Type Ferroelectrics Research Articles

Phase transitions in typical fluorite-type ferroelectrics

While ferroelectric hafnia (HfO2) has become a technically important material for microelectronics, the physical origin of its ferroelectricity remains poorly understood. The tetragonal P42/nmc phase is commonly assigned as its paraelectric mother phase but has no soft mode at the Brillouin zone center. In this work, we propose that the paraelectric—ferroelectric transition in the fluorite-type Pca21 ferroelectric family can be described by a Pcca—Pca21 transition, where the Pcca mother phase will evolve into either the Pca21 ferroelectric phase or the centrosymmetric P21/c monoclinic phase, depending on the strain conditions. The Pcca phase is directly linked to both phases in the context of continuous phase transition. Hafnia is regarded as a special case of this family in that it has accidental atomic degeneracy because all anions are oxygen. The theory is also correlated with the seven-coordination theory that explains the ferroelectricity in hafnia from a chemical perspective. In addition, the strain conditions to promote the ferroelectric phase in hafnia are discussed.

Improved multiferroic properties of lanthanum ferrites through cobalt doping

LaFeO3 is a type II multiferroic material which exhibits improper ferroelectricity induced by magnetic order (antiferromagnetism). However, the antiferromagnetic order limits its applications. This study demonstrates the induction of ferromagnetism in LaFeO3 by doping with cobalt, varying the level of cobalt from 0 to 0.5 mol. Moreover, the effects of cobalt on the crystal structure, magnetic and dielectric properties are described. The XRD results show that at doping concentrations lower than 0.3 mol, LaFeO3 maintains its orthorhombic structure (Pnma), while at higher cobalt concentrations the rhombohedral (R-3c) LaCoO3 phase emerges. In addition, cobalt doping induces ferromagnetism because cobalt occupies iron positions until the solubility limit is reached (0.3 mol). All samples present high relative permittivity values, which increase with increasing cobalt content and temperature. Furthermore, the presence of cobalt results in a decrease in the dielectric dissipation factor for cobalt contents lower than 0.3 mol, due to the inhibition in the formation of vacancies. However, the presence of LaCoO3 at high cobalt contents increases the dielectric losses due to the presence of a mixtures of phases. Moreover, the electric polarization curves exhibited weak ferroelectric behavior for ferrites with cobalt contents lower than 0.3 mol, and lossy improper ferroelectric type behavior for samples with higher contents.

Simulation of neural functions based on organic semiconductor/MXene synaptic transistors

Artificial synaptic devices, which are the basic units of neuromorphic computing systems, can perform signal processing with low power consumption. Organic synaptic transistors have attracted significant attention owing to their lightweight and good compatibility with flexible substrates. As per the current state of research both domestically and internationally, the majority of the existing organic synaptic transistors are based on floating gate structures, electret configurations, ferroelectric types. Furthermore, additional capture layers are required for the preparation of these devices. Two-dimensional MXenes have great potential in the preparation of synaptic transistors owing to their efficient multiple energy storage capabilities, excellent metallic conductivity, abundant surface functional groups, hydrophilicity, and layered structure. Therefore, high-performance synaptic transistors based on the two-dimensional material MXene were developed in this study. These transistors not only exhibited excellent memory performance with a memory window above 20 V, but also successfully simulated typical synaptic behaviors, including excitatory postsynaptic current/inhibitory postsynaptic current (EPSC/IPSC), paired-pulse facilitation/paired-pulse depression (PPF/PPD), and long-term plasticity (LTP). Synaptic transistors based on MXenes represent a promising approach for the preparation of high-performance organic synaptic transistors.

The evolution of 2D vdW ferroelectric materials: Theoretical prediction, experiment confirmation, applications

Since J. Valasek first discovered ferroelectric materials in 1920, researchers have been exploring continuously in various fields through theory and experiments. With the rapid development of the computing technology, energy efficiency and size requirements of semiconductor devices are becoming increasingly demanding. However, the conventional ferroelectric materials, which have been limited by physical size restrictions, can no longer satisfy the above requirements. Two-dimensional (2D) ferroelectric materials can effectively overcome the size limitation of traditional ferroelectrics due to the weak van der Waals force between layers, which is easy to thin while retaining their own unique properties. Currently, a small number of 2D materials have been proved to be ferroelectric properties by experiments and have shown great application potential in nanoscale electrical and optoelectronic devices, expected to become the leaders of next-generation computing. In this review, the current 2D ferroelectric materials are summarized and discussed in detail from seven aspects: theoretical prediction, fabrication methods, ferroelectric characterization methods, principles of typical 2D ferroelectrics, optimization methods of ferroelectric performance, application, and challenges. Finally, the development of 2D ferroelectric materials looks into the future.

