Progress On Polymorph Selection: Structure Specific Coordinates and Dynamics of Structure Formation

The nucleation of crystals with specific structures from solution is important for many applications. In contrast to crystal nucleation from a pure supercooled component, there has been little progress towards simulating crystal nucleation from a supersaturated solution. The impasse is an inability to combine methods for computing the rates of rare events like nucleation with simulation methods that control chemical potential - a necessity for controlling supersaturation. Instead, we focus on key factors that influence polymorph selection: the dynamics of structure formation and the interfacial free energy of nuclei. This presentation illustrates the role of dynamics in a system of oppositely charged colloids that forms either CsCl structures or a charge disordered FCC (DFCC) structures depending on the simulation dynamics. Earlier studies hypothesized that the dynamical effect was a breakdown in the equilibrium distribution of pre-critical nuclei. After developing two structure specific nucleus size coordinates, one for each polymorph, we show how the dynamics alter a strongly anisotropic mobility tensor on a free energy landscape. Kramers-Langer-Berezhkovskii-Szabo theory shows that a high mobility for the formation of nuclei with one of the two structures determines polymorph selection in the oppositely charged colloidal system. Our analysis shows the importance of structure-specific coordinates for understanding polymorph selection mechanisms and suggests a number of promising directions for further research. B. Peters, J. Chem. Phys. 131 244103 (2009).