Diversity Amidst Similarity:  A Multidisciplinary Approach to Phase Relationships, Solvates, and Polymorphs

Description of the red and yellow polymorphs of thallium picrate provides the introduction to this review paper that attempts to set its subject within the framework of classical physical chemistry without losing the connection to the burgeoning activity in academic and industrial laboratories based on more limited sources of information. After a formal definition of polymorphism and a thrust at current trends in nomenclature, the relation of hydrates (solvates) and polymorphs is illustrated in terms of the binary sodium sulfate−water phase diagram, with high-throughput crystallization techniques seen as an important auxiliary when complete phase diagrams are not available. The fundamental thermodynamics is introduced through the polymorphism of tin and, to a lesser degree, adamantane, and this is then connected to energy-temperature diagrams. The polymorphism of the elements (about one-third do not have ambient-pressure polymorphs) is then surveyed, with arrangements of spheres having a predominant (but not exclusive) role. The structural variety found among the elements is a qualitative (but not quantitative) reflection of that in the overall population of crystalline materials. This is followed by a brief look at the polymorphism of inorganics, organics, and biomaterials. First-order enantiotropic phase transformations relate many polymorph pairs, and their mechanism is discussed in terms of Mnyukh's nucleation and growth approach. We close by suggesting that monotropic systems, very important in practice, deserve more attention than they have received. The review is (mostly) restricted to systems at ambient pressure.