Molecular-dynamics investigation of surface-induced melting in sulfur hexafluoride

Abstract

The thermal disordering of the three low-index faces of a molecular crystal: sulfur hexafluoride has been investigated by molecular dynamics simulation. The reliability of the model system used in this work has been tested several times in earlier works. Here, the triple point temperature Tm′ observed in the larger sample (1500 molecules) is equal to 225±5 K as compared to the calorimetric value of Tm=223 K. The following conclusions can be drawn from the temperature dependence of the structural and dynamics properties. A quasi-liquid-layer (QLL), having some liquidlike properties (for instance, translational mobility), but retaining some features of the solid (site-to-site molecular jumps and some long-range order), is observed in the temperature range of TQLL*<T*<TL*. Above TL* (very near the triple point), a truly liquid layer wets the solid surface. The onset of the liquidlike disordering (TQLL*) depends on the orientation of the free surface. More layers are disordered for a less close-packed face near Tm. However, the divergence of the thickness of the liquid layer is similar for three systems having the same size, in agreement with the predictions based on the isotropic nature of the local Van der Waals forces. Our results are basically consistent with the semi-infinite Landau type model of Lipowsky and Speth of surface disordering at a bulk first-order transition. We have shown that surface-induced melting occurs in a realistic polyatomic molecular model system which, on the other hand, does not show vacancy-induced premelting. This provides some evidence that interface-initiated melting not only exists, but might be the dominant mechanism for melting in this kind of material.