Electronic structure of thermoelectric and ferro-electric materials: - Advanced transmission electronmicroscopy studies

Abstract

This thesis describes TEM studies related to the electronic structure of thermoelectric and ferroelectric materials.

Compounds within the thermoelectric skutterudite class of materials were examined using Electron Energy-Loss Near Edge Spectroscopy (ELNES) and Convergent Beam Electron Diraction (CBED) in order to shed light on bonding mechanisms. Using ELNES in combination with two dierent theoretical calculation schemes (density functional theory (DFT) and real-space multiple scattering (RSMS) calculations) on various Phosphoruscontaining skutterudites, it was shown that the compounds are remarkably similar in their Phosphorus bonding environment and the experimental results are well described by both theoretical approaches.

CBED measurements on the skutterudite CoSb3 yielded two re ned structure factors that were compared to and could serve as a reference to DFT calculations and X-ray diraction experiments on structure factors.

ELNES experiments on the ferroelectric BiFeO3 and two isostructural cation substituted compounds have been compared to DFT and RSMS calculations to obtain insight into the electronic structure of these compounds and the role of the two dierent cations. These were found to be interrelated through small, but nite covalent bonds to oxygen.

PbTiO3 nanorods were examined by conventional TEM techniques to reveal the ferroelectric domain structure. Microtomed slices of the nanorods provided insight into the surfaces and internal defects in the nanorods. Further experiments on the nanorods using holography to attempt to image the domain structure as well as a scanning transmission electron microscopy and a high resolution approach to image strain are described. Finally, preliminary ELNES experiments on the ferroelectric HoMnO3 are reported.