Polarized cluster dynamics at the paraelectric to ferroelectric phase transition in BaTiO3

The mechanism of the prototype ferroelectric phase transition in BaTiO(3) is a matter of intense debate and to a large extent still wrapped in mystery. Precursor phenomena in the form of polarized clusters in paraelectric BaTiO(3) are by now believed to represent a key step into the ferroelectric phenomenon. The determination of a slower dynamics of cluster polarization flipping along with a faster order-disorder Ti hopping mode among <111> off-center sites suggests coexistence, instead of mutual exclusion, of displacive and order-disorder types, initially proposed as distinct models. However, no clear picture of the transition state has been proposed so far, which is able to provide insight into the coexistence of the paraelectric and ferroelectric phenomena. Here, by means of a dedicated molecular dynamics approach, we provide a detailed atomistic picture of intermediate regions along the transition. Therein, different time and length scales coexist as they characterize different portions of the same material. From an imbalance of dynamically and more statically polarized clusters in this highly inhomogeneous intermediate, a symmetry breaking step naturally results. Further, we find that ferroelectric nanodomains may host antiferroelectric defects, which appear as an intrinsic feature of the growing BaTiO(3) ferroelectric material.