(599a) Prenucleation Motifs of Various Dimensionality in Entropic Crystallization

In multi-step crystallization pathways, a single or multiple prenucleation state appears during the crystallization process. Such multi-step crystallization pathways occur in many chemical and physical systems, for instance, calcium carbonate (CaCO₃) solutions favor compact pre-nucleation clusters at the early stage of nucleation, and in the crystallization of a gas hydrate, water molecules form amorphous frameworks around guest molecules before clathrate nucleation occurs. The driving force for the appearance of the precursors has been explained by unique characteristics of the system, but universal characteristics shared by the multi-step crystallization process are not well understood. Here, we report three hard particle systems (truncated tetrahedra, pentagonal bipyramids and triangular bipyramids) exhibiting two-step crystallization via a metastable fluid-fluid phase transition. Monte Carlo simulations reveal that these systems first form a high-density fluid with prenucleation motifs in the form of clusters, fibers, and networks from a low-density fluid. Subsequently, complex crystal structures (cF432, oF244 and cP92) nucleate from the high-density fluid. Our observations demonstrate the existence of diverse crystallization pathways in entropic systems and reveal various dimensionalities of prenucleation motifs.