VARIETIES OF POLYMORPHISM

Frank H. Herbstein

Department of Chemistry, TechnionnIsrael Institute of Technology, Haifa, Israel 32000
chr03fh@tx.technion.ac.il

Keywords: Polymorphism, first order phase changes.

Polymorphism, the appearance of different crystal structures for the same chemical entity, is closely related to phase transformations, especially those of first order. Polymorphism is a physical phenomenon with chemical features, and one can set up an approximate ëphysicsnchemistryí scale for different types of polymorph. At the physics limit of this scale, the chemical entity has the same structure in both polymorphs and the crystal structures are often, but not always, similar. At the chemical limit one has the polymorphism of carbon (for example, sp2(p) hybridization in graphite and sp3 hybridization in diamond) and tin (sp3 hybridization in diamond cubic tin and metallic delocalization in tetragonal tin). In between one finds conformational polymorphism and differences in hydrogen bond arrangements. Knowledge of the structures of the various polymorphs provides the essential basis for their study. The importance of thermodynamic information (specific heatntemperature curves, DSC measurements) will be emphasised, as will the value of cell dimension-temperature measurements. These ideas will be illustrated by examples from the literature and recent studies in this laboratory.