(253ah) Mechanism of the Fcc-Hcp Phase Transformation in Solid Ar

We present an atomistic description of the mechanism of the fccâ??toâ??hcp transformation in solid argon obtained from molecular dynamics transition path sampling simulations. We show that the transition barrier at 40 K under ambient pressure is close to the thermal energy per atom. The analysis of the ensemble of reactive trajectories collected during the sampling revealed three transition types. The average coordination number is used as order parameter and to monitor the progress of the transformation. In each transition type, underâ?? and overâ??coordinated atoms form and distribute uniformly throughout the lattice during the transformation. Stacking disorder is discussed to de- scribe the transition process and the cooperative atomic motions. The high activation energy for collective movements of many atoms explains previous experimental observations of a sluggish bulk transformations and difficulties of observation of the bulk transition at lower temperature and ambient pressure. The transition mechanism in bulk Ar is different from Ar clusters as the orthorhombic intermediate structure proposed for the latter is not observed.