Antiferroelectric-ferroelectric Transition Research Articles

Multiple Polarization States in Hf1- xZrxO2 Thin Films by Ferroelectric and Antiferroelectric Coupling.

HfO2-based multi-bit ferroelectric memory combines non-volatility, speed, and energy efficiency, rendering it a promising technology for massive data storage and processing. However, some challenges remain, notably polarization variation, high operation voltage, and poor endurance performance. Here we show Hf1- xZrxO2 (x=0.65 to 0.75) thin films grown through sequential atomic layer deposition (ALD) of HfO2 and ZrO2 exhibiting three-step domain switching characteristic in the form of triple-peak coercive electric field (EC) distribution. This long-sought behavior shows nearly no changes even at up to 125°C and after 1 × 108 electric field cycling. By combining the electrical characterizations and integrated differential phase-contrast scanning transmission electron microscopy (iDPC-STEM), wereveal that the triple-peak EC distribution is driven by the coupling of ferroelectric switching and reversible antiferroelectric-ferroelectric transition. We further demonstrate the 3-bit per cell operation of the Hf1- xZrxO2 capacitors with excellent device-to-device variation and long data retention,by the full switching of individual peaks in the triple-peak EC. The work represents a significant step in implementing reliable non-volatile multi-state ferroelectric devices.

Noncollinear ferroelectric and screw-type antiferroelectric phases in a metal-free hybrid molecular crystal

Noncollinear dipole textures greatly extend the scientific merits and application perspective of ferroic materials. In fact, noncollinear spin textures have been well recognized as one of the core issues of condensed matter, e.g. cycloidal/conical magnets with multiferroicity and magnetic skyrmions with topological properties. However, the counterparts in electrical polarized materials are less studied and thus urgently needed, since electric dipoles are usually aligned collinearly in most ferroelectrics/antiferroelectrics. Molecular crystals with electric dipoles provide a rich ore to explore the noncollinear polarity. Here we report an organic salt (H2Dabco)BrClO4 (H2Dabco = N,N’−1,4-diazabicyclo[2.2.2]octonium) that shows a transition between the ferroelectric and antiferroelectric phases. Based on experimental characterizations and ab initio calculations, it is found that its electric dipoles present nontrivial noncollinear textures with 60o-twisting angle between the neighbors. Then the ferroelectric-antiferroelectric transition can be understood as the coding of twisting angle sequence. Our study reveals the unique science of noncollinear electric polarity.

Giant enhancement and quick stabilization of capacitance in antiferroelectrics by phase transition engineering

The antiferroelectric-ferroelectric phase transition is a basic principle that holds promise for antiferroelectric ceramics in high capacitance density nonlinear capacitors. So far, the property optimization based on antiferroelectric-ferroelectric transition is solely undertaken by chemical composition tailoring. Alternately, here we propose a phase transition engineering tactic by applying pulsed electric stimulus near the critical electric field, which finally results in ~54.3% enhancement and quick stabilization of capacitance density in Pb0.97La0.02(Zr0.35Sn0.55Ti0.10)O3 antiferroelectric ceramics. Ex-situ and in-situ structural characterizations show that electric stimuli can induce the charming successive structural evolution, including domain evolution from multidomain to monodomain state, and modulation period change from 7.49 to 7.73. Structure-property correlation indicates that the antiferroelectric-ferroelectric phase transition engineering mainly stems from the unexpected irreversible recovery of the modulated structures. The present findings would deepen the understanding of the structural phase transition and provoke composition-independent post-treatment property innovation in the incommensurate antiferroelectric materials and devices.

