Contribution of vibronic interactions to the thermodynamics of the phase transition in H-bonded ferroelectrics: Proton-lattice coupling effects in crystals of the KDP family

Abstract

A model Hamiltonian for microscopic description of structural phase transitions in KDP type ferroelectrics was considered in terms of the vibronic theory of heteroligand systems (VTHS). The Hamiltonian contains three terms that describe the proton interactions, the potential energy of lattice oscillators, and the relationship between these subsystems. In the second order perturbation theory, the configuration energies of the Bethe cluster method retained the Ising form, with the pseudospin Hamiltonian parameters explicitly depending on the electronic structure characteristics and the vibronic constants of the AO4-containing structural units. The Ising version of the theory was used for obtaining the numerical estimations of the thermodynamic properties, in particular, the critical temperature Tc as a function of all the parameters of the pseudospin Hamiltonian.