An integral wavy shaped P(VDF-TrFE) transducer for audio directional system

To generate self-demodulated low-frequency acoustic waves with high directivity in audio directional system, an integral piezoelectric P(VDF-TrFE) 80/20 mol% film, proving as a typical ferroelectricity and exhibiting the excellent electromechanical coupling factors, is formed into a regular wavy shaped transducer. The frequency response and directivity performances of the large-size transducer are theoretically simulated using COMSOL software, showing that the response is relative smooth over the audio frequency range. As such, the highest sound pressure level is located at a resonant frequency of 40 kHz, while the sound is highly directional and the 3-dB beam-width becomes increasingly sharp with the raise of size of P(VDF-TrFE) film. Interestingly, the integral wavy shaped P(VDF-TrFE) transducer exhibits a sound pressure of 78 dB at 500 mm and the 3-dB beam-widths of 100 Hz,1 kHz, and 6 kHz are ±2.8º, ±3.1º, and ±2.9º, respectively. Thus, the wavy type P(VDF-TrFE) transducer is duplicatable and easy to facilitate extra-large for emerging applications.

2D Molecular Ferroelectric with Large Out‐of‐plane Polarization for In‐Memory Computing

Abstract2D ferroelectric materials with out‐of‐plane polarization are crucial for future nanoscale logic devices due to the increasing demand for energy‐efficient architectures in artificial intelligence. However, only a few 2D out‐of‐plane ferroelectrics are confirmed experimentally. As an important branch of ferroelectrics, organic–inorganic hybrid perovskite ferroelectrics show flexible structures, making them eligible for constructing multifunctional materials. Here, a 2D organic–inorganic hybrid perovskite ferroelectric (6‐BHA)2CdBr4 (6‐BHA is 6‐bromohexylamine) is designed, which crystallizes in polar point group Cc. It experiences the reversal phase transition at 317.8 K and possesses multiaxial ferroelectric properties. More interestingly, it exhibits a large spontaneous polarization value of 3.26 µC cm−2 in out‐of‐plane direction of the film compared with typical 2D ferroelectrics. Moreover, an inverter based on (6‐BHA)2CdBr4 is fabricated, which serves as a proof of concept for the feasibility for logic‐in‐memory devices. This work not only enriches the family of molecular ferroelectrics but also shows the potential to create the next generation of in‐memory computing devices, nanoelectronics devices, and ultra‐high‐density memories.

Study of Ferroelectric Phase Transition in Triglycine Fluoberyllate Crystal

Triglycine fluoberyllate (TGFB) is an important member of hydrogen bonded order disorder type ferroelectrics, resembling other ferroelectrics which belongs to its group it experiences phase transition at certain temperature. In current theoretical work by utilizing Green’s function (double time thermal dependent) and modified pseudospin two-sublattice mode model we are analyzing ferroelectric behavior of TGFB crystal. For this purpose we are evaluating expressions for shift ( Δ ) , width ( Γ ) , soft mode frequency ( Ω ̂ ) , dielectric constant ( ε ) and loss tangent ( tan δ ) , after that by utilizing model values for TGFB crystal thermal dependence of Ω ̂ , ε and tan δ have been calculated. Comparison illustrates that theoretical results obtained by us are in good harmony with experimental results.

Single Molecular Semi-Sliding Ferroelectricity/Multiferroicity.

In recent years, the unique mechanism of sliding ferroelectricity with ultralow switching barriers has been experimentally verified in a series of 2-dimensional (2D) materials. However, its practical applications are hindered by the low polarizations, the challenges in synthesis of ferroelectric phases limited in specific stacking configurations, and the low density for data storage since the switching process involves large-area simultaneous sliding of a whole layer. Herein, through first-principles calculations, we propose a type of semi-sliding ferroelectricity in the single metal porphyrin molecule intercalated in 2D bilayers. An enhanced vertical polarization can be formed independent on stacking configurations and switched via sliding of the molecule accompanied by the vertical displacements of its metal ion anchored from the upper layer to the lower layer. Such semi-sliding ferroelectricity enables each molecule to store 1 bit data independently, and the density for data storage can be greatly enhanced. When the bilayer exhibits intralayer ferromagnetism and interlayer antiferromagnetic coupling, a considerable difference in Curie temperature between 2 layers and a switchable net magnetization can be formed due to the vertical polarization. At a certain range of temperature, the exchange of paramagnetic-ferromagnetic phases between 2 layers is accompanied by ferroelectric switching, leading to a hitherto unreported type of multiferroic coupling that is long-sought for efficient "magnetic reading + electric writing".

