Melting transition and properties of the plastic crystal and fluid phases ofN2deposited on graphite

A Monte Carlo method using both constant-pressure and constant-surface-density ensembles, with free surface and deformable periodic boundary conditions is employed to examine the melting and orientational behavior of partial and complete monolayers of ${\mathrm{N}}_2$ on graphite. A simple argument concerning the ability of these systems, or lack of it, to fluctuate and expand in the plane of the substrate is sufficient to explain the drastic change in melting temperature ${\mathit{T}}_{\mathit{M}}$, as the adlayer surface density is increased toward monolayer completion. The calculated average orientation of the ${\mathrm{N}}_2$ molecules is parallel to the substrate plane at all temperatures investigated, 30≤T≤95 K, although out-of-plane orientational fluctuations increase substantially with increasing T. The nature of the melting transition of partial and complete monolayers is discussed, and it is shown that a small 256-molecule patch with free-surface boundary conditions exhibits nearly asymptotic behavior. Various order parameters and distributions are calculated to interpret system behavior.