Subangstrom-resolved Aberration-corrected Transmission Electron Microscopic Investigations Of Size Effect In Barium Titanate

The size effect in barium titanate(Ba Ti O3), a typical lead-free displacive ferroelectric, was atomically investigated using aberration-corrected transmission electron microscopy(AC-TEM) combined with first-principle calculations and high resolution image simulation.The surface structures of Ba Ti O3 were studied by density functional theory calculations and AC-TEM. Three possible structures of Ba Ti O3(111) surface were proposed according to the polarity compensation theory. The calculated results showed that atomic shifts were induced by the complicated surface structures near the surface. The three surface structures and the surface-induced atomic shifts were confirmed by AC-TEM. Moreover, surface-induced atomic shifts were also observed near the(110) and(001) surfaces in the AC-TEM experimental images.The phase structures of Ba Ti O3 particles in the size range of 2.5 ~ 10 nm were tested at different temperatures by using AC-TEM with an in-situ heating holder, and the temperature-size effect of Ba Ti O3 was studied. The experimental results showed that each particle was composed of various phases, which with lower symmetry were more observed near the particle surfaces due to the complicated surface structures. A temperature-size phase diagram was proposed, data in which were collected from both reported literatures and our experiment. It was implied that the phase transitions were smeared and the Curie temperature significantly increased as the size decreased to the nanoscale. Size decrease of ferroelectrics will provide new pathways for high temperature device applications.The thickness dependence of 2D freestanding Ba Ti O3 thin film was characterized by AC-TEM, and the dimension-size effect of Ba Ti O3 was studied. Channeling effect and finite element image simulation were used to estimate the thickness of TEM sample. The experimental results showed that the polarization remained when the sample was thicker than 4.8 nm, while disappeared when thinner than 4.4 nm in the 2D freestanding thin film, different from the 3D-0D Ba Ti O3.In addition, dislocations in Ba Ti O3 were atomically studied for the first time.