All Physics Faculty Publications
Document Type
Article
Journal/Book Title/Conference
Physical Review B
Volume
39
Issue
14
Publisher
American Physical Society
Publication Date
5-1989
First Page
10331
Last Page
10341
Abstract
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 N2 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 N2 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 N2-graphite system which is consistent with both heat-capacity and neutron-diffraction data.
Recommended Citation
S.- K. Wang, J. C. Newton, R. Wang, H. Taub, JR Dennison and H. Shechter,"Multi-layer Structure of Nitrogen Adsorbed on Graphite," Phys. Rev. B 39, 10331 (1989).
Comments
http://link.aps.org/doi/10.1103/PhysRevB.39.10331
Published by the American Physical Society in Physical Review B. Publisher PDF is available for download through the link above.