Polymorph selection in charged colloids in the second nucleation step

Abstract

We study polymorph selection in a model of charged colloids, with a focus on the higher-order structure prior to and during nucleation. Specifically, we carry out molecular dynamics simulations of a repulsive Yukawa system with a slightly softened (Weeks–Chandler–Andersen) core. We consider the case where the interaction is long-ranged and the BCC crystal is stable, and also intermediate- and short-ranged cases where the FCC crystal is stable. We use two methods for structure identification, the topological cluster classification (TCC) [A. Malins et al., J. Chem. Phys. 139, 234506 (2013)] and the bond orientational order parameter analysis of Lechner and Dellago [J. Chem. Phys. 129, 114707 (2008)]. Under conditions of high supersaturation appropriate to experiments with colloids, we find that the system forms a precursor state in which the particles are hexagonally ordered. That is to say, the precursors are indistinguishable from an HCP crystal using the bond orientational order parameters. This ordering occurs at state points when the body-centered cubic crystal is the stable phase and also when the face-centered cubic crystal is stable. In all cases, the stable polymorph forms from the precursor phase in a second stage. Although at freezing the fluid is much more ordered when the interactions are short-ranged (when FCC is stable), at the supersaturations where nucleation occurs in our simulations, the higher-order structure of the metastable fluids is almost identical for the long-, short-, and intermediate-ranged systems when measured with the TCC.