triquadratic interactions, single-ion anisotropy and magnetic )eld: exact phase diagrams

We consider a S = 1 kagom! e Ising model with triquadratic interactions around each triangular face of the kagom! e lattice, single-ion anisotropy and an applied magnetic )eld. A mapping establishes an equivalence between the magnetic canonical partition function of the model and the grand canonical partition function of a kagom! e lattice-gas model with localized three-particle interactions. Since exact phase diagrams are known for condensation in the one-parameter lattice-gas model, the mapping directly provides the corresponding exact phase diagrams of the three-parameter S = 1 Ising model. As anisotropy competes with interactions, results include the appearance of con9uent singularities e:ecting changes in the topology of the phase diagrams, phase boundary curves (magnetic )eld vs. temperature) with purely positive or negative slopes as well as intermediate cases showing non-monotonicity, and coexistence curves (magnetization vs. temperature) with varying shapes and orientations, in some instances entrapping a homogeneous phase. c © 2002 Elsevier Science B.V. All rights reserved. PACS: 05.50.+q; 75.10.−b