issue contents

Journal logoSTRUCTURAL SCIENCE
CRYSTAL ENGINEERING
MATERIALS
ISSN: 2052-5206

August 2000 issue

Highlighted illustration

Cover illustration: Perspective view of the structure of the complex of 2,4,6-tris- (4-chlorophenoxy)-1,3,5-triazine with tribromobenzene, determined by neutron diffraction. Courtesy of J. A. K. Howard & C. K. Broder (University of Durham, UK), G. R. Desiraju, A. Nangia & R. K. R. Jetti (University of Hyderabad, India), C. C. Wilson & D. A. Keen (Rutherford Appleton Laboratory, Chilton, UK).

obituaries


topical reviews


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The best measurements of cell dimensions from (small) single crystals and powders on the most favourable specimens have a precision of some parts in 105, which seems to be the maximum attainable with current, and older, techniques. There has been little real improvement over a period of some 60 years. `Routine' measurements, on less favourable specimens, give precisions of some parts in 104 or even 103. Some current assessments of the standard uncertainties of cell dimensions are fictional at best.

research papers


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The structure of RbNbOB2O5 is re-refined as an incommensurate modulated structure using the program JANA98.

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A database search for ammonium ion coordination by anions is used to determine bond-valence parameters. The applicability of these parameters is discussed and exemplified.

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Rietveld refinement of the modulated structure of lead cobalt tungstate. A displacive model involving significant atomic shifts for Pb atoms and quite a complex mixing of tilt and deformation of the oxygen octahedra is proposed.

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The structure of meta-schoepite, UO3·2H2O, is constructed from layers of stoichiometry (UO2)4O(OH)6 separated by water molecules. The material is structurally related to schoepite, UO3·2.25H2O, by the removal and redistribution of molecules in the water layers.

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The Rietveld refinement on synchrotron data was combined with IR data to study three natural hydrated NH4+-exchanged phillipsites with different Si/Al ratios. They all display the typical monoclinic P21/m cell with NH4+, which occupy sites I and II where it acts as a donor in hydrogen bonding with O atoms of the framework and water molecule.

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The electron density and atomic displacement pattern in the virtual ferroelectric KTaO3 studied with accurate X-ray diffraction data gives a comprehensive description of the atomic interactions in this crystal.

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A high-resolution synchrotron X-ray diffraction study on a HfV2 Laves C15 phase conducted at low temperatures has illustrated the structural and lattice changes as a function of temperature and definitely revealed the crystal structure of a tetragonal I41amd phase for the first time.

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Structure solution and refinement details are given for the multi-domain high-temperature (708 \leq T \leq 943 K) phase of LiKSO_4.

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The crystal structures of a new high-temperature phase of zirconium hydrogen phosphate and a new polymorph of zirconium pyrophosphate have been solved and refined from in situ temperature-resolved synchrotron powder diffraction data.

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A new empirical relationship between bond distances and bond valences is derived from the hypothesis that bond valence is proportional to the average electron density between bonding atoms.

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Two rare examples of allotwins are identified in an oxybiotite mica, using the X-ray precession method and geometrical analysis of the minimal unit in reciprocal space. 1M and 2M1 polytypes are attached by rotations of either 120 or 60°, respectively.

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Density functional theory-based calculations have been used to demonstrate that the aplanarity of CO_{3} groups in some carbonates is a ground-state property.

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The structures of λ-Al4Mn, κ-Al177Cr49Ni and ∊-Al4Cr, which have been described in terms of the aggregation of clusters until now, are interpreted as modulated structures. The observed stacking motif of atoms can be found in many phases related to icosahedral and decagonal quasicrystals.

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The crystal structures of B-Ta2O5 and Z-Ta2O5, two high-pressure forms of Ta2O5, have been determined and refined from X-ray powder data. The structures are closely related and an intergrowth between them was indicated by the HRTEM study.

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Dodecylnicotinate bis-adducts of binuclear copper carboxylates, of the general formula Cu2(O2CCn − 1H2n − 1)4(C5H4NCOOC12H25)2, were synthesized for n = 10, 12, 14, 16, 18 and 20 and their crystal structure, thermal behavior and magnetic properties studied. The molecular structure of the decyl derivative has been determined from single-crystal X-ray diffraction data.

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The frequencies of Z′ values (formula units per asymmetric crystal unit) are documented for organic and organometallic crystal structures, using the Cambridge Structural Database.

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The structure of merohedrally twinned 2-methylpyrazine at 123 and 168 K has been solved using SHELXL97.

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Single-crystal photoirradiations of trans-cinnamamide and the 4-methyl and 4-chloro derivatives have been carried out to observe the transformation from a pair of monomers related by a center of symmetry into an α-type dimer. The lower photoreactivity of trans-cinnamamide than those of the other two is not only due to a longer C⋯C distance between the nearest neighboring C=C double bonds, but also due to partial breakdown of the hydrogen-bond network in the process of photodimerization.

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The histidine complex provides an example of the retention of the basic features of amino acid aggregation in different environments and in the presence of different molecules, while the pattern observed in the lysine complex serves as a note of caution and demonstrates how difficult it is to predict crystal structures even though broad generalizations are useful.

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The results are reported of a blind test of crystal structure prediction of four organic molecules using the principal computer programs currently available. Successful predictions for all four molecules were obtained using calculated lattice energy, but no program was successful in all cases.

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The distribution of crystalline racemates with more than one molecule in the asymmetric unit among the common space groups has been determined. The conformational similarities between crystallographically independent enantiomers in 114 non-centrosymmetric racemates were quantified using the r.m.s. deviation for a molecular superposition. The r.m.s. distribution curve shows that the conformations of the crystallographically independent molecules are in general very similar.

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The spontaneous resolution of the racemate DL-allo-isoleucine upon crystallization is rationalized on the basis of an analysis of interatomic distances in the 1:1 DL-isoleucine:DL-allo-isoleucine complex. An unfavorable C⋯C contact prevents the two enantiomers from forming a stable crystal structure with the same hydrogen-bond arrangement as in 1:1 DL-isoleucine: DL-allo-isoleucine and related amino acid racemates.

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The electron density and the electrostatic properties of the title peptide molecules have been determined from high-resolution diffraction data collected at 123 K. The topological characteristics of the (3,−1) critical points of C=O, C—N and C—C bonds are compared in 17 peptide links.

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The dynamical disorder found in the crystal of a myo-inositol derivative is studied using molecular dynamics calculations.

short communications


Acta Cryst. (2000). B56, 744
doi: 10.1107/S0108768100003785
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The space group of bis((phenyl-O,N,N-azoxy)oxy)methane, previously revised from P1 to C2, is further revised to Fdd2.

addenda and errata


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