Ferroelectric Phase Research Articles

Design of polar boundaries enhancing negative electrocaloric performance by antiferroelectric phase-field simulations

Electrocaloric refrigeration which is environmentally benign has attracted considerable attention. In distinction to ferroelectric materials, which exhibit an extremely high positive electrocaloric effect near the Curie temperature, antiferroelectric materials represented by PbZrO3 have a specific negative electrocaloric effect, i.e., electric field decreases the temperature of the materials. However, the explanation of the microscopic mechanism of the negative electrocaloric effect is still unclear, and further research is still needed to provide a theoretical basis for the negative electrocaloric effect enhancement. Herein, the antiferroelectric phase-field model has been proposed to design polar boundaries enhancing antiferroelectric negative electrocaloric performance in PbZrO3-based materials. Based on this, we have simulated the polarization response and domain switching process of the temperature and electric field-induced antiferroelectric—ferroelectric phase transition. It is shown that the temperature range tends to increase as the density of polar boundaries increases from the antiferroelectric stripe domain, polymorphic domain to the nanodomain. Among them, the peak adiabatic temperature change of antiferroelectric nanodomains can reach −13.05 K at 84 kV/cm, and a wide temperature range of about 75 K can be realized at 42 kV/cm. We expect these discoveries to spur further interest in the potential applications of antiferroelectric materials for next-generation refrigeration devices.

Exploring the structural characteristics and electrical conductivity of MnTiO3 across ferroelectric and paraelectric phases

In this study, we present the ceramic's structural, dielectric, and impedance characteristics obtained through the solid-state reaction method synthesis. Initial structural analysis was conducted utilizing X-ray diffraction at room temperature. The XRD results show that MnTiO3 exhibits rhombohedral symmetry. SEM images of MnTiO3 show the presence of both cubic and large agglomerated grains. This demonstrates the effectiveness of the dielectric HN model in controlling the mechanism. The alternating current (ac) conductivity, when examined in relation to frequency, displays a remarkable adherence to the Johnscher's power law. The change in electrical modulus and the width at half maximum (FWHM) of the curve indicate that the as-prepared sample exhibits a non-Debye relaxation mechanism. Both the temperature data and the frequency dependence of the impedance were used to analyze the electrical conductivity of the sample, which showed a negative temperature coefficient resistance (NTCR).

Giant field-induced strain with excellent fatigue performance in (Al0.5Nb0.5)4+-modified 0.85Bi0.5Na0.5TiO3-0.11Bi0.5K0.5TiO3-0.04BaTiO3 piezoelectric ceramics

Utilizing a conventional solid-state reaction procedure, (Al0.5Nb0.5)4+-modified 0.85Bi0.5Na0.5TiO3-0.11Bi0.5K0.5TiO3-0.04BaTiO3 (BNKT-BT) samples were synthesized. The impact of (Al0.5Nb0.5)4+ on the crystal structure, ferroelectric, dielectric, and AC impedance properties was systematically investigated. The XRD patterns demonstrate that the crystal structures of all samples exhibit typical pseudo-cubic features. According to the XPS, the doping of (Al0.5Nb0.5)4+ reduces the concentration of oxygen vacancies inside the ceramics, which significantly affects the relaxation properties. This substitution successfully triggers a change from a non-ergodic relaxor (NR) state to an ergodic relaxor (ER) state, which is ascribed to the weakening of the pinning effect of the oxygen vacancies on the polar nanoregions (PNRs). The ceramics exhibit an enormous monopolar strain of 0.468 % under 60 kV/cm, accompanied by an inverse piezoelectric coefficient (d33*) of 780 pm/V, surpassing the performance of numerous other BNT-based piezoelectric ceramics. After 1×104 switching cycles, the strain value changes slightly, indicating excellent fatigue performance. The considerable strain arises from a reversible transformation of the ER phase to the ferroelectric phase induced by an electric field. This research suggests that (Al0.5Nb0.5)4+-doped BNKT-BT ceramics have good prospects for application in actuators and sensors.

Biodegradable Ferroelectric Molecular Plastic Crystal HOCH2(CF2)7CH2OH Structurally Inspired by Polyvinylidene Fluoride.

