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Title: Theory and simulation of ZrO2/SrTiO3 multilayer structures
Author: Cheah, Wei Li
ISNI:       0000 0004 2732 2667
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2013
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High ionic conductivity of nano-layered heteroepitaxial oxide systems reported in recent literature has been attributed to high interfacial mobility of anions, although this interpretation has remained controversial. My work aims to understand the mechanism of ionic motion in such heterostructures by simulating the atomic structure at the interfaces, specifically for a multilayer system of Y2O3-stabilised ZrO2 (YSZ) and SrTiO3. By probing the energy landscape with a genetic algorithm in which the interatomic potentials were modelled with simple classical pair potentials, possible low energy lattice structures of pure ZrO2 layers in perfectly coherent epitaxy with SrTiO3 layers were explored. These configurations were identified and their energies were evaluated with more accuracy based on density functional theory (DFT). My expectation, based on indirect evidence from published high-resolution transmission electron microscopy, was that the ZrO2 layer has an epitaxial fluorite structure. However, I found that the fluorite ZrO2 structure is completely unstable as an epitaxial layer between layers of SrTiO3. Instead, anatase-, columbite-, rutile- and pyrite-like ZrO2 phases were found to be more stable structures in epitaxy, with the anatase-like epitaxy being the most stable configuration over a wide range of chemical potential of the components. Even with inclusion of Y2O3 doping, the fluorite epitaxial structure could not be stabilised. The genetic algorithm suggested a completely different phase stabilised by the presence of vacancies, whose cation lattice might resemble that of a fluorite or a perovskite. DFT calculations predicted this structure to be even more stable than an anatase YSZ/SrTiO3 structure. Molecular dynamics (MD) simulations of this configuration revealed some relatively low barriers for lateral anion diffusion; nevertheless, the activation energy for anion diffusion within the YSZ layer was predicted to be much higher than that of bulk YSZ. The results of this work therefore indicate that ionic conductivity of an ultra thin YSZ film in epitaxy with SrTiO3 would be suppressed, in contradiction to some of the literature.
Supervisor: Finnis, Mike ; McComb, David Sponsor: Agency for Science, Technology and Research, Singapore
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral