Structure Types and Classifications

The classification of structure types is an invaluable process for revealing and systematizing relationships between different compounds and for understanding basic principles governing chemical stability and physical properties. Moreover, the ab initio computation of atomic arrangements in solids of given stoichiometry remains a key problem in materials science, and although a general predictive method remains elusive, structural classification schemes provide guidance in constraining such calculations. While more than 150 000 entries can be found in the two main databases of inorganic solids and alloys (ICSD and CRYSTMET, respectively), there is a striking contrast between the simplicity of some compounds such as NaCl, which is readily described by simple hard sphere packing, and the apparently overwhelming diversity of more complex structures. It should also be added that intricate atomic patterns do not necessarily predicate involved chemistries. For example, boron offers a rich diversity of complex polymorphs. It is well appreciated that structural diversity originates from manifold atomic interactions – ionic, covalent, van der Waals, and so on – that define bonding characteristics. Even subtle changes in atomic interactions, perhaps as a function of temperature, pressure, or chemistry, can lead to variations in the atomic arrangement that create hurdles for structural cataloging and make the development of systematic taxonomies a challenging task. Nevertheless, the goal of all classification schemes is to establish mutually exclusive categories, while at the same time, striving to be as comprehensive as possible. Practically, all sorting regimes will be complicated, and to some extent limited by intrinsic disturbances such as....