Ferroelectric materials, traditionally comprising inorganic ceramics and polymers, are commonly used in medical implantable devices. However, their nondegradable nature often necessitates secondary surgeries for removal. In contrast, ferroelectric molecular crystals have the advantages of easy solution processing, lightweight, and good biocompatibility, which are promising candidates for transient (short-term) implantable devices. Despite these benefits, the discovered biodegradable ferroelectric materials remain limited due to the absence of efficient design strategies. Here, inspired by the polar structure of polyvinylidene fluoride (PVDF), a ferroelectric molecular crystal 1H,1H,9H,9H-perfluoro-1,9-nonanediol (PFND), which undergoes a cubic-to-monoclinic ferroelectric plastic phase transition at 339 K, is discovered. This transition is facilitated by a 2D hydrogen bond network formed through O-H···O interactions among the oriented PFND molecules, which is crucial for the manifestation of ferroelectric properties. In this sense, by reducing the number of -CF2- groups from ≈5000 in PVDF to seven in PFND, it is demonstrated that this ferroelectric compound only needs simple solution processing while maintaining excellent biosafety, biocompatibility, and biodegradability. This work illuminates the path toward the development of new biodegradable ferroelectric molecular crystals, offering promising avenues for biomedical applications.

Ferroelectric and Relaxor-Ferroelectric Phases Coexisting Boosts Energy Storage Performance in (Bi0.5Na0.5)TiO3-Based Ceramics.

With the intensification of the energy crisis, it is urgent to vigorously develop new environment-friendly energy storage materials. In this work, coexisting ferroelectric and relaxor-ferroelectric phases at a nanoscale were constructed in Sr(Zn1/3Nb2/3)O3 (SZN)-modified (Bi0.5Na0.5)0.94Ba0.06TiO3 (BNBT) ceramics, simultaneously contributing to large polarization and breakdown electric field and giving rise to a superior energy storage performance. Herein, a high recoverable energy density (Wrec) of 5.0 J/cm3 with a conversion efficiency of 82% at 370 kV/cm, a practical discharged energy density (Wd) of 1.74 J/cm3 at 230 kV/cm, a large power density (PD) of 157.84 MW/cm3, and an ultrafast discharge speed (t0.9) of 40 ns were achieved in the 0.85BNBT-0.15SZN ceramics characterized by the coexistence of a rhombohedral-tetragonal phase (ferroelectric state) and a pseudo-cubic phase (relaxor-ferroelectric state). Furthermore, the 0.85BNBT-0.15SZN ceramics also exhibited excellent temperature stability (25-120 °C) and cycling stability (104 cycles) of their energy storage properties. These results demonstrate the great application potential of 0.85BNBT-0.15SZN ceramics in capacitive pulse energy storage devices.

Influence of oxygen pressure on the ferroelectricity of pulsed laser deposition fabricated epitaxial Y-doped HfO2

Ferroelectric properties of hafnium-based thin films have gained significant interest, yet the fundamental mechanisms responsible for the emergence of the ferroelectric phase continue to be inadequately investigated. In contrast with polycrystalline films fabricated by atomic layer deposition or sputter methods, which possess uncertainty in polarization orientation, epitaxial ferroelectric HfO2-based materials are less investigated, especially for factors such as electric field and oxygen vacancy, which are proposed and examined for their potential impacts on phase stability. In this study, Y-doped hafnium oxide (HYO) ferroelectric epitaxial films were fabricated using pulsed laser deposition, with variations in oxygen pressure during the deposition process. Structural and electrical analyses of HYO epitaxial ferroelectric films prepared under differing oxygen pressures revealed a correlation between the ferroelectric properties of the films and the oxygen content. An optimal selection of oxygen pressure was found to be conducive to the formation of HYO epitaxial ferroelectric films, presenting a promising avenue for future ferroelectric memory applications.

Structural evolution, ferroelectric phase transitions, and dielectric behavior of Sn-doped Ba0.95Sr0.05TiO3 ferroelectric ceramics

ABSTRACT In this report, I studied the structural, morphological, and dielectric properties of new ferroelectric compounds prepared according to the formula Ba0.95Sr0.05Ti1-ySnyO3. The solid-state reaction method enhanced by a high-efficiency metal ball mill was used in the preparation process. The quantitative and qualitative analysis of the XRD patterns confirmed the crystallization of the samples y = 0.00, 0.05 in the tetragonal crystal system and the presence of the tetragonal and cubic crystal systems together in the samples y = 0.10, 0.14. Formation of BaSnO3 impurities in very low percentages in the doped samples. Their prevalence increases with increasing tin content, reaching 8.7% at the ratio y = 0.14. SEM images of the ceramic samples showed the formation of nano-ceramics with a high degree of condensation at the doping ratio y = 0.05. The dielectric analyses showed high thermal stability in the dielectric properties of the ceramic samples y = 0.05, 0.10, which was shown by the flatness of the dielectric constant curve over the entire temperature range studied, where this stability is attributed to the structure of the nano-grains that resist the ferroelectric-paraelectric phase transition. Furthermore, the dielectric properties were interpreted by accurately linking the dielectric measurements with the nano-structural and polar properties.

