Two-step crystallization pathways of hard particle systems via a metastable fluid-fluid phase transition

Multi-step crystallization pathways are often found in biomineralization, protein crystallization and tetrahedrally coordinated systems, where a single or multiple intermediate state appear during the crystallization process. Despite the importance of those materials, 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.