Arctic curves of the four-vertex model

We consider the four-vertex model with a special choice of fixed boundary conditions giving rise to limit shape phenomena. More generally, the considered boundary conditions relate vertex models to scalar products of off-shell Bethe states, boxed plane partitions, and fishnet diagrams in quantum field theory. In the scaling limit, the model exhibits the emergence of an arctic curve separating a central disordered region from six frozen ‘corners’ of ferroelectric or anti-ferroelectric type. We determine the analytic expression of the interface by means of the Tangent Method. We supplement this heuristic method with an alternative, rigorous derivation of the arctic curve. This is based on the exact evaluation of suitable correlation functions, devised to detect spatial transition from order to disorder, in terms of the partition function of some discrete log-gas associated to the orthogonalizing measure of the Hahn polynomials. As a by-product, we also deduce that the arctic curve’s fluctuations are governed by the Tracy–Widom distribution.

Evolving electromechanical properties of defect engineered Pb(Zr,Ti)O3-based piezoceramics

Chemical modification is the frequently-adopted strategy to enhance the electromechanical properties of piezoelectric ceramics. However, chemically modified piezoelectric materials will inevitably produce point defects, such as dopant ions, oxygen vacancies, etc. In this study, we investigated the properties of defect-engineered Pb(Zr,Ti)O3 (denoted by PSZT–Fe) polycrystals after aging, quenching, poling, and de-aging processes. At the same time, a general mechanism on the strength of defect dipole is also put forward to explain the corresponding changes. After aging, the spontaneous polarization of PSZT-Fe can be changed instantly upon application of external electric field, but the defect dipole and its defect symmetry can’t change simultaneously. Therefore, after the external electric field is removed, the defective dipole would drive the electric domain back to its original state, resulting in restorable strain effect and macroscopic pinch polarization. Bipolar electric field cycling can effectively redistribute oxygen vacancies to a random state, so that the aged PSZT-Fe can recover the electrical properties of typical ferroelectrics, similar to quenched samples. The spontaneous polarization and defect dipole in the poled PSZT-Fe ceramics are driven by an external electric field parallel to each other, which results in the emergence of internal bias field, accounting for the asymmetric hysteresis loop.

Effect of Milling on Dielectric Properties of PZT

In this study, we investigated the impact of high-energy milling on the structural and dielectric properties of Pb[Zr(1-x)Tix ]O3 [PZT] ceramics synthesized using the solid-state reaction process. The sample was milled for 2, 4 & 6 hours using a high-energy ball milling machine. The unit cell structure for all of the samples was observed to be monoclinic, according to x-ray diffraction measurements (space group: C1m1). A significant reduction in crystallite size was observed, from 132 nm to 46 nm after 6 hours of milling. The dielectric study indicated a classical ferroelectric type behaviour for the un-milled sample and diffused phase transition for all milled samples. However dielectric constant dropped from 940 to 487 after 6 hours of milling.

Synergetic Adsorption‐Catalysis in Potassium Oxide Modified High‐Entropy Relaxor Ferroelectrics for Efficient Dye Removal Strategy

Water scarcity caused by extreme weather events has become a global issue, leading to the necessity of wastewater recycling. Developing new materials with fast pollutant removal efficiency has become a research focus in related fields for producing clean water resources quickly and stably from wastewater. Herein, a relaxor ferroelectric type of piezoelectric high entropy perovskites, Pb(Mg,Nb,Hf,Zr,Ti)O3 (PMNHZT), is used and prepared for industrial dye removal. The synthesized PMNHZT exhibits high dark adsorption of the methylene blue dye (removal efficiency of ≈60% in 30 min) and further catalytic dye degradation (removal efficiency of ≈80–90% in 30 min) through light illumination, sonication, or a combination of both. In the dark adsorption process, K2O compounds from the synthesized environment attached to the surface of PMNHZT play a significant role in the remarkable dark adsorption by promoting a large specific surface area and more negative surface potential. Furthermore, a self‐decomposition of dye into smaller fragments by PMNHZT is also observed during dark adsorption. The piezocatalysis mechanism dominates the dye degradation process in catalytic experiments, where hydroxyl radicals are the main reactive species. Herein, a promising adsorbent and catalyst is disclosed using high‐entropy perovskites for efficient wastewater treatment.

Ferroelectric Polarization in BaTiO3 Nanocrystals Controls Photoelectrochemical Water Oxidation and Photocatalytic Hydrogen Evolution.

