issue contents

ISSN: 2052-5206

June 2012 issue

Highlighted illustration

Cover illustration: A high-angle annular darkfield scanning transmission electron microscopy image of the decagonal quasicrystal Al72Co8Ni20 overlaid with a structure model. Clusters of type 1 (occupied center) highlight transition metal sites (white spheres) and clusters of type 2 (empty center) possess both transition metal and mixed aluminum/transition metal (red) sites. Decagonal-shaped clusters are outlined and exhibit two types of overlap as a unifying building feature of aperiodic quasicrystals of decagonal Al-Co-Ni, Al-Co-Cu and Al-Fe-Ni [Deloudi et al. (2011). Acta Cryst. B67, 1-17, based on Abe & Tsai (2004) J. Non-Cryst. Solids, 334, 198-201]. The 2011 Nobel prize in chemistry was awarded to D. Shechtman for his discovery of quasicrystals.

feature articles

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A specially designed algorithm to predict the structure of complex crystals consisting of well defined molecular units is presented.

research papers

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In three of the nine morphotropic A2MX4 series (nine different tetrahedral anions) the densest structure (spinel) is the stable structure for compounds with A cations of intermediate radius, in contrast to the classical homology rule. In some cases, denser phases have abnormally low overall binding energies.

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The modulated crystal structure and modulated magnetic ordering of the multiferroic CaCuxMn7−xO12 is studied by analysing neutron and synchrotron-radiation powder diffraction data with a model based on the magnetic superspace group R31′(00γ)ts.

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The structure of (Ga2O3)2(ZnO)13 has been determined and a unified description for structures in a homologous series (Ga2O3)2(ZnO)2n + 1 is presented using the (3 + 1)-dimensional superspace formalism.

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The study of C6-arom–chelate versus C6-arom–C6-arom stacking interactions in the crystal structures of square-planar transition metal complexes showed that the C6-arom ring prefers to form stacking contacts with the chelate rings.

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Malachite, Cu2CO3(OH)2, transforms directly into a rosasite structure at high pressure as a result of the pressure-induced reduction of the Jahn–Teller distortion of CuO6 coordination polyhedra.

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Information on the effect of pressure on hydrogen bonds, which could be derived from single-crystal X-ray diffraction at a laboratory source and polarized Raman spectroscopy, has been compared.

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A low-temperature reversible phase transition at 220–230 K in glycine–glutaric acid co-crystals changes the conformation of every second glutaric acid molecule, but preserves the topology of the hydrogen-bond network, despite changes to the hydrogen-bond lengths and the space-group symmetry.

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Salicylideneaniline crystals exhibit photochromism under the following conditions: the dihedral angle between the two terminal benzene rings is greater than 30° and there is no intra- or intermolecular steric hindrance to inhibit a pedal motion within the crystal.

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A relationship between photochromism and non-bonded interactions was investigated with the Voronoi–Dirichlet approach. Crystal structures of five sets of conformational polymorphs with the CwHxNyOz composition have been taken into account. It was demonstrated that each polymorph is characterized by a unique set of inter- and intramolecular contacts.

short communications

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The structure of ceftadizime pentahydrate was determined from a 200 s experiment with synchrotron radiation. Combined with invariom refinement for rapid property calculation, high-throughput electron-density studies of hundreds of compounds become feasible.

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The calculation scheme for determining the bond-valence parameters (r0 and b) resulting in the exact solution of the bond-valence sum rule for a given set of coordination shells is presented.

books received

Acta Cryst. (2012). B68, 321
doi: 10.1107/S0108768112018551
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