Two-dimensional polymorphism and melting in a monolayer of nitric oxide adsorbed on graphite

Abstract

Three two-dimensional (2D) solids γ, δ and β have been observed at low temperature by neutron scattering in the first monolayer of nitric oxide adsorbed on graphite. The structure of two of these solids, y and β, has been determined. In both cases, nitric oxide molecules are dimerized, but in the β phase N2O2 molecules stand up whereas in the y solid they lie down on the substrate. The β and y configurations are analogous to the packing of two planes of the nitric oxide bulk crystal namely the (001) plane for the β phase and the (101) plane for the y solid. The intermediate δ structure is not resolved but a possible unit cell is given. Increasing the temperature results in a melting of the 2D solids. A particular emphasis is made on the β-liquid transition which is shown to be first order. Other first order transitions are also observed like the β solid ⇄ δ solid, liquid ⇄ gas and y solid ⇄ gas transformations. All these results combined with previous adsorption isotherm measurements are put together in a proposed 2D phase diagram displaying three triple points, one critical temperature and one tricritical point