In situ microscopy studies of BaTiO₃

Abstract

This thesis investigates the configurations and thermal mobility of ferroelectric- ferroelastic domains in both poly- and single-crystalline form of the seminal ferroelectric, BaTiO3. These studies have resulted primarily from (S)TEM methods, performed on free-standing thin-film lamellae, with a strong emphasis on in situ TEM techniques. The results here presented are accompanied with relevant theory, discussed in the context of existing literature, and complimented by PFM studies.

To begin, a static study of domain continuity across grain boundaries in polycrystalline lamellae is presented. Domain continuity is speculated to impact the ferroelectric properties of polycrystalline ceramics, however, the link to experimental evidence up to date is limited, greatly hindering the applicability and fundamental understanding of this phenomenon. With careful experimental design, the microstructural arrangements across 11 individual grain boundaries in bi-grain and tri-grain junctions in lamellae samples were analysed by STEM and rationalised with reference to martensite crystallography. A pleasing agreement of minimal strain and polarisation mismatch for a pair of domain variants at the grain boundary was found in all cases where domain wall continuity was observed. Following on from this, the experimental observations were used to investigate theoretical combinations of bi-grain junctions, providing a significant advancement in the understanding of specific cases where domain wall continuity can be expected. The presented method gives a valuable insight into how domain compatibility can be used as a tool to explore and design microstructural arrangement in polycrystalline ferroelectrics and presents, for the first time, a comprehensive experimental data set that analyses domain wall continuity.

Next, whilst switching behaviour under external electric fields has received large literature attention, thermal behaviours have been studied to a much lesser extent and there are still many aspects that are not well understood. Here, dedicated in situ TEM techniques were utilised to investigate the thermal dynamic behaviour of BaTiO3 lamella. In situ heat cycling experiments were performed on a simple <100>pc single- crystal system as an initial check. After optimisation, several interesting behaviours were observed including: (i) TC is pushed to higher temperatures (> 150°C), than bulk BaTiO3 (120°C) (ii) domains have significant thermal mobility far below TC and (iii) vacuum annealing promotes complex metastable domain configurations of competing domain variants. Comparison to <110>pc lamella revealed an interesting orientation dependence on the thermal activities, which was later rationalised by the promotion and subsequent mobility of oxygen vacancies within the different crystalline orientations. Importantly, in situ PFM experiments yielded similar observations, proving that in situ (S)TEM can offer an alternative and complimentary approach for dynamic studies of ferroelectrics. These in situ studies were combined with advanced strain mapping techniques, measuring the local elastic competition between domain variants in singular needle junctions as well as more complex domain structures induced by high temperature annealing. To the authors knowledge, this is the first time that 4D STEM has been combined with in situ heating to probe local strain fields in free-standing ferroelectric single-crystals. Once the behaviour of single crystals was completed, we compared these to in situ STEM heating studies of bi-grain junctions in polycrystalline lamellae and larger scale in situ SEM studies. This comparison indicated that both mechanical boundary conditions of the lamella and high temperature annealing in vacuum can induce interesting thermal behaviours in free- standing thin film ferroelectrics.

Lastly, the observations from an exploratory research project utilising the recent advancement in windowed in situ gas and heating TEM technology are presented. The main aim was to study the ferroelectric-ferroelastic domain behaviour in <100>pc BaTiO3 under distinct chemical environments and temperature including vacuum, oxygen-rich, inert, and reducing atmospheres. This experiment, the first of its kind for free-standing thin film ferroelectrics, was designed to further our understanding of the mechanisms responsible for charge compensation that drives the formation of domains in ferroelectric materials. The results here presented offer a valuable and direct insight into the delicate link between the domain stability and the chemical environment. These observations were related to current literature of the ferroelectric surface, with particular attention placed on the role of adsorbates in determining the polarisation of domains, and the role of polarisation in determining the surface chemistry. These preliminary findings motivate further exploration and use of this technique in the pursuit of novel pathways to tuneable ferroelectric surface chemistry.

Date of Award	Dec 2022
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Engineering & Physical Sciences Research Council
Supervisor	Miryam Arredondo (Supervisor)