A non-mathematical introduction to the superspace description of modulated structures

The basic principles of handling and describing modulated structures with the superspace formalism are given in a non-mathematical way. They are illustrated using the crystal structure of (6R,7aS)-6-(tert-butyl-dimethylsilanyloxy)-1-hydroxy-2-phenyl-5,6,7,7a-tetrahydropyrrolizin-3-one, C₁₉H₂₇NO₃Si.

Complex ceramic structures. I. Weberites

This article focuses on weberite ceramics. The structural features of weberite, its relationship to the fluorite and the pyrochlore structures, and some weberite ceramics with interesting properties are discussed.

Structures of K_0.05Na_0.95NbO₃ (50–300 K) and K_0.30Na_0.70NbO₃ (100–200 K)

The structures of K_xNa_1 − xNbO₃ (x = 0.05, x = 0.3) at low temperatures have been determined by neutron powder diffraction studies. The phase-coexistence region at the transition between rhombohedral and monoclinic phases was also identified, consistent with a first-order phase boundary.

Structures of the ZrZn₂₂ family: suprapolyhedral nanoclusters, methods of self-assembly and superstructural ordering

A number of strict rules are proposed to model the crystal structures of intermetallics as a network of cluster precursors. According to these rules the self-assembly of the ZrZn₂₂-like structures was considered within the hierarchical scheme: primary polyhedral clusters (one AX₁₆ + 2BX₁₂) → zero-dimensional nanocluster precursor (AB₂X₃₇) → one-dimensional primary chain → two-dimensional microlayer → three-dimensional microframework (three-dimensional supraprecursor).

Large, larger, largest – a family of cluster-based tantalum copper aluminides with giant unit cells. I. Structure solution and refinement

The structures of a family of three tantalum copper aluminides with giant unit cells have been determined. Their mutual relationships are emphasized by a comparison of their average structures.

Large, larger, largest – a family of cluster-based tantalum copper aluminides with giant unit cells. II. The cluster structure

The complex cluster structures of a family of three tantalum copper aluminides with giant unit cells are discussed.

Characterization of the pressure-induced second-order phase transition in the mixed-valence vanadate BaV₆O₁₁

The high-pressure behaviour of the mixed-valence vanadate BaV₆O₁₁ was studied with single-crystal X-ray diffraction in a diamond–anvil cell; the pressure-induced transition from P6₃/mmc to P6₃mc was studied in detail.

Structures of the pseudo-trigonal polymorphs of Cu₂(OH)₃Cl

The crystal structure of Cu₂(OH)₃Cl has been refined using two nearly stoichiometric natural specimens. The occurrence of a triclinic polymorph is suggested for pure Cu₂(OH)₃Cl, while clinoatacamite is found for small but significant impurity concentrations.

SiBr₄ – prediction and determination of crystal structures

For SiBr₄ crystal structures were predicted by global lattice-energy minimizations. Two predicted structures could be verified experimentally.

Racemic calcium tartrate tetrahydrate [form (II)] in rat urinary stones

The structure of racemic [Ca(C₄H₄O₆)]·4H₂O [form (II)] is clarified from powder and single-crystal X-ray diffraction data.

Absolute structure determination as a reference for the enantiomeric resolution of racemic mixtures of cyclophosphazenes via chiral high-performance liquid chromatography

The absolute configurations of pairs of enantiomers separated by high-performance liquid chromatography have been confirmed by single-crystal structure analysis and provide a reference system for subsequent assignment of such systems based on elution order. The solid-state structures of the pairs of enantiomers do not fully adhere to expected enantiomorphous crystallization behaviour.

Charge density and electrostatic potential analyses in paracetamol

The charge density of the monoclinic paracetamol drug has been derived from high-resolution X-ray diffraction. Topological features of the electron density and electrostatic potential are presented.

Hantzsch 1,4-di­hydro­pyridine esters and analogs: candidates for generating reproducible one-dimensional packing motifs

Hantzsch 1,4-dihydropyridine ester analogs, including the prototypical hypertension drug Nifedipine, are of pharmaceutical use but are also good candidates for generating a reproducible packing motif that is characterized by intermolecular N—H⋯O hydrogen-bonded C(6) extended chains.

Single-crystal diffuse scattering studies on polymorphs of molecular crystals. I. The room-temperature polymorphs of the drug benzocaine

Simple Monte Carlo models have been developed that reproduce in detail the diffuse diffraction patterns observed for the two room-temperature polymorphs of the drug benzocaine. The analyses reveal highly correlated molecular displacements in both forms, but in polymorph (II) the motions show non-harmonic characteristics that appear to be precursors of a phase transition to a new polymorph which has been found at low temperatures (150 K).

A solution to the observed Z′ = 2 preference in the crystal structures of hydrophobic amino acids

Periodic two-dimensional slabs and full three-dimensional networks were extracted from the X-ray structures of hydrophobic amino acids and then optimized with ab initio methods. The derived interaction energies support the observed propensity for inclusion of two molecules in the asymmetric unit.

On the correlations between the polyhedron eccentricity parameters and the bond-valence sums for the cations with one lone electron pair. Addendum

Systematic variations of the bond-valence sums calculated from the poorly determined bond-valence parameters have been illustrated using a simple graphical scheme.

Structural Crystallography of Inorganic Oxysalts