U_equiv: its past, present and future

Neither of the common definitions of U_equiv yield a parameter of any great use, and its standard uncertainty is even more suspect. The need for a better `one-parameter' measure of the quality of a.d.p.'s is demonstrated.

Architecture of crystal structures from square planes

The ordering of A and B atoms in b.c.c., f.c.c. or p.c. A_xB_y alloys and related compounds is restricted to a small number of thermodynamically stable structures. The structural units of these structures are assembled in different ratios for a variety of attractive or repulsive interactions.

Refinement of framework disorder in dehydrated CaA zeolite from single-crystal synchrotron data

The crystal structure of dehydrated CaA zeolite, solved using single-crystal synchrotron data, shows a disordered distribution of framework oxygen atoms and cations.

X-ray diffraction study of K₃NdSi₇O₁₇: a new framework silicate with a linear Si—O—Si bond

Hydrothermal investigations in the high-silica region of the K₂O–Nd₂O₃–SiO₂ system, carried out in a search for novel fast-ion conductors (FICs), yielded the new compound, K₃NdSi₇O₁₇. Its structure is unusual in that it contains a symmetry-constrained Si—O—Si bond angle of 180°.

Twinning and defects in N-Nb₂O₅

High-resolution electron-microscopy images of N-Nb₂O₅ reveal two different kinds of coherent twinning. The striking similarity between the observed electron-diffraction patterns for multiply twinned N-Nb₂O₅ and that reported for M-Nb₂O₅ can be understood by considering the similar block arrangements in the twin borders of N-Nb₂O₅ and the disordered M-Nb₂O₅.

Synchrotron X-ray analysis of RbTiOAsO₄

An accurate synchrotron X-ray diffraction measurement of a single crystal of the RbTiOAsO₄ compound shows that there is a substantial (dynamic and/or static) disorder of the Rb atoms over additional sites related by pseudosymmetry at room temperature.

Systematic prediction of new ferroelectrics in space group P3

A total of 21 structures reported in the Inorganic Crystal Structure Database under space group P3 satisfy the structural criteria for ferroelectricity. One of the remaining structures is a previously discovered ferroelectric, 19 are probably centrosymmetric and seven are polar but not ferroelectric.

Crystal structure and charge distribution of YbFeMnO₄

The X-ray single-crystal structure and charge distribution analysis of synthetic YbFeMnO₄ are reported. A vacancy of approximately 4% is present at the Yb octahedral site.

Two-dimensional incommensurately modulated structure of (Sr_0.13Ca_0.87)₂CoSi₂O₇ crystals

The structural details of (Sr_0.13Ca_0.87)₂CoSi₂O₇, including the displacement of atoms, rotation and distortion of the structural units, and the partial ordering of the Sr and Ca atoms accompanied with the modulation have been determined. The structural discrepancies in (Sr_0.13Ca_0.87)₂CoSi₂O₇ and in Ca₂CoSi₂O₇ are discussed to find the effects of the partial substitution of Ca by Sr.

Isomorphism and pseudosymmetry in 2,6-di­chloro- and 2,6-di­bromobenzo­nitrile

2,6-Dichloro- and 2,6-dibromobenzonitrile are isomorphous, with two half-molecules in the asymmetric unit of a monoclinic unit cell. The molecules pack in layers with a fourfold pseudosymmetry. The pseudosymmetry appears to be the consequence of halogen–halogen intermolecular interactions.

Two-dimensional packing and pseudosymmetry

It is possible to understand, or at least rationalize, the occurrence of pseudosymmetry elements in some crystals as a consequence of a higher symmetry in a two-dimensional layer being obscured by the three-dimensional packing of the layers.

Packing modes of (R,R)-tartaric acid esters and amides

The packing of primary tartramides is driven by NH⋯O=C hydrogen bonds and supplemented by strong OH⋯O=C and weak NH⋯OH bonds, while in methylated derivatives OH⋯OH⋯O=C patterns dominate. Problems with the packing of methyl groups are overcome by changes in conformation (esters) or by co-crystallization with solvent water molecules (methylamides and dimethylamides).

Intramolecular hydrogen bonds: common motifs, probabilities of formation and implications for supramolecular organization

The probabilities of occurrence of the 50 most common intramolecular hydrogen-bonded (N/O—H⋯O/N) ring motifs in the Cambridge Structural Database are presented and their implications discussed.

Graph-set and packing analysis of hydrogen-bonded networks in polyamide structures in the Cambridge Structural Database

The hydrogen-bond networks and crystal packing arrangements of 81 secondary di- and polyamides in the Cambridge Structural Database are investigated. Graph-set analysis is used to identify and classify network motifs, which are rationalized in terms of the relative dispositions of the amide functional groups; symmetry relationships within and between hydrogen-bonded chains, ladders and sheets in the crystal packing are analysed.

Conformational transformation coupled with the order–disorder phase transition in 2-methyl-1,3-cyclo­hexanedione crystals

The ring conformation transforms between sofa and dynamically disordered half-chair in the crystals of enolized 2-methyl-1,3-cyclohexanedione, C₇H₁₀O₂. The transformation takes place at the continuous phase transition at T_c = 244 K.

N-(6-Amino-3,4-di­hydro-3-methyl-5-nitroso-4-oxopyrimidin-2-yl) derivatives of glycine, valine, serine, threonine and me­thionine: interplay of molecular, molecular–electronic and supramolecular structures

The title compounds are all characterized by very short intermolecular O—H⋯O hydrogen bonds and C—N—O fragments in which the C—N and N—O bond lengths are almost equal. These observations are rationalized by means of database analysis and computational modelling.

Conformational preferences and supramolecular aggregation in 2-nitro­phenylthiolates: di­sulfides and thio­sulfonates

The interplay between molecular conformation and supramolecular aggregation has been investigated in six 2-nitrophenyl disulfides and a 2-nitrophenyl thiosulfonate ester. Hydrogen bonds of the C—H⋯O type are responsible for observed conformations remote from the global energy minimum.

Ferroelastic structures of n-pentyl-, n- hexyl- and n-nonylammonium di­hydrogenphosphate crystals

The title compounds are ferroelastic. There are H atoms in the structures, involved in asymmetric hydrogen bridges, which hop between donor and acceptor O atoms during the ferroelastic switching. The structure of n-pentylammonium dihydrogenphosphate (C5ADP) belongs to the same structure type as C3ADP and differs from C2ADP, C4ADP and C6ADP–C10ADP.

Orthorhombic τ-Zn–Mg–Dy phase related to a Frank-Kasper type decagonal quasicrystal

A model structure of a novel Zn–Mg–Dy compound is proposed based on electron diffraction and high-resolution transmission electron microscopy studies. Local atomic configurations in the structure are shown to be closely related to those in the Frank-Kasper type decagonal quasicrystal and in the Zn₇Mg₄ crystalline compound.

X-ray structure of barium titanate – missed opportunities

The dispersion effects have been calculated for the structure factors of barium titanate. It is stressed that the Friedel pairs should be treated independently in the refinements.

Phosphorus 2000. Chemistry, biochemistry & technology

Direct phasing in crystallography

Diffraction des rayonnements