bluuebunny/crossref_metadata_2025 huggingface dataset:
Small-angle scattering by dislocations
Authors: Thomson Robb , Levine L. E. , Long G. G.
Published: July 2002

It is shown that the small-angle scattering of X-rays or neutrons by dislocations within a deformed metal, which are partially ordered into wall-like structures, is characterized by several factors. Principally these are associated with: (i) a single dislocation or dipole; (ii) the dislocation configuration in the plane of the wall; and (iii) the distribution of dislocations across the wall thickness. With the assumption of isotropic elasticity, small-angle scattering will be sensitive only to the edge components of the dislocations. The scattered intensity is dominated by scattering from dislocations that lie perpendicular to the scattering vector, \bf q, and reaches a maximum when \bf q is normal to the slip plane of these dislocations. Above a particular |\bf q |, the scattered intensity is sensitive only to the total edge dislocation content of the scattering dislocations (i.e. scattering is incoherent), while, below this value, the scattering is dominated by how the dislocations are distributed in walls. For walls normal to their slip planes, the configuration factor will reflect the dislocation distribution in the plane of the wall, while, for walls parallel to their slip planes, the distribution in the thickness direction will be visible. Therefore, even though a deformed material is composed of complicated dislocation structures, only those segments conforming to these rather strict prescriptions will be singled out for scattering, and, by adjusting the beam/slip system geometry, many parameters of the microstructure can be determined experimentally.
Spectral decomposition of the linear elastic tensor for monoclinic symmetry
Authors: Theocaris Pericles S. , Sokolis Dimitrios P.
Published: July 2002

The compliance fourth-rank tensor related to crystalline or other anisotropic media belonging to the monoclinic crystal system is spectrally decomposed for the first time, and its characteristic values and idempotent fourth-rank tensors are established. Further, it is proven that the idempotent tensors resolve the stress and strain second-rank tensors into eigentensors, thus giving rise to a decomposition of the total elastic strain-energy density into non-interacting strain-energy parts. Several examples of representative inorganic crystals of the monoclinic system illustrate the results of the theoretical analysis. It is also proven that the essential parameters required for a coordinate-invariant characterization of the elastic properties of a crystal exhibiting monoclinic symmetry are both the six characteristic values of the compliance tensor and seven dimensionless parameters. These material constants, referred to as the eigenangles, are shown to be accountable for the orientation of the stress and strain eigentensors, when represented in a stress coordinate system. Finally, the restrictions dictated by the classical thermodynamical argument on the elements of the compliance tensor, which are necessary and sufficient for the elastic strain-energy density to be positive definite, are investigated for the monoclinic symmetry.
Exponential Shake-and-Bake: theoretical basis and applications
Authors: Hauptman Herbert A. , Xu Hongliang , Weeks Charles M. , Miller Russ
Published: July 2002

The simple cosine function used in the formulation of the traditional minimal principle and the related Shake-and-Bake algorithm is here replaced by a function of exponential type and its expected value and variance are derived. These lead to the corresponding exponential minimal principle and its associated Exponential Shake-and-Bake algorithm. Recent applications of the exponential function to several protein structures within the Shake-and-Bake framework suggest that this function leads, in general, to significant improvements in the success rate (percentage of trial structures yielding solution) of the Shake-and-Bake procedure. However, only in space group P1 is it presently possible to assign optimal values a priori for the exponential-function parameters.
Neutron Rietveld refinement of the incommensurate phase of the ordered perovskite Pb2CoWO6
Authors: Baldinozzi G. , Calvarin G. , Sciau Ph. , Grebille D. , Suard E.
Published: July 2002

The incommensurate structure of lead cobalt tungstate has been refined by the Rietveld method on neutron data collected at 250 K. The space group is planar monoclinic I2/m(\alpha0\gamma)0s [a = 7.9602 (4), b = 5.6779 (3), c = 5.6967 (3) Å, \beta = 90.047 (5)°, \bf q_{\rm inc} = 0.9000 (9)\bf a^* + 0.1735 (6)\bf c^*]. The use of powder diffraction techniques to investigate ferroelastic modulated phases is discussed and compared with a previous polydomain single-crystal structural analysis. The modulated displacements of light atoms have been determined, allowing an accurate description of the modulation of both the cations and the O-atom framework. The refinement suggests a displacive model for the phase transition, involving significant atomic shifts for Pb atoms and a quite complex mixing of tilt and deformation of the oxygen octahedra. The average character of this modulated structure is antiferroelectric.
A test of crystal structure prediction of small organic molecules
Authors: Lommerse Jos P. M. , Motherwell W. D. Sam , Ammon Herman L. , Dunitz Jack D. , Gavezzotti Angelo , Hofmann Detlef W. M. , Leusen Frank J. J. , Mooij Wijnand T. M. , Price Sarah L. , Schweizer Bernd , Schmidt Martin U. , van Eijck Bouke P. , Verwer Paul , Williams Donald E.
Published: July 2002

A collaborative workshop was held in May 1999 at the Cambridge Crystallographic Data Centre to test how well currently available methods of crystal structure prediction perform when given only the atomic connectivity for an organic compound. A blind test was conducted on a selection of four compounds and a wide range of methodologies representing the principal computer programs currently available were used. There were 11 participants who were allowed to propose at most three structures for each compound. No program gave consistently reliable results. However, seven proposed structures were close to an experimental one and were classified as `correct'. One compound occurred in two polymorphs, but only one form was predicted correctly among the calculated structures. The basic problem with lattice energy based methods of crystal structure prediction is that many structures are found within a few kJ mol−1 of the global minimum. The fine detail of the force-field methodology and parametrization influences the energy ranking within each method. Nevertheless, present methods may be useful in providing a set of structures as possible polymorphs for a given molecular structure.
Structure and complex twinning of dysprosium disilicate (Dy2Si2O7), Type B
Authors: Fleet Michael E. , Liu Xiaoyang
Published: July 2002

