| Popis: |
The authors use synchrotron X-ray diffraction for structural analysis of the behavior of multilayer nitrogen films physisorbed on graphite foam. They provide structural information and concentrations of two- and three-dimensional solid phases at a coverage of {Theta} = 8 ML ({Theta} {equivalent_to} 1 ML for a {radical}3 {times} {radical}3 monolayer structure) for temperatures from below the bulk {alpha}-{beta} transition temperature [T{sub {alpha}-{beta}} = 34 {+-} 0.5 K] to above the bulk triple point [T{sub tp} = 63 K]. Data indicate that layering begins near T{sub {alpha}-{beta}}, with subsequent layering occurring as the temperature is raised; all bulk nitrogen forms disordered film layers by 48 K at {Theta} = 8 mL. 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 {alpha}-{beta} structural transition. The effect of adsorption on a graphite from 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. |
