Structures des phases paraélectrique et ferroélectrique de Pb₂KNb₅O₁₅

The paraelectric phase of Pb₂KNb₅O₁₅ has a tetragonal tungsten-bronze-like structure above the Curie temperature (T_C = 730 K). Below T_C the structure is slightly distorted (orthorhombic) and the phase is both ferroelectric and ferroelastic. A structural study of both phases by X-ray and neutron powder diffraction is presented.

Cluster configurations in modulated EuV_xMo_8±yO₁₄ crystals

The modulated structures of three crystals of EuV_xMo_8±yO₁₄ with different compositions and different modulation waves are studied and compared. The action of the modulation on the distribution and the geometry of the different types of metallic clusters is also studied. Evidence is presented for new types of Mo₉ and Mo₁₀ metallic clusters, formed by capping three or four faces of the octahedral Mo₆ units, and strong metallic bonds between the clusters are implied in some unit cells of the three crystals.

Crystal structure, compressibility and possible phase transitions in -FeSi studied by first-principles pseudopotential calculations

Investigation of the relative stability of the FeSi structure and of some hypothetical polymorphs by first-principles pseudopotential calculations has shown that the observed distortion of the FeSi structure from its idealized form is essential for its stability. It is predicted that FeSi having a CsCl-type structure will be the thermodynamically most stable phase for pressures greater than 13 GPa.

Systematic prediction of new inorganic ferroelectrics in point group 4

Approximately 40% of the crystal structures in point group 4 of the current release of the Inorganic Crystal Structure Database are found to satisfy the structural criteria for potential ferroelectricity. Each prediction of ferroelectricity is associated with a probable phase transition, not all of which are necessarily subgroup–supergroup transitions.

A TEM study of long-period mica polytypes: determination of the stacking sequence of oxybiotite by means of atomic resolution images and Periodic Intensity Distribution (PID)

The stacking sequences of long-period polytypes (including a mixed-rotation 36-layer polytype) of oxybiotite mica have been unambiguously determined by the combination of atomic resolution images recorded down two zone axes and periodic intensity distributions in electron diffraction patterns using TEM.

Structure determination of lithium chloro­acetate, lithium bromo­acetate and lithium iodo­acetate by powder diffraction

The polarizing effect of the small lithium cations determines the crystal structures of three lithium halogenoacetates and is most likely responsible for the absence of the solid-state polymerizability that is found for most other halogenoacetates.

Single-crystal structure analysis of a novel aryl phosphinate diglycidyl ether

The crystal structure of novel 10-[2,5-bis(2,3-epoxy-1-propoxy)phenyl]-9-oxa-10-phosphaphenanthren-10-one has been studied by single-crystal X-ray diffraction. The NMR chemical shifts of the protons of the glycidyl group close to the aryl phosphinate group are reduced by the `ring-current effect'.

Packing similarities of three isosteric molecules: 4,5-di­chlorophthalic anhydride, 4,5-di­bromo­phthalic anhydride and 5,6-di­chlorobenzfurazan 1-oxide, including three polymorphs of 5,6-di­chlorobenzfurazan 1-oxide

4,5-Dichlorophthalic anhydride (CPA), 5,6-dichlorobenzfurazan 1-oxide (CBF) and 4,5-dibromophthalic anhydride (BPA) are approximately isosteric. CBF has three polymorphic forms. Polymorph A of CBF is isomorphous with CPA. Polymorph C of CBF and BPA are both pseudo-isomorphous with CPA, but in different ways.

Modelling the crystal structure of the 2-hydro­nitronylnitroxide radical (HNN): observed and computer-generated polymorphs

The crystal structures of two polymorphs of the title compound are analyzed by packing energy criteria. Other unobserved polymorphic crystal structures are generated using a polymorph predictor package and three different force fields, one of which is without explicit Coulomb-type terms. The X-ray structure of the polymorph was found to be among the most stable ones by the polymorph predictor, even using the chargeless force field.

