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Abstract Piezoresponse force microscopy (PFM) is used for investigation of the electromechanical behavior of the head‐to‐head (H‐H) and tail‐to‐tail (T‐T) domain walls on the non‐polar surfaces of three uniaxial ferroelectric materials with different crystal structures: LiNbO₃, Pb₅Ge₃O₁₁, and ErMnO₃. It is shown that, contrary to the common expectation that the domain walls should not exhibit any PFM response on the non‐polar surface, an out‐of‐plane deformation of the crystal at the H‐H and T‐T domain walls occurs even in the absence of the out‐of‐plane polarization component due to a specific form of the piezoelectric tensor. In spite of their different symmetry, in all studied materials, the dominant contribution comes from the counteracting shear strains on both sides of the H‐H and T‐T domain walls. The finite element analysis approach that takes into account a contribution of all elements in the piezoelectric tensor, is applicable to any ferroelectric material and can be instrumental for getting a new insight into the coupling between the electromechanical and electronic properties of the charged ferroelectric domain walls. 

Abstract Piezoelectricity of ferroelectric crystals is widely utilized in electromechanical devices such as sensors and actuators. It is broadly believed that the smaller the ferroelectric domain size, the higher the piezoelectricity, arising from the commonly assumed larger contributions from the domain walls. Herein, the domain‐size dependence of piezoelectric coefficients of prototypical ferroelectric crystals is theoretically studied based on thermodynamic analysis and phase‐field simulations. It is revealed that the inverse domain‐size effect, i.e., the larger the domain size, the higher the piezoelectricity, is entirely possible and can be just as common. The nature of the domain‐size dependence of piezoelectricity is shown to be determined by the propensity of polarization rotation inside the domains instead of the domain wall contributions. A simple, unified, analytical model for predicting the domain‐size dependence of piezoelectricity is established, which is valid regardless of the crystalline symmetry, the materials chemistry, and the domain structures of a ferroelectric crystal, and thus can serve as a guiding tool for optimizing piezoelectricity of ferroelectric materials beyond the “nanodomain” engineering. In addition, the theoretical approach can be extended to understand the microstructural size effect of multifunctional properties in ferroic and multiferroic materials. 

Abstract Ferroelectric domain walls, topological entities separating domains of uniform polarization, are promising candidates as active elements for nanoscale memories. In such applications, controlled nucleation and stabilization of domain walls are critical. Here, using in situ transmission electron microscopy and phase‐field simulations, a controlled nucleation of vertically oriented 109° domain walls in (110)‐oriented BiFeO₃(BFO) thin films is reported. In the switching experiment, reversed domains that are nucleated preferentially at the nanoscale edges of the “crest and sag” pattern‐like electrode under external bias subsequently grow into a stable stripe configuration. In addition, when triangular pockets (with an in‐plane polarization component) are present, these domain walls are pinned to form stable flux‐closure domains. Phase field simulations show that i) field enhancement at the edges of the electrode causes site‐specific domain nucleation, and ii) the local electrostatics at the domain walls drives the formation of flux closure domains, thus stabilizing the striped pattern, irrespective of the initial configuration. The results demonstrate how flux closure pinning can be exploited in conjunction with electrode patterning and substrate orientation to achieve a desired topological defect configuration. These insights constitute critical advancements in exploiting domain walls in next generation ferroelectronic devices. 

Abstract Both experimental results and theoretical models suggest the decisive role of the filler–matrix interfaces on the dielectric, piezoelectric, pyroelectric, and electrocaloric properties of ferroelectric polymer nanocomposites. However, there remains a lack of direct structural evidence to support the so‐called interfacial effect in dielectric nanocomposites. Here, a chemical mapping of the interfacial coupling between the nanofiller and the polymer matrix in ferroelectric polymer nanocomposites by combining atomic force microscopy–infrared spectroscopy (AFM–IR) with first‐principles calculations and phase‐field simulations is provided. The addition of ceramic fillers into a ferroelectric polymer leads to augmentation of the local conformational disorder in the vicinity of the interface, resulting in the local stabilization of the all‐transconformation (i.e., the polar β phase). The formation of highly polar and inhomogeneous interfacial regions, which is further enhanced with a decrease of the filler size, has been identified experimentally and verified by phase‐field simulations and density functional theory (DFT) calculations. This work offers unprecedented structural insights into the configurational disorder‐induced interfacial effect and will enable rational design and molecular engineering of the filler–matrix interfaces of electroactive polymer nanocomposites to boost their collective properties. 

