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

Area of Science:

• Colloid and Surface Science
• Materials Science
• Computational Physics

Background:

• Homogeneous nucleation is a fundamental process in phase transitions.
• Charged colloids offer a model system for studying nucleation dynamics.
• Ostwald's step rule and the Alexander-McTague mechanism describe precursor ordering in nucleation.

Purpose of the Study:

• To investigate the dynamic process of homogeneous nucleation in charged colloids.
• To determine if the liquid-solid transition involves a metastable body-centered cubic (bcc) phase.
• To understand the factors governing polymorph selection during colloidal crystallization.

Main Methods:

• Brute-force molecular dynamics simulations were employed.
• Simulations were conducted at state points within the thermodynamically stable face-centered cubic (fcc) phase region.
• Analysis focused on the symmetry of preordered precursors and final crystal structures.

Main Results:

• Preordered precursors consistently showed predominant bcc symmetry, aligning with theoretical predictions.
• The final crystal structure was not solely determined by precursor symmetry, varying with state points.
• A narrow region of state points favors the formation of sufficiently large bcc structures.

Conclusions:

• The study confirms the presence of bcc-symmetric precursors during homogeneous nucleation in charged colloids.
• Polymorph selection is complex and depends on specific state point conditions.
• The limited occurrence of large bcc structures explains their rare detection in macroscopic experiments.