Dysprosium disilicate (Dy2Si2O7) is triclinic with a = 6.6158 (2), b = 6.6604 (2), c = 12.0551 (4) Å, α = 94.373 (2), β = 90.836 (2), γ = 91.512 (2)°, V = 529.4 (1) Å3, space group P\overline 1, Z = 4 and D

x
 = 6.156 g cm−3. The structure (single-crystal X-ray, R = 0.033, wR = 0.041) is built from a linear triple tetrahedral group [Si3O10] and isolated [SiO4] tetrahedron cross-linked by Dy3+ in one sixfold and three eightfold coordinated positions, and corresponds to the presently revised type B structure of Ho2Si2O7. The formation of the unusual linear triple tetrahedral group in the type B structure allows for a more continuous transition in the mean size of REE3+O
n
(REE = rare earth element) polyhedra in REE disilicates through the 4f transition metal series. The crystal of Dy2Si2O7 investigated was complexly twinned such that the diffraction pattern was also consistent with a larger dimensionally monoclinic unit cell (a = 22.5354, b = 14.2102, c = 6.6158 Å, β = 91.788°), which resulted in an apparent superstructure of the type B structure in space group C1¯. Lattice coincidence with the type B unit cell appears to have been maintained during crystal synthesis and quenching by the complex sector-zoned growth twin.
Topology of molecular packings in organic crystals
Authors: Peresypkina E. V. , Blatov V. A.
Published: July 2002

Using the methods of coordination sequences and of molecular Voronoi–Dirichlet polyhedra, the topological properties of molecular packings and molecular coordination numbers (MCNs) were determined in the crystal structures of 33 575 monosystem organic compounds within the first three coordination spheres. Numerous examples of disagreement between the topology of molecular packing and the system of intermolecular contacts in a crystal structure were found. It is concluded that within the first coordination sphere most of the molecules tend to arrange with MCN = 14, obeying the model of the thinnest covering of space, but molecular packings as a whole tend to be constructed according to one of the close packings.
Automatic detection of molecular symmetry in the Cambridge Structural Database
Authors: Cole Jason C. , Yao Jing Wen , Shields Gregory P. , Motherwell W. D. S. , Allen Frank H. , Howard Judith A. K.
Published: July 2002

A method for the detection of approximate molecular symmetry in crystal structures has been developed. The point-group symmetry is assigned to each molecule and the relevant symmetry elements can be visualized, superimposed on the molecule. The method has been validated against reference structures with exact symmetry subjected to small random perturbation.
Structural studies on C-amidated amino acids and peptides: structures of hydrochloride salts of C-amidated Ile, Val, Thr, Ser, Met, Trp, Gln and Arg, and comparison with their C-unamidated counterparts
Authors: In Yasuko , Fujii Mayumi , Sasada Yasuhiro , Ishida Toshimasa
Published: July 2002

To elucidate the structural features of amino acids caused by the C-terminal α-amidation, the crystal structures of HCl salts of C-terminal amidated Ile, Val, Thr, Ser, Met, Trp, Gln and Arg were analysed and compared with those of their C-terminal free acids. The bonding parameter of the amide group was little affected by the different chemical properties of the side chains. As for the molecular packing patterns, some structural differences were observed by the C-amidation. The Cα—H...O hydrogen bonds and carbonyl–carbonyl interactions were more strengthened by the salt formation with HCl in C-amides than C-acids. Furthermore, there is a clear difference between the interaction patterns with Cl ions. In most C-amide crystals, Cl ions are bifurcately hydrogen-bonded to two neighbouring amide NH2 groups and the parallel layers of the C-amides and Cl ions are alternatively formed. In the case of the carboxyl OH in C-acid crystals, however, the direct hydrogen bond with the Cl ion is not always observed and is largely dependent on the crystal packing environment. This suggests the superior hydrogen-bonding ability of NH...Cl− compared with OH...Cl−. The difference in hydrogen-bonding ability between the amide and carboxyl groups is considered, based on the spatial dispositions of the hydrogen-bonding polar atoms/groups.
Chiral versus racemic building blocks in supramolecular chemistry: tartrate salts of organic diamines
Authors: Farrell Dorcas M. M. , Ferguson George , Lough Alan J. , Glidewell Christopher
Published: July 2002

In the 1:1 adducts C12H10N2·C4H6O6 formed between 1,2-bis(4′-pyridyl)ethene and racemic tartaric acid [(I), triclinic P\bar 1, Z′ = 1] and (2R,3R)-tartaric acid [(II), triclinic P1, Z′ = 2], the ionic components are linked by hard hydrogen bonds into single sheets, which are further linked by C—H...O hydrogen bonds. In the analogous adducts C10H18N2·C4H6O6 formed by 4,4′-bipyridyl with racemic tartaric acid [(III), triclinic P\bar 1, Z′ = 1] and the chiral acid [(IV), monoclinic P21, Z′ = 1], the hard hydrogen bonds generate bilayers which are again linked by C—H...O hydrogen bonds. Piperazine forms a 1:1 salt [{(C4H10N2)H2}2+]·[(C4H4O6)2−] with (2R,3R)-tartaric acid [(V), monoclinic P21] sheets, which are linked by the cations to form a pillared-layer framework. In each of the 1:2 salts formed by racemic tartaric acid with piperazine [(VI), monoclinic P21/n, Z′ = 0.5] and 1,4-diazabicyclo[2.2.2]octane (DABCO) [(VII), monoclinic P21/n, Z′ = 0.5], the cation lies across a centre of inversion, with the [{HN(CH2CH2)3NH}2+] cation disordered over two sets of sites: in both (VI) and (VII) the anions form a three-dimensional framework encapsulating large voids which accommodate the cations. The salt formed between DABCO and (2R,3R)-tartaric acid [(VIII), orthorhombic P212121, Z′ = 1] has 3:4 stoichiometry and contains four different types of ion, [{HN(CH2CH2)3NH}2+]2·[N(CH2CH2)3NH]+·3(C4H5O6)−·C4H4O6
2−: the hard hydrogen bonds generate a three-dimensional framework.
Polymorphs and pseudopolymorphs of N,N′-dithiobisphthalimide
Authors: Farrell Dorcas M. M. , Glidewell Christopher , Low John N. , Skakle Janet M. S. , Zakaria Choudhury M.
Published: July 2002

