Atomic scale symmetry and polar nanoclusters in the paraelectric phase of ferroelectric materials

TL;DR

This study directly visualizes static polar nanoclusters in paraelectric ferroelectric materials, revealing their size, structure, and implications for understanding atomic-scale disorder and polarization.

Contribution

It provides the first atomic-scale visualization and analysis of static polar nanoclusters in nominally nonpolar phases of ferroelectrics, clarifying their static nature.

Findings

Identification of 2-4 nm static polar nanoclusters

Confirmation of their presence in nominally nonpolar phases

Implications for understanding ferroelectric precursor states

Abstract

The nature of the "forbidden" local- and long-range polar order in nominally nonpolar paraelectric phases of ferroelectric materials has been an open question since the discovery of ferroelectricity in oxide perovskites (ABO3). A currently considered model suggests locally correlated displacements of B-site atoms along a subset of <111> cubic directions. Such offsite displacements have been confirmed experimentally, however, being essentially dynamic in nature they cannot account for the static nature of the symmetry-forbidden polarization implied by the macroscopic experiments. Here, in an atomically resolved study by aberration corrected scanning transmission electron microscopy (STEM) complemented by Raman spectroscopy, we reveal, directly visualize and quantitatively describe static, 2-4 nm large polar nanoclusters in the nominally nonpolar cubic phases of (Ba,Sr)TiO3 and BaTiO3. These results have implications on understanding of the atomic-scale structure of disordered materials, the origin of precursor states in ferroelectrics, and may help answering ambiguities on the dynamic-versus-static nature of nano-sized clusters.