Polymorphic transformations in crystal growth

Polymorphism has critical implications in material design of organic crystals for elucidating specific functions in optics, pharmaceuticals, foods, biology etc. This is because diversified properties are revealed in polymorphic modifications. Depending on the selection of the specific polymorphs, one has to deal with two tasks: crystallization of the specific polymorph in an exclusive manner, and preservation of that form after or even during the crystallization in cases where polymorphic transformation easily occurs. The polymorphic crystallization is primarily determined by the nucleation process, in which chemical potential difference and interfacial energy are predominant factors. Both factors differ between polymorphs, giving rise to complexity in polymorph-dependent nucleation. The polymorphic transformations occur through four schemes: solid state, solution mediation, melt mediation and interface mediation. The last three processes can occur more rapidly than the solid-state transformation, particularly in organic crystals, since steric hindrance for the solid-state transformation is critically serious. Recent studies on polymorphic transformations in organic crystals mediated by melt, solution and interface are reviewed.