Ferroelectric (FE) semiconductors such as BaTiO3 support a remnant polarization after the application of an electric field that can promote the separation of photogenerated charge carriers. Here, we demonstrate FE-enhanced photocatalytic hydrogen evolution and photoelectrochemical water oxidation with barium titanate nanocrystals for the first time. Nanocrystals of the ferroelectric tetragonal structure type were obtained by a hydrothermal synthesis from TiO2 and barium hydroxide in 63% yield. BaTiO3 nanocrystal films on tantalum substrates exhibit water oxidation photocurrents of 0.141 mA cm-2 at 1.23 V RHE under UV light (60 mW cm-2) illumination. Electric polarization at 52.8 kV cm-1 normal to the film plane increases the photocurrent by a factor of 2 or decreases it by a factor of 3.5, depending on the field polarity. It also shifts the onset potential by -0.15 or +0.09 V and it modifies the surface photovoltage signal. Lastly, exposure to an electric field increases the H2 evolution rate of Pt/BaTiO3 by a factor of ∼1.5, and it raises the selectivity of photodeposition of silver onto the (001) facets of the nanocrystal. All FE enhancements can be removed by heating samples above the Curie temperature of BaTiO3. These findings can be explained by FE dipole-induced changes to the potential drop across the space charge layer of the material. The ability to use the ferroelectric effect to enhance hydrogen evolution and water oxidation is of potential interest for the development of improved solar energy for fuel conversion systems.

Periodic Ferroelectric Stripe Domains in α-In2Se3 Nanoflakes Grown via Reverse-Flow Chemical Vapor Deposition.

The two-dimensional (2D) layered semiconductor α-In2Se3 has aroused great interest in atomic-scale ferroelectric transistors, artificial synapses, and nonvolatile memory devices due to its distinguished 2D ferroelectric properties. We have synthesized α-In2Se3 nanosheets with rare in-plane ferroelectric stripe domains at room temperature on mica substrates using a reverse flow chemical vapor deposition (RFCVD) method and optimized growth parameters. This stripe domain contrast is found to be strongly correlated to the stacking of layers, and the interrelated out-of-plane (OOP) and in-plane (IP) polarization can be manipulated by mapping the artificial domain structure. The acquisition of amplitude and phase hysteresis loops confirms the OOP polarization ferroelectric property. The emergence of striped domains enriches the variety of the ferroelectric structure types and novel properties of 2D In2Se3. This work paves a new way for the controllable growth of van der Waals ferroelectrics and facilitates the development of novel ferroelectric memory device applications.

Negative differential resistance behaviour with high PVR value in (CH3)2NH2Fe(HCOO)3 – A multiferroic MOF

We present the NDR behaviour for multiferroic MOF (CH3)2NH2Fe(HCOO)3 crystals with a high peak to valley ratio. The phase pure millimetre sized crystals were grown using hydrothermal process. The multiferrioc conduct of the crystals were confirmed through magnetic and dielectric characteristic measurements, displaying a para-antiferro magnetic transition with TN = 19 K and a para-electric to ferroelectric type dielectric transition at Tc = 171 K. The electronic conduction mechanism was studied through current–voltage I(V) characteristic measurements carried out in temperature range from 100 K to 250 K. The largest peak to valley ratio of 47 was observed at 140 K. This high peak to valley ratio was explained on the basis of onsite coulomb interactions in 1D Hubbard model.

High-Throughput Scanning Second Harmonic Generation Microscopy for Polar Materials.

The Materials Genome Initiative aims to discover, develop, manufacture, and deploy advanced materials at twice the speed of conventional approaches. To achieve this, high-throughput characterization is essential for the rapid screening of candidate materials. In this study, we developed a high-throughput scanning second harmonic generation (HTS-SHG) microscope with automatic partitioning, accurate positioning, and fast scanning that can rapidly probe and screen polar materials. Using this technique, we first investigated typical ferroelectrics, including periodically poled lithium niobate crystals and PbZr0.2 Ti0.8 O3 (PZT) thin films, whereby the microscopic domain structures were clearly revealed. We then applied this technique to a compositional-gradient (100-x)%BaTiO3 -x%SrTiO3 film and a thickness-gradient PZT film to demonstrate its high-throughput capabilities. Since SHG signal is correlated with the macroscopic remnant polarization over the probed region determined by the laser spot, it is free of artifacts arising from leakage current and electrostatic interference, while materials symmetries and domain structures must be carefully considered in the data analysis. We believe this work can help promote the high-throughput development of polar materials, and contribute to Materials Genome Initiative. This article is protected by copyright. All rights reserved.

