Ferroelectricity, Nonlinear Dynamics, and Relaxation Effects in MonoclinicSn2P2S6

Abstract

An ab initio-based model of the temperature-induced ferroelectric phase transition in Sn2P2S6 (SPS) as a prototype of an unconventional ferroelectric is developed. The order parameter in SPS is found as the valley line on a total-energy surface of the zone-center fully symmetrical Ag and polar Bu distortions. Significant nonlinear coupling between order parameter and strain is observed. Monte Carlo simulations describe the additional low-temperature rearrangement in polar structure, which appears in domain boundaries, and describe the relaxation phenomena near the ferroelectric phase transition.

Atomic-scale evolution of modulated phases at the ferroelectric–antiferroelectric morphotropic phase boundary controlled by flexoelectric interaction

“…Second, we note that the observation of regular, pronounced modulations in the polarization and strain fields is inconsistent with the adaptive phase model of coexistent domains of different phases. Rather, the formation of the modulated structures as seen in Figs 2 and 3 is consistent with the behaviour of ferroics with negative wall energy, such as those in the incommensurate phases [37][38][39][40][41] . Here, we explore possible origins and implications of this behaviour.…”

“…While the notion of an interface with a negative energy is at the first glance paradoxical, similar concepts were previously used for description of incommensurate ferroic phases [37][38][39][40][41] , intercalation compounds 42,43 , crystallographic shear phases 44 and charge density waves. Essentially, an intrinsic wall instability predicts transition to the modulated phase where interfaces are uniformly distributed in space (that is, emergence of the long-range modulated phase as the thermodynamic ground state of the system), as opposed to formation of two-phase mixtures [45][46][47] .…”

Valence fluctuations in Sn(Pb)2P2S6 ferroelectrics

“…Earlier [1,2], for Sn 2 P 2 S 6 ferroelectrics the stereoactivity of Sn 2+ cations, which are placed inside sulfur octahedron, was only considered as origin of local dipoles that exist already in paraelectric phase, because tin cations occupy general position in the monoclinic elementary cell. This stereoactivity is determined by covalency between tin atomic orbitals and molecular orbitals of P 2 S 6 groups, and can be considered as second order Jahn-Teller effect, which involve phosphorous orbitals also.…”

“…Continuous phase transition can be described as second order Jahn-Teller (SOJT) effect which is related to the stereoactivity of Sn 2+ cations placed inside of sulfur ions polyhedron [1,2]. For the Sn 2 P 2 S 6 crystal lattice ( Fig.…”

“…For Sn 2 P 2 S 6 ferroelectrics, as follows from the first principles calculations [1], the spontaneous polarization is determined by three-well local potential for the order parameter fluctuations. Continuous phase transition can be described as second order Jahn-Teller (SOJT) effect which is related to the stereoactivity of Sn 2+ cations placed inside of sulfur ions polyhedron [1,2].…”