Slow and fast relaxation times of quantum lattice model with local multi-well potentials: phenomenological dynamics for Sn${}_2$P${}_2$S${}_6$ ferroelectric crystals

As a continuation of the previously published work [Velychko O. V., Stasyuk I. V., Phase Transitions, 2019, 92, 420], a phenomenological framework for the relaxation dynamics of quantum lattice model with multi-well potentials is given in the case of deformed Sn${}_2$P${}_2$S${}_6$ ferroelectric lattice. The framework is based on the combination of statistical equilibrium theory and irreversible thermodynamics. In order to study these dynamics in a connected way we assume that the dipole ordering or polarization ($\eta$) and volume deformation ($u$) can be treated as fluxes and forces in the sense of Onsager theory. From the linear relations between the forces and fluxes, the rate equations are derived and characterized by two relaxation times (${\tau }_S,{\tau }_F$) which describe the irreversible process near the equilibrium states. The behaviors of ${\tau }_S$ and ${\tau }_F$ in the vicinity of ferroelectric phase transitions are studied.