Structural transformations of nitrogen adsorbed on graphite: Monte Carlo studies of spatial heterogeneity in multilayer system.

We present numerical Monte Carlo studies of nitrogen multilayers adsorbed on the basal plane of graphite. The analysis is focused on the system spatial heterogeneity and its influence on structures and phase transitions. The simulations have been carried out for surface coverage from monolayer to four layers, in canonical ensemble, in the temperature range from 5 to 100 K. An intricate phase situation is observed due to the competition between intermolecular and N(2)-graphite interactions. The commensurate monolayer is stabilized by the graphite corrugation. The multilayer commensurate structure is only metastable at low temperatures. Its stable structure is triangular, 1.08 times denser than the commensurate one stabilized by the N(2)-N(2) interactions. The multilayer structure is strongly spatially nonuniform, the individual layer structure changes from herringbone in the first layer to pinwheel arrangement in the fourth one. Two structural phase transitions, orientational order-disorder, and melting, are observed in each layer. Their mechanism and transition temperatures show strong variations depending on the position of the layer and the number of layers in the system.