Ferroelectric Characteristics Research Articles

Unveiling the Racemization in Ferroelectrics with Coupled Ferroelectricity and Optical Activity.

The most distinctive features of ferroelectrics are spontaneous polarization and depolarization. Ferroelectricity was first observed in Rochelle salt, a compound with a chiral component in which the molecular chirality is not affected by depolarization. For structurally chiral ferroelectrics (SCFs), such as triglycine sulfate, which lacks chiral components, the depolarization effect on chirality presents an intriguing and unexplored topic. In this study, we focus on a newly proposed chiral optical topic in optically active ferroelectrics with coupled ferroelectricity and optical activity. We examined the depolarization effect on the optical activity in these ferroelectric materials and employed chiral-environment crystallization and solid-state circular dichroism (CD) techniques to elucidate the racemization-depolarization mechanism in an SCF for the first time. For polar and nonchiral optically active ferroelectrics, optical activity is averaged out in the powder state, resulting in no CD signal. Additionally, we experimentally validated the consistency between CD measurements and optical activity theory across the four optically active point groups (m, mm2, 4̅, 4̅2m). These findings could advance the development of chiral spintronic devices by leveraging coupled ferroelectricity and optical activity to tune the spin selectivity.

Investigation of electric field-induced phase transitions in unfilled tungsten bronze relaxor ceramics designed by the high entropy concept

Electric field-induced phase transition behaviour, extensively studied in perovskite-structured ceramics, has not been previously reported in unfilled tetragonal tungsten bronze (TTB) structured ceramics. In this work, we present the first investigation of electric field-induced phase transitions in high entropy designed unfilled TTB structured Ca0.25Sr0.25Ba0.25Pb0.25Nb2O6 (CSBP) ceramics using dielectric and ferroelectric characterization techniques. The findings reveal that field-induced polarization in the CSBP ceramics evolve from irreversible to reversible with increasing temperature. Furthermore, relaxor ferroelectric behaviour was observed in the ceramics, attributed to the A-site cation disorders in the unfilled TTB structure, facilitated by the high entropy design. The absence of non-180° domain switching was indicated by microstructural observations and analysis of the strain-electric field (S-E) response. In-situ poling synchrotron studies and experimental S-E response measurements revealed an electrostrictive behaviour characterized by an electrostrain not originating from macroscopic structural transformations or long-range domain switching but more likely contributed by the reorientation of polar nanoregions. The results obtained provide a foundation for future studies investigating the electric field-induced phase transition and domain switching behaviour in the unfilled TTB structured ceramics.

Effect of Lanthanum-Aluminum Co-Doping on Structure of Hafnium Oxide Ferroelectric Crystals.

Hafnium oxide (HfO2)-based devices have been extensively evaluated for high-speed and low-power memory applications. Here, the influence of aluminum (Al) and lanthanum (La) co-doping HfO2 thin films on the ferroelectric characteristics of hafnium-based devices is investigated. Among devices with different La/Al ratios, the Al and La co-doped hafnium oxide (HfAlAO) device with 4.2% Al and 2.17% La exhibited the excellent remanent polarization and thermostability. Meanwhile, first principal analyses verified that hafnium-based thin films with 4.2% Al and 2.17% La promoted the formation of the o-phase against the paraelectric phase, providing theoretical support for supporting experimental results. Furthermore, a vertical ferroelectric HfO2 memory based on 3D macaroni architecture is reported. The devices show excellent ferroelectric characteristics of 22 µCcm-2 under 4.5 MVcm-1 and minimal coercive field of ≈1.6V. In addition, the devices exhibit great memory performance, including the response speed of device can achieve 20ns and endurance characteristic can achieve 1010 cycles.

