Crystallization is a naturally occurring or artificially initiated process for the solidification of substances or materials into a structured form, where the atoms or molecules are highly organized into a structure known as a crystal. Some materials, such as drug substances in pharmaceuticals, can form several different crystal structures, called crystal modifications or crystal forms or polymorphs.
Crystallization occurs at a certain temperature and is accompanied by a certain amount of energy released (exothermal process), which is known as the heat or enthalpy of crystallization. Furthermore, the crystallization procedure also has a kinetic component (rate of crystal growth), which should be taken into account in crystallization studies. Crystallization as a thermally initiated process often occurs in two major steps: The first step is nucleation, whereas the second step is known as crystal growth; the latter is also dependent upon the conditions of heat treatment.
With the help of DSC (differential scanning calorimetry), the crystallization temperatures and the energy released can be determined with a high degree of reliability. It should be mentioned that the temperature at which crystallization takes place at DSC experiments might shift to lower temperature values due to cooling or super-cooling effects, which may occur during the crystallization process. In the below illustrated indium measurement, the crystallization occurred at 155°C, which is approximately 2 Kelvin lower than the melting temperature of 156,6°C.