MS-12-O-2802 Enhanced Polarization Propagation by Mixing Polar and Nonpolar Phases in Piezoelectric Oxides

Properties of new functional materials strongly depend on the composition and atomic structure down to the level of single atoms, and thus characterization at the atomic scale has been a key technology in materials science. Recently, we found via aberration-corrected scanning transmission electron microscopy (STEM) that the giant electro-strain was induced by the polar core – nonpolar shell model wherein electric field-induced strain can be enhanced by the electric field induced polarization propagations from the polar core to nonpolar shell region, as described in Fig. 1, offering a new mechanism to achieve large electromechanical coupling in non-Pb based ceramics. It has been expected that the propagation of polarization can be promoted by the mixture of polar and nonpolar phases such as in relaxor ferroelectric materials; or by using the nanocomposite materials embedding the ferroelectric nanoparticles, each of which exhibits the flexible single domain. As polarization configuration at the interface between polar and nonpolar phases in nanocomposite have a completely behave different with that of interior domains, peculiar types of polarization configuration, such as nanoscale rotational vortices, can be dominant in the nanometric dimension, where the large strain effect should be necessarily considered.

Since there is still poor discussion about the details of polarization behavior in those peculiar materials systems, we utilize aberration-corrected STEM to determine the local polarization giving rise to atomic displacement (Fig. 2) and also in-situ TEM technique to dynamically observe the polarization by biasing the electric field or mechanical stress (Fig. 3). We successfully analyzed the electrical response when a single crystalline BaTiO₃ nanoparticle was mechanically compressed by a conductive indenter. Moreover, ~10 nm-BaTiO₃ particles, consisting of polar core and nonpolar shell, exhibit the flexo-polarization behavior, contrary to the conventional prediction that BaTiO₃ nanoparticles undergo the phase transition from ferroelectric to paraelectric. In this study, by combining the HAADF-STEM and in-situ TEM skills, the surfacial charge and strain have a crucial impact on the formation of unusual domain structures, suggesting plausible flexoelectricity in the piezoelectric oxides.