N,N′-Dithiobisphthalimide, C16H8N2O4S2 (I), forms a wide range of polymorphs and solvates (pseudopolymorphs). When (I) is crystallized from methanol it yields a solvent-free polymorph (4), in Pna21 with Z′ = 1, in which the molecules are linked into chains by a single C—H...O hydrogen bond: crystallization from either acetonitrile or dimethylformamide produces a monoclinic polymorph (5), in P21/c with Z′ = 2, also solvent-free, in which the molecules are linked into molecular ladders. Nitromethane forms a monosolvate, C16H8N2O4S2·CH3NO2 (6), in P21/c with Z′ = 1, in which the solvent molecules are linked to the molecules of (I) not only via a conventional C—H...O hydrogen bond but also via a polarized multicentre interaction involving all three C—H bonds of the solvent molecule. Chlorobenzene forms a precise hemisolvate, C16H8N2O4S2·0.5C6H5Cl (7), in P{\bar 1 } with Z′ = 1, while ethylbenzene forms an approximate hemisolvate 2C16H8N2O4S2·0.913C6H5C2H5·0.087H2O (8), in P21/c with eight molecules of (I) per unit cell. In both solvates the molecules of (I) are linked, in (7) by π...π stacking interactions augmented by weak C—H...O hydrogen bonds and in (8) by stronger C—H...O hydrogen bonds: the solvent molecules lie in isolated cavities, disordered across inversion centres in (7) and fully ordered in general positions in (8). Crystallization of (I) either from tetrahydrofuran or from wet tert-butanol yields isomorphous solvates (9) and (10), respectively, in C2/c with Z′ = 0.5, in which molecules of (I) lie across twofold rotation axes and are linked by π...π stacking interactions and very weak C—H...O hydrogen bonds, forming a framework enclosing continuous channels: highly disordered solvent molecules lie within these channels. p-Xylene and toluene form isomorphous hemisolvates (11) and (12) with unit cells metrically very similar to those of (9) and (10), but in P21/n with Z′ = 1: in these two solvates the molecules of (I) are linked into a framework by very short C—H...O hydrogen bonds; the solvent molecules lie within continuous channels, but they are localized across inversion centres so that the toluene is disordered across an inversion centre.
Modulated structure of Bi2Sr2CaCu2O8+δ, a high-T
c superconductor with monoclinic symmetry
Authors: Gladyshevskii R. E. , Flükiger R.
Published: July 2002

The structure of the bismuth strontium calcium cuprate Bi2.09Sr1.90Ca1.00Cu2O8.22 (Tc
= 90 K) was studied by single-crystal X-ray diffraction. A two-component modulation vector was found with respect to the C-centred orthorhombic cell of the average structure [a = 5.4112 (7), b = 30.873 (7), c = 5.4161 (20) Å, q = 0.21
c

* + 0.14
b

*]. Refinement was performed in a ninefold supercell [Mr
= 902, monoclinic, mS548-0.4, Cc (No. 9), a = 37.754 (7), b = 5.4109 (8), c = 41.070 (9) Å, β = 103.58 (2)°, V = 8155 (2) Å3, Z = 36, Dx
= 6.61 Mg m−3] to wR = 0.057. As in orthorhombic Bi2Sr2CaCu2O8+δ, the structure of this new monoclinic variant contains perovskite-derived slabs delimited by a BiO layer on each side. All metal atoms show a transverse displacement wave which has the largest amplitude for the Cu and Ca atoms. In addition, the Bi and Sr atoms are displaced longitudinally. Bi-rich regions with a distorted rocksalt-type atom arrangement and Bi-poor regions with chains of corner-linked BiO3
ψ-tetrahedra are formed. One additional oxygen site per translation unit of the modulation wave was detected in the Bi-poor regions of the chains, giving the composition Bi9O10. Approximately 10% of Bi substitution was found on the Ca sites, but no clear indication of preferential sites was observed. A partial substitution by Ca was refined at Sr sites situated near the regions where the additional O atoms are located. The monoclinic symmetry corresponds to a systematic shift of the modulation waves of consecutive slabs by π/9 with respect to orthorhombic Bi2Sr2CaCu2O8+δ.
Occurrence of a monoclinic distortion in β-Fe2PO5
Authors: Elkaïm E. , Berar J. F. , Gleitzer C. , Malaman B. , Ijjaali M. , Lecomte C.
Published: July 2002

