High-pressure crystal structure of the non-linear optical compound BiB₃O₆ from two-dimensional powder diffraction data

A new non-centrosymmetric high-pressure phase of the NLO compound α-BiB₃O₆ has been analyzed with the aid of a new method for automatic detection, calibration and evaluation of Debye–Scherrer ellipses using pattern-recognition techniques and advanced signal filtering.

Compounds with a `stuffed' anti-bixbyite-type structure, analysed in terms of the Zintl–Klemm and coordination-defect concepts

Compounds with a `stuffed anti-bixbyite' structure, such as Li<sub>3</sub>AlN<sub>2</sub>, were analysed in terms of both the extended Zintl–Klemm concept and the coordination-defect concept. For the first time, inorganic crystal structures are seen as a set of `multiple resonance structures' (Klemm pseudo-structures) which co-exist as the result of unexpected electron transfers between any species pair comprising either like or unlike atoms, cations or anions. If this is the driving force controlling crystal structures, the conventional oxidation states assigned to cations and anions lose some of their usefulness.

Structural study of K_xNa_1 − xNbO₃ (KNN) for compositions in the range x = 0.24–0.36

The reported morphotropic phase boundary of K_xNa_1 − xNbO₃ at x = 0.3 has been studied with powder X-ray and neutron diffraction.

Polymorphism of K₂Co₂Mo₃O₁₂: variations in the packing schemes and changes in molybdenum coordination under high pressure

Three polymorphs of K₂Co₂Mo₃O₁₂ are compared: the ambient-conditions phase α, a so-called `high-temperature' phase β, and a high-pressure phase II, formed at ca 6.7 GPa. Coordination polyhedra and packing schemes are compared for the three different network structures.

Dynamic proton disorder and the II–I structural phase transition in (NH₄)₃H(SO₄)₂

Using X-ray powder diffraction, differential scanning calorimetry (DSC)/thermogravimetry (TG) and single-crystal neutron diffraction, dynamic proton disorder and structural phase transition studies were carried out on (NH)H(SO₄)₂ between 293 and 493 K.

Crystallochemical formula as a tool for describing metal–ligand complexes – a pyridine-2,6-dicarb­oxylate example

The capabilities of the `method of crystallochemical analysis' are demonstrated with an example of all the pyridine-2,6-dicarboxylate containing complexes known so far; this method reproduces the structure and topology of any complex concisely.

Planar packing of tetra­chlorodi­cyano­benzenes. II

Equimolar mixtures of ortho and meta, or ortho and para, or meta and para tetrachlorodicyanobenzenes form structures containing layers of disordered molecules with both kinds of molecule in the same layer, but which are otherwise similar in structure to one of the polymorphs of the pure meta isomer.

Following an isosymmetric phase transition by changes in bond lengths and anisotropic displacement parameters: the case of meta-carboxyphenyl­ammonium phosphite

The evolution of the bond lengths, hydrogen bonds and thermal displacement have been accurately investigated over a wide temperature range (100–345 K) in order to better understand an original isosymmetric first-order phase transition of an m-CPAMP single crystal.

Persistent hydrogen bonding in polymorphic crystal structures

New automated methods are applied to analyse polymorphic structures in the Cambridge Structural Database, revealing that a large proportion of chemically equivalent hydrogen bonds persist in multiple structural forms. The roles of conformational variation, hydrogen-bond geometries and chemical functionality are systematically explored.

Probing the shapes of chiral bis-(o-naphthalimido­benzoyl) systems using X-ray and circular dichroism methods

X-ray and circular dichroism (CD) investigations on diesters and diamides containing two ortho-(1,8-naphthalimido)benzoyl (NAB) bichromophores reveal the high complexity of the system and the conformational variety of the molecules, which makes reliable prediction of their shape difficult. The types of intermolecular interactions experienced by these molecules in a crystal have been summarized quantitatively by means of a Hirshfeld surface analysis.

On the optimization of bond-valence parameters: artifacts conceal chemical effects

Systematic differences between calculated bond-valence sums and stoichiometric valences, ^stoichV, which are e.g. observed in structures containing lone-pair cations, are due to differences between the values of structural valence, ^structV, and ^stoichV, caused by electronic effects. Optimization of bond-valence parameters should be based on values of ^structV instead of ^stoichV.

Alternative presentation of the Brown–Wu bond-valence parameters for some s² cation/O²⁻ ion pairs

Using the least-squares refinement procedure, high-performance Brown–Wu bond-valence parameters, r₀ and n, reported for the Sn²⁺/O²⁻, Sb³⁺/O²⁻, Te⁴⁺/O²⁻ and I⁵⁺/O²⁻ ion pairs have been converted into the commonly used bond-valence parameters, r₀ and b. The r₀ and b values obtained are recommended as a replacement for the conventional bond-valence parameters based on the `universal constant' b = 0.37 Å.

Notes for authors 2009