Direct Visualization of Irreducible Ferrielectricity in Crystals

TL;DR

This paper uses advanced electron microscopy to directly observe a unique form of ferrielectricity in BaFe2Se3, revealing irreducible ferrielectricity with distinct temperature-dependent polarization behavior, thus clarifying the difference from ferroelectricity.

Contribution

It provides the first direct visualization of irreducible ferrielectricity in a crystal, demonstrating a nontrivial structural evolution and distinguishing it from ferroelectricity at the microscopic level.

Findings

Visualization of nontrivial ferrielectric structural evolution.

Identification of irreducible ferrielectricity with out-of-sync polar sub-lattices.

Observation of non-monotonic temperature-dependent polarization behavior.

Abstract

In solids, charge polarity can one-to-one correspond to spin polarity phenomenologically, e.g. ferroelectricity/ferromagnetism, antiferroelectricity/antiferromagnetism, and even dipole-vortex/magnetic-vortex, but ferrielectricity/ferrimagnetism kept telling a disparate story in microscopic level. Since the definition of a charge dipole involves more than one ion, there may be multiple choices for a dipole unit, which makes most ferrielectric orders equivalent to ferroelectric ones, i.e. this ferrielectricity is not necessary to be a real independent branch of polarity. In this work, by using the spherical aberration-corrected scanning transmission electron microscope, we visualize a nontrivial ferrielectric structural evolution in BaFe2Se3, in which the development of two polar sub-lattices is out-of-sync, for which we term it as irreducible ferrielectricity. Such irreducible ferrielectricity leads to a non-monotonic behavior for the temperature-dependent polarization, and even a compensation point in the ordered state. Our finding unambiguously distinguishes ferrielectrics from ferroelectrics in solids.