Uncompensated Polarization in Incommensurate Modulations of Perovskite Antiferroelectrics

Clamping enables enhanced electromechanical responses in antiferroelectric thin films

Thin-film materials with large electromechanical responses are fundamental enablers of next-generation micro-/nano-electromechanical applications. Conventional electromechanical materials (for example, ferroelectrics and relaxors), however, exhibit severely degraded responses when scaled down to submicrometre-thick films due to substrate constraints (clamping). This limitation is overcome, and substantial electromechanical responses in antiferroelectric thin films are achieved through an unconventional coupling of the field-induced antiferroelectric-to-ferroelectric phase transition and the substrate constraints. A detilting of the oxygen octahedra and lattice-volume expansion in all dimensions are observed commensurate with the phase transition using operando electron microscopy, such that the in-plane clamping further enhances the out-of-plane expansion, as rationalized using first-principles calculations. In turn, a non-traditional thickness scaling is realized wherein an electromechanical strain (1.7%) is produced from a model antiferroelectric PbZrO3 film that is just 100 nm thick. The high performance and understanding of the mechanism provide a promising pathway to develop high-performance micro-/nano-electromechanical systems.

Room-temperature stabilizing strongly competing ferrielectric and antiferroelectric phases in PbZrO3 by strain-mediated phase separation

PbZrO3 has been broadly considered as a prototypical antiferroelectric material for high-power energy storage. A recent theoretical study suggests that the ground state of PbZrO3 is threefold-modulated ferrielectric, which challenges the generally accepted antiferroelectric configuration. However, such a novel ferrielectric phase was predicted only to be accessible at low temperatures. Here, we successfully achieve the room-temperature construction of the strongly competing ferrielectric and antiferroelectric state by strain-mediated phase separation in PbZrO3/SrTiO3 thin film. We demonstrate that the phase separation occurs spontaneously in quasi-periodic stripe-like patterns under a compressive misfit strain and can be tailored by varying the film thickness. The ferrielectric phase strikingly exhibitsa threefold modulation period with a nearly up-up-down configuration, which could be stabilized and manipulated by the formation and evolution of interfacial defects under applied strain. The present results construct a fertile ground for further exploring the physical properties and applications based on the novel ferrielectric phase.

Emergent Three-Dimensional Electric Dipole Sinewave in Bulk Perovskite Oxides

The magnetic and electric dipoles of ferroics play a central role in their fascinating properties. In particular, topological configurations have shown promising potential for use in novel electromechanical and electronic devices. Magnetic configurations from simple collinear to complex topological are well-documented. In contrast, many complex topological features in the electric counterpart remain unexplored. Here, we report the first example of three-dimensional electric dipole sinewave topological structure in a PbZrO3-based bulk perovskite, which presents an interesting triple-hysteresis loop macroscopically. This polar configuration consists of two orthogonal sinewave electric dipole modulations decoded from a polar incommensurate phase by advanced diffraction and atomic-resolution imaging techniques. The resulting topology is unraveled to be the competition between the antiferroelectric and ferroelectric states, stabilized by the modulation of the Pb 6s2 lone pair and the antiferrodistortive effect. These findings further reinforce the similarity of the magnetic and electric topologies.

In-Plane Ferrielectric Order in van der Waals β'-In2Se3

van der Waals ferroic materials exhibit rich potential for implementing future generation functional devices. Among these, layered β'-In2Se3 has fascinated researchers with its complex superlattice and domain structures. As opposed to ferroelectric α-In2Se3, the understanding of β'-In2Se3 ferroic properties remains unclear because ferroelectric, antiferroelectric, and ferroelastic characteristics have been separately reported in this material. To develop useful applications, it is necessary to understand the microscopic structural properties and their correlation with macroscopic device characteristics. Herein, using scanning transmission electron microscopy (STEM), we observed that the arrangement of dipoles deviates from the ideal double antiparallel antiferroelectric character due to competition between antiferroelectric and ferroelectric structural ordering. By virtue of second-harmonic generation, four-dimensional STEM, and in-plane piezoresponse force microscopy, the long-range inversion-breaking symmetry, uncompensated local polarization, and net polarization domains are unambiguously verified, revealing β'-In2Se3 as an in-plane ferrielectric layered material. Additionally, our device study reveals analogous resistive switching behaviors of different types owing to polarization switching, defect migration, and defect-induced charge trapping/detrapping processes.

