Perovskite-based oxides are of vital importance in the modern society due to their excellent properties, such as superconduction, ferroelectricity, colossal magnetic resistance. These properties attribute to their complicated structures. With the development of high resolution transmission electron microscopy and high spatial resolution analytical microscopy, it is highly possible to study the microstructures at atomic scale, thus improve our understanding of the relationships between structures and properties.
In this dissertation, microstructures of several perovskite-based oxides were investigated by means of high resolution (scanning) transmission electron microscopy, high-angle annular dark-field imaging, X-ray energy dispersive spectroscopy. The main results are as follows:
An intergranular layer was found in 0.4 mol % Y-doped BaTiO3, which was sintered below the eutectic temperature in air. The layer possessed a crystal structure similar to Ba6Ti17O40 and a thickness of 0.7 nm. Plate-like second phases Ba6Ti17O40 were also observed at triple-grain junctions and an orientation relationship of (1 )t // (001)m, (1 2)t // (60 )m and [110]t // [0 0]m was determined between the tetragonal BaTiO3 and the monoclinic Ba6Ti17O40. Detailed structure of the interface between BaTiO3 and Ba6Ti17O40 was resolved by scanning transmission electron microscopy and high-resolution transmission electron microscopy investigations.
The microstructure of abnormal grains in BaTiO3 based ceramics suggests that 2-D nucleation and lateral growth take place in the material. High-resolution transmission electron microscopy investigation reveals that nucleation takes place at triple junctions of abnormal grain and adjacent matrix grains, subsequently form steps and the steps grow laterally along {111} planes of the abnormal grains. The abnormal grain boundaries are faceted in the growth direction, however, the ledges, which possess boundaries deviated from low-index crystallographic planes of the abnormal grains, could provide high mobility necessary for abnormal grain growth.
Asymmetrical twin boundaries and highly dense antiphase domains are found by means of transmission electron microscopy in the Perovskite-based BaNb0.3Ti0.7O3 thin film grown by laser molecular beam epitaxy on SrTiO3 (001) substrate. The microstructural characteristic of the BaNb0.3Ti0.7O3 film was clarified in terms of lamellar {111} twins and antiphase domains whose domain boundaries are 1/2 <110> stacking faults. It is proposed that the intersections of (111) twinning with the antiphase domain boundaries result in the asymmetrical twin boundaries.
The microstructural characteristics of PbZr0.52Ti0.48O3 thin films grown epitaxially on (001) SrRuO3-buffered (001) SrTiO3 substrates by pulsed laser deposition were analyzed by transmission electron microscopy. Through analysis of the ferroelectric domain structures, tetragonal and rhombohedral phases were found coexisting in the PZT film. In order to affirm the coexisting of tetragonal and rhombohedral phases, the misfit strain status and misfit dislocations of the films were studied. In addition to the phase coexisting, threading dislocations with Burgers vector a<100> also were identified. It is proposed that threading dislocations can pin the domains effectively.
Multilayers consisting of two tetragonal compositions PbZr0.2Ti0.8O3 and PbZr0.4Ti0.6O3 were deposited onto a SrRuO3 electrode grown on a (001) SrTiO3 substrate. It has been shown that 90ºa-c ferroelectric domains and threading dislocations formed at the first interface between PbZr0.2Ti0.8O3 and PbZr0.4Ti0.6O3 of the both multilayers. However, the densities of the a-domain and the threading dislocation are highetr in the multilayer with thicker single layers than that in the multilayer with thin single layers. Further more, some a-domains confined to PbZr0.2Ti0.8O3 single layers in the multilayer with thicker single layers and all a-domains propagated the whole film in the multilayer with thinner single layers. This indicates that the strain state of the multilayer with thinner single layers is much more homogenous than that of the multilayer with thicker single layers.
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