IS THERE A VACANCY-INDUCED PREMELTING IN A MOLECULAR-CRYSTAL

Molecular-dynamics simulations performed on a crystalline sample of sulphur hexafluoride with periodic boundary conditions and containing up to 6% of point defects suggest that, at least in this case, the answer to the question posed in the title is no. Melting of our surface-free systems is observed at temperatures above the thermodynamic melting point and occurs through a thermoelastic instability of the crystalline phase. Insertion of vacancies affects the elastic properties of the crystal and thus lowers the melting point but no premelted regions (local liquidlike disorder) are observed. This supports the idea that melting in true crystals is solely initiated at extended defects, such as grain boundaries or free surfaces.