Crystallography Reviews, 23(2), 118-151, 2017.
To produce substances with required physico-chemical properties demands the recognition of structural features of materials. It includes polymorphism and isostructurality which are strongly related with intermolecular interactions and crystal symmetries. Fine tuning of structural properties can be achieved by the application of substituents or in the case of multi-component systems by the introduction of molecules of different sizes, shapes and chemical compositions. How far can a crystal structure tolerate small molecular changes? The molecular conformation of flexible molecules may adjust to the supramolecular features both in the preserved and in the new lattices. Packing motifs conserving the supramolecular arrangement may still remain whereas the space groups may be different. This is morphotropy which may follow isostructurality in the course of stepwise chemical modification before a completely different packing arrangement appears. What is the degree of structurally decreasing similarity in fine-tuned systems that we may still consider
to be isostructural and when do we say the structures are different?