Significant enhancement of ferroelectric performance in lead-free NaNbO3 ceramics

The comparable free energy between antiferroelectric (AFE) and ferroelectric (FE) phases in NaNbO3 (NN) leads to unstable ferroelectricity, restricting future applications for energy storage devices. In this work, lead-free NN ceramics based on different sintering aids have been rigorously synthesized and the microstructural, dielectric, and ferroelectric properties of this system have been thoroughly examined. It is found that a tiny amount of sintering aids effectively lowers the sintering temperature and manipulates the grain growth, as well as plays an essential role in the stability of FE phase in NN ceramics. The tunable ferroelectricity is observed in NN: CuO as sintering aids results in a hardening effect, whereas MnO2 addition promotes softening behavior. More interestingly, introducing 1 wt% MnO2 as sintering aids results in strong ferroelectricity with high polarization (Pm) of 84.6 μC/cm2 stabilized by a stimulated electric field (Ef) as low as 60 kV/cm. This is significantly better than other NN-based ceramics (Pm is smaller than 40 μC/cm2 and Ef is higher than 90 kV/cm in the NN-based systems reported earlier). Our findings demonstrate the potential of sintering aids in tailoring the ferroelectric performance of lead-free NN ceramics, offering insights into the development of environmentally friendly ferroelectric materials.

Revealing the antipolar order in the antiferroelectric SmZA phase by means of circular alignment

Many ferroelectric nematic liquid crystals, like one of the archetype materials, DIO, do not have a direct paraelectric N to ferroelectric NF phase transition, but exhibit yet another phase between N and NF. This phase has recently been proposed to be antiferroelectric, with a layered structure of alternating polarization normal to the average director and is sometimes referred to as Smectic ZA (SmZA). We have examined the SmZA phase in circularly rubbed (CR) cells, known to discriminate between the polar NF and the non-polar N phase from the configuration of disclination lines formed. We find that the ground state of SmZA has the same disclination configuration as the non-polar N phase, demonstrating that the SmZA phase is also non-polar, i.e., it has no net ferroelectric polarization. At the same time, the SmZA texture generally has a grainy appearance, which we suggest is partly a result of the frustration related to layered order combined with the imposed twist in CR cells. We discuss possible orientations of the smectic layers, depending on the alignment conditions. While a horizontal SmZA layer structure is always compatible with surface-induced twist, a vertical layer structure would tend to break up in a twisted bookshelf structure to match non-parallel alignment directions at the two surfaces.

Field-induced effects and ferroelectric critical endpoint in (K0.95Li0.05)(Nb1–x Ta x )O3 single crystals

Dielectric permittivities under dc biasing fields in the temperature range between –169 °C and 75 °C have been investigated in (K0.95Li0.05)(Nb1–x Ta x )O3 (KLNT–x, x = 71% and 74%). Field-induced ferroelectric phase transitions in KLNT–x have been clarified in the dc field applied along the [001] c direction (in the cubic coordinate). The ferroelectric critical endpoints in KLNT–71% and KLNT–74% have been determined to be 43.5 °C, 3.0 kV cm−1, and 30.3 °C, 2.0 kV cm−1, respectively. The tricritical point at which the first-order phase transition changes to the second-order phase transition has been estimated at 10.4 °C and x = 77.4% in the temperature–concentration phase diagram. The field-induced phase transitions and dielectric tunabilities have been discussed on the basis of the Landau-type free energy density.

Modeling the ferroelectric phase transition in barium titanate with DFT accuracy and converged sampling

Modeling the ferroelectric phase transition in barium titanate with DFT accuracy and converged sampling

Ultra-fast perovskite electro-optic modulator and multi-band transmission up to 300 Gbit s−1

The gap between the performance of optoelectronic components and the demands of fiber-optic communications has narrowed significantly in recent decades. Yet, the expansion of data communications traffic remains substantial, with fiber-link speeds increases anticipated in the near future. Here, we demonstrate an ultra-high-speed electro-optic waveguide modulator constructed using a thin film of lanthanum-modified lead zirconate titanate with a ferroelectric phase exhibiting a strong Pockels effect. The modulator has a wide optical window; thus, the modulation was demonstrated for 1550 and 1310 nm wavelengths. This device showed electro-optical intensity signaling with line rates of 172 Gbit s−1, in conjunction with on–off keying modulation; this performance could be increased to 304 Gbit s−1 using four-level pulse modulation. The signaling performance of this modulator was found to be robust, with stable performance at temperatures as high as 100 °C. This technology is expected to have applications in a wide range of classical optoelectronic devices and in quantum science and technology.

