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Monoclinic C2/c (monoclinic + c)

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Chemistry100%

Terms modified by Monoclinic C2/c

  • monoclinic c space group
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    Selected Abstracts

    Structures of 6H perovskites Ba3CaSb2O9 and Ba3SrSb2O9 determined by synchrotron X-ray diffraction, neutron powder diffraction and ab initio calculations

    ACTA CRYSTALLOGRAPHICA SECTION B, Issue 2 2008
    Budwy Rowda
    The structures of the 6H perovskites Ba3B2+Sb5+2O9, B = Ca and Sr, have been solved and refined using synchrotron X-ray and neutron powder diffraction data. Ba3CaSb2O9 and Ba3SrSb2O9 have monoclinic C2/c and triclinic space-group symmetries, respectively, while Ba3MgSb2O9 has ideal hexagonal P63/mmc space-group symmetry. The symmetry-lowering distortions are a consequence of internal `chemical pressure' owing to the increasing effective ionic radius of the alkaline-earth cation in the perovskite B site from Mg2+ (0.72,Å) to Ca2+ (1.00,Å) to Sr2+ (1.18,Å). Increasing the effective ionic radius further to Ba2+ (1.35,Å) leads to decomposition at room temperature. The driving force behind the transition from P63/mmc to C2/c is the need to alleviate underbonding of Ba2+ cations in the perovskite A site via octahedral rotations, while the transition from C2/c to is driven by the need to regularize the shape of the Sb2O9 face-sharing octahedral dimers. Ab initio geometry-optimization calculations were used to find a triclinic starting model for Ba3SrSb2O9. [source]

    Polymorphs of DABCO monohydrate as structural analogues of NaCl

    ACTA CRYSTALLOGRAPHICA SECTION C, Issue 6 2010
    Barbara Wicher
    A new crystalline form of 1,4-diazabicyclo[2.2.2]octane (DABCO) monohydrate, C6H12N2·H2O, crystallizing in the space group P31, has been identified during screening for cocrystals. There are three DABCO and three water molecules in the asymmetric unit, with two DABCO molecules exhibiting disorder over two positions related by rotation around the N...N axis. As in the monoclinic C2/c (Z, = 2) polymorph, the molecular components are connected via O,H...N hydrogen bonds into a polymeric structure that consists of linear O,H...N(CH2CH2)3N...H,O segments, which are approximately mutually perpendicular. The two polymorphic forms of DABCO monohydrate can be considered as structural analogues of NaCl, with the nearly globular DABCO molecules showing distorted cubic closest packing and all octahedral interstices occupied by water molecules. [source]

    Single-crystal diffuse scattering studies on polymorphs of molecular crystals.

    ACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2009

    The drug benzocaine (ethyl 4-aminobenzoate), commonly used as a local anaesthetic, is a bimorphic solid at room temperature. Form (I) is monoclinic P21/c, while the metastable form (II) is orthorhombic P212121. Three-dimensional diffuse X-ray scattering data have been collected for the two forms on the 11-ID-B beamline at the Advanced Photon Source (APS). Both forms show strong and highly structured diffuse scattering. The data have been interpreted and analysed using Monte Carlo (MC) modelling on the basis that the scattering is purely thermal in origin and indicates the presence of highly correlated molecular motions. In both forms (I) and (II) broad diffuse streaks are observed in the 0kl section which indicate strong longitudinal displacement correlations between molecules in the ,031, directions, extending over distances of up to 50,Å. Streaks extending between Bragg peaks in the hk0 section normal to [100] correspond to correlated motions of chains of molecules extending along a that are linked by N,H...O=C hydrogen bonds and which occur together as coplanar ribbon pairs. The main difference between the two forms is in the dynamical behaviour of the ribbon pairs and in particular how they are able to slide relative to each other. While for form (I) a model involving harmonic springs is able to describe the motion satisfactorily, as simple excursions away from the average structure, there is evidence in form (II) of anharmonic effects that are precursors of a phase transition to a new low-temperature phase, form (III), that was subsequently found. [source]

    Polymorphism of 4-bromobenzophenone

    ACTA CRYSTALLOGRAPHICA SECTION B, Issue 2 2007
    Mikhail A. Strzhemechny
    A combination of single-crystal and powder X-ray diffractometry was used to study the structure of two polymorphs of 4-bromobenzophenone over the temperature range from 100 to 300,K. One of the polymorphs of the title compound was known previously and its structure has been determined at room temperature [Ebbinghaus et al. (1997). Z. Kristallogr.212, 339,340]. Two crystal growth methods were employed, one of which (a modification of the Bridgman,Stockbarger technique) resulted in single crystals of a previously unknown structure. The basic physical properties of the stable polymorph are: growth method, from 2-propanol solutions or gradient sublimation; space group, monoclinic P21/c; melting point, Tm = 355.2,K; X-ray density (at 100,K), Dx = 1.646,g,cm,3. The same properties of the metastable polymorph (triclinic ) are: growth method, modified Bridgman,Stockbarger method; X-ray density (at 100,K), Dx = 1.645,g,cm,3; Tm = 354,K. Thermograms suggest that the melting of the metastable form is accompanied by at least a partial crystallization presumably into the monoclinic form; the transformation is therefore monotropic. Analysis of short distances in both polymorphs shows that numerous weak hydrogen bonds of the C,H,, type ensure additional stabilization within the respective planes normal to the longest dimension of the molecules. The strong temperature dependence of the lattice constants and of the weak bond distances in the monoclinic form suggest that the weak bond interactions might be responsible for both the large thermal expansion within plane bc and the considerable thermal expansion anisotropy. [source]