Ferroics are characterized by the existence of domains with uniform order parameters such as polarization, magnetization and strain. Domain boundaries (DBs), which separate these domains, have been attractive topics in recent years thanks to the development of experimental techniques. Many exotic properties which are different from bulk have been observed at DBs. The enhancement of electric conductivity, photovoltaic effect and the emergence of charged DBs are typical examples and reported in various oxides. Among these properties, we are interested in the appearance of polarity in non-polar materials. We used a second harmonic generation microscope (SHGM) to confirm the polarity at DBs in various ferroic materials and found that all ferroelastic materials that we have investigated so far exhibit polar nature at DBs. Recently, we extend this idea to antiferroelectrics and performed SHGM observations and x-ray diffuse scattering experiments. It turns out that antiphase boundaries in antiferroelectrics possesses polar nature, which is originated from the displacement of the atom inside the antiphase boundary. The application of a stress or an electric field can enhance the polar nature at antiphase boundary, which has a great potential for domain boundary engineering.
We also observe the possibility of polar nature at DBs in ferroaxial materials which are represented by an electric toroidal moment.

The study of domain patterns in ferroelectrics is crucial for understanding their macroscopic properties. Imaging these patterns, especially in three dimensions (3D) and especially in ferroelectric single crystals, is essential due to the possible complex geometry of the domain walls. The domain pattern throughout the volume of a ferroelectric sample can differ significantly from the domain pattern observed at the surface of the sample. The 180° tilted charged domain walls respond differently to the applied electric field than the neutral walls and can exhibit unique physical quantities such as conductivity. This paper presents two digital Mach-Zehnder holographic interferometers designed for optical imaging of domain patterns in ferroelectric single crystals. The first, a free-space interferometer, provides improved axial resolution for 3D imaging. The second, an on-chip digital holographic microscope, enables super-resolution methods for excellent lateral resolution. We demonstrate 3D imaging of ferroelectric domains in transparent single crystals using digital holographic microscopy (DHM) and tomography (DHT). Our tomographic approach involves acquiring multiple projections of domain patterns from different directions and numerically reconstructing a 3D image using our original method called curvilinear filtered back-projection. We validate our method by imaging different domain patterns in lithium niobate single crystals. This approach allows rapid and accurate 3D observation of ferroelectric domain patterns across the entire volume of single crystals at the centimeter scale. We provide insight into the benefits and potential applications of our DHM and DHT systems by discussing their advantages and challenges. Our study contributes to the advancement of the characterization of ferroelectric materials and opens avenues for further research in this field.

Ferroelectric oxides host technologically-relevant properties, and their integration onto the CMOS-compatible silicon platform is key for the next generation of oxide electronics.
However, the interplay of thermal coefficient mismatch and strain relaxation during the growth of ferroelectric epitaxial thin films on silicon results in a multi-domain configuration in the films, deviating from the application-relevant single-domain state. Here, taking BaTiO₃ (BTO) as our ferroelectric model system, we directly investigate the evolution of the polarization state of our films during the pulsed laser deposition growth on conventional single crystalline SrTiO₃ (STO) and STO-buffered silicon substrates using in-situ optical second harmonic generation (ISHG). We shed light on the role of the growth conditions on the final polarization in our films and stabilize a single domain, out-of-plane polarized state in films grown on STO substrates. For films grown on STO-buffered silicon templates, we reveal a thickness-dependent in-plane polarized domain formation induced by interfacial strain relaxation. Finally, ISHG measurements during post-growth cooldown isolate the effect of thermal expansion coefficient mismatch on the final domain architecture of the films. Hence, our work provides new insights into the controlled integration of ferroelectric oxides on silicon templates, a necessary step for the realization of energy-efficient technologies.

The increasing need for small-scale electrical devices drives the demand for multifunctional materials capable of storing energy, rapidly releasing the stored energy, and harvesting the heat generated during energy production. Relaxor ferroelectrics (RFEs) are particularly promising due to their intrinsic polarization and the synergy of their electrical, mechanical, and thermal properties. RFEs can store and release charge efficiently, responding to the application and removal of an electric field. Additionally, they can convert waste heat into electricity and enable advanced cooling technologies through their pyroelectric and electrocaloric effects. Furthermore, the use of RFEs in thin film structure offer advantages such as lower thermal mass, higher breakdown strength, and enhanced functionalities. Importantly, their ability to withstand high electric fields at elevated temperatures boosts the electrocaloric and pyroelectric effects. Enhancing these properties can be achieved by introducing compositional disorder, such as doping, which creates chemical and structural heterogeneity. Recently, Sm-doped Pb(Mg1/3Nb2/3)O₃–PbTiO₃ (Sm-PMN–PT) bulk materials have revealed outstanding ferroelectric and piezoelectric properties due to enhanced local structural heterogeneity. Here, by employing epitaxial thin films using pulsed laser deposition (PLD), we demonstrate that the Sm-doping enhances the energy storage, piezoelectric, electrocaloric and pyroelectric properties of PMN–PT thin films. Fatigue-free and thermally-stable energy storage properties, as well as a colossal electrocaloric effect (59.4K) and pyroelectric energy density (40 J/cm³), along with a remarkable pyroelectric energy efficiency of 85.5% relative to Carnot, were obtained. By using scanning transmission electron microscopy and phase-field modeling, we found that these giant properties arise from the increased local structural heterogeneity and strong local electric fields along spontaneous polarization directions facilitating nucleation of slush-like polar structure with extremely small domain sizes (2–5 nm). Our findings suggest that Sm-PMN–PT films have significant potential for capacitive energy storage and electrothermal energy conversion, offering a strategy applicable to other RFEs for developing robust, multifunctional materials.