Structural derivation and crystal chemistry of apatites

The crystal chemistries of [A]₅[(BO₄)]₃X, [A]₅[(BO₃)]₃X and [A]₅[(BO₅)]₃X apatites are reviewed. The empirical relationship between the A(1)O₆ metaprism twist angle and average crystal radius is proposed as an indicator of thermodynamic equilibrium.

The incommensurate structure of K₃In(PO₄)₂

The incommensurately modulated (3+1)-dimensional structure of K₃In(PO₄)₂ has been refined from single-crystal X-ray diffraction data. The cation–cation interactions are used to explain the origin of the structure modulation. It is shown that this incommensurately modulated structure is the only phase that has been unequivocally identified at room temperature.

The modulated structure of Ba_0.39Sr_0.61Nb₂O₆. I. Harmonic solution

The structure of a crystal of Sr_0.61Ba_0.39Nb₂O₆ has been solved and refined as an incommensurate structure in five-dimensional superspace. The modulation affects mainly the positions of the O atoms, which are displaced by as much as 0.5 Å, and the site 4c that is occupied by Sr and Ba atoms. Only a simplified model, in which this atomic position is occupied by an effective atom Sr/Ba, could be refined from the data set. The modulation of displacement parameters has been used to account for the modulated distribution of Sr and Ba.

Determination of the crystal structure of the π-AlFeMgSi phase using symmetry- and site-sensitive electron microscope techniques

The crystal structure of the π-AlFeMgSi phase has been determined by combining several electron-microscope techniques. Electron-microprobe analysis revealed the composition; convergent-beam electron diffraction (CBED) was used for the space-group determination and refinement of the atomic positions; and electron channelling was used for the determination of the atomic arrangement.

22 Space-group changes

22 space groups are revised to higher symmetry and discussed in connection with the physico-chemical behaviour. In particular, two novel changes P2₁ → P4₃ and the uncommon changes P2₁ → C222₁, P2₁ → Cmc2₁ and P2₁ → P2₁2₁2₁ are reported.

Crystal chemistry and binding of NO₂, SCN and SeCN to Co in cobalamins

The accurate crystal and molecular structures of NO₂Cbl·2LiCl, NO₂Cbl·NaCl, NCSCbl and NCSeCbl, based on synchrotron diffraction data collected at 100 K, are described. The first X-ray structural determination of a cobalamin with a Co—Se bond is reported. The crystal chemistry of cobalamins is discussed in terms of packing of roughly spherical molecules. The axial distances in several cobalamins and cobaloximes are compared and discussed in terms of the cis influence of the equatorial ligand.

Influence of the molecular structures on the high-pressure and low-temperature phase transitions of plastic crystals

X-ray powder diffraction with synchrotron radiation is used to study the temperature dependence between 295 and 105 K and the high-pressure behavior between 0 and 5 GPa of the crystal structures of Si[C(CH)][Si(CH)] and of Si[C(CH)][Si(CH)].

Urotropin azelate: a rather unwilling co-crystal

The superposition and order–disorder structure of the 1:1 co-crystal of urotropin and azelaic acid are described. A simulation of the crystal's complex diffraction pattern is performed based on a three-dimensional Monte Carlo model that involves occupational random variables and random displacements of the chains along their axes. The low-temperature phase transitions are explained by the action of two thermally activated slip systems.

Phosphono­acetic acid as a building block in supramolecular chemistry: salts with organic polyamines

The adducts of phosphonoacetic acid with a range of amines form two- and three-dimensional hydrogen-bonded structures, which contain substructures of the acid component only in the form of simple chains, chains of rings or sheets.

Salts of maleic and fumaric acids with organic polyamines: comparison of isomeric acids as building blocks in supramolecular chemistry

Maleic acid and fumaric acid both readily form adducts with organic diamines; some of these adducts are salts and some contain only neutral molecular components. A combination of O—H⋯N, N—H⋯O and C—H⋯O hydrogen bonds generates a rich variety of supramolecular structures in two and three dimensions, none of which could readily be predicted from a knowledge of all the others.

(1R,3S)-Camphoric acid as a building block in supramolecular chemistry: adducts with organic polyamines

The adducts of (1R,3S)-camphoric acid with a range of amines form hydrogen-bonded structures in two and three dimensions, in which the substructures formed only by the hard hydrogen bonds take the form of chains, sheets or three-dimensional frameworks.

Hydrogen bonding in enantiomeric versus racemic mono-carboxylic acids; a case study of 2-phen­oxy­propionic acid

Structures of the optically active compound 2-phenoxypropionic acid have been solved in the pure state and with n-alkanes cocrystallized. These structures are included in an analysis of hydrogen-bond motifs in optically active mono-functional carboxylic acids. The results are related to the thermodynamic data for 2-phenoxypropionic acids.

An X-ray diffraction study of partially ordered electron density in clathrates of Dianin's compound that include simple carboxylic acids

Difference electron-density maps within the cavities in clathrates of Dianin's compound with carboxylic acids have been interpreted in terms of symmetry-related partially occupied sites. The size and shape of the entrapped acid determine the packing and mode of interaction with the host.

Single-crystal-to-single-crystal photodimerization of 4-chlorocinnamoyl-O,O'-dime­thyl­dopamine

In crystals of 4-chlorocinnamoyl-O,O′-dimethyldopamine, pairs of molecules related by centers of symmetry have parallel C=C bonds at a C⋯C distance of 4.155 (17) Å. Irradiation with a mercury lamp achieves 100% conversion via [2+2]-photodimerization in a single crystal. The same solid-state reaction is induced by irradiation with X-rays.

Twinning in natural melilite simulating a fivefold superstructure

The presence of additional reflections suggesting a fivefold superstructure in melilite is considered in the light of twinning.

Electron density study of 2H-chromene-2-thione

Erratum to Acta Cryst. (2002), B58, 1011–1017.

Notes for authors 2003