PHASE-TRANSITIONS IN A SPIN-1 MODEL WITH PLAQUETTE INTERACTION ON THESQUARE LATTICE

An extension of the Blume-Emery-Griffiths model with a plaquette four- spin interaction term, on the square lattice, is investigated by means of the cluster variation method in the square approximation. The grou nd state of the model, for negative plaquette interaction, exhibits se veral new phases, including frustrated ones. At finite temperature we obtain a quite rich phase diagram with two new phases, a ferrimagnetic and a weakly ferromagnetic one, and several multicritical points.