Translational Antiphase Boundaries in NaNbO3 Antiferroelectrics

Planar defects are known to be of importance in affecting the functional properties of materials. Translational antiphase boundaries (APBs) in particular have attracted considerable attention in perovskite oxides, but little is known in lead-free antiferroelectric oxides that are promising candidates for energy storage applications. Here, we present a study of translational APBs in prototypical antiferroelectric NaNbO3 using aberration-corrected (scanning) transmission electron microscopy (TEM) techniques at different length scales. The translational APBs in NaNbO3 are characterized by a 2-fold-modulated structure, which is antipolar in nature and exhibits a high density, different from the polar nature and lower density in PbZrO3. The high stability of translational APBs against external electric fields and elevated temperatures was revealed using ex situ and in situ TEM experiments and is expected to be associated with their antipolar nature. Density functional theory calculations demonstrate that translational APBs possess only slightly higher free energy than the antiferroelectric and ferroelectric phase energies with differences of 29 and 33 meV/f.u., respectively, justifying their coexistence down to the nanoscale at room temperature. These results provide a detailed atomistic elucidation of translational APBs in NaNbO3 with antipolar character and stability against external stimuli, establishing the basis of defect engineering of antiferroelectrics for energy storage devices.

Translational Boundaries as Incipient Ferrielectric Domains in Antiferroelectric PbZrO_{3}

In the archetypal antiferroelectric PbZrO_{3}, antiparallel electric dipoles cancel each other, resulting in zero spontaneous polarization at the macroscopic level. Yet in actual hysteresis loops, the cancellation is rarely perfect and some remnant polarization is often observed, suggesting the metastability of polar phases in this material. In this work, using aberration-corrected scanning transmission electron microscopy methods on a PbZrO_{3} single crystal, we uncover the coexistence of the common antiferroelectric phase and a ferrielectric phase featuring an electric dipole pattern of ↓↑↓. This dipole arrangement, predicted by Aramberri et al. to be the ground state of PbZrO_{3} at 0 K, appears at room temperature in the form of translational boundaries. The dual nature of the ferrielectric phase, both a distinct phase and a translational boundary structure, places important symmetry constraints on its growth. These are overcome by sideways motion of the boundaries, which aggregate to form arbitrarily wide stripe domains of the polar phase embedded within the antiferroelectric matrix.

Polarization Switching and Correlated Phase Transitions in Fluorite-Structure ZrO2 Nanocrystals

Unconventional ferroelectricity in fluorite-structure oxides enables tremendous opportunities in nanoelectronics owing to their superior scalability and silicon compatibility. However, their polarization order and switching process remain elusive due to the challenges of visualizing oxygen ions in nanocrystalline films. In this work, the oxygen shifting during polarization switching and correlated polar-nonpolar phase transitions are directly captured among multiple metastable phases in freestanding ZrO₂ thin films by low-dose integrated differential phase-contrast scanning transmission electron microscopy (iDPC-STEM). Bidirectional transitions between antiferroelectric and ferroelectric orders and interfacial polarization relaxation are clarified at unit-cell scale. Meanwhile, polarization switching is strongly correlated with Zr-O displacement in reversible martensitic transformation between monoclinic and orthorhombic phases and two-step tetrahedral-to-orthorhombic phase transition. These findings provide atomic insights into the transition pathways between metastable polymorphs and unravel the evolution of polarization orders in (anti)ferroelectric fluorite oxides.

Atomic Insight into the Successive Antiferroelectric-Ferroelectric Phase Transition in Antiferroelectric Oxides

Antiferroelectrics characterized by voltage-driven reversible transitions between antiparallel and parallel polarity are promising for cutting-edge electronic and electrical power applications. Wide-ranging explorations revealing the macroscopic performances and microstructural characteristics of typical antiferroelectric systems have been conducted. However, the underlying mechanism has not yet been fully unraveled, which depends largely on the atomistic processes. Herein, based on atomic-resolution transmission electron microscopy, the deterministic phase transition pathway along with the underlying lattice-by-lattice details in lead zirconate thin films was elucidated. Specifically, we identified a new type of ferrielectric-like dipole configuration with both angular and amplitude modulations, which plays the role of a precursor for a subsequent antiferroelectric to ferroelectric transformation. With the participation of the ferrielectric-like phase, the phase transition pathways driven by the phase boundary have been revealed. We provide new insights into the consecutive phase transformation in low-dimensional lead zirconate, which thus would promote potential antiferroelectric-based multifunctional devices.

