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Journal logoSTRUCTURAL SCIENCE
CRYSTAL ENGINEERING
MATERIALS
ISSN: 2052-5206

August 2007 issue

Highlighted illustration

Cover illustration: The cover illustration represents the high- and incommensurately modulated room-temperature phases of Na2CO3 and its reconstructed reciprocal (h2lm) layer [[gamma]-Na2CO3; Dusek et al. (2003). Acta Cryst. B59, 337-352]. Main reflections (open circles) and satellites aligned along parallel lines are indicated. A comparison of the two structures represented by slices parallel to the vertical axis c illustrates their relationship [Arakcheeva & Chapuis (2005). Acta Cryst. B61, 601-607]. [beta]-Na2CO3 at 746 K (C2/m; pseudohexagonal) is shown on the left. All C and Na atoms closer than 3.12 Å are linked. The corresponding non-periodic slice of [gamma]-Na2CO3 indicates how the modulation creates additional close contacts (Ivan Orlov's design is kindly acknowledged).

research papers


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A simple formula is derived for the analysis of chirally sensitive convergent-beam electron diffraction (CBED) patterns based on second-order scattering between all visible beams. The results agree with experiment and the limits of the formula are explored. Comparisons are made between patterns taken near major zone axes and those exactly at minor zone axes. The experimental aspects of chiral CBED are discussed.

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The structure of β-NH4Fe2(PO4)2 exhibits complex disorder phenomena preventing an accurate structural description. An artificially ordered structure has been generated using a five-dimensional approach and the diffuse streaks observed on the diffraction patterns. The analysis of this artificial model allows a description of the actual compound and opens the route to the synthesis of polytype compounds.

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The atomic volume of non-metallic elements depends on the packing density of atoms as a feature of the crystal structure, and on a reduced atomic volume which is characteristic of each element and its valence. Different valences of one element are recognized by different reduced atomic volumes.

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The structural parameters of BiFeO3 have been determined by high-resolution neutron powder diffraction for temperatures between 293 and 923 K. It was found that there is a local minimum for the rhombohedral angle αrh, near the Néel temperature TN ≃ 640 K, a gradual rotation of the FeO6 octahedra and an increase of the Fe—O—Fe angle.

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The structure of NaTaOGeO4 at T = 98 K has been determined. It is of the distorted titanite type with space-group symmetry P21/c. A phase transition to the titanite aristotype structure with space-group symmetry C2/c occurs close to T = 116 K. Anisotropic diffuse scattering in layers of constant l = 2n is observed above Tc and up to ambient temperatures.

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The crystal structure and local ordering of Co6Al11−xSi6+x was determined by single-crystal X-ray diffraction and transmission electron microscopy. The crystal structure consists of planar layers with the composition [Co6Al10Si4], which are penetrated by perpendicular (Si—Si—Al) chains.

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The structures of two phases of unsolvated Mg(BH4)2, a material of interest for hydrogen storage, were solved by synchrotron powder diffraction. Both structures are more complicated than other similar metal hydrides and theoretical predictions.

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Synchrotron X-ray diffraction has determined that β-Bi2O3 is the dominant oxide phase covering hexagonal bismuth nanoclusters produced in an inert gas aggregation source. High-resolution transmission electron microscopy of bismuth nanoclusters has shown the oxide thickness and orientation, and has been compared with the synchrotron diffraction measurements.

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Probability density functions of mobile ions in hollandite ionic conductors were obtained from a new structure model considering the local structure around a vacancy.

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The high-temperature structure of Ba3BaSb2O9 has been solved and Rietveld-refined as an ideal (P63/mmc) 6H perovskite, using synchrotron X-ray powder diffraction data. The size difference between SbO6 and BaO6 octahedra is greater than for any other reported 6H perovskite.

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The crystal structures of two new cobaltites, Ba6Co5BrO14 and Ba7Co6BrO17, have been solved. They are built by the stacking of [BaO3] and new [BaOBr] layers. Disorder has been observed in the vicinity of the [BaOBr] layers and characterized by the maximum entropy method for the two compounds.

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Framework oxygens in La10(GeO4)5(GeO5)O2 apatite provide a reservoir of ions that migrate between the large channels in a process that converts some GeO4 tetrahedra to GeO5 distorted trigonal bipyramids.

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Distortions in M = Pt and Pd complexes of planar aromatic bidentate ligands (L) coordinating by two pyridyl ring N atoms are caused by repulsive interligand interactions involving the substituents on the pyridyl C atoms adjacent to these N atoms. Whereas the highly distorted structures observed for LMCl2 with bulky L (e.g. 6,6′-dimethyl-2,2′-bipyridine) are generally similar, the nearly equal occurrence of two different types of distortions (canting and bow-incline) for [L2M]X2 complexes suggests that these distortions have similar energy and that subtle solid-state effects dictate which type of distortion occurs.

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Cluster analysis is used to analyse and classify metal coordination geometry in four-coordinate bis-salicylaldimato transition-metal complexes available in the Cambridge Structural Database. Structures are automatically clustered into groups based purely on the geometric similarity of the fragments and these groups can then be interpreted by the structural chemist.

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The systematic variation of two substituents results in a set of 100 related compounds and 133 crystal structures whose interrelations are analysed. A subset of 22 isostructures is discussed in detail.

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The dependence of electron densities obtained by the maximum entropy method on the measurement temperature (100 versus 20 K) and the resolution of X-ray diffraction data are analysed for the tripeptides Ala-Pro-Ala and trialanine.

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The single-crystal-to-single-crystal EZ photoisomerization of tiglic acid occurs in the supramolecular crystals CECR–HTA·2MeOH·1.5H2O (1) (where HTA = tiglic acid, CECR = C-ethylcalix[4]resorcinarene) with the preservation of the crystal lattice, indicating the `scaffolding effect' of the molecular framework, which holds the crystal together notwithstanding the change in shape of the embedded guest molecules.

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The quantum-mechanically derived reaction coordinates (QMRC) for the proton transfer in (N—H⋯N)+ hydrogen bonds have been derived from ab initio calculations of potential-energy surfaces. A comparison is made between QMRC and the corresponding bond-order reaction coordinates derived by applying the Pauling bond-order concept together with the principle of conservation of bond order.

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Monte Carlo computer simulation has been used to interpret and model observed single-crystal diffuse X-ray scattering data for pentachloronitrobenzene, C6Cl5NO2. The strong, detailed and very distinctive diffraction patterns can be accounted for almost entirely on the assumption of the random occupancy of each molecular site, but with very large local relaxation displacements that tend to increase the neighbouring distances for contacts involving NO2⋯NO2 and NO2⋯Cl with a corresponding reduction for those involving Cl⋯Cl.

addenda and errata


Acta Cryst. (2007). B63, 674
doi: 10.1107/S0108768107034052
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