Invariant elastic constants and eigentensors of orthorhombic, tetragonal, hexagonal and cubic crystalline media

The spectral decomposition of the compliance fourth-rank tensor of orthorhombic, tetragonal, hexagonal and cubic crystals.

On the origin of topological differences between experimental and theoretical crystal charge densities

Topological analysis of molecular – and crystal – charge densities of p-nitroaniline and p-amino-p′-nitrobiphenyl reveals considerable discrepancies between experimental and theoretical topologies at the bond critical points. Analysis of the differences shows the discrepancies to be mainly due to the nature of the radial functions used in the experimental multipole model.

Resonant X-ray diffraction: `forbidden' Bragg reflections induced by thermal vibrations and point defects

In general, the local atomic environment becomes less symmetric owing to point defects and thermal vibrations of atoms in crystals. It is shown that, as a result of this phenomenon, the extra Bragg reflections can be excited, providing a novel method to study point defects (Ge is one of the discussed examples).

Diffuse X-ray scattering and disorder in p-methyl-N-(p-chloro­benzyl­idene)aniline, C₁₄H₁₂ClN (ClMe): analysis via automatic refinement of a Monte Carlo model

A recently developed method for fitting a Monte Carlo computer simulation model to observed single-crystal diffuse X-ray scattering data has been used to study the disorder in p-methyl-N-(p-chlorobenzylidene)aniline, C₁₄H₁₂ClN (ClMe), at 295 K. The short-range-order (SRO) correlation structure observed within the layers of molecules in the ab plane is typical of a frustrated lattice, where nearest- and second-nearest-neighbour effects oppose each other. Local `size-effect' strains operating on different intermolecular vectors generally reflect the tendency for distances involving Cl—Cl interactions to be longer than the observed average distances.

Distinction of space groups (I23 and I2₁3) and (I222 and I2₁2₁2₁) using coherent convergent-beam electron diffraction

A new method using the coherent CBED method is proposed to distinguish the special pairs of space groups (I23 and I2₁3) and of (I222 and I2₁2₁2₁), which have so far been indistinguishable.

Subperiodic groups isomorphic to factor groups of reducible space groups

A relationship exists between factor groups of space groups and subperiodic groups. This relationship, an isomorphism between factor groups of reducible space groups and subperiodic groups, can be used in the derivation of higher-dimensional space groups, of lattices of space groups, and of irreducible representations of space groups.

Connectivity properties of high-density regions and ab initio phasing at low resolution

A property of well phased low-resolution Fourier syntheses to reveal macromolecules as `blobs' may be successfully used for ab initio phasing of low-resolution diffraction data.

Elucidation of simple pathways for reconstructive phase transitions using periodic equi-surface (PES) descriptors. The silica phase system. I. Quartz–tridymite

A novel method for modelling solid–solid reconstructive phase transitions, based on periodic nodal surfaces (PNS), is presented. The method allows exploration of such topological pathways in a very general fashion, and is applied to the quartz-to-tridymite transition.

The phenomenon of polarization suppression of X-ray Umweg multiple waves in crystals

The phenomenon of the polarization suppression of X-ray Umweg multiple waves in Renninger scans of crystals, showing intensity decrease due to properly chosen wavelength and polarization of incident radiation, is observed. With elliptically or linearly polarized synchrotron radiation, partial suppression of the strong Umweg interfered component provides an alternative way of enhancing the visibility of multiple-wave interference in crystals for direct phase determination.

Group theoretical methods and applications to molecules and crystals