title: Dislocation-Based Functionality in Ferroelectric Perovskites
Creators: name: Höfling, Marion Anita
Publisher: name: UNSPECIFIED
Year: 2021

description: Dislocations have been severely underestimated as a tool for tailoring the functional properties of ferroelectric materials, which are essential electroceramic components in many applications. Recently, numerical simulations have predicted strong ferroelectric domain wall–dislocation interactions. However, little is known about the plastic deformability of bulk ferroelectrics to introduce dislocations and the ability of dislocations to tune electromechanical properties. In this work, these issues are addressed for the ferroelectric single crystal materials KNbO3 and BaTiO3. Furthermore, the obtained knowledge is applied to fine-grained polycrystalline BaTiO3. The anisotropic deformation behavior of 〈010〉_pc and 〈101〉_pc-oriented KNbO3 single crystals is studied by uniaxial compression at room temperature. The activation of the {110}〈110〉 slip system and the observed low critical resolved shear stress of 20–30 MPa cause a large plastic deformation of 4.6% and allow the determination of the resulting slip band structures. A reduction in domain size from the initial 1.8 µm to 0.5 µm and local switching experiments confirm that dislocations act as nucleation and pinning sites for domain walls. The dislocation-induced change in functional behavior was examined in detail using 〈001〉-oriented BaTiO3 single crystals. Creep experiments at high temperature cause a mechanical imprint, leading to rhombic domain structures. Moreover, a huge increase in dielectric and electromechanical response (εr ≈ 5800 and d33* ≈ 1890 pm/V) was observed in the subcoercive field region. Based on these results, we propose an extension of the concept of local domain wall pinning at dislocations towards a macroscopic restoring force acting against the domain wall motion. The mechanical imprint and the resulting pinning force (Peach Koehler force) stabilize an a-c-domain configuration. The application of an electric field causes strain incompatibilities between pinned and switched domains, which result in the macroscopic restoring force preventing the switching of further domains or causing already switched domains to switch back. Since plastic deformation is anisotropic, a brief outlook to the deformability of 〈110〉-oriented BaTiO3 is given, indicating that BaTiO3 and SrTiO3 behave similarly at high temperatures and high stresses. This mechanism provides a deeper understanding of the domain wall-dislocation interaction on a macroscopic level and paves the way for anisotropy studies in single crystals, as well as the transfer to polycrystalline ceramics. A creep deformation map for polycrystalline BaTiO3 is then developed. The influence of dislocation- and diffusion-based creep mechanisms on the microstructural and electromechanical properties is discussed. An increase in the phase transition temperature of ΔT = 5 °C, a decrease in the maximum polarization by 10% and an electromechanical strain by 30% were observed. Moreover, the different creep mechanisms are discussed. In this work, the foundations are laid to better understand the plastic deformability of KNbO3 and BaTiO3 single crystals and the resulting consequences of the mechanical imprint on the ferroelectric domain structure and the dielectric and ferroelectric behavior. Thus, the dislocation-based functionality approach was established on the macroscopic scale, which could facilitate the further implementation of dislocations as functional property-tuning defects in ferroelectric bulk materials.