The structure of the mixed valence iron oxyphosphate β-Fe2PO5 has been reinvestigated using powder synchrotron X-ray data. A previous study based on the similarity with the powder diagrams of NiCrPO5 concluded that the structure existed in a tetragonal cell (I41/amd). In this space group both Fe atoms, Fe2+ and Fe3+, are sharing the same crystallographic site and, therefore, cannot be distinguished. Parts of the pattern have been measured at high resolution on the LURE D23 powder station. The instrumental resolution there clearly shows the splitting of some Bragg peaks. Among the subgroups of I4/amd, the monoclinic group I2/a permits a description of the spectra if the twofold b' of the new unit cell is chosen as the diagonal of the square base of the tetragonal cell. A Rietveld refinement was then performed against the full powder diffraction diagram. This reveals two iron sites with slightly different Fe—O distances. Finally, study as a function of temperature shows a monoclinic to tetragonal transition above 415 K, which may be related to a magnetocrystalline effect at TN
= 408 K.
Conformational analysis from crystallographic data using conceptual clustering
Authors: Conklin D. , Fortier S. , Glasgow J. I. , Allen F. H.
Published: July 2002

The rapid growth of crystallographic databases has created a demand for novel and efficient techniques for the analysis of molecular conformations, in order to derive new concepts and rules and to generate useful classifications of the available data. This paper presents a conceptual clustering approach, termed IMEM (image memory), which discovers the conformational diversity present in a dataset of crystal structures. In contrast to numerical clustering methods, IMEM views a molecular structure as comprising qualitative relationships among its parts, i.e. the structure is viewed as a molecular scene. In addition, IMEM does not require the user to have any a priori knowledge of an expected number of conformational classes within a given dataset. The IMEM approach is applied to several datasets derived from the Cambridge Structural Database and, in all cases, chemically correct and sensible conformational classifications were discovered. This is confirmed by a rigorous comparison of IMEM results with published conformational data obtained by energy-minimization and numerical clustering methods. Conformational analysis tools have an important part to play in the conversion of raw molecular databases to knowledge bases.
Structural, kinetic and thermodynamic aspects of the conformational dimorphism of diethyl 3,6-dibromo-2,5-dihydroxyterephthalate
Authors: Näther C. , Nagel N. , Bock H. , Seitz W. , Havlas Z.
Published: July 2002

The crystal structures of the title compound and of its ethanol solvate have been determined by X-ray diffraction. The compound crystallizes in two different crystal forms (I: P21/n, high-temperature stable modification, II: P21/c, room-temperature stable modification), which differ in molecular conformation and hydrogen bonding. Ab initio calculations show that both observed conformations correspond to local energy minima and predict the conformation in polymorph (II) to be energetically more favourable. Packing energy calculations confirm that form (II) should be more stable than form (I). Crystals of both modifications and a solvate, containing 2 equiv, of ethanol, can be obtained by slow crystallization from ethanolic solution. Modification (II) transforms to (I) at 391 K, as shown by optical microscopy, powder diffraction and differential thermal analysis (DTA), whereas (I) melts at 429 K without previous transformation.
Simple averaging procedure for aromatic ring system: mean molecular fragment of tryptophan metabolites
Authors: Wakahara A. , Ishida T.
Published: July 2002

A simple averaging procedure of bond lengths and angles applicable to an aromatic ring system was applied to the crystal structures of the tryptophan metabolites analyzed so far, as an example. Statistical analysis showed no marked effect of the 5-substituent on the molecular dimensions of the 3-substituted indole ring in tryptophan metabolites. The standard bond lengths and angles of the 3-substituted indole ring were calculated according to the statistical averaging procedure outlined here and using data from 35 tryptophan metabolites; the interatomic correlations of the indole ring have been discussed based on these values. The result suggests the validity of the D
2 statistic for averaging the planar ring structure in aromatic systems.
Electron crystallography
Authors: Dorset D. L.
Published: July 2002

The idea of solving unknown crystal structures from experimental electron-diffraction intensities and high-resolution electron micrographs has remained a controversial topic in the 60 year history of electron crystallography. In this review it will be shown that the application of modern direct phasing techniques, familiar to X-ray crystallographers, has decisively proven that such ab initio determinations are, in fact, possible. This statement does not, by any means, refute the existence of the several significant scattering perturbations identified by diffraction physicists. Rather, it does affirm that experimental parameters can be controlled to ensure that a `quasi-kinematical' data set can be collected from many types of specimens. Numerous applications have been made to various types of specimens, ranging from small organics to proteins, and also some inorganic materials. While electron crystallography may not be the optimal means for determining accurate bonding parameters, it is often the method of choice when only microcrystalline specimens are available.
4f-Electron Density Distribution in Crystals of CeB6 at 165 K and its Analysis Based on the Crystal Field Theory
Authors: Tanaka K. , Kato Y. , Onuki Y.
Published: July 2002

4f-Electron density in single crystals of CeB6, cerium hexaboride, was measured at 165 K by X-ray diffractometry. Significant peaks 2.0 e Å−3 high were found along the \langle100\rangle directions at 0.41 Å from Ce on the deformation density map. Analysis based on the crystal field theory removed the peaks, confirming that they were due to the Ce 4f-electrons on the t
1u
-orbital. The deformation density in the B6 octahedron was also markedly improved by the analysis and coincides qualitatively with a theoretical molecular orbital (MO) calculation. It also coincides with the model deformation density map of CaB6 composed of only light atoms. These facts guarantee the accuracy of the intensity measurement for the present study. Since Ce3+ has only one 4f-electron, a highly accurate intensity measurement is necessary to detect its 4f-electron density distribution. A ψ-scan technique was therefore employed to avoid multiple diffraction (MD) and to measure the intensities at ω and χ angles with minimum fluctuation of temperature at the sample position. Relativistic radial functions for orbitals of Ce3+ and the corresponding scattering factors, which take the aspherical electron density distribution of 4f-electrons into account, were used for the analysis. CeB6 is a typical dense Kondo material. The Kondo effect occurs in CeB6 from low temperature to above room temperature. X-ray analysis of the f-electron density based on atomic orbitals (AO) revealed that 1.5 (2) electrons are donated from B6 to Ce and a total of 2.5 (2) electrons localize on the 4f-orbital. κ values are consistent with the 4f-orbitals being highly contracted and thus stabilized. These may be related to the Kondo effect.
Octahedral Tilting in Perovskites. II. Structure Stabilizing Forces
Authors: Woodward P. M.
Published: July 2002

