Acta Crystallographica Section B-structural Science最新文献

The structural relation between malachite and the brochantite MDO (maximum degree of order) polytypes is discussed. It is demonstrated that the same building blocks which form the basis of brochantite polytypism also occur in malachite. The different arrangements of these building blocks in the two mineral structures are rationalized as a result of the different coordination geometries required by the respective non-metal atoms acting as linkers. The compound stoichiometries are discussed in light of a common structured formula scheme, in which pairs of H atoms can play a similar role as single non-H atoms. An overview on the occurrence of malachite-like building blocks in several other crystal structures is given.

The crystal structures of four polymorphs of the pesticide dithianon (5,10-dihydro-5,10-dioxonaphtho[2,3-b]-1,4-dithiine-2,3-dicarbonitrile) have been solved from powder diffraction data and refined using the Rietveld method. Three polymorphs crystallize in non-centrosymmetric space groups. Two polymorphs have Z' > 1. The structures are assembled via interactions between carbonyl groups of quinoid fragments into layers which further interact only by weak interactions.

The electron-density distribution of a new crystal form of coumarin-102, a laser dye, has been investigated using the Hansen-Coppens multipolar atom model. The charge density was refined versus high-resolution X-ray diffraction data collected at 100 K and was also constructed by transferring the charge density from the Experimental Library of Multipolar Atom Model (ELMAM2). The topology of the refined charge density has been analysed within the Bader `Atoms In Molecules' theory framework. Deformation electron-density peak heights and topological features indicate that the chromen-2-one ring system has a delocalized π-electron cloud in resonance with the N (amino) atom. The molecular electrostatic potential was estimated from both experimental and transferred multipolar models; it reveals an asymmetric character of the charge distribution across the molecule. This polarization effect is due to a substantial charge delocalization within the molecule. The molecular dipole moments derived from the experimental and transferred multipolar models are also compared with the liquid and gas-phase dipole moments. The substantial molecular dipole moment enhancements observed in the crystal environment originate from the crystal field and from intermolecular charge transfer induced and controlled by C-H···O and C-H···N intermolecular hydrogen bonds. The atomic forces were integrated over the atomic basins and compared for the two electron-density models.

5,6-Dimethylbenzofurazan 1-oxide (Me2BF), C(8)H(8)N(2)O(2), occurs in four polymorphic forms that are polytypes of each other. Each polymorph of Me2BF contains molecules disordered about pseudo-twofold axes and arranged head-to-tail in ribbons, with the ribbons forming approximately planar layers held together by weak C-H···N and C-H···O interactions. Adjacent layers interact in different ways in the different polymorphs. In addition to twinning in the individual polymorphs, four examples of allotwining, that is, oriented overgrowths between different polymorphs, were found.

The crystal structures of two very close, but distinct complex minerals of the lead sulfosalt group have been solved: sterryite, Cu(Ag,Cu)(3)Pb(19)(Sb,As)(22)(As-As)S(56), and parasterryite, Ag(4)Pb(20)(Sb,As)(24)S(58). They are analyzed and compared according to modular analysis. The fundamental building block is a complex column centred on a Pb(6)S(12) triangular prismatic core, with two additional long and short arms. The main chemical and topological differences relate to the short arm, which induces a relative a/4 shift (~2 Å along the elongation parameter) of the constitutive rod layers, as illustrated by distinct cell settings within the same space group (P2(1)/n and P2(1)/c, respectively). Selection of the shortest (i.e. strongest) (Sb,As)-S bonds permitted to enhance the polymeric organization of (Sb,As) atoms with triangular pyramidal coordination. These two quasi-homeotypic structures are expanded derivatives of owyheeite, Ag(3)Pb(10)Sb(11)S(28). The hierarchy of organization levels from zero- to three-dimensional entities is subordinated to building operators, which appear as the driving force for the construction of such complex structures. Minor cations (Ag, Cu) or the As-As pair in sterryite secure the final locking, which favours the formation of one or the other compound.

A complex, disorder-free structure in the space group P1 has been established for L-tryptophan, for which no crystal structure has previously been available. The 16 molecules in the asymmetric unit can be divided into two groups of eight; one where the side chains have gauche orientations and one with trans orientations. Molecules within each group have almost identical molecular geometries. The unit-cell parameters mimic a hexagonal cell, but deviations from 90° for the cell angles α = 84.421 (4) and β = 87.694 (4)° give a small tilt that rules out hexagonal symmetry. The hydrogen-bonding pattern resembles that found in the crystal structure of the racemic structure of DL-tryptophan, but a lower density combined with longer hydrogen bonds and inter-aromatic interactions show that the enantiomeric structure is less efficiently packed.

