  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.
