Structure of incommensurate ammonium tetra­fluoroberyllate studied by structure refinements and the maximum entropy method

The incommensurately modulated structure of ammonium fluoroberyllate is found to result from correlated rotations and displacements of the rigid complex ions according to a first harmonic modulation wave. The resulting changes in the network of the hydrogen bonds lead to the stabilization of the structure.

Calcium–lead fluoro-vanadinite apatites. I. Disequilibrium structures

X-ray diffraction and high-resolution electron microscopy show that crystallochemical equilibration of calcium–lead fluoro-vanadinite requires annealing at 1073 K for 2 weeks. The approach to equilibrium is controlled by the redistribution of lead ions into the apatite channels which expand to accommodate the larger cation.

Calcium–lead fluoro-vanadinite apatites. II. Equilibrium structures

Structure refinement of calcium–lead fluoro-vanadinites by powder X-ray diffraction confirmed these apatites to be one-dimensional microporous solids containing tunnels that expand to accommodate larger cations through reduction in the metaprism twist angle. This trend can be modelled quantitatively by derivation from a regular triangular oxygen network.

The surprisingly elusive crystal structure of sodium metabisulfite

The structure of Na₂S₂O₅ has been determined from a twin domain isolated by cleavage along a twin boundary. The metabisulfite ion does not adopt the C_s conformation observed in all other reported structures containing the anion.

Pressure and temperature effects on the degree of symmetry and chirality of the molecular building blocks of low quartz

Quantitative symmetry and quantitative chirality measures were used to etablish correlations between pressure and temperature (PT) and the magnitude of these structural properties for the molecular building blocks of low quartz.

New bond-valence parameters for lanthanides

The values of the lanthanide–oxygen bond-valence parameters calculated for coordination compounds, taking the coordination numbers into consideration, are presented.

An investigation of H-atom positions in sulfuric acid crystal structures

When determining crystal structures using X-ray diffraction methods, the H atoms are often left out in the dark because of their weak ability to interact with X-rays. In this article, a theoretical method of positioning H atoms is utilized.

Atomic volumes and charges in a system with a strong hydrogen bond: L-tryptophan formic acid

The charge density of L-tryptophan formic acid has been studied from a high-resolution synchrotron experiment with CCD area detection at 100 K. The results were compared with those of previous experimental studies and theoretical calculations. The topology of a strong hydrogen bond is discussed.

Two tautomers in one crystal: 4(5)-nitro-5(4)-methoxyimidazole

The crystal structure appears as disordered and centrosymmetric, but the hydrogen-bond pattern shows that in reality it is the mixture of two tautomers that crystallize in one crystal with an exact 1:1 ratio.

2-Bi­phenylol revisited

The disordered structure of 2-biphenylol shows no unusual behavior when cooled. At all temperatures above 90 K the drive for efficient packing is more important than the drive for the formation of O—H⋯O bonds.

Crystal structure of the supramolecular linear polymer formed by the self-assembly of mono-6-de­oxy-6-adamantylamide-β-cyclodextrin

In C₅₃H₈₅NO₃₅·C₃H₇NO·15H₂O, mono-6-deoxy-6-adamantylamide-β-cyclodextrin, units form a supramolecular linear polymer via self-assembly. Adjacent macrocycles in a column are linked by further hydrogen bonds. The water molecules, which reside in channels surrounding the columns of macrocycles, act as bridges between the cyclodextrin units.

Structural studies of (N-phenylthioureido­alkyl- and -aryl)phosphonates

The crystal structures of three (N-phenylthioureidoalkyl- and -aryl)phosphonates have been determined by X-ray diffraction. The conformations of the title molecules, the geometry of the thioureide fragments and the molecular packing arrangements are analyzed and compared with literature data.

Structures of piperazine, piperidine and morpholine

The crystal structures of piperazine, piperidine and morpholine have been determined at 150 K. All three structures are characterized by the formation of NH⋯N hydrogen-bonded chains. In piperazine these are linked to form sheets, but the chains are shifted so that the molecules interleave. In morpholine there are in addition weak CH⋯O interactions. Packing in these three structures is compared to that observed in cyclohexane.

Model for the crystal packing and conformational changes of bi­phenyl in incommensurate phase transitions

Global lattice-energy minimization with empirical potentials for the intermolecular and conformational energy predicts the ground state of biphenyl (phase III) to be a high-order superstructure, with periodic modulation of the twist angle of the molecule. The high-temperature phase I with a planar conformation is predicted to be a stationary point of the energy function, at a small height above the ground state.

Hydrogen-bonded adducts of tri­phenylsilanol with di­amines: a finite ten-molecule aggregate, and chain, sheet and framework structures built from O—H⋯O, O—H⋯N and C—H⋯π(arene) hydrogen bonds

Hydrogen-bonded adducts of triphenylsilanol, Ph₃SiOH, with organic diamines form supramolecular structures containing isolated (zero-dimensional) aggregates or continuous structures in the form of a chain (one-dimensional), a sheet (two-dimensional) or a framework (three-dimensional).

Structure for perovskites with layered ordering of A-site cations

It is noted that La_1/3NbO₃ and La_2/3TiO₃, in which successive layers of A-sites are occupied by cations alternately to a greater and lesser degree, adopt the same orthorhombic structure. It is argued that a number of other perovskites, of the type Ln_2/3TiO₃, Ln_1/3NbO₃ and Ln_1/3TaO₃, where Ln is a lanthanoid, adopt this same structure.

Space group Cc: an update

A recent survey of the Cambridge Structural Database shows that the percentage of incorrect assignments of the space group Cc has remained at about 10% since the last survey in 1997.