Synchrotron X-ray study of the electron density in RFeO₃ (R = Nd, Dy)

Synchrotron X-ray diffraction images of the deformation electron density (Δρ) for two rare-earth orthoferrites RFeO₃ (R = Nd, Dy) show a high degree of correlation between aspherical electron densities around magnetic R and Fe atoms in each compound, which can be related to the magnetic properties of these orthoferrites.

Temperature-dependent neutron powder diffraction evidence for splitting of the cationic sites in ferroelectric PbHf_0.4Ti_0.6O₃

The ferroelectric-to-paraelectric phase transition in the lead hafnate titanate PbHf_0.4Ti_0.6O₃ has been studied by temperature-dependent neutron powder diffraction. Particular attention has been directed towards the splitting of the Hf/Ti cationic site and the distortion of the oxygen octahedra.

Neutron structures of ammonium tetra­fluoroberyllate

Neutron structures of ammonium tetrafluoroberyllate at 200, 163 and 20 K reveal that both the BeF and the NH ions rotate and shift positions upon going from the paraelectric to the ferroelectric phase, resulting in stronger hydrogen-bonding interactions in the ferroelectric phase accompanied by a doubling of the crystallographic a axis.

High-temperature phases of NaNbO₃ and NaTaO₃

Diffraction patterns of the high-temperature phases of the perovskites sodium niobate and sodium tantalate have been collected at intervals of 5 K using the high-resolution powder diffractometer HRPD at the ISIS neutron spallation source. This has allowed the structural changes with temperature within a single phase to be determined for the first time.

Residual short-range order in the heavy fermion compound CeInCu₂

From the analysis of X-ray diffuse scattering, short-range order is indicated in CeInCu₂. A superstructure-like disorder between Ce and In atoms is clarified; the feature is probably related to the fact that the structural disorder hardly affects the residual resistivity at low temperature.

Disorder determined by high-resolution powder diffraction: structure of penta­methylcyclo­pentadienyllithium

Modelling and calculations by the maximum entropy method (MEM) have been used to solve the crystal structure of pentamethylcyclopentadienyllithium (LiCp*) from a high-resolution powder pattern. The structure was found to comprise polymeric `multidecker' chains. The pentamethylcyclopentadienyl anions are coplanar with each other and show threefold disorder.

Electron density study of urea using TDS-corrected X-ray diffraction data: qu­antitative comparison of experimental and theoretical results

The electron-density distribution in urea obtained by X-ray diffraction is compared quantitatively in direct and reciprocal space with the outcome of Hartree–Fock and density-functional solid-state calculations with different correlation and exchange potentials.

Crystal and molecular structures of di­azapyrenes and a study of ππ interactions

The crystal structures of 5,10-dimethyl-4,9-diazapyrene and the new compound 2,7-dimethyl-4,9-diazapyrene are presented and the experimental bond lengths are compared with those calculated by molecular mechanics and semi-empirical methods and values predicted by valence-bond and variable-electronegativity self-consistent field (VESCF) methods. The quantitative parameters of ππ interactions have been calculated for two diazapyrenes and other pyrene-like molecules selected from the Cambridge Structural Database and used for classification of their crystal-packing types; the analysis is based on geometrical parameters and elements of electronic structure.

Low-temperature crystallization and structure determination of N-(tri­fluoro­methyl)formamide, N-(2,2,2-tri­fluoro­ethyl)formamide and 2,2,2-tri­fluoro­ethyl isocyanide

The in situ crystallization and structure determination of the three title compounds is presented. The molecular geometries are in each case compared with the results of ab initio studies.

Experimental charge density and electrostatic potential in nicotinamide

The accurate crystal structure of nicotinamide was determined from X-ray and neutron diffraction experiments. The electron distribution determined by the maximum entropy method indicates that the electrostatic properties depend on the rotation of the carboxamide group out of the pyridine plane.

Structures of 2,6-disubstituted naphthalenes

The crystal structures of 2,6-naphthalenedicarboxylic acid and dimethyl 2,6-naphthalenedicarboxylate have been solved an initio, and refined (along with that of their precursor, 2,6-dimethylnaphthalene) using powder diffraction data.

Three conformational polymorphs and order–disorder phase transition in a calix[4]arene bis­(crown)

Three polymorphic modifications of a calix[4]arene bis(crown) including an azobenzene unit have been structurally characterized. The phase transition between two of them was investigated by X-ray diffraction and adiabatic calorimetry.

Symmetry, pseudosymmetry and packing disorder in the alcohol solvates of L-leucyl-L-valine

L-Leu-L-Val forms non-isomorphous 1:1 solvates with methanol, ethanol (LVE) and 2-propanol (LV2P). LVE has four peptide molecules in the asymmetric unit, with intricate pseudosymmetry. LV2P retains the same hydrogen-bonding pattern as LVE with only half the unit-cell volume. This is owing to packing disorder resulting from four alternative peptide conformations.

Structure of mono-acid even-numbered β-tri­acylglycerols

The crystal structure of tripalmitin (PPP), a C_nC_nC_n-type (n = even) triacylglycerol (TAG), was determined and used to build an overlap model for this type of TAG. From the model the crystal structures of the unknown members of this series are predicted.

3,4:9,10-Perylene­tetra­carboxylic dianhydride (PTCDA) by electron crystallography

The crystal structures of the α and β modifications of PTCDA were analyzed as projected structures on the (102) planes by electron crystallography using an imaging plate.

Theoretical prediction of bond-valence networks. II. Comparison of the graph-matrix and resonance-bond approaches. Erratum

The correct version of Fig. 2> in the paper by Rutherford [Acta Cryst. (1998), B54, 204–210] is given.

Notes for Authors 1999