Structural aspects and charge ordering in perovskite -based materials studied by transmission electron microscopy.

摘要

Perovskite-based materials are one of the most studied materials due to their enormous variety of physical properties. These properties are closely related with the structure of these materials. In this work the structure of some particular perovskite-based structures is characterised by means of transmission electron microscopy.;Part I deals with general introduction to the perovskite structure and the electron microscopy techniques that are used within this work.;Part II is devoted to the charge ordering process in manganites and its relationship with the physical properties. Manganites are perovskite-based structures with general formula (Ln1-x AEx)MnO 3 with Ln a lanthanide element (in this study Nd3+ and La3+) and AE an alkaline earth (in this study Ca2+). Studying the structure of these materials and relating this structure to the physical properties allow to characterise the charge ordering process in function of temperature. Especially the influence of Cr-doping of the B-site of the perovskite and the Mn3+/Mn4+ ratio on the structure and the physical properties is highlighted. Also the coexistence of ferromagnetism and charge ordering is discussed.;In Part III the structure of two other perovskite-based materials is characterised. First the hexagonal perovskite Ba4Ca0.9Mn 3.1O12-delta is studied by high resolution microscopy. This technique allows to exactly determine the layer sequence within this structure. Finally the exact space group of the Sr0.5Ca0.5 TiO3 perovskite is determined by a combination of microdiffraction and convergent beam electron microscopy. There where other techniques as X-ray diffraction and neutron diffraction fail, TEM allows to completely characterise this structure.