Transmission Electron Microscopy Study of Domains in Ferroelectrics

It was demonstrated that the relaxation of the out-of-plane constraint gives rise to an undetermined set of linear equations which can be solved for compatible domain wall orientations and volume fractions of domains, indicating that groups of adjacent grains can form stress-free domain patterns. STEM in-situ heating cycle experiments, heating and cooling through T_C, showed that the re-configuration of the domain structure (domain density, favourable domain orientations and presence of domain bundles) was directly influenced by the rate and continuous/dis-continuous nature of the performed heating cycles.<br>Furthermore, this material was explored with focus on the functionality of its positive temperature coefficient of resistivity (PTCR) effect.

Aberration-corrected STEM and EELS revealed a grain boundary PbTiO₃-like region (~10-15 nm), which was associated with an increased local polarisation in that region. The chemical and electronic heterogeneity of the ceramic was linked to the changes in potential barrier at the grain boundary, theorised by the Heywang-Jonker model. It was inferred that the confined PbTiO₃ rich grain boundary region would have a higher spontaneous polarisation (than BaTiO₃), thus reducing the grain boundary barrier potential further below T_C, augmenting electronic transport and enhancing the magnitude of resistivity jump at T_C, and so justifying the optimised PTCR effect exhibited by this ceramic.<br>For the study on phase reversibility in BiFeO₃ thin films, the native polymorphs, known as T and R, were initially identified.

The thermal activation phase transformation was investigated by STEM in-situ heating cycle experiments; showing a lateral growth of the highly-strained T phase above 400°C. Additionally, an AFM tip was used to locally apply electric field and stress, demonstrating reversible switching between the native mixed-phase and a pure T phase state. Energy-based effective Hamiltonian simulations verified phase competition under the application of electric field and stress, comparable to experimental data.

The stress-written phase boundaries (R’/T’) were investigated via c-AFM showing enhanced conductivity. TEM analysis of cross-sectional lamellae from pre-written AFM regions revealed that the stress-written R’ and T’ polymorphs differ in structure from the native polymorphs and, the R’/T’ boundaries have higher in-plane strain gradients compared to the native R/T boundaries, rationalising the enhanced conductivity as a strain mediated effect.<br>