In-plane distribution of huge local permittivity of KTa1-xNbxO3 estimated from local phase transition temperatures and spatially averaged permittivity.

Potassium tantalate niobate (KTa1-xNbxO3, KTN) single crystals have a very large relative permittivity εr (>104) just above the paraelectric to ferroelectric phase transition temperature (TC). The quadratic electro-optic coefficient and the electro-strictive coefficient are also very large because of their proportionality to εr2. However, the local relative permittivity can easily vary spatially due to the incongruently melting nature of KTN. In this study, we quantitatively estimated the in-plane distribution of the huge local relative permittivity of KTN. First, we measured the spatial distribution of TC using scanning nonlinear dielectric microscopy, then deposited the electrodes and measured the temperature dependence of the spatially averaged permittivity using an LCR meter. Following that, we evaluated the spatial distribution of the huge local permittivity from the combination of the spatial distribution of TC and the spatially averaged permittivity. Finally, we measured the deflection angle of light to confirm the validity of the εr estimation procedure. The maximum error for the estimated permittivity was estimated to be around 3.3%.

Accelerating ferroelectric materials discovery through high-throughput first-principles screening and experimental validation

We conducted high-throughput screening of ferroelectrics using first-principles calculations based on an existing crystal structure database. We focused on nonpolar structures with polar instability, to efficiently screen materials for their potential to undergo ferroelectric phase transitions from oxide materials in crystal structure databases. Our screening criteria included computational feasibility (excluding partial occupation), the absence of hazardous elements, and a maximum of 250 atoms in the conventional cell. Through this screening, we identified 47 ferroelectric candidates, 8 of which have already been reported as ferroelectrics. To validate our screening approach, we synthesized and evaluated several candidate materials with Dion–Jacobson-type structures, and measured their dielectric and ferroelectric properties. Although the ferroelectric behavior was not initially identified in these materials, our experiments confirmed their properties. Finally, we discovered a new ferroelectric material, CsCa2Nb3O10, which exhibited a ferroelectric phase transition at 28 K, clearly demonstrating the effectiveness of our screening strategy.

Giant overall energy density performances via introducing aliovalent cations in BNT-based ferroelectric ceramics

The strategy of introducing aliovalent cations in the ferroelectric perovskites is the most promising for obtaining excellent overall energy storage density performances, simultaneously with good efficiency and thermal stability in dielectric ceramics for high pulse power technologies. Aliovalent cations could greatly enhance the short-range ferroelectric order (Polar nanoregions, PNRs), and local random field caused by vacancies, mismatch between size, electronic configuration, and other physical and chemical properties of aliovalent cations with the host perovskite. PNRs and local random field make the ceramic relaxor like with excellent overall energy storage density results such as total energy storage density (WT = 10.55 J/cm3, recoverable energy storage density Wrec = 7.82 J/cm3), power density (PD = 197 MW/cm3), current density (CD = 1312 A/cm2), and ultra-fast discharge rate (t = 24 ns) simultaneously with good working efficiency of 74 % by introducing 14 mol% of Bi(Zn1/3Nb2/3) aliovalent cations in the morphotropic phase boundary (MPB) (Bi0.47Na0.47)TiO3–0.06BaTiO3 composition. In addition, the same optimum sample also showed good thermal stability up to 240 °C, with <10–13 % variation in recoverable energy storage density and working efficiency respectively. The reason of excellent overall energy storage density, and their thermal stability in a broad temperature range might be due to short-range ferroelectric order, local random field, relaxor ferroelectric polymorphic phases, highly dense microstructure with small and uniform grains, as well as large band gape. Our study provides a good clue to obtain giant overall energy density results with good working efficiency and thermal stability in relaxor ferroelectric ceramics for high pulse power technology.

Chiral ferroelectric nematic liquid crystals as materials for versatile laser devices

We present a liquid-crystal laser device based on the chiral ferroelectric nematic phase (NF*). The laser medium is obtained by mixing a ferroelectric nematic material with a chiral agent and a small proportion of a fluorescent dye. Notably, in the NF* phase very low electric fields perpendicular to the helical axis are able to reorient the molecules, giving rise to a periodic structure whose director profile is not single harmonic but contains the contribution of various Fourier components. This feature induces the appearance of several photonic bandgaps whose spectral ranges depend on the field, which can be exploited to build tunable laser devices. Here we report the characterization of home-made NF* lasers that can be tunable under low electric fields and present laser action in two of the photonic bands of the material. The obtained results open a promising route for the design of new and more versatile liquid-crystal based lasers.

Dimensional Scaling of Ferroelectric Properties of Hafnia-Zirconia Thin Films: Electrode Interface Effects.

