ATOMIC STRUCTURE REORGANIZATION OVER BaTiO₃ NANOPARTICLES SURFACE

Ferroelectrics are attractive for low-power application at the nanoscale thanks to the reversible spontaneous polarization they exhibit. Polarization stability, which is essential for the realization of nano-ferroic technologies, is possible thanks to the organization of compensating charged ions at the surface, following Gauss’s law. In addition to electrical contribution, the ion arrangement at the surface is expected to influence chemical as well as mechanical characteristics of the material. However, to-date, understanding and controlling the surface ion organization and reorganization mechanism has remained elusive.

Here, we report on the direct observation of a mechanism combining cation vacancies and surface ion redistribution that helps stabilize the ferroelectric polarization. Using various high-resolution TEM methods, we found that tetragonal BaTiO₃ (ca. 50-nm particle size) contains a thin (≲ 1.5nm) TiO surface of a cubic structure. Not only does the existence of such a cubic surface allow mechanical stabilization of the tetragonal structure, but also the flexible oxidation state of the Ti ion in this structure, which was observed by means ionic displacement, supports the unique electrical characteristics of the ferroelectrics. The content and structure of the shell were determined by high-angle annular dark-field (HAADF) and 4 quadrants detectors for Z contrast as well as iDPC-STEM technique. Moreover, by showing that the shell evolves dynamically under electro-mechanical excitations that were induced by the electron beam, we demonstrated the dynamic surface-bulk stabilization mechanism.