Rationalizing the polymorph selection process during the crystallization of model systems

Using molecular dynamics simulations, we study the crystallization of supercooled liquids in three simple systems. First, we study an atomic fluid modeled by the Lennard-Jones potential. We show that growth proceeds through the successive cross nucleations of the metastable hexagonal close-packed (hcp) polymorph on the stable face-centered (fcc) polymorph and of the stable fcc polymorph on the hcp metastable polymorph. Moreover we are able by varying the conditions of crystallization (i) to prevent cross nucleation and (ii) to control polymorphism e.g. by varying temperature at fixed pressure and by varying pressure at fixed supercooling, respectively. We form large crystallites either of the stable fcc form or of the metastable bcc form and even fine-tune the fractions of stable and metastable polymorphs in the crystallite. The second system modeled by the Yukawa (screened-Coulomb) potential allows us to understand when and how polymorph selection takes place during the crystallization of charge-stabilized colloidal suspensions. By modifying the value of the screening parameter lambda, we are able to invert the stability of the body-centered cubic (bcc) and face-centered cubic (fcc) polymorphs. We show that the crystallization mechanism strongly depends on the value of lambda. When bcc is the stable polymorph (lambda = 3), the crystallization mechanism is straightforward. Both kinetics and thermodynamics favor the formation of the bcc particles and polymorph selection takes place early during the nucleation step. When fcc is the stable polymorph (lambda = 10), the molecular mechanism is much more complex. First, kinetics favors the formation of bcc particles during the nucleation step. The growth of the post-critical nucleus proceeds through the successive cross nucleations of the stable fcc polymorph on the metastable hcp polymorph as well as of the hcp polymorph on the fcc polymorph. Finally, we simulate the entire crystallization process for a metal. We consider the example of Aluminum. We shed light on the molecular mechanisms underlying the nucleation and growth steps and find significant differences with those followed by simple fluids. First, Al nucleates into a random packing of the hcp and fcc phases. Bcc clusters, which usually form during the nucleation of simple fluids, are not observed during the crystallization of Aluminum. Second, the crystallites formed are strongly faceted. This is in sharp contrast with the ellipsoidal crystallites observed during the crystallization of simple fluids. We add that we did not find any icosahedral signature within the crystallite. The concentration in icosahedral atoms in the liquid was constant throughout the growth step, showing that the icosahedral structures do not play an active role in the crystallization process of Al.