Giant energy storage density, high efficiency and excellent stability achieved in lead-free KNN-based ceramic via a composition optimization strategy

The (1-x)K0.5Na0.5NbO3-xBi(Zn2/3Sb1/3)O3 ceramics, abbreviated as (1-x)KNN-xBZS with x values of 0.05, 0.10, 0.15, and 0.20, were prepared via solid-state reaction technique. Their structural and electrical characteristics, including dielectric and ferroelectric characteristics, were systematically examined. The achievement of an optimized phase transition-field and the broadening of the bandgap in the 0.85KNN-0.15BZS ceramics contribute to the delay of polarization and improvement in the breakdown field strength. Dielectric measurements indicate that the introduction of BZS disrupts the original large domain structure of KNN ceramics, giving rise to small domains and polar nano regions (PNRs). Specifically, the 0.85KNN-0.15BZS ceramic exhibits exceptional energy storage density (Wrec = 5.90 J/cm3) and an ultra-high energy efficiency (η = 79.9 %) at an applied electric field of 570 kV/cm. Furthermore, this ceramic displays excellent frequency stability in the range of 1–100 Hz and temperature stability between 30 and 150 °C. The remarkable energy storage properties of (1-x)KNN-xBZS ceramics position them as highly promising materials for future pulse power capacitors and various energy storage applications.

Establishing room-temperature multiferroic behaviour in bismuth-based perovskites

In the search of single-phase multiferroic materials at room temperature, a ceramic system with composition 0.5(0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-0.5BiFe0.8Mn0.2O3 (BNT-6BT-5BFO2M) was fabricated via the solid-state reaction route, and its crystal structure, dielectric, ferroelectric, and magnetic properties were studied. The results indicate that the ceramic can be considered a single-phase perovskite system with ferroelectric and ferromagnetic characteristics at room temperature. The ferroelectricity is evidenced by the switching of ferroelectric domains, as imaged by piezoresponse force microscopy (PFM). The presence of a weak ferromagnetism is manifested by a non-negligible remnant magnetization in the magnetization-magnetic field loops. The spontaneous net magnetization is mediated by the presence of Mn4+ ions, which may introduce ferromagnetic Fe3+-O-Mn4+ double-exchange interactions in the system. The PFM images taken during the application of a magnetic field of 2000 Oe revealed that the ferroelectric domain structure at room temperature can be significantly influenced by the magnetic field, reflecting the presence of a magnetoelectric effect that allows the occurrence of magnetic field-induced polarization reorientation.

Ferroelectric Perovskite/MoS2 Channel Heterojunctions for Wide-Window Nonvolatile Memory and Neuromorphic Computing.

Ferroelectric materials commonly serve as gate insulators in typical field-effect transistors, where their polarization reversal enables effective modulation of the conductivity state of the channel material, thereby realizing non-volatile memory. Currently, novel 2D ferroelectrics unlock new prospects in next-generation electronics and neuromorphic computation. However, the advancement of these materials is impeded by limited selectivity and narrow memory windows. Here, new concepts of 2D ferroelectric perovskite/MoS2 channel heterostructures field-effect transistors are presented, in which 2D ferroelectric perovskite features customizable band structure, few-layered ferroelectricity, and submillimeter-size monolayer wafers. Further studies reveal that these devices exhibit unique charge polarity modulation (from n- to p-type channel) and remarkable nonvolatile memory behavior, especially record-wide hysteresis windows up to 177V, which enables efficient imitation of biological synapses and achieves high recognition accuracy for electrocardiogram patterns. This result provides a device paradigm for future nonvolatile memory and artificial synaptic applications.

Oxygen reservoir effect of Tungsten trioxide electrode on endurance performance of ferroelectric capacitors for FeRAM applications

The effect of W and WO3 electrodes on the ferroelectric characteristics of HZO (Zr-doped HfO2)-based MFM (metal-ferroelectric-metal) capacitors was investigated. During the deposition of tungsten, the W electrode was formed using only Ar gas, while the WO3 electrode was formed using a mixture of Ar and O2 gases. The W-based MFM capacitors exhibited superior remnant polarization (2Pr of 107.9 µC/cm2 at 700 oC) compared to the WO3-based capacitors; however, their endurance performance was degraded. In contrast, the WO3-based capacitors showed endurance performance enhanced by three orders of magnitude due to the oxygen-rich reservoir effect. The oxygen introduced during the deposition of WO3 prevented the oxygen scavenging effect of tungsten. Consequently, excessive generation of oxygen vacancies in the HZO layer was suppressed, resulting in improved endurance performance. These results were quantitatively confirmed through TEM, XPS, and XRD analyses.

