MOLECULAR-DYNAMICS SIMULATIONS OF THE MELTING OF CF4 ADSORBED ON GRAPHITE

A potential model has been developed to study the behaviour of CF, ads orbed on graphite. To avoid the normal problems in simulating commensu rate/incommensurate phase transitions, we have studied a patch of 400 molecules in the centre of a periodically repeating box at a total cov erage of 1/2. The model patch melts, dose to the experimentally predic ted temperature, from the correct (2 x 2) commensurate solid structure . At the melting temperature the average intermolecular separation in the centre of the patch is 4.92 Angstrom, which is that of the commens urate (2 x 2) structure. The order parameter which measures order with respect to the perfect (2 x 2) lattice fans sharply at 75 K. The ener gy minimisations have shown that an important characteristic of this s ystem is the low barrier to translation, for tripod-down molecules, be tween the bridge and atop sites. Variations in the order parameter O-1 , which measures the order of the molecules with respect to the surfac e, indicate that a patch of molecules is not strongly pinned to the su rface. The molecules at the centre of the patch are observed to make c oncerted moves between the edge and bridge sites during the course of simulations. At all temperatures the molecules are observed to be orie ntationally ordered. Below the melting temperature there is orientatio nal order both in the plane and out of the plane of the surface. Molec ules prefer to be tripod-down, and there is a preferred direction in t he plane of the surface for the CF bonds pointing towards the surface. The order parameter which measures in-plane order decays to zero appr oximately 3 K below the observed translational melting temperature, in dicating that any orientationally disordered phase must exist over a s mall temperature range just prior to translational melting. Above the melting temperature the in-plane distribution curves shown some in-pla ne ordering of the molecules, which is induced by the surface corrugat ion 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 orientat ion. The simulations show no evidence of the phase transitions below 7 3 K observed in the calorimetric studies.