Thermodynamic behavior of the structures and magnetic order of O2 monolayers on graphite

A Monte Carlo procedure with continuously deformable boundary conditions is used to calculate the thermodynamic behavior of O2 monolayers on graphite. At temperatures 0≤T≤11.9 K, the predicted structural and magnetic properties of the equilibrium high density monolayer is exactly as is observed for ε-O2 and, at T=11.9±0.1 K, a structural and magnetic phase transition into the zeta phase triangular structure with quasihelical magnetic order is predicted. The calculated low density structure is identical to the observed δ phase in the range 0≤T≤14 K, above which orientational disorder occurs, apparently contrary to experimental observations. It is shown that the magnetic character can be understood in terms of a series of nearly uncorrelated, linear, antiferromagnetic chains along the monolayer b axis. A discussion of the success and failures of this calculation and the conclusions that can be drawn from it, is presented.