On the structure of grain/interphase boundaries and interfaces.

Grain/interphase boundaries/interfaces of varying misorientations, free volume fractions, curvatures and irregularities are present in materials, both 3D and 2D, regardless of whether these materials are crystalline or amorphous/glassy. Therefore, a question arises about the central idea on which a general description of grain/interphase boundaries/interfaces can and should be based. It is suggested that a generalized model of a structural/basic unit (crystalline, non-crystalline or of any scale), which depends on the interatomic (including electronic) interactions, the spatial distribution of the atoms and electrons, the number of atoms and free volume fraction present in the structural/basic unit and the experimental conditions should serve the purpose. As the development of a quantitative model, which reflects the effects of all these variables is difficult, slightly defective material boundaries are often modeled by treating the entire boundary as planar and by using the concepts of crystallography. For highly disordered boundaries, a description in terms of a representative volume, made up of a non-crystalline basic unit or a combination of such units, which depend on interatomic (including electronic) interactions and forces, is advocated. The size, shape, free volume fraction and number of atoms in the representative volume could differ with material composition and experimental conditions. In the latter approach, it is assumed that all processes connected to a problem on hand is contained within this representative volume. The unresolved issues are identified.

Isostructurality, polymorphism and mechanical properties of some hexahalogenated benzenes: the nature of halogen...halogen interactions.

The nature of intermolecular interactions between halogen atoms, X...X (X = Cl, Br, I), continues to be of topical interest because these interactions may be used as design elements in crystal engineering. Hexahalogenated benzenes (C6Cl(6-n)Br(n), C6Cl(6-n)I(n), C6Br(6-n)I(n)) crystallise in two main packing modes, which take the monoclinic space group P2(1)/n and the triclinic space group P1. The former, which is isostructural to C6Cl6, is more common. For molecules that lack inversion symmetry, adoption of this monoclinic structure would necessarily lead to crystallographic disorder. In C6Cl6, the planar molecules form Cl...Cl contacts and also pi...pi stacking interactions. When crystals of C6Cl6 are compressed mechanically along their needle length, that is, [010], a bending deformation takes place, because of the stronger interactions in the stacking direction. Further compression propagates consecutively in a snakelike motion through the crystal, similar to what has been suggested for the motion of dislocations. The bending of C6Cl6 crystals is related to the weakness of the Cl...Cl interactions compared with the stronger pi...pi stacking interactions. The triclinic packing is less common and is restricted to molecules that have a symmetrical (1,3,5- and 2,4,6-) halogen substitution pattern. This packing type is characterised by specific, polarisation-induced X...X interactions that result in threefold-symmetrical X3 synthons, especially when X = I; this leads to a layered pseudohexagonal structure in which successive planar layers are inversion related and stacked so that bumps in one layer fit into the hollows of the next in a space-filling manner. The triclinic crystals shear on application of a mechanical stress only along the plane of deformation. This shearing arises from the sliding of layers against one another. Nonspecificity of the weak interlayer interactions here is demonstrated by the structure of twinned crystals of these compounds. One of the compounds studied (1,3,5-tribromo-2,4,6-triiodobenzene) is dimorphic, adopting both the monoclinic and triclinic structures, and the reasons for polymorphism are suggested. To summarise, both chemical and geometrical models need to be considered for X...X interactions in hexahalogenated benzenes. The X...X interactions in the monoclinic group are nonspecific, whereas in the triclinic group some X...X interactions are anisotropic, chemically specific and crystal-structure directing.

Structural basis for bending of organic crystals.

Bending is observed in organic crystals when the packing is anisotropic in such a way that strong and weak interaction patterns occur in nearly perpendicular directions.