Topography and topology in solid-state chemistry

Abstract

A survey of recent activity and current trends in solid-state chemistry reveals that several novel and seemingly unrelated physicochemical phenomena, together with some bio-organic, mineralogical and metallurgical attributes of the solid-state, can now be interconnected. Four main topics are discussed. First, so far as the behavioural patterns of organic solids are concerned, topochemical control is often crucial, and it transpires that the reactivity of many types of unsaturated molecules is governed more by the precise molecular orientation and stacking sequences within the crystal structure than by the intrinsic electronic properties of the molecule itself. This fact has led to the synthesis, by the strategem of ‘crystal engineering’ (Schmidt 1971), of stereochemically-pure, organic products and also of single-crystal, extended-chain polymers both of which are difficult if not impossible to prepare by conventional (solution) techniques. It has also led to absolute asymmetric syntheses, under abiotic conditions, from optically inactive materials, by inducing solid-state or surface reactions in chiral crystals. The selectivity and high yields of gas reactions with organic solids, and the facile conversion of polymer crystals from their folded-chain to extended-chain forms are other manifestations of topochemical control in organic solid-state chemistry. Structural defects, too, can play a dominant role, and they account for the not infrequent production of topochemically ‘forbidden’ molecules in photo-induced reactions. The existence of stress-induced and photo-induced phase transformations and of structure-mimicry (in which guest species assume the molecular structure of the host) are further recent discoveries in this general area. Second, topochemical control, which needs to be distinguished from topotaxy, is often important in the reactions of inorganic solids, particularly in intercalation (or its reverse) in which charged or neutral guest species are accommodated between the individual sheets of layered compounds, thereby resulting in expansion of the interlayer separation distances and, generally, some modification of the stacking sequence. Exceptionally selective organic reactions may be carried out in the interlamellar spaces of silicate minerals, and crystal engineering, in the sense that reactant molecules are locked in well defined orientations conducive for subsequent reaction within inorganic matrixes, again becomes a feasible proposition. The third topic of discussion is stacking sequences and stacking faults, which serve as the nexus between the chemistry of layered solids and the structural principles of inorganic solids in general. The relations between the structural characteristics of a range of inorganic solids, and plausible mechanisms of interconversion based on martensitic transformation, follows logically; and changes of coordination number (in going from NaC1- to CsC1-type structures for example) may be rationalized in terms of continuous topological variation. Stacking faults are intimately connected both with partial dislocations and antiphase boundaries, and these, in turn, figure as important concepts in the understanding of the ultramicrostructure of grossly non-stoicheiometric solids, which is the main theme for the fourth topic discussed here. The chemical consequences of linear and planar faults are surveyed, there being a summarizing account of their role in the reactivity of solids, and a fuller one of single, double and pivoting crystallographic shear planes. Relations between shear and block structures are adumbrated; and the merit of introducing the notion of cylindrical fault boundaries, so as inter alia to relate ReO 3 -type to tungsten bronze structures, is outlined. The considerable analogical value of dislocation theory in interpreting the ultramicrostructural characteristics of non-metallic solids, especially as revealed by electron microscopy, is emphasized throughout.