Phase diagram of ferroelectrics with tricritical and Lifshitz points at coupling between polar and antipolar fluctuations

TL;DR

This paper investigates the complex phase behavior of certain ferroelectrics with tricritical and Lifshitz points, using a combined BEG-ANNNI model to describe critical phenomena and phase coexistence.

Contribution

It introduces a combined BEG-ANNNI model to describe the critical behavior and phase diagram of ferroelectrics with tricritical and Lifshitz points, including the coexistence of multiple phases.

Findings

Critical behavior described as a crossover between Ising and XY universality classes.

Presence of a chaotic state with coexistence of ferroelectric, paraelectric, and modulated phases.

Strong development of antipolar fluctuations in the paraelectric phase.

Abstract

Static and dynamic critical behavior of Sn$_2$P$_2$S$_6$ type ferroelectrics and (Pb$_y$Sn$_{1-y}$)$_2$P$_2$(Se$_x$S$_{1-x}$)$_6$ mixed crystals with line of tricritical points and line of Lifshitz points on the $T-x-y$ phase diagram, which meet at the tricritical Lifshitz point, can be described in a combined BEG-ANNNI model. Such the model considers first and second neighbor interactions for pseudospins in a local three-well potential. Below the temperature of tricritical Lifshitz point, the "chaotic" state accompanied by the coexistence of ferroelectric, metastable paraelectric and modulated phases is expected. In addition to the frustration of polar fluctuations near the Brillouin zone center, in Sn$_2$P$_2$S$_6$ crystal the antipolar fluctuations also strongly develop in the paraelectric phase at cooling to continuous phase transition temperature $T_0$. Here, critical behavior can be described as a crossover between Ising and XY universality classes, what is expected near bicritical points with coupled polar and antipolar order parameters and competing instabilities in $q$-space.