The 23 Glazer tilt systems describing octahedral tilting in perovskites have been investigated. The various tilt systems have been compared in terms of their A-cation coordination and it is shown that those tilt systems in which all the A-cation sites remain crystallographically equivalent are strongly favored, when all the A sites are occupied by the same ion. Calculations based on both ionic and covalent models have been performed to compare the seven equivalent A-site tilt systems. Both methods predict that when the tilt angles become large, the orthorhombic a
+
b
−
b
− tilt system will result in the lowest energy structure. This tilt system gives the lowest energy structure because it maximizes the number of short A—O interactions. The rhombohedral a
−
a
−
a
− tilt system gives a structure with a slightly lower Madelung energy, but increased ion–ion repulsions destabilize this structure as the tilt angles increase. Consequently, it is stabilized by highly charged A cations and small to moderate tilt angles. The ideal cubic a
0
a
0
a
0 tilt system is only observed when stabilized by oversized A cations and/or M—O π-bonding. Tilt systems with nonequivalent A-site environments are observed when at least two A cations with different sizes and/or bonding preferences are present. In these compounds the ratio of large-to-small cations dictates the most stable tilt system.
Zeolite Rho Loaded with Methylamines. III. Trimethylamine Loadings
Authors: Weidenthaler C. , Fischer R. X. , Abrams L. , Hewat A.
Published: July 2002

Samples of two differently prepared zeolites rho loaded with different amounts of trimethylamine (TMA) were studied in their hydrated and dehydrated forms by X-ray and neutron diffraction experiments. Both zeolites are partially dealuminated, as indicated by nonframework Al, which is assumed to be Al2O3 or AlOOH. Series I was prepared from dry-calcined NHn-rho at 873 K, series II from steam-calcined NHn-rho at 773 K. The samples were loaded with different amounts of deuterated TMA. Rietveld refinements yielded the following results for series I: (1) H3.8(H-TMA)5Cs0.2Al9 Si39O96.Al2O3.22H2O, X-ray data collection at room temperature, Im\overline 3m, a = 15.0542 (2) Å, R

wp
= 0.094; (2) composition as in (1) (anhydrous), neutron data collection at 5 K of dehydrated and deuterated sample, Im3m, a = 15.0467 (4) Å, R

wp
= 0.034. Series II: (3) H0.3(H-TMA)5Cs0.7Al6Si42O96.2.5Al2O3.22H2O, X-ray data collection at room temperature, Im\overline 3m, a = 15.0574 (2) Å, R

wp
= 0.118; (4) composition as in (3) (anhydrous), neutron data collection at 5 K of dehydrated and deuterated sample, lm\overline 3m, a = 15.0761 (5) Å, R

wp
= 0.037. In all determinations the TMA molecules reside with the N and H (D in neutron diffraction analysis) atoms on the central axes (x, 0, 0) pointing towards the center of the single eight-ring, whilst the three methyl groups point away to the center of the \alpha-cage.
Prediction of ferroelectricity in recent inorganic crystal structure database entries under space group pba2. Erratum
Authors: Abrahams S. C. , Mirsty K. , Nielson R. M.
Published: July 2002

The x coordinate for N8 in Table 1 on p. 807 [Abrahams, Mirsky & Nielson, Acta Cryst. (1996), B52, 806–809] was incorrectly quoted as 0.567 (9). The correct value is 0.0567 (9). The correct value is 0.0567 (9).
Structural Studies of Rutile-Type Metal Dioxides
Authors: Bolzan A. A. , Fong C. , Kennedy B. J. , Howard C. J.
Published: July 2002

The structures of the four metal dioxides GeO2, SnO2, RuO2 and IrO2 (germanium, tin, ruthenium and iridium dioxides, respectively) have been redetermined by Rietveld refinement from neutron diffraction powder data. The four dioxides all adopt the rutile-type structure, space group P42/mnm (no. 136), with a = 4.4066 (1), 4.7374 (1), 4.4968 (2) and 4.5051 (3), c = 2.8619 (1), 3.1864 (1), 3.1049 (1) and 3.1586 (2) Å, and x = 0.3060 (1), 0.3056 (1), 0.3053 (1) and 0.3077 (3), respectively. These results are compared with those for other metal dioxides that adopt the rutile structure and trends in structural and thermal vibrations for a series of 11 metal dioxides which adopt the rutile-type structure are described.
Cleavage and Twinning in CuInSe2 Crystals
Authors: Shukri Z. A. , Champness C. H.
Published: July 2002

A study was made of the cleavage and twinning character in single crystals of the chalcopyrite CuInSe2, copper indium diselenide, grown in the laboratory by a vertical Bridgman method. In this material, with a c/a ratio of 2.006, the two main cleavage orientations were found to be {101} and {112} (corresponding to the descriptions {201} and {111}, respectively, in a cubic lattice). The plane identifications were made by measuring angles between adjacent cleaved surfaces and verifying the orientations by X-ray Laue and diffractometry. Cleavage was also found less frequently in a {110} plane, but here microscopic examination of the surface revealed it to consist of ridges in a <110> direction, where the microplanes on either side of the ridge edge were {112} planes. Twinning in the grown crystals was found, by angle measurements and X-ray diffraction, to occur along {112} planes, which is similar to the result in face-centred cubic, diamond and zinc blende lattices, where they are the corresponding {111} planes.
Mixed Alkali Systems: Structure and 29Si MASNMR of Li2Si2O5 and K2Si2O5
Authors: de Jong B. H. W. S. , Supèr H. T. J. , Spek A. L. , Veldman N. , Nachtegaal G. , Fischer J. C.
Published: July 2002

