Microdomain dynamics in single-crystal BaTiO₃ during paraelectric-ferroelectric phase transition measured with time-of-flight neutron scattering

Microscopic polar clusters can play an important role in the phase transition of ferroelectric perovskite oxides such as BaTiO₃, which have shown coexistence of both displacive and order-disorder dynamics, although their topological and dynamical characteristics are yet to be clarified. Here, we report sharp increases in the widths and intensities of Bragg peaks from a BaTiO₃ single crystal, which are measured in situ during heating and cooling within a few degrees of its phase transition temperature T_C, using the neutron time-of-flight Laue technique. Most significantly sharper and stronger increases in peak widths and peak intensities were found to occur during cooling compared to that during heating through T_C. A closer examination of the Bragg peaks revealed their elongated shapes in both the paraelectric and ferroelectric phases, the analysis of which indicated the presence of microdomains that have correlated ⟨111⟩-type polarization vectors within the {110}-type crystallographic planes. No significant increase in the average size of the microdomains (∼10nm) near T_C could be observed from diffraction measurements, which is also consistent with small changes in the relaxation times for motion of Ti ions measured with quasielastic neutron scattering. The current observations do not indicate that the paraelectric-ferroelectric phase transition in BaTiO₃ is primarily caused by an increase in the size of the microscopic polar clusters or critical slowing down of Ti ionic motion. The sharp and strong increases in peak widths and peak intensities during cooling through T_C are explained as a result of microstrains that are developed at microdomain interfaces during paraelectric-ferroelectric phase transition.