  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.