Crystal and molecular structures of 2-[1-(2-amino­ethyl)-2-imidazolidinyl­idene]-2-nitroaceto­nitrile (C₇H₁₁N₅O₂) and 2,6-di­amino-5-hy­droxy-3-nitro-4H-pyrazolo[1,5-a]pyrimidin-7-one monohydrate (C₆H₆N₆O₄·H₂O) from X-ray, synchrotron and neutron powder diffraction data

The unknown crystal and molecular structures of two organic compounds have been determined from X-ray, synchrotron and neutron powder diffraction data using various methods. The results obtained demonstrate the high level of reproducibility and reliability of various powder software and equipment.

Topological analysis of the electron density in hydrogen bonds

Topological analysis of the electron density at the hydrogen bond critical point (CP) on a large number of crystals studied by experimental [X − X and X − (X + N)] methods has been investigated. A new classification of the hydrogen bond is proposed involving the λ₃ electron density curvature along the bonding direction.

Crystal engineering using bis­phenols: interwoven ladders, sheet and framework structures in the binary adducts of 4,4′-sulfonyldiphenol with pyrazine (2/1), 4,4′-bi­pyridyl (1/1), trans-1,2-bis­(4-pyridyl)ethene (1/1), 1,2-bis­(4-pyridyl)ethane (1/1) and 4,4′-tri­methylenedi­pyridine (1/1), and in 4,4′-sulfonyldiphenol–4,4′-tri­methylenedi­piperidine–water (2/2/1)

The 2:1 adduct of 4,4′-sulfonyldiphenol and pyrazine takes the form of continuously interwoven molecular ladders; the 1:1 adducts of 4,4′-sulfonyldiphenol with 4,4′-bipyridyl, 1,2-bis(4-pyridyl)ethene, 1,2-bis(4-pyridyl)ethane and 4,4′-trimethylenedipyridine all consist of chains, further cross-linked by C—H⋯O hydrogen bonds. The aquated adduct with 4,4′-trimethylenedipiperidine is a salt in which the components form a hydrogen-bonded three-dimensional framework.

Crystal engineering using bis­phenols and tris­phenols. Complexes with 1,10-phenanthroline: hydrogen-bonded chains in adducts with 4,4′-biphenol (1/1) and 4,4′-sulfonyldiphenol (2/3), π–π stacked chains in the (1/2) adduct with 4,4′-thiodiphenol, and pairwise-interwoven nets in 1,1,1-tris­(4-hydroxyphenyl)ethane–1,10-phenanthroline–methanol (1/1/1)

In its 1:1 adduct with 4,4′-biphenol, 1,10-phenanthroline act as a builder of hydrogen-bonded chains; in its 1:2 adduct with 4,4′-thiodiphenol the diamine acts as a terminator of such chains, and in its 2:3 adduct with 4,4′-sulfonyldiphenol, both roles are manifest. In the methanol-solvated 1:1:1 adduct with 1,1,1-tris(4-hydroxyphenyl)ethane the 1,10-phenanthroline units act as chain terminators and the structure contains pairwise-interwoven nets.

The single-crystal structure of the organic superconductor β_CO-(BEDT-TTF)₂I₃ from a powder grain

A synchrotron radiation study has been carried out on a single crystal of dimensions 12 × 10 × 2 µm of the organic superconductor β_CO-(BEDT-TTF)₂I₃. The triiodide site population deviates slightly from unity and the conformations and occupancies of the disordered ethylene groups have been determined.

Absolute configurations of Emycin D, E and F; mimicry of centrosymmetric space groups by mixtures of chiral stereoisomers

A mixture of stereoisomers can be misinterpreted as a disordered centrosymmetric structure, but only the ordered structure in a chiral space group is consistent with the anomalous dispersion and chemical evidence.

A mediated hydrogen bond in an α-hydroxycarboxyl group: X-ray structure of (R,R)-N-methyltartramic acid monohydrate and an ab initio study of model systems

A mediated three-centre hydrogen bond is one of the packing motifs involving the α-hydroxycarbonyl moiety in (R,R)-tartaric acid and (R,R)-N-methyltartramic acid monohydrate. The stability of such bonding in model complexes is studied via theoretical calculations.

Anhydrous ammonioguanidinium(2+) and dihydrated bis­[aminoguanidinium(1+)] hexa­fluoro­silicates: new co-products of preparing ferroelectric ammonioguanidinium(2+) hexa­fluoro­zirconate. Erratum

An error in printing in the paper by Ross et al. [Acta Cryst. (1999), B55, 246–254] is reported.