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.