A Monte Carlo study of carbon monoxide layers adsorbed on ionic substrates: Structures and phase transitions

We have studied the order-disorder phase transitions of carbon monoxide lay ers adsorbed on sodium chloride and lithium flouride substrates using the M etropolis Monte Carlo method. The simulations have been performed in the te mperature range from 5 K to 60 K. At low temperature and monolayer coverage , both of these systems form ordered phases which disorder as the temperatu re is increased. The transition temperature (T-c) is between 30 K and 35 K for CO/NaCl, and from 40 K to 45 K for CO/LiF. Below T-c, both systems have an ordered p(2 x 1) type structure due to correlated azimuthal orientation s. Above T-c, both systems undergo a phase transition to an azimuthally dis ordered p(1 x 1) structure, i.e. one with no preferred orientation in the s urface plane. The heat capacity shows a characteristic divergence at the tr ansition temperature. Coverages of less than a monolayer of the CO/NaCl sys tem have also been studied. The CO molecules are found to aggregate and for m islands with an ordered structure in the middle of the islands. These isl ands also undergo an order-disorder transition but at lower temperatures. M ultilayer systems were found to destabilize the p(2 x 1) structure of the b ottommost layer in favor of a p(1 x 1) structure with the upper layers adop ting the bulk structure.