TWO-DIMENSIONAL POLYMORPHISM AND MELTING IN A MONOLAYER OF NITRIC-OXIDE ADSORBED ON GRAPHITE

Three two-dimensional (2D) solids gamma , delta and beta were observed at low temperature by neutron scattering in the first monolayer of nitric oxide adsorbed on graphite. The structure of two of these solids, gamma and beta , was determined. In both cases, nitric oxide molecules are dimerized, but in the beta phase N//2O//2 molecules stand up whereas in the gamma solid they lie down on the substrate. The beta and gamma configurations are analogous to the packing of two planes of the nitric oxide bulk crystal. The intermediate delta 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 beta -liquid transition which is shown to be first order. Other first order transitions are also observed like the beta solid reversible reaction delta solid, liquid reversible reaction gas and gamma solid reversible reaction 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.