84.41 (2), β = 74.81 (2), γ = 87.34 (2)° with Z = 1, also has the [Hg3Cl10]4− stoichiometry and the same type of {([HgCl4]2−)2[HgCl2]}
n
anionic chains that were found in compound (II), but the formal HgCl2 and [HgCl4]2− moieties are more discrete with much weaker contacts linking the individual units. Bifurcated hydrogen bonds with the cations cross-link the anionic chains to form an infinite two-dimensional network. Second forms of the 3- and 4-chloropyridinium salts were also obtained. 3-Chloropyridinium trichloromercurate(II), (IV), (C5H5ClN)[HgCl3], monoclinic, P21/c, a = 7.243 (5), b = 22.145 (8), c = 12.320 (3) Å, β = 99.52 (3)° with Z = 8, has the [HgCl3]− stoichiometry, but the anions are infinite chains composed of distorted [Hg2Cl6]2− moieties. Bifurcated hydrogen bonds from the cations cross-link the anionic chains to form infinite two-dimensional layers. Bis(4-chloropyridinium) hexachlorodimercurate(II), (V), (C5H5ClN)2[Hg2Cl6], monoclinic, C2/m, a = 13.447 (3), b = 7.534 (2), c = 9.939 (2) Å, β = 97.48 (2)° with Z = 2, contains highly symmetrical discrete [Hg2Cl6]2−anions. Bifurcated hydrogen bonds from the cations interconnect the anions to form infinite one-dimensional chains.
Preparation and characterization of tetrachlorocobaltates(II) of α,ω-alkylenediammonium. Magnetic and thermal properties. Crystal structure of [NH3(CH2)5NH3]CoCl4
Authors: Criado J. J. , Jiménez-Sánchez A. , Cano F. H. , Sáez-Puche R. , Rodríguez-Fernández E.
Published: July 2002

Tetrachlorocobaltates(II) of diprotonated α,ω-diaminoalkanes with the formula [NH3(CH2)
n
NH3]CoCl4, where n = 5 (cadaverine; 1,5-pentanediammonium tetrachlorocobaltate), 8 (1,8-octanediammonium tetrachlorocobaltate) and 10 (1,10-decanediammonium tetrachlorocobaltate), were prepared. The compounds were studied by mass spectrometry, FT-IR and visible spectroscopy, magnetic susceptibility techniques and thermal analysis. The compounds contain the tetrahedral tetrachlorocobaltate(II) ion and the corresponding diprotonated diamine (cadaverine, 1,8-octamethylenediamine and 1,10-decamethylenediamine). The compound corresponding to cadaverine crystallizes in the monoclinic space group P21/c, with lattice parameters a = 7.1633 (7), b = 15.940 (3), c = 11.137 (2) Å, β = 98.44 (1)° and Z = 4. Its crystal structure contains slightly distorted tetrahedral CoCl_4^{2-} ions: the largest difference in Co—Cl bond lengths is 0.029 Å and the largest difference in Cl—Co—Cl angles is 7.76°. The compound also contains diprotonated cadaverine ions. An extensive hydrogen-bonding network connects these ions. The slightly positive deviations of the magnetic susceptibility from the Curie–Weiss law are in agreement with the 4
A
2 ground state for the tetrachlorocobaltate anion. The compounds with eight and ten C atoms show phase transitions in the solid state and a greater complexity is observed in their differential scanning calorimetry curves. Unfortunately, no suitable single crystals of these could be obtained.
Graph-set analysis of hydrogen-bond patterns: some mathematical concepts
Authors: Grell J. , Bernstein J. , Tinhofer G.
Published: July 2002

To provide a foundation for further theoretical and software development of the application of graph sets to patterns of hydrogen bonding and other intermolecular interactions a number of mathematical concepts and tools are defined, developed and demonstrated. Following a review of the basic definitions and uses of graph sets, the directional properties of hydrogen bonds are now included in the treatment. The concepts of a constructor graph and covalent distance matrix have been developed to aid in the generation of a qualitative descriptor for the straightforward, consistent and ultimately automatic (with appropriate software) definition of patterns. An additional mathematical tool, the arrowed T-labeling, has been developed to deal with situations in which pattern-forming moieties are located on crystallographic special positions. To demonstrate the utility and various features of these concepts they are applied in detail to two particular structures, polymorphic iminodiacetic acid [N-(carboxymethyl)glycine] and trans-tetraamminedinitrocobalt(III) acetate. To facilitate the application and use of graph sets many of these developments have already been incorporated into the software of the Cambridge Structural Database, as described in the accompanying paper.
The use of Karle–Hauptman determinants in small-structure determinations
Authors: Vermin W. J. , de Graaf R. A. G.
Published: July 2002

This paper illustrates the potential of Karle-Hauptman determinants for the solution of the phase problem in a 'difficult' small structure. An outline of the procedure followed is given. Suggestions for the future are presented, together with a discussion of the results obtained.
Ferroelasticity and phase transformation in Rb2Hg(CN)4 spinel
Authors: Haussühl S.
Published: July 2002

The low-temperature form of Rb2Hg(CN)4 [space group R\bar 3c, Z = 12, a
1 = 9.076, a
3 = 46.050 Å (273 K)] possesses a slightly distorted spinel structure with parameters similar to those of cubic cyanospinels. It transforms into a cubic spinel modification at ca 398 K (a = 13.140 Å at 399 K). Below the transition temperature the crystals exhibit ferroelastic properties, which allow the switching of the distortion directions into any one of four equivalent states. The critical stress is less than 1 x 10-3 kp mm-2. The resulting deformations do not exceed 2 x 10-2. Larger twinned crystals return to their primary state after the release of stress. They behave like extremely soft elastic springs. The crystals belong to the ferroelastic Aizu species m3m
F\bar 3m. The transition shows a constant hysteresis of ca 12 K in all crystals investigated. The pressure derivatives of the upper and lower transition temperatures are observed to be almost equal and constant over a wide temperature range (dT/dP ≈ O. 245 K bar-l). The observed enthalpy of transformation is 1.03 J g-1 in fair agreement with the prediction of the Clapeyron-Clausius relation. Crystals of K2Hg(CN)4 exhibit similar properties; their transition temperature, hysteresis, enthalpy of transformation, and dT/dP are considerably less.
Clarification of discussion on constrained refinement
Authors: Price P. F.
Published: July 2002

Pawley's [Acta Cryst. (1976). A32, 921-922] disproof of the C & D method of applying constraints in crystallographic refinements [Chesick & Davidon (1975). Acta Cryst. A31, 586-591] is incorrect. Nevertheless, the C & D method is incorrect. Both the method and its disproof clearly fail for the particular case of linear least-squares with linear constraints. The method can be corrected in this case but not for the non-linear case.