Li2Si2O5,
M

r
= 150.05, λ(Mo Kα) = 0.71073 Å, 293 K, orthorhombic, Ccc2, a = 5.807 (2), b = 14.582 (7), c = 4.773 (3) Å, V = 404.2 (3) Å3, μ(Mo Kα) = 0.78 mm−1, Z = 4, D

x
= 2.466 Mg m−3, R
1 = 0.045 for 249 reflections with I > 2.0σ(I), wR
2 = 0.1138 for all 261 reflections. K2Si2O5, M

r
= 214.36, λ(Mo Kα) = 0.71073 Å, 150 K, monoclinic, Cc, a = 16.322 (2), b = 11.243 (2), c = 9.919 (1) Å, β = 115.97 (1)°, V = 1636.4 (4) Å3, μ(Mo Kα) = 2.11 mm−1, Z = 12, D

x
= 2.610 Mg m−3, R
1 = 0.0385 for 2370 reflections with I > 2.0σ(I), wR
2 = 0.0785 for all 2711 reflections. The crystal structures of Li2Si2O5 and K2Si2O5 have been determined and refined. The lithium phyllosilicate sheet topology and structure based on three-dimensional data and refined in the orthorhombic space group Ccc2 confirm the previous determination by Liebau [Acta Cryst. (1961), 14, 389–395] from two-dimensional data, but described in the lower monoclinic Cc space-group symmetry. Potassium disilicate, on the other hand, is not a phyllosilicate but forms a three-dimensional defect cristobalite structure built from Q
3 silica units. In this structure, one Si—O—Si bridge, common to four six-membered T-rings, i.e. rings consisting of six silica tetrahedra in the chair conformation, is missing, resulting in 14-membered T-rings. Continuous sheets built up from these rings form a layer connected to layers above and below by six-membered T-rings in the chair conformation. 29Si MASNMR shows a good correlation between Si—O—Si angle and 29Si chemical shift for K2Si2O5, Li2Si2O5 and KLiSi2O5. The 29Si spin lattice relaxation times, T
1, are 335, 689 and 〈1256〉 s for Li2Si2O5, KLiSi2O5 and K2Si2O5, respectively. The pronounced non-linearity in hygroscopicity in glassy potassium–lithium disilicates is not observed in the osmolality of 0.8M potassium–lithium chloride aqueous solutions.
A New Global Parameterization of Perovskite Structures
Authors: Thomas N. W.
Published: July 2002

A new method is proposed for the quantitative description of all perovskite structures, ABX
3. It extends earlier work on the relationship between polyhedral volume ratio VA/VB
and octahedral tilt angles [Thomas (1996a). Acta Cryst. B52, 16–31] by defining additional parameters, \Gamma_i, which describe the planes formed by chains of octahedral stalks. The number of parameters required to define a particular perovskite structure, i.e. the number of structural degrees of freedom (d.o.f.), may be deduced from its atomic coordinates and cell parameters. Relationships between space group and these d.o.f. are examined for all known space groups, with particular attention paid to the group Pnma (No. 62), which is commonly found. Here the variation of the eight anion-related d.o.f. with VA/VB
is investigated for: (i) different compositions; (ii) NaMgF3 at different temperatures; (iii) MgSiO3 at different pressures. Furthermore, the ability of the volume ratio VA/VB
to act as a structural determinant permits a prediction of the structures (i.e. cell parameters and atomic coordinates) of novel chemical compositions. It is anticipated that this method of predicting structures will find application in deriving good starting models for Rietveld refinements.
Phase Transitions and Structures at 10, 92 and 293 K in Eight Trigonal Double Salts M
I[M
III(NH3)6](ClO4)2
X
2
Authors: Figgis B. N. , Sobolev A. N. , Reynolds P. A.
Published: July 2002

The crystal structures of eight hexaammine-M
III double salts have been determined by X-ray diffraction at 293 K, where M
I, M
III and X are NH4, Co, Cl (I), ammonium hexaamminecobalt(III) diperchlorate dichloride; Cs, Co, Cl (II), caesium hexaamminecobalt(III) diperchlorate dichloride; NH4, Ru, Cl (III), ammonium hexaammineruthenium(III) diperchlorate dichloride; K, Ru, Cl (IV), potassium hexaammineruthenium(III) diperchlorate dichloride; Rb, Ru, Cl (V), rubidium hexaammineruthenium(III) diperchlorate dichloride; Cs, Ru, Cl (VI), caesium hexaammineruthenium(III) diperchlorate dichloride; Cs, Cr, Cl (VII) caesium hexaamminechromium(III) diperchlorate dichloride; Cs, Cr, Br (VIII), caesium hexaamminechromium(III) diperchlorate dibromide. The structures of (I), (IV), (V) and (VI) have also been determined at 92 K and those of (V), (VII) and (VIII) at 10 K. At room temperature all are isomorphous, trigonal R3¯m, Z = 3, with only seven positional parameters for the seven unique non-H atoms. The ammine H atoms are disordered only in the Cs+ and NH_4^+ salts, and the NH_4^+ H atoms in the ammonium salts. At lower temperatures a reversible phase change is observed in all except the K+ member, by the lowering of symmetry to a twinned, but atomically ordered, R3¯ phase. In this R3¯ phase the hexaammine-M
III and perchlorate ions have rotated about the threefold axis removing the former mirror plane in the crystal. Displacement parameters, unit cells and observed structures all change smoothly with changes in M
I ionic radii and M
III—N distances.
Investigation of the Cs
x
Rb1−x
TiOAsO4 Series. I. Crystal Structure Analysis and Pseudo-symmetry
Authors: Womersley M. N. , Thomas P. A. , Corker D. L.
Published: July 2002