What Determines the Critical Electric Field of AFE-to-FE in Pb(Zr,Sn,Ti)O3-Based Perovskites?

Electric-field-induced antiferroelectric-ferroelectric (AFE-FE) phase transition is a prominent feature of antiferroelectric (AFE) materials. The critical electric field of this phase transition is crucial for the device performance of AEFs in many applications, but the determining factor of the critical electric field is still unclear. Here, we have established the correlation between the underlying structure and the critical electric field by using in situ synchrotron X-ray diffraction and high-resolution neutron diffraction in Pb(Zr,Sn,Ti)O₃-based antiferroelectrics. It is found that the critical electric field is determined by the angle between the average polarization vector in the incommensurate AFE state and the [111]_P polarization direction in the rhombohedral FE state. A large polarization rotation angle gives rise to a large critical electric field. Further, density functional theory (DFT) calculations corroborate that the lower energy is required for driving a smaller angle polarization rotation. Our discovery will offer guidance to optimize the performance of AFE materials.

Antiferroelectric PbSnO3 Epitaxial Thin Films

In condensed matter physics, oxide materials show various intriguing physical properties. Therefore, many efforts are made in this field to develop functional oxides. Due to the excellent potential for tin-based perovskite oxides, an expansion of new related functional compounds is crucial. This work uses a heteroepitaxial approach supported by theoretical calculation to stabilize PbSnO₃ thin films with different orientations. The analyses of X-ray diffraction and transmission electron microscopy unveil the structural information. A typical antiferroelectric feature with double hysteresis and butterfly loops is observed through electrical characterizations consistent with the theoretical prediction. The phase transition is monitored, and the transition temperatures are determined based on temperature-dependent structural and electrical characterizations. Furthermore, the microscopic antiferroelectric order is noticed under atomic resolution images via scanning transmission electron microscopy. This work offers a breakthrough in synthesizing epitaxial PbSnO₃ thin films and comprehensively understanding its anisotropic antiferroelectric behavior.

Phase Competition in High-Quality Epitaxial Antiferroelectric PbZrO3 Thin Films

Antiferroelectric PbZrO₃ has attracted renewed interest in recent years because of its unique properties and wide range of potential applications. However, the nature of antiferroelectricity and its evolution with the electric field and temperature remain controversial, mostly due to the difficulty of obtaining high-quality single-crystal samples. The lack of consensus regarding the phase transition in PbZrO₃ is not only important on a fundamental side but also greatly hinders further applications. Herein, high-quality PbZrO₃ epitaxial thin films are successfully fabricated by pulsed laser deposition. The structural and physical properties of the films are systematically studied via a combination of electric property measurements, X-ray diffraction, scanning transmission electron microscopy imaging, and second-harmonic generation studies. Our studies unveil the noncentrosymmetric nature of PbZrO₃ films at room temperature. Moreover, the Curie temperature increased to 270°, ∼40° higher than that in the bulk, and no intermediate ferroelectric phase was observed. Besides, an incipient ferroelectric with relaxor-like behavior above the Curie temperature due to the existence of a local polar cluster in the high-temperature paraelectric phase is experimentally observed for the first time. Our studies provide a better understanding of PbZrO₃ thin films and pave the way for practical applications of antiferroelectric material in modern electronic devices.

