BLUME-EMERY-GRIFFITHS MODEL ON 3-DIMENSIONAL LATTICES - CONSEQUENCES FOR THE ANTIFERROMAGNETIC POTTS-MODEL

Using the cluster-variation method we study the phase diagram of the Blume-Emery-Griffiths (BEG) model on simple-cubic and face-centered cubic lattices. For the simple-cubic lattice the main attention is paid to reentrant phenomena and ferrimagnetic phases occurring in a certain range of coupling constants. The results are in close agreement with Monte Carlo data, available for parts of the phase diagram. Several ferrimagnetic phases are obtained in the vicinity of the line in parameter space, at which the model reduces to the antiferromagnetic three-state Potts model. Our results imply the existence of three phase transitions in the antiferromagnetic Potts model on the simple-cubic lattice. The phase diagrams for the BEG model on the face-centered cubic lattice are obtained in the region of antiquadrupolar ordering. Also the several ordered phases of the antiferromagnetic Potts model on this lattice are discussed.