Effect of oxygen octahedral tilt on ferroelectric-antiferroelectric transition of PZT95/5 induced by hydrostatic pressure and electric field: A phenomenological thermodynamic analysis

Ceramic PZT95/5 lies near the boundary between the ferroelectric and antiferroelectric phases and undergoes complex domain switching and various phase transitions in response to temperature, pressure, and electric fields. This phenomenon often leads to unusual performances, making it challenging to characterize and control the electromechanical properties of the material. In particular, the phase transition from ferroelectric rhombohedral to antiferroelectric orthorhombic is caused by the combined effect of polarization switching and oxygen octahedral tilt. However, the underlying mechanism of their interaction is still unclear, resulting in a lack of thermodynamic models for ferroelectric-antiferroelectrics in multifield settings. In this study, a thermodynamic model was established by considering the combined effect of oxygen octahedral tilt, polarization, and stress, which can describe the polarity characteristics of phase transitions in PZT95/5 under multiple fields. This model comprehensively accounts for all phases of PZT95/5 and successfully simulates the experimental phase diagram of PZT95/5 under hydrostatic pressure. The tilt angle of the oxygen octahedra and the polarization were incorporated to reveal the special polarity microstructure of the intermediate phase between the ferroelectric and antiferroelectric phases and determine the effect of pressure on the incommensurate modulations. Exploring the application of antiferroelectric materials in smart devices via the pressure-induced transition of incommensurate modulation phases may become a new possibility.

Low-temperature stable ferroelectric–antiferroelectric transition for cryogenic energy storage application

The capacitors are in rising demand for cryogenic applications. As for now, it still remains an ongoing challenge for simultaneously achieving high energy storage density and cryogenic temperature stability. Herein, the strategy of stable backward phase transition was demonstrated in the antiferroelectric composition of (Pb0.9175La0.055)(Zr0.975Ti0.025)O3. As a result, we achieved high recoverable energy density about 10 J/cm3 with exceptional low-temperature stability from −160 to 25 °C. Multi-layer ceramic capacitors designed for pulse discharge applications also demonstrated high performance in cryogenic conditions, with the peak current fluctuations of less than 4%. Through in situ characterizations using x-ray diffraction, Raman spectra, and transmission electron microscopy, we discovered that the anisotropic structural evolution is responsible for a stable backward phase transition, providing the material with robust stability at cryogenic temperatures. These results offer a good paradigm for improving the temperature stability of antiferroelectric multi-layer capacitors to meet the rigorous demands of energy storage applications.

Superb energy density of PbHfO3-based antiferroelectric ceramics via regulating the antiferroelectric–ferroelectric transition energy barrier

By coordinating the positions of the AFE and FE states in the energy path, a high Pmax was achieved while stabilizing the AFE nature, resulting in a remarkable Wrec of 16.05 J cm−3.

Compromise Optimized Superior Energy Storage Performance in Lead-Free Antiferroelectrics by Antiferroelectricity Modulation and Nanodomain Engineering.

Lead-free antiferroelectrics with excellent energy storage performance can become the core components of the next-generation advanced pulse power capacitors. However, the low energy storage efficiency caused by the hysteresis of antiferroelectric-ferroelectric transition largely limits their development toward miniaturization, lightweight, and integration. In this work, an ultrahigh recoverable energy storage density of ≈11.4 J cm-3 with a high efficiency of ≈80% can be realized in La-modified Ag0.5 Na0.5 NbO3 antiferroelectric ceramics at an ultrahigh breakdown electric field of ≈67kV mm-1 by the compromise optimization between antiferroelectricity enhancement and nanodomain engineering, resulting in the transformation of large-size ferrielectric antipolar stripe domains into ultrasmall antiferroelectric nanodomains or polarization nanoregions revealing as Moiré fringe structures. In addition, the enhanced transparency with increasing La content can also be clearly observed. This work not only develops new lead-free antiferroelectric energy storage materials with high application potential but also demonstrates that the strategy of compromise optimization between antiferroelectricity modulation and nanodomain engineering is an effective avenue to enhance the energy storage performance of antiferroelectrics.