Abstract The availability of materials with high electrocaloric (EC) strengths is critical to enabling EC refrigeration in practical applications. Although large EC entropy changes, ΔS_EC, and temperature changes, ΔT_EC, have been achieved in traditional thin‐film ceramics and polymer ferroelectrics, they require the application of very high electric fields and thus their EC strengths ΔS_EC/ΔEand ΔT_EC/ΔEare too low for practical applications. Here, a fundamental thermodynamic description is developed, and extraordinarily large EC strengths of a metal‐free perovskite ferroelectric [MDABCO](NH₄)I₃(MDABCO) are predicted. The predicted EC strengths: isothermal ΔS_EC/ΔEand adiabatic ΔT_EC/ΔEfor MDABCO are 18 J m kg⁻¹K⁻¹MV⁻¹and 8.06 K m MV⁻¹, respectively, more than three times the largest reported values in BaTiO₃single crystals. These predictions strongly suggest the metal‐free ferroelectric family of materials as the best candidates among existing materials for EC applications. The present work not only presents a general approach to developing thermodynamic potential energy functions for ferroelectric materials but also suggests a family of candidate materials with potentially extremely high EC performance. 

Abstract Nanoelectronic devices based on ferroelectric domain walls (DWs), such as memories, transistors, and rectifiers, have been demonstrated in recent years. Practical high‐speed electronics, on the other hand, usually demand operation frequencies in the gigahertz (GHz) regime, where the effect of dipolar oscillation is important. Herein, an unexpected giant GHz conductivity on the order of 10³S m⁻¹is observed in certain BiFeO₃DWs, which is about 100 000 times greater than the carrier‐induced direct current (dc) conductivity of the same walls. Surprisingly, the nominal configuration of the DWs precludes the alternating current (ac) conduction under an excitation electric field perpendicular to the surface. Theoretical analysis shows that the inclined DWs are stressed asymmetrically near the film surface, whereas the vertical walls in a control sample are not. The resultant imbalanced polarization profile can then couple to the out‐of‐plane microwave fields and induce power dissipation, which is confirmed by the phase‐field modeling. Since the contributions from mobile‐carrier conduction and bound‐charge oscillation to the ac conductivity are equivalent in a microwave circuit, the research on local structural dynamics may open a new avenue to implement DW nano‐devices for radio‐frequency applications. 

Abstract Application of scanning probe microscopy techniques such as piezoresponse force microscopy (PFM) opens the possibility to re‐visit the ferroelectrics previously studied by the macroscopic electrical testing methods and establish a link between their local nanoscale characteristics and integral response. The nanoscale PFM studies and phase field modeling of the static and dynamic behavior of the domain structure in the well‐known ferroelectric material lead germanate, Pb₅Ge₃O₁₁, are reported. Several unusual phenomena are revealed: 1) domain formation during the paraelectric‐to‐ferroelectric phase transition, which exhibits an atypical cooling rate dependence; 2) unexpected electrically induced formation of the oblate domains due to the preferential domain walls motion in the directions perpendicular to the polar axis, contrary to the typical domain growth behavior observed so far; 3) absence of the bound charges at the 180° head‐to‐head (H–H) and tail‐totail (T–T) domain walls, which typically exhibit a significant charge density in other ferroelectrics due to the polarization discontinuity. This strikingly different behavior is rationalized by the phase field modeling of the dynamics of uncharged H–H and T–T domain walls. The results provide a new insight into the emergent physics of the ferroelectric domain boundaries, revealing unusual properties not exhibited by conventional Ising‐type walls. 

Abstract Domain wall nanoelectronics is a rapidly evolving field, which explores the diverse electronic properties of the ferroelectric domain walls for application in low‐dimensional electronic systems. One of the most prominent features of the ferroelectric domain walls is their electrical conductivity. Here, using a combination of scanning probe and scanning transmission electron microscopy, the mechanism of the tunable conducting behavior of the domain walls in the sub‐micrometer thick films of the technologically important ferroelectric LiNbO₃is explored. It is found that the electric bias generates stable domains with strongly inclined domain boundaries with the inclination angle reaching 20° with respect to the polar axis. The head‐to‐head domain boundaries exhibit high conductance, which can be modulated by application of the sub‐coercive voltage. Electron microscopy visualization of the electrically written domains and piezoresponse force microscopy imaging of the very same domains reveals that the gradual and reversible transition between the conducting and insulating states of the domain walls results from the electrically induced wall bending near the sample surface. The observed modulation of the wall conductance is corroborated by the phase‐field modeling. The results open a possibility for exploiting the conducting domain walls as the electrically controllable functional elements in the multilevel logic nanoelectronics devices.