The calculation of the hardness of Mo and W disulfides using a crystallo-chemical model provides a unique opportunity to obtain separate quantitative information on the maximum hardness H(max) governed by strong intra-layer covalent bonds acting within the (0001) plane versus the minimum hardness H(min) governed by weak inter-layer van der Waals bonds acting along the c-axis of the hexagonal lattice. The penetration hardness derived from fundamental crystallo-chemical data (confirmed by experimental determinations) proved to be far lower in MS(2) (M = Mo, W) than in graphite and hexagonal BN, both for H(max) (H(graph)/H(MoS2) = 3.85; H(graph)/H(WS2) = 3.60; H(hBN)/H(MoS2) = 2.54; H(hBN)/H(WS2) = 2.37) as well as for H(min) (H(graph)/H(MoS2) = 6.22; H(graph)/H(WS2) = 5.87; H(hBN)/H(MoS2) = 4.72; H(hBN)/H(WS2) = 4.46). However, the gap between H(max) and H(min) is considerably larger in MS(2) (M = Mo,W), as indicated by H(max)/H(min) being 279 in 2H-MoS(2), 282 in 2H-WS(2), 173 in graphite and 150 in hBN. The gap was found to be even larger in MS(2) (M = Mo, W) nanostructures. These findings help to explain the excellent properties of MS(2) (M = Mo, W) as solid lubricants in high tech fields, either as bulk 2H crystals (inter-layer shear and peeling off lubricating mechanisms), or especially as onion-like fullerene nanoparticles (rolling/sliding mechanisms).

BNC nanotubes and nanofibers have been synthesized in the high isostatic pressure apparatus in Ar at 1923 K and 1.5 MPa in the presence of yttrium aluminium garnet. Some of the nanotubes obtained were filled with Al(2)O(3). Transmission electron microscopy (TEM) studies have shown that the nanotubes and nanofibers have a polygonal cross-section (prismatic shape), and most often they are twisted, which is due to the transversal instability of the nanotubes originating under the growth conditions, including temperature treatment. Twisting also revealed itself in the appearance of the moiré fringes during the TEM observation of some of the nanotubes and nanofibers. Analysis of these fringes has shown that the facets of these nanotubes represent the slightly misoriented hexagonal BN and/or C plates. An Al(2)O(3) filling of the nanotube makes it harder to twist when subjected to torque, which conforms to the tube deformation theory.

The reaction of 5-nitroisophthalic acid (H(2)NIA) with Gd(NO(3))(3)·6H(2)O in DMF afforded three new metal-organic frameworks: [Gd(NIA)(1.5)(DMF)(2)]·DMF (I), [Gd(2)(NIA)(3)(DMF)(4)]·xH(2)O (II) and [Gd(4)(NIA)(6)(DMF)(5.5)(H(2)O)(3)]·4DMF·H(2)O (III). These compounds can be prepared through a variety of methods. Compounds (I) and (II) are more reproducibly formed than compound (III). Network analysis revealed (I) to have a (4(12).6(3))-pcu topology, while (II) displays a (4(2).8(4))(4(2).8(4))-pts topology. Compound (III) was found to present the uncommon 4,5,6T11 topological net, which combines aspects of both the pcu and pts topologies. The short symbol of this net is (4(4).6(2))(4(6).6(4))(2)(4(8).6(6).8).

One of the main phases formed at the beginning of the carbonation reaction of cementitious building materials is the calcium hemicarboaluminate (abbreviated as Hc). This AFm (shorthand for hydrated calcium aluminate phases structurally related to hydrocalumite) phase was synthesized, crystallized and then studied by synchrotron X-ray powder diffraction and micro-Raman spectroscopy. At room temperature and standard experimental conditions two major cementitious phases were detected, the Hc phase (as a major phase) and carbonated calcium hemicarboaluminate (abbreviated as cHc). By increasing the temperature the Hc form transforms into cHc. The crystal structures of these important AFm phases were successfully solved and refined in the R3c space group of the trigonal crystal system. Hc has the unit-cell parameters a = 5.7757 (1) and c = 48.812 (2) Å, and cHc the unit-cell parameters a = 5.7534 (1) and c = 46.389 (1) Å. The two crystal structures are composed of positively charged main layers, [Ca(4)Al(2)(OH)(12)](2+), and negatively charged interlayers, [OH(2n)(CO(3))(1 - n)·4H(2)O](2-). The structure of the main layers is typical of the AFm family. Conversely, the interlayer region has a characteristic structure built up from water molecules and statistically distributed anions. In the interlayer, the Hc carbonate and hydroxyl anions are distributed in a 0.25:0.5 ratio, whereas the ratio of the anions in the cHc interlayers is 0.4:0.2.