Local and long-range order in ferroelastic lead phosphate at high pressure

Neutron and X-ray diffraction studies indicate that at high pressure the monoclinic to trigonal phase transition in lead phosphate is accompanied by the development of static disorder of the Pb atoms. This disorder in the trigonal phase is suppressed at pressures more than 1 GPa above the transition pressure at room temperature.

Jahn–Teller distortions, cation ordering and octahedral tilting in perovskites

Cooperative Jahn–Teller distortions and their interplay with octahedral tilting and cation ordering have been examined systematically in AMX₃ perovskites containing a Jahn–Teller ion on the octahedral site, as well as in quaternary A₂MM′X₆ perovskites containing a Jahn–Teller ion on one-half of the octahedral sites.

Superspace description of NaCa₄Nb₅O₁₇ (a perovskite-related compound of the type A_nB_nO_3n+2) as a modulated layered structure

An alternative superspace description of the perovskite-related compound NaCa₄Nb₅O₁₇ as a commensurate modulated structure with discontinuous atomic domains is proposed. This approach highlights the layered character of the structure and can be applied to predict the space group and structure of other members of the [Na,Ca]_nNb_nO_3n+2 compound series.

Structures and phase transitions of trigonal ZrMo₂O₈ and HfMo₂O₈

This paper describes the structures and thermal expansion properties of trigonal ZrMo₂O₈ and HfMo₂O₈. Both materials have been shown to undergo displacive phase transitions at high temperature to a new trigonal structure with space group Pm1.

An X-ray powder diffraction study of the spin-crossover transition and structure of bis­(2,6-dipyrazol-1-ylpyrazine)iron(II) perchlorate

The crystal structure of an iron(II) spin-crossover compound, [Fe(C₁₀H₈N₆)₂](ClO₄)₂, is solved from X-ray powder diffraction data and the thermal spin transition is monitored using the change in unit-cell parameters with temperature.

Hydrated metal(II) complexes of N-(6-amino-3,4-di­hydro-3-methyl-5-nitroso-4-oxopyrimidin-2-yl) derivatives of glycine, glycylglycine, threonine, serine, valine and me­thionine: a monomeric complex and coordination polymers in one, two and three dimensions linked by hydrogen bonding

The title ligands form hydrated complexes with Ca²⁺, Sr²⁺ and Ba²⁺ in which the metal–ligand coordination may give finite (zero-dimensional) entities or coordination polymers on one, two or three dimensions.

Adducts of 1,4,8,11-tetra­aza­cyclo­tetra­decane with carboxylic acids: hydrogen-bonded supramolecular structures in two or three dimensions

With a variety of carboxylic acids, 1,4,8,11-tetraazacyclotetradecane forms solvated salts that contain hydrogen-bonded supramolecular structures in two or three dimensions.

Symmetrically 4,6-disubstituted 2-amino­pyrimidines and 2-amino-5-nitroso­pyrimidines: interplay of molecular, molecular–electronic and supramolecular structures

In symmetrically 4,6-disubstituted 2-aminopyrimidines, hard hydrogen bonds can generate supramolecular structures in zero, one, two or three dimensions.

Supramolecular assembling using synthons with NH—CO(S)—CS—NH and NH—CO—CO—NH functionalities: crystal structures of (S,S)-N,N′-mono­thiooxalyldileucine methyl ester and its di­thio analogue

A comparison of the structures of (S,S)-dileucine methyl esters with oxalamide, monothiooxalamide and dithiooxalamide bridges is presented. Asymmetric substitution of one oxygen by sulfur leads to packing of the helical hydrogen-bonded supramolecular assemblies with non-crystallographic, approximate 4₁ symmetry along b in the non-polar space group P2₁2₁2₁. Molecules with chemically symmetrical bridges (oxalamide and dithiooxalamide) are packed in the polar space group P4₁ with similar hydrogen-bonded supramolecular assemblies.

Different packing in three polymorphs of 2,4,6-tri­meth­oxy-1,3,5-triazine

2,4,6-Trimethoxy-1,3,5-triazine is found to exhibit three different polymorphs. The major difference between these polymorphs is the mode of packing of the molecules in the crystal.

Geometry at the aliphatic tertiary carbon atom: computational and experimental test of the Walsh rule

Bond lengths and angles in two series of compounds with a tertiary C atom, calculated at the DFT level, conform qualitatively to the Walsh rule. In the presence of an electron-attracting substituent the angles are usually widened and bonds shortened.

Quininium (R)-mandelate, a structure with large Z′ described as an incommensurately modulated structure in (3+1)-dimensional superspace

A single-crystal X-ray structure analysis is presented for the incommensurate structure of quininium (R)-mandelate. The structure was solved in two superstructure approximations, with Z′ = 15 and Z′ = 18, and then refined in (3+1)-dimensional space using the superspace approach.

Notes for authors 2004