Antiferroelectric Phase Diagram Enhancing Energy-Storage Performance by Phase-Field Simulations

Antiferroelectric materials have shown potential applications in energy storage. However, controlling and improving the energy-storage performance in antiferroelectric remain challenging. Here, a domain structure and energy-storage performance diagram for Pb(Zr_1-xTi_x)O₃ (x ≤ 0.1) single crystal are investigated via phase-field simulations. Controlling the ratio of domain wall coefficients λ and g can tune the periodicities of the antiferroelectric stripe domain and generate a complicated topological domain. By decreasing the antiferroelectric domain periodicity, one can achieve high recoverable energy-storage density (W_rec = 30.24 J/cm³) with an efficiency of 80.9%. In addition, Pb(Zr_1-xTi_x)O₃ (x ≤ 0.1) thin-film system has also been investigated. Positive equiaxial misfit strain significantly enhances recoverable energy-storage density up to 21.96 J/cm³ with an efficiency of 84.9%. Our results offer another train of thought to tune antiferroelectric domain structure, which provides the idea to design high-energy-density materials in experiments.

Tunable Domain Switching Features of Incommensurate Antiferroelectric Ceramics Realizing Excellent Energy Storage Properties

An incommensurate modulated antiferroelectric phase is a key part of ideal candidate materials for the next generation of dielectric ceramics with excellent energy storage properties. However, there is less research carried out when considering its relatively low polarization response. Here, the incommensurate phase is modulated by stabilizing the antiferroelectric phase and the energy storage performance of the incommensurate phase under ultrahigh electric field is studied. The tape-casting method is applied to construct dense and thin ceramics. La³⁺ doping induces a room-temperature incommensurate antiferroelectric orthorhombic matrix. With little Cd²⁺ , the extremely superior energy storage performances arose as follows: when 0.03, the recoverable energy storage density reaches ≈19.3 J cm^-3 , accompanying an ultrahigh energy storage efficiency of ≈91% (870 kV cm^-1 ); also, a giant discharge energy density of ≈15.4 J cm^-3 emerges during actual operation. In situ observations demonstrate that these superior energy storage properties originate from the phase transition from the incommensurate antiferroelectric orthorhombic phase to the induced rhombohedral relaxor ferroelectric one. The adjustable incommensurate period affects the depolarization response. The revealed phase-transition mechanism enriches the existing antiferroelectric-ferroelectric transition. Attention to the incommensurate phase can provide a reference for the selection of the next generation of high-performance antiferroelectric materials.

Discovery of electric devil's staircase in perovskite antiferroelectric

The devil's staircase, describing step-like function for two competing frequencies, is well known over a wide range of dynamic systems including Huyghens' clocks, Josephson junction, and chemical reaction. In condensed matter physics, the devil's staircase has been observed in spatially modulated structures, such as magnetic ordering. It draws widespread attentions because it plays a crucial role in the fascinating phenomena including phase-locking behaviors, commensurate-incommensurate phase transition, and spin-valve effect. Here, we report the observation of polymorphic phase transitions consisting of several steps in PbZrO₃-based system-namely, electric devil's staircase-originated from competing ferroelectric and antiferroelectric interactions. We fully characterize a specific electric dipole configuration by decomposing this competitive interaction in terms of basic structure and modulation function. Of particular interest is that the occurrence of many degenerate electric dipole configurations in devil's staircase enables superior energy storage performance. These observations are of great significance for exploring more substantive magnetic-electric correspondence and engineering practical high-power antiferroelectric capacitors.

Structural Phase Transition and In-Situ Energy Storage Pathway in Nonpolar Materials: A Review

Benefitting from exceptional energy storage performance, dielectric-based capacitors are playing increasingly important roles in advanced electronics and high-power electrical systems. Nevertheless, a series of unresolved structural puzzles represent obstacles to further improving the energy storage performance. Compared with ferroelectrics and linear dielectrics, antiferroelectric materials have unique advantages in unlocking these puzzles due to the inherent coupling of structural transitions with the energy storage process. In this review, we summarize the most recent studies about in-situ structural phase transitions in PbZrO₃-based and NaNbO₃-based systems. In the context of the ultrahigh energy storage density of SrTiO₃-based capacitors, we highlight the necessity of extending the concept of antiferroelectric-to-ferroelectric (AFE-to-FE) transition to broader antiferrodistortive-to-ferrodistortive (AFD-to-FD) transition for materials that are simultaneously ferroelastic. Combining discussion of the factors driving ferroelectricity, electric-field-driven metal-to-insulator transition in a (La_1−xSr_x)MnO₃ electrode is emphasized to determine the role of ionic migration in improving the storage performance. We believe that this review, aiming at depicting a clearer structure–property relationship, will be of benefit for researchers who wish to carry out cutting-edge structure and energy storage exploration.

