Résumé:

Mechanism of adsorption of nitrogen multilayers on the basal plane of graphite has been studied using Monte Carlo method. To account for multi-phase equilibrium large scale (~6000 molecules) simulations have been carried out in canonical ensemble, for surface coverage equivalent to more than four layers. To get more insight in the metastability aspect of the simulated situation and better understanding of the experimental phase diagram, the simulations were performed starting from liquid-like disordered configurations and progressively lowering the temperature from 100 K to 10 K. The analysis is focused on the system spatial heterogeneity and its influence on structures and phase transitions. An intricate phase situation is observed due to the competition between intermolecular and N2-graphite interactions. The multilayer structure is strongly spatially non-uniform, the molecular arrangement in each individual layer changing from herringbone in the first layer to pinwheel in the forth one. The commensurate multilayer structure is only metastable at low temperatures. The stable phase has still triangular symmetry within each layer, but with molecular packing that is 1.08 times denser than the commensurate one and is stabilized by the N2-N2 interactions.. 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 layer position with respect to graphite. We observed that layers formed on freezing had higher than commensurate density. Certainly, such an effect can be observed only if surface coverage is larger than monolayer: a single layer is stabilized by the strong N2-graphite interaction and is always commensurate with graphite: this structure seems to be the most stable one. However, in the multilayer systems the intermolecular interactions overcome the influence of graphite corrugation. The final structure results from the competition between the interaction with graphite (which tries to stabilize the herringbone structure) and the 3D intermolecular forces (which favor cubic -phase). In consequence, the pinwheel-like arrangement is observed in higher layers as a part of molecular order in the <111> crystallographic planes of cubic structure. These numerical results concord for the first time and explain quantitatively the multi-structure arrangements observed in the experiments.