Twin phase symmetry and multifunction features of PLZT (7/60/40) ceramics

Here, we report the effect of 7 wt% La substitution at the Pb-site of PZT (60/40), resulting in PLZT (7/60/40) prepared by solid-state reaction technique. The powder X-ray diffraction (XRD) taken at room temperature revealed the coexistence of tetragonal and rhombohedral (T+R) crystal structures having P4mm and R3c space groups. Scanning electron micrograph (SEM) showed the inhomogeneous distribution of quasi-cubical grains with a grain size range of 0.5–1.15 μm. Energy dispersive X-ray analysis (EDS) showed well-resolved peaks of all elements present, and a dielectric peak observed at 220 °C exhibited diffused ferroelectric-paraelectric phase transition of PLZT (7/60/40). The AC conductivity, impedance studies, and the conduction mechanism based on large and small polaronic movements are discussed. The semiconductor to metal transition and a ferroelectric relaxor kind feature were observed in the studied compound. The observed value of bipolar strain is ∼0.2 %, and unipolar is ∼0.18 %. The remnant polarization (Pr) and coercive field (Ec) are ∼ 20 (μC/cm2) and ∼ 13.5 (kV/cm), respectively, in both loops. We calculated large-signal piezoelectric coefficient d33* of ∼600pmV–1. A compact multilayer actuator was fabricated by a stacking method using pellets of PLZT.

Peierls Distortion Induced Giant Linear Dichroism, Second-Harmonic Generation, and In-Plane Ferroelectricity in NbOBr2.

The Peierls distortion plays an essential role in governing the in-plane ferroelectricity and nonlinear optical characteristics of anisotropic niobium oxide dihalides, such as NbOCl2 and NbOI2. Despite its significance, experimental investigation into the structural, optical, and ferroelectric properties of NbOBr2 has been lacking. Here, the successful fabrication of centimeter-sized, high-quality NbOBr2 single crystals, enabling direct observation of Peierls distortion using aberration-corrected scanning transmission electron microscopy, is reported. Notably, the magnitude of Peierls distortion in NbOBr2 falls between that observed in NbOCl2 and NbOI2. Furthermore, the existence of giant linear dichroism absorption and second-harmonic generation (SHG) phenomena associated with Peierls distortion is confirmed. Exfoliated NbOBr2 flakes exhibit single-domain ferroelectricity, with the polarization switchable by an electric field. Temperature-dependent SHG measurements reveal a Curie temperature of ≈502K for ferroelectric NbOBr2. This work demonstrates that NbOBr2 holds promise for polarized nonlinear optical devices, as well as for nonvolatile information processing and storage devices.

High energy storage capacity and relaxation ferroelectric characteristics of fine-grained Ba0.99Bi0.01TiO3@MnO core-shell nanoceramics

A novel core-shell structured Ba0.99Bi0.01TiO3@xMnO (BBT@MnO) (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 mol%) relaxation ferroelectric ceramics were prepared by co-precipitation method. The structures, insulating, dielectric, and energy storage properties of the BBT@MnO ceramics were systematically investigated. According to TEM, the particles had a diameter of about 430 nm, high uniformity, and high dispersity. They were fabricated using a coating technique to simultaneously improve both the dielectric breakdown strength (BDS) and densification of the ceramics. The thickness of the MnO layers in the BBT@MnO particles averaged about 19 nm. Complex impedance testing of BBT@MnO ceramics revealed that only a one-grain boundary response existed for all ceramics, with the best insulating properties at x = 0.4 mol%. Furthermore, MnO coating increased lattice distortion and polarization intensity, altering the crystal structure and microstructure morphology while increasing energy storage density. The ceramics with 0.4 mol% MnO coating showed thin P-E hysteresis loops, with an optimal dielectric constant of 3610, a dielectric loss of 0.01, and the discharged energy density (Jd) of 0.26 J/cm3 and efficiency (η) of 76.5 %. The results showed that MnO coating is beneficial for reducing dielectric loss and improving insulation performance. This study provided valuable insights for the research of lead-free dielectric ceramic capacitors, and the BBT@MnO ceramics present good development prospects in high-power pulse energy storage systems.

