Competing Interactions and the Origins of Polytypism

Abstract

Crystal structures can be analysed at many levels. At the most fundamental level, they are described in terms of the relative distribution of their constituent atoms, or of the coordination polyhedra of the component cations and anions. It is becoming increasingly apparent, however, that many families of structures can be usefully described in terms of larger basic structural units or modules. If such an approach to the description of crystal structures is adopted, many complex solids may be systematised in terms of series of stacking variants of the simple subunits; these phases are known as polytypes. This relatively broad definition of polytypism closely follows that of THOMPSON [1] and is discussed at length by ANGEL [2]. The definition removes any chemical constraints upon stacking variants, and allows more than one type of module to be present in a given polytype family. Polytypism is however a special form of polymorphism, and although there is no constraint upon the chemistry or structure of the modules involved, to be considered polymorphic the various modes of module stacking should not affect the composition of the phase as a whole. It should also be noted that this definition of polytypism allows for the existence of 2- and 3-dimensional polytypic structures, comprised of prismatic (rod-like) and block modules respectively, as well as the classically more familiar layer modules of the 1-dimensional polytypes.