2D Atomic Displacement Map of Ferroelectric Domain Walls from HREM Images and Local Complex Dielectric Function from PEELS in Barium-Titanate Based Ceramics

摘要

Dielectric properties and mechanisms of ferroelectricity of barium-titanate-based ceramics have been investigated through complementary approaches in order to have a better and more fundamental understanding of material behaviour. This was done with an extensive use of transmission electron microscopy and associated techniques. Two local methodologies are proposed in order to study low-frequency (static) and very-high-frequency phenomena involved in the ferroelectricity of BaTiO_3: First, a quantitative processing of high-resolution TEM images of ferroelectric domain walls have been developed. Structure of domain walls is then described by 2D atomic displacement maps and comparison with simulations based on Landau-Ginzburg theory is then possible. Second, the analysis of the plasmon region in electron energy loss spectra (which has given rise to the development of software) allows one to determine the complex dielectric function (Kramer-Kronig analysis) with a spatial resolution of a few nanometres. In this study evidence is given that a local radial gradient of dopant concentration in small grains gives rise to different times of relaxation. This is tentatively interpreted in terms of electronic band structure.