Title: THE UNIFIED MODEL DESCRIPTION OF ORDER-DISORDER AND DISPLACIVE STRUCTURAL PHASE TRANSITIONS
Authors: S.Stamenkovic (Institute of Nuclear Sciences, Laboratory for Theoretical Physics and Condensed Matter Physics, Belgrade, P.O.Box 522, Yugoslavia)

A series of co-authors' studies [1-7] devoted to the unified model description of structural phase transitions (SPT) in ferroelectrics and related materials are reviewed and partly innovated. Starting from a general Hamiltonian of pair-coupled anharmonic (quartic) oscillators, together with the concept of local normal coordinates, a unified model description of both order-disorder and displacive types of SPT-systems is proposed. Within the framework of the standard variational procedure, a hybridized pseudospin-phonon Hamiltonian is formulated by introducing variables corresponding to phonon, magnon-like (flipping) and nonlinear (domain-wall-like) displacements of atoms participating in SPT. This is achieved by representing the cooperative atomic motion onto several quasiequilibrium positions (in the simplest case, two) as slow tunnelling displacement (decomposed into magnon-like and soliton-like deviations), in addition to comparatively fast phonon oscillations around inhomogeneous momentary rest positions, in turn induced by domain-wall-like (soliton) excitations. The qualitative and quantitative analyses show that SPT (of the first or second order) can be either of a displacive (governed by a phonon soft mode), order-disorder (governed by a tunnelling-magnon-like soft mode) or of a mixed type, depending on both the coupling energy between atoms and their zero-point vibrational energy. In the critical temperature region, the domain-wall-like excitations bring on the formation of microdomains (precursor clusters of the ordered phase) which induce SPT of the Ising type universality class. The incomplete softening of the phonon or pseudomagnon mode occurs and a central peak due to slow cluster relaxation appears in the spectral density of excitations.

Key words: structural phase transitions, order-disorder,displacive transition
Comments: Figs. 9, Refs. 36, Tabs. 3.