A series of co-authors’ studies [1-7] devoted to the unified model descriptionudof structural phase transitions (SPT) in ferroelectrics and related materialsudare reviewed and partly innovated.udStarting from a general Hamiltonian of pair-coupled anharmonic (quartic)udoscillators, together with the concept of local normal coordinates, audunified model description of both order-disorder and displacive types ofudSPT-systems is proposed. Within the framework of the standard variationaludprocedure, a hybridized pseudospin-phonon Hamiltonian is formulated byudintroducing variables corresponding to phonon, magnon-like(flipping) andudnonlinear(domain–wall-like) displacements of atoms participating in SPT.udThis is achieved by representing the cooperative atomic motion onto severaludquasiequilibrium positions (in the simplest case, two) as slow tunnellinguddisplacement (decomposed into magnon-like and soliton-like deviations), inudaddition to comparatively fast phonon oscillations around inhomogeneousudmomentary rest positions, in turn induced by domain–wall-like (soliton) excitations.udThe qualitative and quantitative analyses show that SPT (of the first orudsecond order) can be either of a displacive (governed by a phonon softudmode), order-disorder (governed by a tunnelling–magnon-like soft mode)udor of a mixed type, depending on both the coupling energy between atomsudand their zero-point vibrational energy. In the critical temperature region,udthe domain–wall-like excitations bring on the formation of microdomainsud(precursor clusters of the ordered phase) which induce SPT of the Isingudtype universality class. The incomplete softening of the phonon or pseudomagnonudmode occurs and a central peak due to slow cluster relaxationudappears in the spectral density of excitations.
展开▼
