Two-dimensional ice-type vertex model with two types of staggered sites II. A system of two interacting modified KDP models

An exact solution is obtained in the thermodynamic limit for the free energy of a system of two interacting modified KDP models of a ferroelectric. Addition of an interaction between the models does not change the nature of the phase transition, but leads to a shift of the transition temperature by an amount proportional to the interaction. It is shown that there exists a prefluctuation region in which the interaction qualitatively changes the temperature dependence of the specific heat. The obtained exact results confirm the predictions of the phenomenological theory on the influence of perturbations on phase transitions.