Critical and triple point temperatures of the first adsorbed layer of nitric oxide on graphite,boron nitride and lamellar halides

The two-dimensional critical temperature T_c(2D) of the first layer of nitric oxide adsorbed on homogeneous surfaces of graphite, boron nitride, cadmium halides (CdI₂, CdBr₂, CdCl₂) and magnesium bromide has been determined as accurately as possible. A correlation curve between T_c(2D) and the adsorbent crystalline parameter is compared to the correlation curve of this parameter with the two-dimensional triple point temperature T_t(2D) of the same layer on the above substrates and on those previously investigated by Enault and Larher. It appears from this comparison that T_c(2D) is less sensitive than T_t(2D) to the nature of the substrate: the difference between the extreme values of T_c(2D) is 25 K, whereas it is larger than 70 K for T_t(2D). Moreover the lowest values of T_c(2D) correspond to the substrates for which the lowest values of T_t(2D) have also been observed, namely graphite and boron nitride, both standing out by the smoothness of the adsorption potential of their cleavage faces. Furthermore we are led to reconsider the nature of the less dense phase (α phase) which hitherto has been inferred to be a 2D solid essentially composed of dimers lying flat on the surface.