issue contents

ISSN: 2052-5206

February 2004 issue

Highlighted illustration

Cover illustration: Crystal packing diagram of 1,4-benzoquinone with Hirshfeld surfaces. The translucent gray surfaces illustrate the nature of these novel surfaces and how they envelop each molecule and reflect the presence of neighbouring molecules. Colours mapped on the central surface represent the distance from the surface to the nearest nucleus in an adjacent molecule: red (shortest), through green, to blue (longest) (McKinnon, Spackman & Mitchell, Acta Cryst. B, in preparation).

research papers

Acta Cryst. (2004). B60, 1-9
doi: 10.1107/S0108768103026582
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Neutron and X-ray diffraction studies indicate that at high pressure the monoclinic to trigonal phase transition in lead phosphate is accompanied by the development of static disorder of the Pb atoms. This disorder in the trigonal phase is suppressed at pressures more than 1 GPa above the transition pressure at room temperature.

Acta Cryst. (2004). B60, 10-20
doi: 10.1107/S0108768103026661
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Cooperative Jahn–Teller distortions and their interplay with octahedral tilting and cation ordering have been examined systematically in AMX3 perovskites containing a Jahn–Teller ion on the octahedral site, as well as in quaternary A2MMX6 perovskites containing a Jahn–Teller ion on one-half of the octahedral sites.

Acta Cryst. (2004). B60, 21-31
doi: 10.1107/S010876810302901X
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An alternative superspace description of the perovskite-related compound NaCa4Nb5O17 as a commensurate modulated structure with discontinuous atomic domains is proposed. This approach highlights the layered character of the structure and can be applied to predict the space group and structure of other members of the [Na,Ca]nNbnO3n+2 compound series.

Acta Cryst. (2004). B60, 32-40
doi: 10.1107/S0108768103025138
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This paper describes the structures and thermal expansion properties of trigonal ZrMo2O8 and HfMo2O8. Both materials have been shown to undergo displacive phase transitions at high temperature to a new trigonal structure with space group P\bar 3m1.

Acta Cryst. (2004). B60, 41-45
doi: 10.1107/S0108768103027897
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The crystal structure of an iron(II) spin-crossover compound, [Fe(C10H8N6)2](ClO4)2, is solved from X-ray powder diffraction data and the thermal spin transition is monitored using the change in unit-cell parameters with temperature.

Acta Cryst. (2004). B60, 65-75
doi: 10.1107/S0108768103026739
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With a variety of carboxylic acids, 1,4,8,11-tetraazacyclotetradecane forms solvated salts that contain hydrogen-bonded supramolecular structures in two or three dimensions.

Acta Cryst. (2004). B60, 76-89
doi: 10.1107/S0108768103024856
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In symmetrically 4,6-disubstituted 2-aminopyrimidines, hard hydrogen bonds can generate supramolecular structures in zero, one, two or three dimensions.

Acta Cryst. (2004). B60, 90-96
doi: 10.1107/S0108768103029173
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A comparison of the structures of (S,S)-dileucine methyl esters with oxalamide, monothiooxalamide and dithiooxalamide bridges is presented. Asymmetric substitution of one oxygen by sulfur leads to packing of the helical hydrogen-bonded supramolecular assemblies with non-crystallographic, approximate 41 symmetry along b in the non-polar space group P212121. Molecules with chemically symmetrical bridges (oxalamide and dithiooxalamide) are packed in the polar space group P41 with similar hydrogen-bonded supramolecular assemblies.

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2,4,6-Trimethoxy-1,3,5-triazine is found to exhibit three different polymorphs. The major difference between these polymorphs is the mode of packing of the molecules in the crystal.

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Bond lengths and angles in two series of compounds with a tertiary C atom, calculated at the DFT level, conform qualitatively to the Walsh rule. In the presence of an electron-attracting substituent the angles are usually widened and bonds shortened.

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A single-crystal X-ray structure analysis is presented for the incommensurate structure of quininium (R)-mandelate. The structure was solved in two superstructure approximations, with Z′ = 15 and Z′ = 18, and then refined in (3+1)-dimensional space using the superspace approach.

international union of crystallography

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