Atomic dynamics and local structural disorder during ultrafast melting of polycrystalline Pd

The primary distinction between solid and liquid phases is mechanical rigidity, with liquids having a disordered atomic structure that allows flow. While melting is a common phase transition, its microscopic mechanisms still remain unclear. This study uses molecular dynamics simulations to investigate ultrafast melting in polycrystalline palladium, focusing on the relationship between atomic dynamics quantified by the root-mean-squared displacement (RMSD) and local structural disorder characterized by the deviation from centrosymmetry. In the crystal bulk, melting is preceded by a gradual rise in the RMSD and local disorder. As the Lindemann limit for the RMSD is approached, the increasing concentration of lattice defects is manifested by a discontinuous rise in disorder. On melting, the rise is followed by a rapid increase in displacement, indicative of atomic flow. In contrast, the grain boundaries undergo melting through a continuous increase of both the displacement and the disorder, resembling a glass transition on heating.