Phase transitions in lattice gases of hard-core molecules having two orientations

A number of hard-core lattice gases in which a lattice site can be occupied by a molecule in either of two possible orientations are proved to undergo order-disorder phase transitions. Examples include lattice gases of trigonal planar molecules on a triangular lattice, tetrahedral molecules on a bodycentered cubic lattice, and linear molecules on a square lattice.This research was supported by The Robert A. Welch Foundation Grant P-446 and by the TCU Research Foundation.