Spatial buffer-area assisted antiferroelectric-ferroelectric transition in NaNbO3

Lead-free antiferroelectric materials can be used in environmental-friendly energy applications and are receiving tremendous attention owing to their high energy-storage density, which is achieved through a phase transition between antiferroelectric and ferroelectric state. Here, by slowing down the antiferroelectric-ferroelectric transition via controllable beam irradiation in a transmission electron microscope, we demonstrate that such a transition in NaNbO3 undergoes by means of forming a buffer-area (about 40–150 nm wide) at the on-going transition front. The spatial buffer-area approximately maintains its shape during moving and are characterized to be the antiferroelectric phase but exhibits a gradual variation of lattice spacing along the [010] proceeding direction of phase transition. With help of such buffer-area, the overall stress or barrier between the two phases could be considerably reduced compared to the direct change between the two structures. Therefore, our findings provide new insights for understanding the dynamical energy-storage process in antiferroelectric materials.

Ferroelectric-Antiferroelectric Transition of Hf1-x Zr x O2 on Indium Arsenide with Enhanced Ferroelectric Characteristics for Hf0.2Zr0.8O2.

The ferroelectric (FE)-antiferroelectric (AFE) transition in Hf1-x Zr x O2 (HZO) is for the first time shown in a metal-ferroelectric-semiconductor (MFS) stack based on the III-V material InAs. As InAs displays excellent electron mobility and a narrow band gap, the integration of ferroelectric thin films for nonvolatile operations is highly relevant for future electronic devices and motivates further research on ferroelectric integration. When increasing the Zr fraction x from 0.5 to 1, the stack permittivity increases as expected, and the transition from FE to AFE-like behavior is observed by polarization and current-voltage characteristics. At x = 0.8 the polarization of the InAs-based stacks shows a larger FE-contribution as a more open hysteresis compared to both literature and reference metal-ferroelectric-metal (MFM) devices. By field-cycling the devices, the switching domains are studied as a function of the cycle number, showing that the difference in FE-AFE contributions for MFM and MFS devices is stable over switching and not an effect of wake-up. The FE contribution of the switching can be accessed by successively lowering the bias voltage in a proposed pulse train. The possibility of enhanced nonvolatility in Zr-rich HZO is relevant for device stacks that would benefit from an increase in permittivity and a lower operating voltage. Additionally, an interfacial layer (IL) is introduced between the HZO film and the InAs substrate. The interfacial quality is investigated as frequency-dependent capacitive dispersion, showing little change for varying ZrO2 concentrations and with or without included IL. This suggests material processing that is independently limiting the interfacial quality. Improved endurance of the InAs-Hf1-x Zr x O2 devices with x = 0.8 was also observed compared to x = 0.5, with further improvement with the additional IL for Zr-rich HZO on InAs.

Ultrahigh Piezoelectric Strains in PbZr1-xTixO3 Single Crystals with Controlled Ti Content Close to the Tricritical Point.

Intensive investigations of PbZr1-xTixO3 (PZT) materials with the ABO3 perovskite structure are connected with their extraordinary piezoelectric properties. Especially well known are PZT ceramics at the Morphotropic Phase Boundary (MPB), with x~0.48, whose applications are the most numerous among ferroelectrics. These piezoelectric properties are often obtained by doping with various ions at the B sites. Interestingly, we have found similar properties for undoped PZT single crystals with low Ti content, for which we have confirmed the existence of the tricritical point near x~0.06. For a PbZr0.95 ± 0.01Ti0.05 ∓ 0.01O3 crystal, we describe the ultrahigh strain, dielectric, optical and piezoelectric properties. We interpret the ultrahigh strain observed in the region of the antiferroelectric–ferroelectric transition as an inverse piezoelectric effect generated by the coexistence of domains of different symmetries. The complex domain coexistence was confirmed by determining optical indicatrix orientations in domains. The piezoelectric coefficient in this region reached an extremely high value of 5000 pm/V. We also verified that the properties of the PZT single crystals from the region near the tricritical point are incredibly susceptible to a slight deviation in the Ti content.