Observation of negative capacitance in antiferroelectric PbZrO3 Films

Negative capacitance effect in ferroelectric materials provides a solution to the energy dissipation problem induced by Boltzmann distribution of electrons in conventional electronics. Here, we discover that besides ferroelectrics, the antiferroelectrics based on Landau switches also have intrinsic negative capacitance effect. We report both the static and transient negative capacitance effect in antiferroelectric PbZrO₃ films and reveal its possible physical origin. The capacitance of the capacitor of the PbZrO₃ and paraelectric heterostructure is demonstrated to be larger than that of the isolated paraelectric capacitor at room temperature, indicating the existence of the static negative capacitance. The opposite variation trends of the voltage and charge transients in a circuit of the PbZrO₃ capacitor in series with an external resistor demonstrate the existence of transient negative capacitance effect. Strikingly, four negative capacitance effects are observed in the antiferroelectric system during one cycle scan of voltage pulses, different from the ferroelectric counterpart with two negative capacitance effects. The polarization vector mapping, electric field and free energy analysis reveal the rich local regions of negative capacitance effect with the negative dP/dE and (δ²G)⁄(δD²), producing stronger negative capacitance effect. The observation of negative capacitance effect in antiferroelectric films significantly extends the range of its potential application and reduces the power dissipation further.

The observation of negative capacitance effect is focused on the ferroelectrics, while the antiferroelectrics based on Landau switches may have negative capacitance effect. Here, the authors report the static and transient negative capacitance effect in antiferroelectric PbZrO₃ and reveal its origin.

An Intriguing Polarization Configuration of Mixed Ising- and Néel-Type Model in the Prototype PbZrO3-Based Antiferroelectrics

Antiferroelectrics are of great academic and technological interest due to their excellent strain and energy storage properties, while the incommensurate modulation-related complex configuration of polarization remains unclear. In this study, an intriguing polarization configuration of mixed Ising- and Néel-type has been established in the prototype PbZrO₃-based antiferroelectric materials via the combination of high-resolution synchrotron X-ray and neutron diffraction investigations. Such unusual polarization configuration of antiferroelectrics shows the internal coupling, which mainly contributes from the displacement of the A-site and the distortion of the AO₁₂ and BO₆ polyhedra. The electric dipole oscillates in the (001)_P plane, and its magnitude and direction show obvious modulation characteristics resulting in a neither antiparallel nor fully compensated polarization configuration, which is completely different from the traditional view of antiferroelectricity. There is an antiferroelectric feature along the [110]_P direction, while there is a ferrielectric one along the [1̅10]_P direction in which the net polarization can reach 12 μC/cm². The maximum local polarization can reach 4.3 μC/cm² in the [110]_P direction and 27.6 μC/cm² in the [1̅10]_P direction. This work will be very helpful for the development of antiferroelectric theories and the design of novel energy storage materials.

Unveiling the ferrielectric nature of PbZrO3-based antiferroelectric materials

Benefitting from the reversible phase transition between antiferroelectric and ferroelectric states, antiferroelectric materials have recently received widespread attentions for energy storage applications. Antiferroelectric configuration with specific antiparallel dipoles has been used to establish antiferroelectric theories and understand its characteristic behaviors. Here, we report that the so-called antiferroelectric (Pb,La)(Zr,Sn,Ti)O₃ system is actually ferrielectric in nature. We demonstrate different ferrielectric configurations, which consists of ferroelectric ordering segments with either magnitude or angle modulation of dipoles. The ferrielectric configurations are mainly contributed from the coupling between A-cations and O-anions, and their displacement behavior is dependent largely on the chemical doping. Of particular significance is that the width and net polarization of ferroelectric ordering segments can be tailored by composition, which is linearly related to the key electrical characteristics, including switching field, remanent polarization and dielectric constant. These findings provide opportunities for comprehending structure-property correlation, developing antiferroelectric/ferrielectric theories and designing novel ferroic materials.

The large family PbZrO₃-based solid solutions are usually considered as antiferroelectric materials with specific antiparallel polarization configuration. Here, the authors demonstrate the PbZrO₃-based material has ferrielectric dipoles ordering and configure a clear structure-property relationship.