Two-dimensional polymorphism and melting in a monolayer of nitric oxide adsorbed on graphiteReportar como inadecuado




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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.

Keywords : adsorbed layers graphite melting monolayers nitrogen compounds polymorphism





Autor: J.P. Coulomb J. Suzanne M. Bienfait M. Matecki A. Thomy B. Croset C. Marti

Fuente: https://hal.archives-ouvertes.fr/



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