First-order phase transition in order-disorder ferroelectrics

A four-spin coupling term is introduced to the pseudospin model Hamiltonian to account for the recently observed first-order phase transition in order-disorder ferroelectrics. The critical behavior is investigated and characteristic temperatures are calculated. We have also expressed the free energy in terms of microscopic variables and demonstrated that, in the limit of small mean spin 〈$S^2$〉, its behavior near the critical point is identical to what is expected in thermodynamical theory.