Nucleation and polymorph selection in a model colloidal fluid

Understanding and controlling polymorph selection is important because the structure of a crystal is critical to its properties. We have explored polymorph selection during nucleation in the hard-core repulsive Yukawa fluid. This model has two polymorphs (bcc and fcc), the relative stability of which can be adjusted by changing the potential parameters. We used seeded molecular-dynamics simulations to follow the evolution of the metastable bcc structure in the fcc stable region. It was found that the degree of undercooling affected the development of the bcc seeds. At small undercoolings, the bcc seed became a bcc crystal, but at large undercoolings bcc seeds became fcc crystals. Homogenously nucleated simulations were also compared to colloidal experiments. The simulations yielded nuclei of similar shape to the experimentally observed nuclei and a similar minimum size of growth. The simulated nuclei had more bcc signature than the experimental nuclei, but upon further growth the bcc signature faded, indicating that the difference between simulations and experiments may be due to the experimental nuclei having more time to resolve their structure.