Ostwald Rule of Stages of Close-packed Spherical Colloids

The initial structure of nucleating and growing crystal domains of equal spherical colloids has been reported fluctuation-induced randomly-stacked layers of two-dimensional hexagonal close-packed (RHCP) arrays until the crystal domains adopt face-centered cubic (FCC) symmetry with sufficiently large translational entropic gains. Contrary to this belief, our experimental investigation of metastable close-packed structures of soft micellar colloids reveals that the initial close-packed structure at the nucleation step is close to hexagonally close-packed (HCP) structures, gradually transforming to FCC through intermediate RHCP states as the crystal grains grow. This observation suggests that growing domains of a new phase pass through a series of thermodynamic states over a temperature or density pathway in the phase space.