Role of bond orientational order in the crystallization of hard spheres

TL;DR

This study uses computer simulations to demonstrate that in hard sphere crystallization, bond orientational order fluctuations, rather than density fluctuations, drive nucleation, challenging the two-step nucleation paradigm.

Contribution

It reveals that bond orientational order, not density, is the primary factor in nucleation, contradicting previous two-step models for fluids without liquid-liquid phase separation.

Findings

Nucleation driven by orientational order fluctuations

Orientational order precedes positional order in nuclei

Challenges the two-step nucleation mechanism

Abstract

With computer simulations of the hard sphere model, we examine in detail the microscopic pathway connecting the metastable melt to the emergence of crystalline clusters. In particular we will show that the nucleation of the solid phase does not follow a two-step mechanism, where crystals form inside dense precursor regions. On the contrary, we will show that nucleation is driven by fluctuations of orientational order, and not by the density fluctuations. By considering the development of the pair-excess entropy inside crystalline nuclei, we confirm that orientational order precedes positional order. These results are at odd with the idea of a two-step nucleation mechanism for fluids without a metastable liquid-liquid phase separation. Our study suggests the pivotal role of bond orientational ordering in triggering crystal nucleation.