Defect-mediated melting in superheated noble gas crystals.
Molecular dynamics simulations have been used to investigate the mechanisms governing the homogeneous melting of pure noble gases at the limit of superheating. For each chemical species considered, the heterogeneous melting point was estimated by monitoring the thermal behavior of crystalline systems containing a high-angle grain boundary. To determine the limit to superheating, calculations were instead carried out on a perfect crystalline bulk. The temperature was gradually increased to bring the systems within the metastable region above the equilibrium melting point. The static order parameter was employed to monitor the structural disordering during the slow temperature increase and to determine the temperature at which the crystalline lattice collapses to a liquid. Structural disorder was further characterized by studying the appearance of atoms with defective coordination. Their relative number and spatial correlation appeared to play a fundamental role in destabilizing the crystalline lattice bulk and triggering the homogeneous melting. The fraction of atoms with defective coordination and the total length of the stringlike clusters they form in the vicinity of the homogeneous melting point were found to be approximately the same for all of the chemical species considered. These findings have been compared with theoretical predictions.
Delogu
Francesco
F
Dipartimento di Ingegneria Chimica e Materiali, Università di Cagliari, piazza d'Armi, I-09123 Cagliari, Italy. delogu@dicm.unica.it