Single-crystal ferroelectric LiNbO3 thin film-based synaptic devices enabled with tunable domain wall current for neuromorphic computing

Neuromorphic devices can emulate the human brain to process information, which receives lots of attention in the field of artificial intelligence. Synaptic devices based on ferroelectric thin films feature low-power consumption, multifunctionality, and scalability. Among them, ferroelectric charged domain wall (CDW) devices have attracted intensive interest for the implementation of memristive devices due to their ultrahigh integration ability inherited from the nanoscale domain wall thickness. In particular, the preparation of wafer-scale single-crystalline ferroelectric thin films via ion-sliced heterogeneous wafer bonding lays a good foundation for large-scale integration of ferroelectric devices with functional circuits. However, the biomimic synaptic characteristics and the systematic demonstration of synaptic devices are largely unexplored for this material system. Here, we demonstrate a model synaptic device based on a single-crystal ferroelectric LiNbO3 thin film, which provides the desired characteristics for neuromorphic computing. The conductance modulation demonstrates good linearity for efficient neuromorphic computing applications. Simulations using the Modified National Institute of Standards and Technology handwritten recognition dataset prove that LiNbO3-based synaptic devices can operate with an online learning accuracy of 95.1%. The injection and annihilation of the CDW are proposed as the basis of the conductivity modulation by combining with the piezoresponse force microscopy and conductive atomic force microscopy mapping measurements. With the mature fabrication process of the ultrathin high-quality ferroelectric thin films, LiNbO3-based synaptic devices have an extensive application prospect for future neuromorphic computing systems.

Development of the magnetodielectric properties of BaTiO3-CoFe2O4 multiferroic composites

The lead-free x CoFe2O4 – (1-x) BaTiO3 (x = 0, 0.075, 0.15, 0.225, and 1) composites have been prepared via the solid-state process. The composites' microstructure, dielectric, ferroelectric, ferromagnetic, and magnetodielectric (MD) properties were investigated. Notably, XRD analysis indicates a decrease in the tetragonality factor (cT/aT) of the BTO phase, accompanied by an expansion of the unit cell as x increases from 7.5 to 22.5 wt%. According to the FESEM images, the CoFe2O4 (CFO) grains are uniformly distributed in the matrix with the formation of no impurity phase. The optimal dielectric properties (dielectric constant (εr) ∼ 2018 and dielectric loss factor (tan δ) ∼ 0.16) and ferroelectric characteristics (saturation polarization (PS) ∼ 12 μC/cm2 and remnant polarization (Pr) ∼ 10 μC/cm2) are achieved at x = 7.5 wt%, attributed to the minimal presence of the non-ferroelectric CFO phase. By increasing the ferrite content (up to 22.5 wt%), the ferroelectric properties of the samples showed a gradual diminution. Also, from x = 7.5 to 22.5 wt%, a sharp increase in the saturation magnetization (from 3.75 to 11.3 emu/g) and remnant magnetization (from 0.93 to 2.35 emu/g) values was observed, while the coercivity diminished from 475.8 to ∼ 272 Oe. The simultaneous observation of two ferroic characteristic hysteresis loops validated the multiferroicity of the composites (x = 7.5, 15, and 22.5 wt%). The maximum MD constant (4.68 %) was measured for x = 22.5 wt% composite at the applied magnetic field of 7 kOe.

Polarization pinning at antiphase boundaries in multiferroic YbFeO3

Abstract The switching characteristics of ferroelectrics and multiferroics are influenced by the interaction of topological defects with domain walls. We report on the pinning of polarization due to antiphase boundaries in thin films of the multiferroic hexagonal YbFeO3. We have directly resolved the atomic structure of a sharp antiphase boundary (APB) in YbFeO3 thin films using a combination of aberration-corrected scanning transmission electron microscopy (STEM) and total energy calculations based on density-functional theory (DFT). We find the presence of a layer of FeO6 octahedra at the APB that bridges the adjacent domains. STEM imaging shows a reversal in the direction of polarization on moving across the APB, which DFT calculations confirm is structural in nature as the polarization reversal reduces the distortion of the FeO6 octahedral layer at the APB. Such APBs in hexagonal perovskites are expected to serve as domain-wall pinning sites and hinder ferroelectric switching of the domains.

Large Room-Temperature Electrocaloric Effect in Lead-Free Relaxor Ferroelectric Ceramics with Wide Operation Temperature Range.