Dielectric, piezoelectric and electrostrictive properties of antiferroelectric lead-zirconate thin films

Dielectric, piezoelectric and electrostrictive properties of antiferroelectric lead zirconate thin films, elaborated by sol gel on alumina substrates, have been studied as a function of the driving field magnitude EAC. Measurement of the displacement shows that the strain for applied fields below the antiferroelectric-ferroelectric transition is relatively small and that its main contribution arises from the electrostrictive effect. At the same time, due to the presence of a residual ferroelectricity, the piezoelectric effect also contributes to the displacement. At higher fields, a large strain is visible which comes mainly from the antiferroelectric to ferroelectric phase transition, the electrostrictive contribution however still being present. Similar to what has been shown for ferroelectric materials, strain versus the square of polarization loops S(P2) of the studied antiferroelectric material, exhibit hysteresis character due to the 180∘ domain walls which contribute to polarization but not to strain. Simultaneous measurement of polarization and displacement enables the extraction of the electrostrictive coefficient and a value Q = 0.082 ± 0.009 m4 C−2 has been obtained. Subtraction of the pure electrostrictive contribution from the displacement curve allows evidencing that the piezoelectric activity coexists with the electrostrictive effect at low fields. Maximum values of the equivalent piezoelectric coefficients are respectively 93 ± 3 pm V−1 and 100 ± 3 pm V−1 for the positive and the negative parts of the curve for a driving field magnitude EAC = 700 kV cm−1.

Li+ and Sm3+ co-doped AgNbO3-based antiferroelectric ceramics for high-power energy storage

Ag1-x-3yLixSmyNbO3 (x≤0.05, y≤0.05) (ALSN) antiferroelectric ceramics were successfully prepared via conventional solid-state reaction and sinter routes in oxygen atmosphere for improving the energy storage characteristic of pure AgNbO3. The results indicate that all of the studied compositions display a pure orthorhombic antiferroelectric (AFE) perovskite structure, while their key parameters of electric-field-induced antiferroelectric-ferroelectric transition can be affected by Li+ or/and Sm3+ doping contents. The Sm3+ doping can enhance the stability of antiferroelectric state, giving rise to higher antiferroelectric-ferroelectric transition electric-field (EF and EB), while Li+ doping can reduce EF and EB for Sm3+ doped AgNbO3 with low Sm3+ content (y≤0.03). When co-doping the same amounts of Li+ and Sm3+ at x=y≤0.03, both EF and EB almost remain unchanged. At x=y=0.05, the diffuse phase transition (DPT) behavior of antiferroelectric-paraelectric (AFE-PE) phase transition occurred, resulting in a “slim-like” double-polarization hysteresis with significantly enhanced EF. Due to these features, both Wrec and η are improved compared with pure AgNbO3. The Wrec and η with composition at x=y=0.05 is 2.33 J/cm3 and 58% under applying electric field of 240 kV/cm, respectively. The results suggest that building DPT behavior of AFE-PE phase transition could be an alternative strategy to improve the energy storage characteristic of AgNbO3.

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% (870kV 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.

Decoding the Double/Multiple Hysteresis Loops in Antiferroelectric Materials.

Antiferroelectric materials has become one of the most promising candidates for pulsed power capacitors. The polarization versus electric-field hysteresis loop is the key electrical property for evaluating their energy-storage performance. Here, we applied in situ biasing transmission electron microscopy to decode two representative energy-storage behaviors-namely, multiple and double hysteresis loops-in (Pb,La)(Zr,Sn,Ti)O3 system. Simultaneous structural examination and domain/defects observation establish a direct relationship between phase transitions and hysteresis loops. Sustaining a smaller period of modulated structure to a certain applied electric field and then undergoing additional transitions among varying antiferroelectric phases with increasing modulation periods before the final antiferroelectric-ferroelectric transition leads to the favorable multiple-loop configuration that realizes a high energy-storage performance. Such phenomenon is described by a model in terms of defect-driven phase transition. The distinctive mechanisms of multiple phase transition will inspire future composition-design for high switch-fielding antiferroelectric materials.

