Two-dimensional phase changes of nitric oxide adsorbed on lamellar halides. Two-dimensional triple points

Adsorption isotherms of nitric oxide on the cleavage face of fourteen lamellar halides (NiCl2, CoCl2, FeCl2, NiBr2, CoBr2, FeBr2, MgBr2, CdCl2, Col2, CdBr2, MnI2, CdI2, CaI2 and PbI2) have been determined. Adsorbents having a uniform surface have been prepared by sublimation in a rapid current of dry nitrogen. The isotherms show vertical steps corresponding to two-dimensional phase transitions. Except for CaI2, a crystalline monomolecular layer β constituted of dimers oriented perpendicularly to the surface of the substrate, is formed. At low temperatures this phase is formed directly during a two-dimensional condensation. Above the temperature of a two-dimensional triple point, a crystalline layer α, where the dimers lie flat on the surface, appears as an intermediate phase. The variations of the two-dimensional triple point temperature and of the free energy of the β phase with the nature and the crystal parameter of the adsorbent are explained