On the Landau theory of phase transitions

TL;DR

This paper re-examines the Landau theory of phase transitions in ferroelectrics, revealing the roles of lattice vibrations and nematic phases, and explaining deviations from experiments near the critical point.

Contribution

It introduces a hierarchical dynamic model involving lattice vibrations and nematic phases to refine the Landau theory for ferroelectric phase transitions.

Findings

Identification of two atomic movements involved in ferroelectric transitions

Modulation of phonon mode softening by nematic phase dynamics near the Curie temperature

Explanation of deviations from Landau theory predictions in experiments

Abstract

The Landau theory of phase transitions has been re-examined under the framework of a modified mean field theory in ferroelectrics. By doing so, one can see that there are two atomic movements involved in the ferroelectric phase transition; the first corresponds to the vibration of crystalline lattice, which will render phonon mode softening at the critical point, and the second represents the slow evolution of a partially ordered nematic phase formed by the cooperative behavior of high-temperature structure precursors. In this hierarchical dynamic structure, the former fast dynamics could be significantly modulated by the latter slow dynamics in the vicinity of the Curie temperature; it then turns out that it is the behavior of the nematic phase on approaching the critical point that makes the Landau theory deviate from experimental observations.