Real‐Space Observation of Potential Reconstruction at Metallic/Insulating Oxide Interface

AbstractElectric field reconstruction at interfaces plays a crucial role in device performances controlling, for example, Schottky potential barrier and interfacial Rashba effect. Here, scanning transmission electron microscopy (STEM) and ab‐initio calculation are used to estimate the atomic‐scale and large‐scale potential reconstruction at the interface between a metallic oxide SrRuO3 (SRO) thin film and an insulating DyScO3 (DSO) substrate. The intensity and the symmetry of the large‐scale electrostatic reconstruction at the interface is probed by 4D‐STEM discussing the center‐of‐mass shift for different angular ranges detection. Numerical simulations indicate that thermal diffuse scattered (TDS) electrons can be sensitive to large‐scale electric field and experiments based on these diffused electrons near the interface confirm that the electric field extends more in the insulating DyScO3 (DSO) side. The magnitude of the electrostatic drop at the interface estimated by the 4D‐STEM experiment is in accordance with the ab‐initio values for a p‐type reconstruction of the interface plane. Furthermore, an atomically resolved TDS potential asymmetry is observed in real‐space at the SRO/DSO interface by 4D‐STEM. This asymmetry is associated with the formation of a local ferroelectric type dipole at the interfacial unit‐cell revealing unambiguously the balance evolution between antiferrodistortive and ferroelectric instabilities at the interface between a metallic SRO and an insulating DSO.

Dielectric Layer Design of Bilayer Ferroelectric and Antiferroelectric Tunneling Junctions Toward 3D NAND-Compatible Architecture

The 3D vertical ferroelectric tunneling junction (FTJ) of bilayer antiferroelectric (AFE) <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathrm {Hf}_{1-x}Zr_{x}\text{O}$ </tex-math></inline-formula> 2(HZO) and Al2O3 has been demonstrated for NAND-compatible feasibility. A bilayer-type FTJ is explored for the designs of the dielectric interlayer Al2O3 0 nm to 4 nm and the ferroelectric type, while the current mechanism is revealed. The multilevel AFE-FTJ is exhibited for both the Program and Erase operations and realizes a synaptic device. High-density emerging memory and computing-in-memory (CiM) are in high demanded for the future era and can be feasible by the proposed vertical FTJ.

Origin of ferroelectricity in magnesium-doped zinc oxide

Recent experiments demonstrated robust ferroelectricity in Mg-doped ZnO (ZMO) of the wurtzite structure, hinting at a promising strategy to substantially expand the list of ferroelectrics by doping conventional piezoelectrics. We investigate the origin of ferroelectricity in ZMO with first-principles density functional theory (DFT). The general argument that the Mg alloying could soften the ionic potential energy surface of ZMO for polarization reversal is overly simplified. Our DFT calculations reveal that even at a high Mg concentration, the energy difference ($\Delta U$) between the polar and nonpolar phases remains prohibitively large for ZMO systems when the strain is fixed to the polar phase. Interestingly, the magnitude of $\Delta U$ becomes substantially smaller when the strain relaxation is allowed, approaching the value of typical perovskite ferroelectrics such as PbTiO$_3$ with increasing Mg doping concentrations. The enabled switchability of ZMO systems is attributed to a hexagonal phase of MgO that is much lower in energy than its wurtzite counterpart. Detailed orbital and bonding analysis supports that the intra-atomic $3d_{z^2}$-$4p_z$ orbital self-mixing of Zn plays an important role in stabilizing the polar wurtzite phase, the lack of which is responsible for the low-energy nonpolar hexagonal phase of MgO.

Ferroelectricity-Driven Phonon Berry Curvature and Nonlinear Phonon Hall Transports.

Berry curvature (BC) governs topological phases of matter and generates anomalous transport. When a magnetic field is applied, phonons can acquire BC indirectly through spin-lattice coupling, leading to a linear phonon Hall effect. Here, we show that polar lattice distortion directly couples to a phonon BC dipole, which causes a switchable nonlinear phonon Hall effect. In a SnS monolayer, the in-plane ferroelectricity induces a phonon BC and leads to the phononic version of the nonvolatile BC memory effect. As a new type of ferroelectricity-phonon coupling, the phonon Rashba effect emerges and opens a mass gap in tilted Weyl phonon modes, resulting in a large phonon BC dipole. Furthermore, our ab initio non-equilibrium molecular dynamics simulations reveal that nonlinear phonon Hall transport occurs in a controllable manner via ferroelectric switching. The ferroelectricity-driven phonon BC and corresponding nonlinear phonon transports provide a novel scheme for constructing topological phononic transport/memory devices.