Phase Transition in a Sixteen-Vertex Lattice Model

We consider a sixteen-vertex ferroelectric model defined on a square lattice and deduce the nature of the phase transition in this model from its equivalence with an Ising model with a nonzero magnetic field. We find that, as the value of a parameter which occurs linearly in the Hamiltonian is varied, the model may exhibit a first-order transition, a second-order transition with an infinite specific heat, or no phase transition.