Multilayer structure of nitrogen adsorbed on graphite

Elastic neutron diffraction has been used to study the structure and layering of nitrogen films adsorbed on the (002) surfaces of an exfoliated graphite substrate. The neutron-diffraction pattern of the fully compressed monolayer at a coverage Θ=1.67 layers and a temperature ≲11 K which we reported earlier has been reanalyzed (unity coverage corresponds to a complete layer having the commensurate √3 × √3 structure). We now find it to be consistent with a four-sublattice pinwheel structure as well as the two-sublattice herringbone structure which we found previously. Below 11 K, we infer crystallization of the bilayer at Θ between 2.6 and 3.3 layers. As the coverage is increased further, diffraction peaks from bulk particles are first observed at Θ≃3.7 layers, and their intensity increases linearly with Θ up to 10 layers. It is suggested that two amorphous or highly disordered layers of ${\mathrm{N}}_2$ may adsorb above the fully compressed monolayer prior to bilayer crystallization and a single such layer may adsorb above the bilayer prior to bulk nucleation. From analysis of the diffraction pattern at Θ=3.3 layers, we infer that the bilayer crystal, like the fully compressed monolayer, is slightly distorted from hexagonal symmetry. The two ${\mathrm{N}}_2$ layers are commensurate with each other and have the same density, which is slightly less than that of the fully compressed monolayer. While we are unable to reach a definitive conclusion on the type of orientational ordering in the bilayer, we present some arguments favoring a pinwheel structure, possibly with more orientational disorder in the first layer than in the second. At coverages Θ=6.4 and 8.0 layers, we find no change in the bulk peak intensities at 23.4 K, where a large heat-capacity peak has been observed. We conclude that a layering transition does not occur at this temperature and suggest instead that the heat-capacity peak may result from the melting of the second layer. Based on this interpretation, we propose a new phase diagram for the ${\mathrm{N}}_2$-graphite system which is consistent with both heat-capacity and neutron-diffraction data.