Refinements of five crystals in the Cs2x
Rb2−2x
Ti2O2As2O8 series, caesium rubidium titanyl arsenate, with x = 0.07, 0.31, 0.58, 0.71 and 0.86, which are compositional analogues of KTiOPO4 (KTP), have been completed at 293 K and two (x = 0.71, 0.86) at low temperature. All the structures are found to be orthorhombic (space group Pna21) and are isostructural with KTP, although there is evidence of some Cs disorder over additional sites in the framework, particularly at the Cs-rich end of the series, as discussed in Part II [Thomas & Womersley (1998). Acta Cryst. B54, 645–651]. Unusually large U
33 parameters for shared Cs/Rb sites are observed and are shown to be the result of the existence of separate sites for Cs and Rb within the structural framework, although the coordinates of these sites cannot be resolved convincingly. The structural changes in the TiO6/AsO4 framework required to accommodate an increasing fraction of the larger Cs cation across the series and under cooling to 120 K are elucidated. Finally, the deviations of the room-temperature and low-temperature structures from the high-temperature prototypic structure (space group Pnan) are examined and suggest that the phase-transition temperature should increase linearly from CsTiOAsO4 to RbTiOAsO4.
Isotope Effect in TlH2PO4 and TlD2PO4
Authors: Ríos S. , Paulus W. , Cousson A. , Quilichini M. , Heger G.
Published: July 2002

The crystal structure of the antiferroelectric phase of TlD2PO4, deuterated thallium dihydrogenphosphate, has been determined from single-crystal neutron diffraction data collected at room temperature. The para-antiferroelectric transition (T_c^d = 353 K) of TlD2PO4 is analysed from a structural point of view and compared with the phase transition of TlH2PO4 at TI
= 357 K, already characterized. The distinct phase sequences observed in the two compounds when decreasing temperature from that of the high-temperature prototype phase (prototype phase/room-temperature phase/low-temperature phase) are discussed and associated with the different ordering of the two crystallographically inequivalent H (D) atoms existing in the prototype phase.
Charge distribution as a tool to investigate structural details: meaning and application to pyroxenes
Authors: Nespolo Massimo , Ferraris Giovanni , Ohashi Haruo
Published: July 2002

The charge distribution (CD) method, previously introduced as a development of the bond-valence (BV) approach, is applied for the first time to mineral structures, and specifically to pyroxenes. CD essentially involves the distribution of the Effective Coordination Number (ECoN) of a cation among all the neighboring anions. This distribution is then interpreted in terms of distribution of `charges', where `charge' represents the formal oxidation state. Differently from BV, the CD description depends upon the geometry of each coordination polyhedron, which is characterized through ECoN (a non-integer number). The contribution of each cation–oxygen bond to ECoN, labelled `bond weight', corresponds to the bond strength in the BV method, but it is defined in terms of bond-length ratio in each polyhedron and not as a function of the cation–oxygen pair. The ratio q/Q of the formal oxidation number to the computed charge can be interpreted as a measure of the correctness of the structure (cation ratio) and of the degree of over- or under-bonding (anion ratio). A similar interpretation is not possible for the analogous quantities obtained through the BV approach. The analysis in terms of CD of the pyroxene chains (from 101 structures) shows different trends as a function of composition, temperature and pressure; in particular it shows a different behaviour of the two crystallographically independent chains of orthopyroxenes and of P21/c clinopyroxenes.
Incommensurately modulated phase of Rb2CoBr4 at 295 and 200 K
Authors: Friese K. , Madariaga G. , Breczewski T.
Published: July 2002

The crystal structure of Rb_2CoBr_4 at 295 and 200 K has been determined. At these temperatures Rb_2CoBr_4 exhibits an incommensurately modulated structure with wavevector {\bf q} = (1/3+\delta){\bf a}^*. At room temperature only the average structure was refined. Lattice parameters are a = 9.732 (3), b = 13.328 (4), c = 7.654 (3) Å, space group Pnam. The R(F) value was 0.0414 for 286 observed reflections (0.0778 for all 477 reflections). At 200 K the lattice parameters are a = 9.691 (4), b = 13.278 (5), c = 7.630 (6) Å, superspace group P:Pnam:\overline{1}ss. Main reflections and satellite reflections of first order were measured. The refinement converged at R(F) = 0.052 for 309 observed reflections (255 main reflections and 54 satellites) and 0.2971 for all reflections (1849; 695 main reflections and 1154 satellites). Amplitudes and phases of the modulation function as well as bond distances show close relationships to those observed in the incommensurately modulated phase of Rb_2ZnBr_4.
Deux nouvelles structures polytypiques dans le système titane–soufre: 16H et 18H
Authors: Legendre J. J. , Huber M.
Published: July 2002

The structures of two recently discovered polytypes of titanium sulphide have been determined. The structure of 18H, mainly built on a stacking sequence similar to that of 8H (3212), confirms the formation mechanism formerly suggested for TiS1.7 hexagonal polytypes. In the same way, the stacking sequence of 16H provides some evidence of a 4H → 12H transformation producing rhombohedral polytypes. The experimental data supporting these conclusions are presented in this paper.
Ab initio low-resolution phasing in crystallography of macromolecules by maximization of likelihood
Authors: Petrova T. E. , Lunin V. Y. , Podjarny A. D.
Published: July 2002

