Nonclassical Nucleation

Nucleation determines the essential properties of a population of growing crystals including the number of crystals, their size distribution, location, and polymorphs. In this chapter, we review the basic tenets of classical nucleation theory, which provide a fundamental framework to understand crystal nucleation. We discuss several recently uncovered deviations from the pathways assumed in classical theory. Among these scenarios are two-step nucleation, barrier-free nucleation, nuclei with nonequilibrium shapes, and nuclei with diffuse and variable interfaces. We also discuss nucleation control strategies. Two-step nucleation hosted by disordered phases can be controlled and suppressed by modifying the properties of the precursors. By contrast, nucleation along the classical pathway can only be boosted by lower barriers or heterogeneous substrates but not suppressed. The abundance of materials and environments where crystallization and aggregation dominate crucial architectures or pernicious pathologies will bring about even richer nucleation behaviors.