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Enhanced susceptibilities in ferroelectrics often arise near phase boundaries between competing ground states. While chemically-induced phase boundaries have enabled ultrahigh electrical and electromechanical responses in lead-based ferroelectrics, precise chemical tuning in lead-free alternatives, such as (K,Na)NbO₃ thin films, remains challenging due to the high volatility of alkali metals. Here, we demonstrate strain-induced morphotropic phase boundary-like polymorphic nanodomain structures in chemically simple, lead-free, epitaxial NaNbO₃ thin films. Combining ab initio simulations, thin-film epitaxy, scanning probe microscopy, synchrotron X-ray diffraction, and electron ptychography, we reveal a labyrinthine structure comprising coexisting monoclinic and bridging triclinic phases near a strain-induced phase boundary. The coexistencemore » of energetically competing phases facilitates field-driven polarization rotation and phase transitions, giving rise to a multi-state polarization switching pathway and large enhancements in dielectric susceptibility and tunability across a broad frequency range. Our results open new possibilities for engineering lead-free thin films with enhanced functionalities for next-generation applications.« less

Abstract Antiferroelectrics, which undergo a field‐induced phase transition to ferroelectric order that manifests as double‐hysteresis polarization switching, exhibit great potential for dielectric, electromechanical, and electrothermal applications. Compared to their ferroelectric cousins, however, considerably fewer efforts have been made to understand and control antiferroelectrics. Here, it is demonstrated that the polarization switching behavior of an antiferroelectric can be strongly influenced and effectively regulated by point defects. In films of the canonical antiferroelectric PbZrO ₃ , decreasing oxygen pressure during deposition (and thus increasing adatom kinetic energy) causes unexpected “ferroelectric‐like” polarization switching although the films remain in the expected antiferroelectric orthorhombic phase.more » This “ferroelectric‐like” switching is correlated with the creation of bombardment‐induced point‐defect complexes which pin the antiferroelectric–ferroelectric phase boundaries, and thus effectively delay the phase transition under changing field. The effective pinning energy is extracted via temperature‐dependent switching‐kinetics studies. In turn, by controlling the concentration of defect complexes, the dielectric tunability of the PbZrO ₃ can be adjusted, including being able to convert between “positive” and “negative” tunability near zero field. This work reveals the important role and strong capability of defects to engineer antiferroelectrics for new performance and functionalities.« less

Here, we report on the high temperature thin film growth of BaTiO₃ on Ti₃C₂T_x MXene flakes using van der Waals epitaxy on a degradable template layer. MXene was deposited on amorphous and crystalline substrates by spray- and dip-coating techniques, while the growth of BaTiO₃ at 700 °C was accomplished using pulsed laser deposition in an oxygen rich environment. We demonstrate that the MXene flakes act as a temporary seed layer, which promotes highly oriented BaTiO₃ growth along the (111) direction independent of the underlying substrate. The lattice parameters of the BaTiO₃ films are close to the bulk value suggesting thatmore » the BaTiO₃ films remains unstrained, as expected for van der Waals epitaxy. The initial size of the MXene flakes has an impact on the orientation of the BaTiO₃ films with larger flake sizes promoting a higher fraction of the polycrystalline film to grow along the (111) direction. The deposited BaTiO₃ film adopts the same morphology as the original flakes and piezoresponse force microscopy shows a robust ferroelectric behavior for individual grains. Transmission electron microscopy results indicate that the Ti₃C₂T_x MXene fully decomposes during the BaTiO3 deposition and the surplus Ti atoms are readily incorporated into the BaTiO₃ film. Electrical measurements show a similar dielectric constant as a BaTiO₃ film grown without the MXene seed layer. The demonstrated process has the potential to overcome the longstanding issue of integrating highly oriented complex oxide thin films directly on any desired substrate.« less