Mechanism of the fcc-hcp Phase Transformation in Solid Ar

TL;DR

This study uses molecular dynamics simulations to elucidate the atomistic mechanisms of the fcc-hcp phase transition in solid argon, revealing defect accumulation, stacking disorder, and high energy barriers that explain experimental inaccessibility.

Contribution

It provides a detailed atomistic pathway and identifies the role of stacking faults and defects in the phase transition, which was previously not well understood.

Findings

Three transition types identified based on lattice deformation.

High enthalpy barrier explains sluggish transition at low pressure.

Transition mechanism differs from that in argon nanoclusters.

Abstract

We present an atomistic description of the {\it fcc}--to--{\it hcp} transformation mechanism in solid argon (Ar) obtained from transition path sampling molecular dynamics simulation. The phase transition pathways collected during the sampling for an 8000--particle system reveal three transition types according to the lattice deformation and relaxation details. In all three transition types, we see a critical accumulation of defects and uniform growth of a less ordered transition state, followed by a homogeneous growth of an ordered phase. Stacking disorder is discussed to describe the transition process and the cooperative motions of atoms in \{111\} planes. We investigate the nucleation with larger system. In a system of 18000--particles, the collective movements of atoms required for this transition are facilitated by the formation and growth of stacking faults. However the enthalpy barrier is still far beyond the thermal fluctuation. The high barrier explains previous experimental observations of the inaccessibility of the bulk transition at low pressure and its sluggishness even at extremely high pressure. The transition mechanism in bulk Ar is different from Ar nanoclusters as the orthorhombic intermediate structure proposed for the latter is not observed in any of our simulations.