Polymorphism

This chapter assesses polymorphism, which is the ability of some molecules to adopt more than one crystal structure. When preparing materials by crystallization, it is important to recognize and to be able to control this phenomenon, because each polymorph has its own unique combination of mechanical, thermal, and physical properties. The production of specific and well-defined polymorphs is therefore crucial in chemical manufacture. In the chemical and pharmaceutical industry, the demand for high yields and high production rates has forced chemists and engineers to operate processes far from equilibrium, so exacerbating the tendency to form polymorphic structures that, for a given temperature, pressure, and composition, are not the most stable. Such unstable structures will eventually undergo phase transitions to more stable phases. This transformation process and its structural and thermodynamic basis are the subjects of the chapter.