issue contents

ISSN: 2052-5206

June 2003 issue

Highlighted illustration

Cover illustration: Orange polytypically disordered crystals of HgI2 are built from layers of corner-linked Hg4I10 supertetrahedra. The stacking disorder has been quantitatively analyzed with a Markov chain model. Two probabilities describing next-nearest-layer interactions were adjusted to observed diffuse intensity profiles extracted from image-plate detector data. Results show nearly equal volumes of the two maximum degree of order structures, with an average domain thickness of about 5 layers or 30 Å [Hostettler, Birkedal & Schwarzenbach (2002), Acta Cryst. B58, 903-913].

research papers

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The structures of 94 ternary aluminates are reinterpreted on the basis of the Zintl–Klemm concept and Pearson's generalized octet rule. In aluminates of highly electropositive metals such as alkali, alkaline-earth and rare-earth metals, the Al atoms form three-dimensional skeleta which can be interpreted as if the Al atoms would behave as Zintl polyanions, adopting the structure of either main-group elements or Zintl polyanions showing the same connectivity.

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The single-crystal investigation of the β-Ta self-hosting structure at 293 and 120 K shows that this σ-structure is a two-component [the host (H) and the guest (G)] composite with the same (or a multiple) lattice constants, but different space groups. The phase transition revealed in the thermal process 293 → 120 → 293 K is reversible for H and irreversible for G.

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A single-crystal X-ray structure analysis is presented for the incommensurate room-temperature phase γ and for the commensurate phase δ at 110 K of anhydrous sodium carbonate. A previous study of phase δ included only first-order satellites; this study included higher-order satellites. Both phases are compared within the unified superspace group approach.

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An accurate structure model of sodium-doped KTiOPO4 at 10.5 K has been determined by single-crystal X-ray diffraction. The rigid crystal lattice of TiO6 octahedra and PO4 tetrahedra has not been influenced by the temperature change, from room temperature to 10.5 K, and no phase transformation has been detected. However, both potassium cations have moved 0.033 (2) Å along the polar direction.

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Rules to predict the crystal structures of complex fluorides M1nM2mM3F6 were formulated using the uniformity criterion for ionic sublattices. The essential role of cationic sublattices and the relationship between the composition of an ionic compound, and the size and charge of ions were analysed under normal conditions, with thermal and high-pressure polymorphic transitions.

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The structure of lithium benzilate hemihydrate has been determined from synchrotron powder diffraction data using a simulated-annealing global-minimization algorithm and the long-established difference Fourier maps.

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The short-range order of both uranium ions and iodine chains [I_3^-]n in two crystals of uranium iodine phthalocyanine has been analysed by RMC simulation of X-ray diffuse scattering. We have shown that arbitrarily regarding diffuse reflections as Bragg reflections in the refinement may produce ambiguous and incorrect results.

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Unlike the α-modifications of platinum phthalocyanine and metal-free phthalocyanine, the α-modification of copper phthalocyanine grown on KCl does not have a herringbone-type molecular arrangement. A classification of the polymorphs of planar phthalocyanines is proposed.

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The mechanism of the first-order solid-to-solid phase transition of 1-ethyl-3-(4-methylpentanoyl)urea crystal was clarified by analysing the structure of intermediate stages of the transformation with a detailed temperature-resolved single-crystal diffraction method.

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The structure of the 1:1 adduct of hexamethylenetetramine and undecanedioic acid is investigated. The disorder observed in the O atoms of the carboxylic group is analyzed as a function of the temperature.

addenda and errata

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