On the possibility of quantum‐chemical cluster approach to the description of structural phase transition in H‐bonded materials on the example of KDP

Abstract

Vibronic theory of heteroligand systems is applied to deal with the model pseudospin Hamiltonian (PSH) that describes structural phase transitions of the KDP-family ferroelectrics. In this approach, the PSH consists of three terms, describing proton-proton interactions, the potential energy of lattice oscillators, and the interaction between these two subsystems. The Ising form for the configuration energies of the Bethe cluster method (where the PSH's parameters depend explicitly on electronic structure and on vibronic constants of the AO 4 -containing structural units) remains valid up to the second order of perturbation theory. The Ising theory with the PSH parameters calculated by means of quantum chemical methods is applied further to obtain the estimates of thermodynamic characteristics, in particular, the critical temperature T c of the structural phase transition.