A key challenge in guiding experiments toward materials with desired properties is to effectively navigate the vast search space comprising the chemistry and structure of allowed compounds. Here, it is shown how the use of machine learning coupled to optimization methods can accelerate the discovery of new Pb-free BaTiO (BTO-) based piezoelectrics with large electrostrains. By experimentally comparing several design strategies, it is shown that the approach balancing the trade-off between exploration (using uncertainties) and exploitation (using only model predictions) gives the optimal criterion leading to the synthesis of the piezoelectric (Ba Ca )(Ti Zr Sn )O with the largest electrostrain of 0.23% in the BTO family. Using Landau theory and insights from density functional theory, it is uncovered that the observed large electrostrain is due to the presence of Sn, which allows for the ease of switching of tetragonal domains under an electric field.

Topological defects of spontaneous polarization are extensively studied as templates for unique physical phenomena and in the design of reconfigurable electronic devices. Experimental investigations of the complex topologies of polarization have been limited to surface phenomena, which has restricted the probing of the dynamic volumetric domain morphology in operando. Here, we utilize Bragg coherent diffractive imaging of a single BaTiO3 nanoparticle in a composite polymer/ferroelectric capacitor to study the behavior of a three-dimensional vortex formed due to competing interactions involving ferroelectric domains. Our investigation of the structural phase transitions under the influence of an external electric field shows a mobile vortex core exhibiting a reversible hysteretic transformation path. We also study the toroidal moment of the vortex under the action of the field. Our results open avenues for the study of the structure and evolution of polar vortices and other topological structures in operando in functional materials under cross field configurations.

Nanocomposites of ferroelectric ceramic filler and polymer matrix show considerable promise as high energy storage dielectric capacitors. However, the influence of microstructure of the ferroelectric filler on the electric energy storage performance in the nanocomposite has not been quantitatively studied, yet it is a key element in understanding the methods employed to improve the performance of capacitors. We demonstrate an innovative strategy to enhance the energy storage density with topological vortex structures in nanocomposites. Using three dimensional phase field calculations, we show that multi-vortex structures can exist in ferroelectric nanowires without charge defects or free charges at the interface between the filler and matrix. The switching behavior of the topological structure (vortex and anti-vortex pair) under external electric field is calculated in nanocylinder wires. The small remnant polarization and very narrow hysteresis loop due to the vortex structure in the nanocomposites can lead to a large enhancement of energy density, as high as 5 J/cm 3 compared to 1-2 J/cm 3 for commercial capacitors, and high energy storage efficiency (over 95%) at a relatively low electric field of 140 MV/m.