Nucleation and structural growth of cluster crystals

TL;DR

This study investigates the microscopic mechanisms of cluster crystal nucleation in a generalized exponential model, revealing a preference for bcc structures during nucleation and analyzing particle mobility in different phases.

Contribution

It provides detailed molecular dynamics analysis of cluster crystal nucleation, comparing free energy barriers with classical theory, and introduces methods to distinguish crystal structures.

Findings

Nucleation favors bcc cluster crystals over fcc, even when fcc is thermodynamically stable.

Free energy calculations align with classical nucleation theory predictions.

Particle mobility in cluster crystals can be modeled by a reaction-diffusion process.

Abstract

We study the nucleation of crystalline cluster phases in the generalized exponential model with exponent n=4. Due to the finite value of this pair potential for zero separation, at high densities the system forms cluster crystals with multiply occupied lattice sites. Here, we investigate the microscopic mechanisms that lead to the formation of cluster crystals from a supercooled liquid in the low-temperature region of the phase diagram. Using molecular dynamics and umbrella sampling, we calculate the free energy as a function of the size of the largest crystalline nucleus in the system, and compare our results with predictions from classical nucleation theory. Employing bond-order parameters based on a Voronoi tessellation to distinguish different crystal structures, we analyze the average composition of crystalline nuclei. We find that even for conditions where a multiply-occupied fcc crystal is the thermodynamically stable phase, the nucleation into bcc cluster crystals is strongly preferred. Furthermore, we study the particle mobility in the supercooled liquid and in the cluster crystal. In the cluster crystal, the motion of individual particles is captured by a simple reaction-diffusion model introduced previously to model the kinetics of hydrogen bonds.