Phase boundary and temperature driven enhanced piezoelectric and electrostrictive strain in (1−2x) Bi0.5Na0.5TiO3-xBaTiO3-xBa0.7Ca0.3TiO3 solid solution

Bismuth-based piezoelectric ceramics are presently of immense interest to researchers as they are believed to be Pb-free alternatives to well-known lead zirconate titanate-based piezoceramics. Herein, the author reports a lead-free ternary solid solution (1−2x)Na0.5Bi0.5TiO3-xBaTiO3-xBa0.7Ca0.3TiO3 (x = 0.01, 0.03, 0.05, 0.07, and 0.09: BNT-BT-BCT) synthesized through a standard solid state reaction route. All the samples crystallized to a complete perovskite structure studied through the powder x-ray diffraction analysis. Rietveld analysis of x-ray diffraction data revealed a structural transformation from monoclinic (Cc) to tetragonal phase (p4mm) with the co-existence of monoclinic (Cc) and tetragonal (p4mm) phases in the samples of x ≥ 0.03. The temperature-dependent dielectric analysis of (x = 0.03 and x = 0.05) systems suggests relaxor characteristics near ferroelectric–antiferroelectric phase transition temperature (Td). A changeover from relaxor to a near normal ferroelectric character was realized for x ≥ 0.07. Furthermore, the existence of polar nano-regions (PNRs) was studied through HR-TEM. Interestingly, a low electric field (±25 kV/cm) driven enhanced piezoelectric [(with 0.22% of strain; Smax/Emax = 850 pm/V for x = 0.03) and (with 0.17% strain and Smax/Emax of 714 pm/V for x = 0.07)] and an electrostrictive [with 0.20% of strain; Smax/Emax = 820 pm/V for x = 0.05] was achieved around Td. This can be attributed to the combined effects of phase boundary, ferroelectric–antiferroelectric transition, and the existence of PNRs.

A Novel Combinatorial Approach to the Ferroelectric Properties in HfxZr1−xO2 Deposited by Atomic Layer Deposition

Since HfO2‐based ferroelectric films were reported in 2011, several doping/alloying elements have been introduced to secure interesting ferroelectric/antiferroelectric properties of Hf‐based fluorite‐structured thin films. Using a conventional approach by atomic layer deposition (ALD), an enormous number of experiments would be required to reveal the compositional effects of doping/alloying components in ternary and quaternary systems. Therefore, for a comprehensive study of the ferroelectric properties of HfxZr1−xO2, a novel combinatorial ALD technique that enables the fabrication of multicomponent films with a different composition ratio and thickness via saturated/non‐saturated ALD without changing the supercycle of each material is reported. The gradient of the amount of HfO2 over a 100 mm substrate is achieved using a lower Hf‐precursor temperature that results in an insufficient Hf‐precursor dose for saturated deposition. Systematic study on the HfxZr1−xO2 combinatorial library is carried out by spectroscopic ellipsometry, X‐ray photoelectron spectroscopy, and X‐ray diffraction. Furthermore, TiN/HfxZr1−xO2/TiN capacitors are fabricated to measure the remnant polarization. The obtained results clearly show the ferroelectric–antiferroelectric transition according to continuous composition change instead of discrete research. Therefore, a highly productive method for studying ferroelectricity changes in terms of a doping and a composition of HfO2‐based ferroelectric thin films through a novel ALD combinatorial approach is proposed.

