Slow and fast relaxation times of quantum lattice model with local multi-well potentials: phenomenological dynamics for Sn2P2S6 ferroelectric crystals

Abstract

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 Sn2P2S6 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 (η) 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 (τS , τF) which describe the irreversible process near the equilibrium states. The behaviors of τS and τF in the vicinity of ferroelectric phase transitions are studied.

Description of CuInP₂S₆ ferrielectrics in a mixed Ising model

“…The temperature behavior of the Sn 2 P 2 S 6 ferroelectric was explained within the Blume-Capel model with pseudospin 𝑆 = 1 related to the local three-well potential for the spontaneous polarization fluctuations [1][2][3]. In the case of the CuInP 2 S 6 crystal, the mixed Ising model [4,5] with spins 𝑠 = 1/2 and 𝑆 = 1, that are related to the ordering dynamics of Cu + cations in the double-well potential and to the ordering/displacive dynamics of In 3+ cations in the three-well local potential can be applied.…”