Calculated properties of monolayer and multilayerN2on graphite

The properties of monolayer and multilayer deposits of ${\mathrm{N}}_2$ on graphite are calculated using a pattern optimization of the total lattice energy and a Monte Carlo procedure with continuously deformable periodic boundary conditions. It is confirmed that the monolayer ground state is a 2 √3 × √3 registered in-plane herringbone structure and the molecular orientations with respect to the substrate are in good agreement with experiment, as is the predicted orientational order-disorder transition at T=25±2 K. As the surface density is increased above its registered value (ρ=1), a uniaxial incommensurate (UI) phase is found which persists until ρ≃1.06, at which point the monolayer is in coexistence with the bilayer. The top layer of this bilayer, which is complete at ρ=2.2, forms a ‘‘pinwheel’’ structure much like the (111) plane of bulk α-${\mathrm{N}}_2$, and the bottom layer is a somewhat distorted herringbone. It is established that the common tangent between the UI monolayer and the bilayer does not intercept the monolayer pinwheel phase, which is therefore never physically realized. It is also shown that the bilayer, trilayer, and bulk are essentially in coexistence with one another and thus bulk formation should occur at densities above bilayer completion.