Polymorph selection and nucleation pathway in the crystallization of Hertzian spheres

Abstract

The crystallization process of Hertzian spheres is studied by means of molecular dynamics simulations in an NPT ensemble where the total number of particles N, the pressure P, and the temperature T are kept constant. It has been observed that the bond orientational ordering rather than the translational ordering (density) plays a primary role. The crystal polymorphs are determined by the state points. Under the conditions of small supercooling, the system is likely to be nucleated into crystals that have a preference for the metastable bcc structure, which can be regarded as a manifestation of the Alexander-McTague mechanism. In contrast, small nuclei are found to have a preference for fcc symmetry under conditions of a high degree of supercooling. Prestructured precursors that act as seeds and wet on the nuclei during nucleation always have a high degree of bcc-like ordering, despite different state points. The results above may provide a clue to the understanding of the crystallization process in core-softened particles.

Crystal nucleation and metastable bcc phase in charged colloids: A molecular dynamics study

“…As a fundamental process of crystallization, the formation of precursors has been observed in various systems. 4,[8][9][10][11][12][13]16,17 In the picture of precursor-mediated crystallization, precursors act as the seeds of nucleation. Before the nucleation takes place, some clusters of precursors appear first in the system.…”

“…7). As previous studies have suggested that there are correlations between the location of critical nucleus and Q 6 order parameter, 13,16,18,20,31 we define the liquid particles with the character of Q 6 ≥ 0.3 as the precursors. Notice here that the threshold of Q 6 for precursors is just chosen arbitrarily, and that it can of course be chosen as other values (e.g., 0.28 and 0.29) close to 0.3.…”

“…Finally we would like to mention that such a relation between the presence of a fcc-bcc boundary and the crystallization process also happens in other systems, e.g., a similar competition in rare gas systems, 49 Gaussian core-model system, 50 and Hertzian spheres. 16…”

“…Among those previous studies discussing the roles of precursors on crystal nucleation, there are basically three types of preordering: 29 crystallization involving composition fluctuations, [4][5][6][7][8][9] the role of density fluctuations in crystal nucleation, 10,11 and the roles of orientational ordering in crystal nucleation. 12,13,[16][17][18]20,30,31 Tanaka and co-workers, based on their observations of crystal nucleation, even suggested further that a typical crystallization process was not a discrete two-step process but a continuous one at the microscopic level. 20 In order to study the nucleation mechanism, here we consider the charged colloidal suspensions, as they have larger temporal and spatial scales than atoms and molecules making it easier to observe the nucleation process due to much more measurement means available.…”

Polymorph selection during crystallization of a model colloidal fluid with a free energy landscape containing a metastable solid

“…FCC-like particles are those withw 6 ≤ 0 and w 4 ≤ 0. This useful assignment criteria to distinguish different solid-like local environments in the metastable fluid phase has been used extensively in computer simulations of model atomic and molecular systems [47][48][49][50].…”