| Popis: |
A potential model has been developed to study the behaviour of CF4 adsorbed on graphite. To avoid the normal problems in simulating commensurate/incommensurate phase transitions, we have studied a patch of 400 molecules in the centre of a periodically repeating box at a total coverage of 1 2 . The model patch melts, close to the experimentally predicted temperature, from the correct (2 × 2) commensurate solid structure. At the melting temperature the average intermolecular separation in the centre of the patch is 4.92 A, which is that of the commensurate (2 × 2) structure. The order parameter which measures order with respect to the perfect (2 × 2) lattice falls sharply at 75 K. The energy minimisations have shown that an important characteristic of this system is the low barrier to translation, for tripod-down molecules, between the bridge and atop sites. Variations in the order parameter O1, which measures the order of the molecules with respect to the surface, indicate that a patch of molecules is not strongly pinned to the surface. The molecules at the centre of the patch are observed to make concerted moves between the edge and bridge sites during the course of simulations. At all temperatures the molecules are observed to be orientationally ordered. Below the melting temperature there is orientational order both in the plane and out of the plane of the surface. Molecules prefer to be tripod-down, and there is a preferred direction in the plane of the surface for the CF bonds pointing towards the surface. The order parameter which measures in-plane order decays to zero approximately 3 K below the observed translational melting temperature, indicating that any orientationally disordered phase must exist over a small temperature range just prior to translational melting. Above the melting temperature the in-plane distribution curves shown some in-plane ordering of the molecules, which is induced by the surface corrugation potential. Energy minimisation calculations indicate that the low-temperature structure for this model is an incommensurate, hexagonally close-packed structure with the molecules in the tripod-down orientation. The simulations show no evidence of the phase transitions below 73 K observed in the calorimetric studies. |
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