Hafnia-based ferroelectric (FE) thin films are promising candidates for semiconductor memories. However, a fundamental challenge that persists is the lack of understanding regarding dimensional scaling, including thickness scaling and area scaling, of the functional properties and their heterogeneity in these films. In this work, excellent ferroelectricity and switching endurance are demonstrated in 4 nm-thick Hf0.5Zr0.5O2 (HZO) capacitors with molybdenum electrodes in capacitors as small as 65 nm × 45 nm in size. The HZO layer in these capacitors can be crystallized into the ferroelectric orthorhombic phase at the low temperature of 400 °C, making them compatible for back-end-of-line (BEOL) FE memories. With the benefits of thickness scaling, low operation voltage (1.2 V) is achieved with high endurance (>1010 cycles); however, a significant fatigue regime is noted. We observed that the bottom electrode, rather than the top electrode, plays a dominant role in the thickness scaling of HZO ferroelectric behavior. Furthermore, ultrahigh switched polarization (remanent polarization 2Pr ∼ 108 μC cm-2) is observed in some nanoscale devices. This study advances the understanding of dimensional scaling effects in HZO capacitors for high-performance FE memories.

Phase Transformations Driving Biaxial Stress Reduction During Wake‐Up of Ferroelectric Hafnium Zirconium Oxide Thin Films

AbstractBiaxial stress is identified to play an important role in the polar orthorhombic phase stability in hafnium oxide‐based ferroelectric thin films. However, the stress state during various stages of wake‐up has not yet been quantified. In this work, the stress evolution with field cycling in hafnium zirconium oxide capacitors is evaluated. The remanent polarization of a 20 nm thick hafnium zirconium oxide thin film increases from 9.80 to 15.0 µC cm−2 following 106 field cycles. This increase in remanent polarization is accompanied by a decrease in relative permittivity that indicates that a phase transformation has occurred. The presence of a phase transformation is supported by nano‐Fourier transform infrared spectroscopy measurements and scanning transmission electron microscopy that show an increase in ferroelectric phase content following wake‐up. The stress of individual devices field cycled between pristine and 106 cycles is quantified using the sin2(ψ) technique, and the biaxial stress is observed to decrease from 4.3 ± 0.2 to 3.2 ± 0.3 GPa. The decrease in stress is attributed, in part, to a phase transformation from the antipolar Pbca phase to the ferroelectric Pca21 phase. This work provides new insight into the mechanisms controlling and/or accompanying polarization wake‐up in hafnium oxide‐based ferroelectrics.

Effect of A-site excess on the shape memory effect of sodium bismuth titanate ceramics

The impact of A-site excess on the shape memory effect of Na0.5Bi0.5TiO3 ceramics was investigated. We found that the recoverable plastic-like strain was increased by more than 10 % by A-site excess. The enhancement is attributed to the change in crystal structure from a rhombohedral phase to a monoclinic one, which further changes the phase transition behavior between the ferroelectric and nonpolar phases. Suppression of grain size and the generation of defects after the addition of excess Bi and Na were proposed for the appearance of monoclinic phase. This investigation proposes a viable approach to modulate the shape memory effect of Na0.5Bi0.5TiO3-based ceramics through the manipulation of the crystal structure and the associated phase transition processes, ultimately influencing the shape memory characteristics.

Control of intrinsic ferroelectricity and structural phase in pure HfO2 films via crystalline orientation

The ferroelectricity of hafnia-based thin films has garnered considerable attention in both academic researches and industrial applications. However, the fundamental properties, such as high coercivity, the wake-up effect, and the mechanism of ferroelectricity have not been fully elucidated. Here we report the crystallization orientation control of structural phase and ferroelectricity in pure HfO2 thin films. Both (001)- and (111)-oriented HfO2 thin films exhibit a mixture of ferroelectric orthorhombic and non-ferroelectric monoclinic phases. With decreasing film thickness, the orthorhombic phase ratio increases for both orientations, with a consistently higher proportion for (111)-oriented film. Consequently, the ferroelectricity is significantly enhanced in thinner (111)-oriented film. Remarkably, both (001)- and (111)-oriented pure HfO2 thin films demonstrate an intrinsic ferroelectricity. Moreover, the coercive field of the (001)-oriented film appears to be lower than that of the (111)-oriented film. Additionally, oxygen ions migrate more easily in the (001)-oriented film, which exhibits distinct electronic structure and local atomic ordering compared to the (111)-oriented film. These results provide valuable insights into the ferroelectricity of HfO2 and suggest that crystalline orientation is an effective approach to explore the ferroelectric properties in hafnia-based films.