The interplay between domains, defects and electric field induced phase transformations in bulk ferroelectrics

The manipulation of local morphological features, such as domains, domain walls and defects, is reported to have a dramatic influence on the local and global properties of electro-active materials. The majority of local techniques for characterizing and quantifying have been limited to two dimensions (2D), such as surface and thin films, leaving no methods sufficient for investigating the bulk.

The role of morphological features in structural dynamics is not fully understood and thus validation and guidance for bulk and 3D models does not yet exist. This work has produced an experimental approach to overcome these limitations based on imaging of the structural dynamics in situ with dark field x-ray microscopy, accompanied by relevant analytical methods for the data analysis.

As a first step, we have designed and manufactured the Stable Temperature and Electric Field (STEF) holder system capable of applying an electric field and keeping a stable temperature in the range 26-200 ◦C with stability of 0.01 K. This facilitates spatial resolution of the electric field induced phase transformation morphology in deeply embedded material.

We observe domain-like structures well above the Curie temperature, indicating bulk domain imprinting previously only observed near surfaces. Combined with density functional theory (DFT) calculations, our results suggests that point defects, such as oxygen vacancies, plays an important role in determining morphology both above and below the Curie temperature.

In an effort to quantify this role of oxygen vacancies, I then developed a method for detecting defect clusters in bulk material based on their volume expansion. The combined observations throughout this thesis consistently points towards a complex model of the interactions between the various morphological features, which is necessary to understand and predict phase transformation dynamics and related phenomena, such as the heterogeneous nucleation of ferroelectric domains.

Here, I present the hypothesis for such a complex interaction behaviour, and our findings and framework to further deepen the knowledge of such claims.