Motion of phase boundary during antiferroelectric–ferroelectric transition in a PbZrO3 -based ceramic

The in situ biasing transmission electron microscopy technique is employed to investigate the nucleation and growth of the ferroelectric phase during the electric field-induced phase transition in ${\mathrm{Pb}}_{0.99}{{\mathrm{Nb}}_{0.02}{[{({\mathrm{Zr}}_{0.57}{\mathrm{Sn}}_{0.43})}_{0.94}{\mathrm{Ti}}_{0.06}]}_{0.98}}{\mathrm{O}}_{3}$, a ${\mathrm{PbZrO}}_{3}$-based antiferroelectric ceramic. The first-order displacive phase transition is found to be highly reversible with the initial antiferroelectric domain configuration almost completely recovered upon removal of the applied field. In the forward transition from the antiferroelectric to ferroelectric phase, ${{100}}_{\mathrm{c}}$ facets are dominant on the phase boundary; while in the reverse transition from the ferroelectric to antiferroelectric phase during bias unloading, the phase boundary is segmented into ${{101}}_{\mathrm{c}}$ and ${{121}}_{\mathrm{c}}$ facets. The motion of the phase boundary is nonuniform, taking the form of sequential sweeping of facet segments. The elastic distortion energy and the depolarization energy at the antiferroelectric/ferroelectric phase boundary is suggested to dictate the facet motion.

High pyroelectric performance due to ferroelectric–antiferroelectric transition near room temperature

Pyroelectric materials have a huge market in daily life applications and high pyroelectric performances near room temperature are highly desired.

Effect of fluorinated achiral chain length on structural, dielectric and electro-optic properties of two terphenyl based antiferroelectric liquid crystals

Two antiferroelectric liquid crystals of different fluorinated chain length have been studied. Although both exhibit antiferroelectric (SmCA*), ferroelectric (SmC*) and paraelectric (SmA*) phases, melting point decreases with increasing achiral chain length but clearing point shows opposite trend. Thermal ranges of SmCA* and SmC* phases exhibit odd-even effect like the dipole moments and spontaneous polarization. Layer spacings, average intermolecular distances and correlation lengths across the smectic planes also increase with chain length. Temperature dependence of layer spacings, x-ray tilt, optical tilt and spontaneous polarization suggest tricritical nature of SmC*–SmA* transition. Dielectric increments decrease with increased achiral chain. Both soft mode and Goldstone mode critical frequencies decrease with decreasing temperature in the lower derivative but opposite behaviour is observed in the higher derivative. Both the compounds show Curie–Weiss behaviour in soft mode near SmC*–SmA* transition temperature (Tc). Goldstone mode critical frequency is much higher in the higher derivative. Both in-phase and anti-phase antiferroelectric modes are observed in SmCA* phase in the higher derivative but in the lower derivative only anti-phase antiferroelectric mode is observed. Optical tilts suggest orthoconic nature of the SmCA* phase of both the compounds. Ferroelectric-antiferroelectric transition temperature decreases with increasing ac field in both compounds.

Large energy storage density, low energy loss and highly stable (Pb0.97La0.02)(Zr0.66Sn0.23Ti0.11)O3 antiferroelectric thin-film capacitors

In this work, high performance (Pb0.97La0.02)(Zr0.66Sn0.23Ti0.11)O3 polycrystalline antiferroelectric thin-film was successfully fabricated on (La0.7Sr0.3)MnO3/Al2O3(0001) substrate via a cost-effectively chemical solution method. A large recoverable energy storage density (Wre) of 46.3 J/cm3 and high efficiency (η) of 84% were realized simultaneously under an electric field of 4 MV/cm by taking full advantage of the linear dielectric response after the electric field induced antiferroelectric-ferroelectric transition. Moreover, the PLZST thin-film displayed high temperature stability. With increasing temperature from 300 K to 380 K, the Wre decreased only 1.3%. The film also exhibited good fatigue endurance up to 1 × 105 cycling under an electric field of 2.2 MV/cm. Our work underlines the importance of the interface quality between the film and the substrate and the important role of linear dielectric answer after saturation in the improvement of the energy storage density and efficiency of antiferroelectric materials.