Two-stage crystallization of charged colloids at low supersaturations

TL;DR

This study uses simulations to reveal that charged colloids undergo a two-stage crystallization process involving liquid to bcc and then bcc to fcc/hcp phases, driven by order fluctuations and influenced by contact energy.

Contribution

It demonstrates the two-stage nucleation pathway and the role of free energy barriers, highlighting the heterogeneous nature of the bcc-fcc transition in homogeneous systems.

Findings

Order fluctuations drive nucleation, not density.

Two-stage transition: liquid to bcc, then bcc to fcc/hcp.

Barrier heights depend on contact energy, affecting transition spontaneity.

Abstract

We report simulations on the homogeneous liquid-fcc nucleation of charged colloids for both low and high contact energy values. As a precursor for crystal formation, we observe increased local order at the position where the crystal will form, but no correlations with the local density. Thus, the nucleation is driven by order fluctuations rather than density fluctuations. Our results also show that the transition involves two stages in both cases, first a transition liquid-bcc, followed by a bcc-hcp/fcc transition. Both transitions have to overcome free energy barriers, so that a spherical bcc-like cluster is formed first, in which the final fcc-like structure is nucleated mainly at the surface of the crystallite. This means that the bcc-fcc phase transition is a heterogeneous nucleation, even though we start from a homogeneous bulk liquid. The height of the bcc-hcp/fcc free energy barrier strongly depends on the contact energies of the colloids. For low contact energy this barrier is low, so that the bcc-hcp/fcc transition happens spontaneously. For the higher contact energy, the second barrier is too high to be crossed spontaneously by the colloidal system. However, it was possible to ratchet the system over the second barrier and to transform the bcc nuclei into the stable hcp/fcc phase. The transitions are dominated by the first liquid-bcc transition and can be described by Classical Nucleation Theory using an effective surface tension.