We present a study of the phase diagram of molecules adsorbed on a triangular lattice, with quadrupole-quadrupole interactions and including the presence of vacancies. We analyze the effects of the short-range correlations with use of the cluster variational method, and we find that the pinwheel phase, which was only marginally stable in the mean-field approximation, is stabilized in a finite region of the temperature-density phase diagram. However, the long-range correlations, which are still missing in our approximation, may produce important changes in the phase diagram.

• Received 20 June 1990

DOI:https://doi.org/10.1103/PhysRevB.42.8571