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Synchrotron X-ray Diffraction Studies of Layering Transitions of Multi-layer Nitrogen Physisorbed on Graphite

T. E. Burns
JR Dennison, Utah State University
S. N. Ehrlich

Published by the American Chemical Society in Langmuir. Publisher PDF is available through the link above. Publisher requires a subscription to access the article.


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.