Synchrotron X-ray Diffraction Studies of Layering Transitions of Multi-layer Nitrogen Physisorbed on Graphite
We use synchrotron X-ray diffraction for structural analysis of the behavior of multilayer nitrogen films physisorbed on graphite foam. We provide structural information and concentrations of two- and three-dimensional solid phases at a coverage of ϑ = 8 ML (ϑ ≡ 1 ML for a √3 × √3 monolayer structure) for temperatures from below the bulk α−β transition temperature [Tα-β = 34 ± 0.5 K] to above the bulk triple point [Ttp = 63 K]. Our data indicate that layering begins near Tα-β, with subsequent layering occurring as the temperature is raised; all bulk nitrogen forms disordered film layers by 48 K at ϑ = 8 mL. Our results are consistent with ellipsometry studies of nitrogen on highly oriented pyrolytic graphite which found multilayer nitrogen on single-crystal graphite to undergo a series of layering transitions above the bulk nitrogen α−β structural transition. [Volkmann, U. G.; Knorr, K. Phys. Rev. Lett. 1991, 66, 473.] The effect of adsorption on a graphite foam substrate, which results in capillary condensation and finite size effects, is limited to a broadening and overlap of the discrete transition temperatures observed on a single-crystal substrate. A phase diagram for coverages above 2 ML is proposed, summarizing this and previous work.