Unified theory of ferroelectric phase transitions

The unified model theory describing both the "order-disorder" and "displacive" ferroelectric phase transitions is proposed by introducing the model pseudospin-phonon Hamiltonian. On the basis of the self-consistent phonon-field and molecular-field approximations a complete system of self-consistent equations for two order parameters (average displacement of active atoms and average population of atomic equilibrium positions) is obtained. The qualitative discussion as well as the detailed numerical analysis show that the ferroelectric phase transition (first or second order) can be either the order-disorder, displacive, or mixed type, depending on the dimensionless coupling energy between the atoms in the model Hamiltonian.