issue contents

ISSN: 2052-5206

June 2018 issue

Highlighted illustration

Cover illustration: VESTA plot of the Ia{\bar 3}d crystal structure of dry-synthesized K2ZnSi5O12 looking down [111], see Bell & Henderson [(2018). Acta Cryst. B74, 274-286].

research papers

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The differences in crystallographic parameters of various phosphine–borane pairs have been described. It was found that truly frustrated Lewis pairs could not be isolated. This report also suggests a revising of the nomenclature of the pairs.

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A theory of order–disorder phase transitions of B-cations in AB1/2B′′1/2O3 perovskites is developed which allows reliable estimation of the phase transition temperatures and of the reduced lattice constants of such compounds.

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Crystal structures for synthetic analogues of the tectosilicate mineral leucite (KAlSi2O6) with stoichiometries of K2X2+Si5O12 (X = Fe2+, Co, Zn) and Rb2X2+Si5O12 (X = Mn) were refined from X-ray powder diffraction data. Structures refined in P21/c have ordered Si and X cations on the tetrahedral sites, whereas structures refined in Ia{\overline 3}d have disordered Si and X cations. These leucite structures show an inverse relationship between the tetrahedral T—O distances and the intertetrahedral T—O—T angles.

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The solid-state structure of a star-shaped triazine compound is determined from a powder sample by exploiting the respective strengths of single-crystal electron diffraction and powder X-ray diffraction data, confirming that the symmetry of the crystal packing allows for nonlinear optical effects.

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The structure of the low-temperature polar (orthorhombic) phase of russellite (Bi2WO6) was re-examined with single-crystal X-ray diffraction. The position of the stereochemically-active lone pair of electrons of bis­muth was found on a line running through the Bi sites in the b-axis direction. The ferroelectricity of the compound was ascribed to the strong tendency of bis­muth to form shorter Bi—O bonds, and this resulted in the cooperative shift of tungsten.

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The dehydration of the monohydrate salt of pridinol mesylate is strongly dependent on the heating rate, slow rates leading to a solid–solid process, intermediate rates to coexistence with the hydrate and faster heating to complete melting, without generation of the anhydrous solid in the water-loss process.

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The crystal structure of L-valyl-L-leucine aceto­nitrile solvate presented here completes a systematic investigation of dipeptides constructed from the five nonpolar amino acids alanine, valine, isoleucine, leucine and phenyl­alanine. The crystal packing arrangements and hydrogen-bonding preferences of this unique group of substances are reviewed.

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