Molecular-dynamics studies of crystal defects and melting.

Abstract

The technique of repeated quenching and relaxation is used to reveal the equilibrium structure of a superheated crystal undergoing the melting transition. The structure evolution seems to start with an atom of unusually large Voronoi volume in the hot configuration. At an appropriate moment, the position of this atom becomes vacant in the quenched configuration with the atom occupying the site of its nearest neighbor. This begins a chain of nearest-neighbor displacements. The displacement chain usually starts with a vacancy and ends with a pair of split interstitials. It can grow from both ends and can be very long, such as 64 atoms. Sometimes, the chain can be closed when the two ends meet with the annihilation of the interstitial-vacancy pair. New displacement chains can be also formed. No line defects appear in the quenched structure. Eventually, a critical configuration is reached after which the quenched structure is suddenly liquidlike instead of crystalline. The critical configuration is characterized by the Lindemann ratio (average displacement to lattice spacing) of 0.12 in the hot configuration. At this ``point of no return'' the system is already thermodynamically ``liquid,'' but the quenched structure has not revealed that until now. The point of no return seems to appear suddenly. Two femtoseconds earlier, the quenched structure is still a single crystal with only a few point defects. The hotter the superheated crystal, the shorter is the total time required for melting.