Density dependent structural phase transition for confined copper: origin of the layering.

Confinement presents the opportunity for novel structural transition scenarios not observed in three-dimensional systems. Here, we report a comprehensive molecular dynamic (MD) study of the structural phase transition induced by density for an ordinary metal copper (Cu) confined between two parallel panel walls. At 4.19 g cm-3 < ρ < 4.66 g cm-3, a notable structural phase transition occurs between the triangle unit cell structure and quasi-square unit cell structure upon densification. Both the bond order parameter (BOP) and angular distribution function (ADF) can provide evidence for the transition. We highlight the fact that when the sole decrease of the atom distance cannot adapt to the further densification, the system starts to adjust the neighboring bond angle and promote the layering transition, thus inducing the structural phase transition. At the metastable coexistence zone, the viscosity exhibits a remarkable drop and the diffusion coefficient shows a notable increase, both facilitating the accomplishment of the structural transition. Our results will trigger more interest on the phase transition under confinement in a metallic system.