Statistical likelihood criteria were tested to select the true (or closest to true) structure-factor phases from an ensemble of phase sets. To define the criterion value for a given trial phase set, the trial `molecular region' is defined as a region consisting of the points with the highest values in the Fourier synthesis calculated with the observed magnitudes and the trial set of phases. The structure studied is considered as composed of atoms randomly placed inside the trial molecular region. The figure of merit is defined as the likelihood corresponding to this hypothesis, i.e. the probability that the structure-factor magnitudes calculated (from the positions of atoms randomly placed into the trial region) are equal to the observed magnitudes. The concept of generalized likelihood is introduced to make the calculations more straightforward. The tests performed for known structures with the use of experimentally observed magnitudes show that in general it is impossible to unambiguously determine the best phases among a `population' of trial phase sets. Nevertheless, the random generation of a great number of phase sets and the selection of phase sets with high likelihood values give a collection of variants with a higher concentration of `good' phase sets than those found in the original population. Averaging the selected phase sets gives a starting solution of the low-resolution phase problem.
Fast Fourier feature recognition
Authors: Cowtan Kevin
Published: July 2002

Various approaches have been demonstrated for the automatic interpretation of crystallographic data in terms of atomic models. The use of a masked Fourier-based search function has some benefits for this task. The application and optimization of this procedure is discussed in detail. The search function also acquires a statistical significance when used with an appropriate electron-density target and weighting, giving rise to improved results at low resolutions. Methods are discussed for building a library of protein fragments suitable for use with this procedure. These methods are demonstrated with the construction of a statistical target for the identification of short helical fragments in the electron density.
The Globalization of Crystallographic Knowledge
Authors: Murray-Rust Peter
Published: July 2002

The rapid growth of the World Wide Web provides major new opportunities for distributed databases, especially in macromolecular science. A new generation of technology, based on structured documents (SD), is being developed which will integrate documents and data in a seamless manner. This offers experimentalists the chance to publish and archive high-quality data from any discipline. Data and documents from different disciplines can be combined and searched using technology such as eXtensible Markup Language (XML) and its associated support for hypermedia (XLL), metadata (RDF) and stylesheets (XSL). Opportunities in crystallography and related disciplines are described.
Crystallization and preliminary X-ray crystallographic analysis of a vancomycin–N-acetyl-D-Ala-D-Ala complex
Authors: McPhail Deborah , Cooper Alan , Freer Andy
Published: July 2002

A vancomycin–N-acetyl-D-Ala-D-Ala complex has been crystallized by the sitting-drop vapour-diffusion method using imidazole maleic buffer at pH 7.6. The novel crystals obtained belong to the space group P6322 with unit-cell parameters a = b = 73.43 (1), c = 277.17 (4) Å, γ = 120°. The crystal density was determined as 1.106 g cm−3 which gives a supercell of 24 molecules (12 dimers) per asymmetric unit for an acceptable Matthews number and an estimated solvent content of 42%. Data were collected at room temperature to 2.8 Å.
Discrimination of solvent from protein regions in native Fouriers as a means of evaluating heavy-atom solutions in the MIR and MAD methods
Authors: Terwilliger Thomas C. , Berendzen Joel
Published: July 2002

An automated examination of the native Fourier is tested as a means of evaluation of a heavy-atom solution in MAD and MIR methods for macromolecular crystallography. It is found that the presence of distinct regions of high and low density variation in electron-density maps is a good indicator of the correctness of a heavy-atom solution in the MIR and MAD methods. The method can be used to evaluate heavy-atom solutions during MAD and MIR structure solutions and to determine the handedness of the structure if anomalous data have been measured.
Automated production of small-molecule dictionaries for use in crystallographic refinements
Authors: Greaves Richard B. , Vagin Alexei A. , Dodson Eleanor J.
Published: July 2002

Many macromolecules are now being studied crystallographically in complexes with a range of ligands and other associated molecules. It is necessary to have templates describing the expected geometry of such molecules before refinement and model building can be carried out. This paper describes a method for generating templates beginning from the SMILES description of the molecule, the final format of the molecular template being based on the mmCIF definitions for chemical composition. Additionally, the program SMILE2DICT, which converts the SMILES string to a more extended format, is described. The description details the input required, the output produced and how the program relates to attempts to automate the procedure of model building for crystallographic refinement. Examples of input to and output from the program are given.
Crystallization and preliminary X-ray diffraction studies of monomeric isocitrate dehydrogenase from Corynebacterium glutamicum
Authors: Audette Gerald F. , Quail J. Wilson , Hayakawa Koto , Bai Cheng , Chen Ridong , Delbaere Louis T. J.
Published: July 2002

A monomeric isocitrate dehydrogenase has been crystallized for the first time. This enzyme catalyzes the conversion of isocitrate to oxalosuccinate and subsequently to α-ketoglutarate and CO2; the coenzyme NADP+ is reduced to NADPH during the reaction. Polyethylene glycol 2000 monomethyl ether was used to crystallize the enzyme in space group C2 with unit-cell parameters a = 137.1, b = 54.6, c = 126.4 Å, β = 108.2°. The very small crystal (0.05 × 0.20 × 0.05 mm) diffracted to 3.5 Å d spacing using synchrotron radiation.
The molecular-replacement solution of an intermediate-sized helical polypeptide, antiamoebin I
Authors: Snook C. F. , Wallace B. A.
Published: July 2002

The successful use of molecular-replacement methods for the solution of the intermediate-sized helical polypeptide antiamoebin I required the careful consideration of a number of parameters and exhibited some unusual characteristics when compared with molecular-replacement solutions of globular proteins. High-resolution data were required owing to several features, including the comma-like shape of the molecule (which results in a pseudo-symmetric structure at low resolution), the relative uniformity of the structure in the direction along the helix axis and the small differences between the two independent molecules in the P1 asymmetric unit. Other parameters which were important for the solution of this relatively low solvent content closely packed cell included the radius of integration, the use of normalized structure factors and especially the choice of starting model.