The unified model description of order-disorder and displacive structural phase transitions

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.