Crystallization is a chemical process by which atoms and molecules in solution are arranged in a highly organized structure (normally solid) called crystal. This precipitation process can be triggered by a number of physical triggers, such as evaporation or freezing, although deposition can also occur directly from gas.

Crystallization occurs in two steps:

  • Nucleation: formation of molecular proto-aggregates, also known as nuclei, seeds, or embryos, which will evolved into larger macroscopic crystals. Nucleation is triggered when the solution in which the atoms or molecules are reaches conditions of supersaturation or supercooling. However, as this process occurs at a molecular level, the characterization of the mechanisms that govern nucleation is not fully understood. This situation is worsened because this first phase can suffer from long induction times. Nucleation can be started by the presence of templates (surface catalysis), physical structures that may resemble the structure of the crystals and can act as an anchor point.
  • Crystal growth: once the crystal seed has formed, new atoms, ions, or molecules can attach, expanding the crystal in an orderly repeating pattern, which extends in all three space dimensions. This second step occurs much faster than the nucleation. Crystals are characterized by structural rigidity and a notable resistance to deformation.
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