Entropy-driven Two-step Nucleation of a Clathrate Colloidal Crystal via a Metastable Fluid-Fluid Phase Transition

Tetrahedral bonding is known to be key to metastable liquid-liquid phase transitions in water, silica, and elements of the carbon family. There, the intermediate liquid phase is believed to be responsible for subsequent nucleation of the crystal. Here we present the first report of an entropy-driven fluid of hard particles following a two-step nucleation pathway mediated by a metastable fluid-fluid phase transition. Monte Carlo simulations reveals that the fluid separates into a low density phase and a high density fluid phase that possesses locally ordered structures mappable to a tetrahedral network (step 1). In turn, a clathrate-like crystal nucleates and grows at the interface of the two fluid phases (step 2). The structural, thermodynamic, and dynamic discontinuities are observed across the fluid-fluid phase transition, and a structural order parameter is developed to clarify each crystallization step. Our observations suggest that the existence of tetrahedral order is the primary reason for the pathway and kinetics of the complex phase behavior, regardless of the origin of the tetrahedral bonding.