A lattice gas model for carbon tetrachloride

A lattice gas model of tetrahedral molecules on a bcc lattice was considered in which a lattice site can be either vacant or occupied by a molecule in either of the two orientations in which the bonds of the molecule point toward neighboring lattice sites. If two molecules occupy neighboring lattice sites, they repel with a steric bond–bond repulsion γ if each points a bond toward the other, they attract with an energy ε if exactly one points a bond toward the other, and they attract with an energy δ if neither points a bond toward the other. The Peierls argument was used to prove that an orientationally ordered phase exists in the model at sufficiently low temperatures if 2εmax {4ε,12ε−4δ}. The equation of state and critical point were calculated using a Guggenheim approximation. The parameters in the model were fitted to experiment using the properties of carbon tetrachloride at the critical point. The condensed phase in the model exhibits the same type of short-range orientational ordering proposed by Nishikawa and Murata to exist in liquid carbon tetrachloride.