Adsorption of gases on SnS2: trace of order–disorder phase transition of a honeycomb lattice gas

We have made volumetric measurements of adsorption isotherm of Ar, Kr, O2 and N2 on a powder sample of tin disulfide SnS2 in the temperature range of 77–90 K. Ar, Kr and O2 exhibited pressure isotherms unusually bending at the coverage of 0.3–0.4 monolayer, while N2 did not. The isotherms are analyzed by a quasi-chemical approximation theory developed for a honeycomb lattice gas with first nearest neighbor exclusion and with a small adsorption potential difference between the two (1 × 1)-1/2 sublattices. The unusual bending of the pressure isotherms is successfully explained as a trace of the order–disorder phase transition of second order that would take place in the honeycomb lattice gas if the two sublattices had the same adsorption potential. The adsorption potential difference is found to be at most of order of magnitude of the second nearest neighbor interaction energy.