Crystal Polymorphism Challenges at the Crossroads of Science and Technology

Crystal Polymorphism Challenges at the Crossroads of Science and Technology 

Materials are traditionally classified in three states of matter gases, liquids and solids distinguished by their properties. However, the solid phase of a material can exhibit different structures, which in turn can show different properties. In addition to different crystal structures, called polymorphs, which are characterized by long-range order, a material may appear as an amorphous solid, characterized by the lack of long-range order. The polymorphism of calcium carbonate (calcite, vaterite and aragonite) was identified more than 200 years ago by Klaproth in 1788 

From thermodynamic principles, under specified conditions only one polymorph is the stable form (except at a transition point) [5], In practice, however, due to kinetic considerations, metastable forms can exist or coexist in the presence of more stable forms. Such is the case for diamond, which is metastable with regard to graphite, the thermodynamically stable form of carbon under ambient conditions. In practice, the relative stability of the various crystal forms and the possibility of interconversion between crystal forms, between crystals with a different degree of solvation and between an amorphous phase and a crystalline phase, can have very serious consequences on the life and effectiveness of a polymorphic product and the persistence over time of the desired properties (therapeutic effectiveness in the case of a drug, chromatic properties in the case of pigment, etc). 

Conversions between different crystal forms are possible and often take place. Among the many possibilities for conversion, depending on variables such as temperature, pressure, relative humidity, etc. specific examples are  

a metastable form can convert to a thermodynamically more stable crystal form with very slow kinetics  

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