Controlling polymorphism during the crystallization of an atomic fluid.

We use molecular dynamics simulations to shed light on polymorph selection during the crystallization of the Lennard-Jones fluid. By varying pressure at fixed supercooling, we form large crystallites either of the stable face centered cubic form or of the metastable body centered cubic form and even fine-tune the fractions of stable and metastable polymorphs in the crystallite. We demonstrate that the conditions of crystallization, leading to large bcc crystallites, lie within the occurrence domain of the metastable bcc polymorph. We also find that the predominantly fcc crystallites contain a notable amount of the hexagonal close packed form, due to the cross nucleation of the hcp form on the fcc form. By varying temperature at fixed pressure, we prevent cross nucleation and form pure fcc crystallites. Our results reveal that polymorph selection may take place, and be controlled, during the growth step.