In order to obtain large room-temperature electrocaloric effect (ECE) and wide operation temperature range simultaneously in lead-free ceramics, we proposed designing a relaxor ferroelectric with a Tm (the temperature at which the maximum dielectric permittivity is achieved) near-room temperature and glass addition. Based on this strategy, we designed and fabricated lead-free 0.76NaNbO3-0.24BaTiO3 (NN-24BT) ceramics with 1wt.% BaO-B2O3-SiO2 glass addition, which showed distinct relaxor ferroelectric characteristics with strongly diffused phase transition and a Tm near-room temperature. Based on a direct measurement method, a large ΔT (adiabatic temperature change) of 1.3 K was obtained at room temperature under a high field of 11.0 kV mm-1. Additionally, large ECE can be maintained (>0.6 K@6.1 kV mm-1) over a broad temperature range from 23 °C to 69 °C. Moreover, the ECE displayed excellent cyclic stability with a variation in ΔT below ±7% within 100 test cycles. The comprehensive ECE performance is significantly better than other lead-free ceramics. Our work provides a general and effective approach to designing lead-free, high-performance ECE ceramics, and the approach possesses the potential to be utilized to improve the ECE performance of other lead-free ferroelectric ceramic systems.

Influence of RE2O3 (Dy2O3, Yb2O3, and Lu2O3) buffer layers on the structural and ferroelectric characteristics of BiFeO3 thin films

This study investigates the impact of RE2O3 (Dy2O3, Yb2O3, and Lu2O3) buffer layers on the structural and ferroelectric properties of BiFeO3 thin films grown using a spin-coating method. BiFeO3 thin films with various RE2O3 buffer layers were analyzed using x-ray diffraction, atomic force microscopy, secondary ion mass spectrometry, and x-ray photoelectron spectroscopy to determine their crystalline structures, surface topographies, depth profiles, and chemical compositions, respectively. The RE2O3-buffered BiFeO3 film showed better electrical properties compared to the control BiFeO3 film. The buffer layer composed of Yb2O3 showed the lowest leakage current of 6.82 × 10−6 A cm−2, highest remnant polarization of 44.1 μC cm−2, and smallest coercive field of 189 kV cm−1 because of the incorporation of Yb3+ ions into the BiFeO3 film, high degree of (110) preferred orientation, high Fe3+ content, low surface roughness, reduction of Fe3+ valence fluctuation to Fe2+ ions, and decrease in oxygen vacancies. Such BiFeO3 thin films with various RE2O3 buffer layers using spin-coating method pave a pathway toward practical applications of spintronic, sensor and memory.

High-performance PSZT-PSMI-PSZS ceramics: Piezoelectric and ferroelectric insights for advanced applications

High-performance ferroelectric materials have garnered increased attention due to their exceptional dielectric, piezoelectric, and electrostrictive properties. A solid-state reaction method was used to prepare the perovskite Pb(1-x)Smx[(Zr0.52Ti0.48)0.9(Mo1/3In2/3)0.05(Zn1/3Sb2/3)0.05]O3 (where x = 0, 0.02, 0.04, 0.06, and 0.08) ceramics, abbreviated PSZT-PSMI-PSZS. Energy-dispersive X-ray spectroscopy (EDX) and Fourier-transform infrared (FT-IR) spectroscopy were employed to verify the elemental composition and molecular structure, respectively. The results showed good agreement between nominal and measured compositions, and indicated structural changes post-calcination, suggesting successful formation of the perovskite phase. Piezoelectric properties were evaluated, revealing the highest piezoelectric coefficient (d33 = 310 pC/N) at x = 0.02, attributed to optimal morphological features and the morphotropic phase boundary effect. This sample also demonstrated the highest electromechanical coupling factors (kp = 60 %, k31 = 35 %) and the largest impedance resonance frequency difference (Δf = 15.05 kHz). Ferroelectric testing indicated excellent ferroelectric characteristics, with the maximum remanent polarization (Pr = 17.71 μC/cm2) and saturation polarization (Ps = 22.75 μC/cm2) observed at x = 0.02, along with the lowest coercive field (Ec = 10.16 kV/cm). Additionally, this composition exhibited the highest unipolar strain (Smax = 0.17 %) and the inverse piezoelectric coefficient (d∗33 = 427.57 p.m./V). This comprehensive analysis emphasizes the potential of Sm-doped PZT-PMI-PZS ceramics for advanced piezoelectric and ferroelectric applications, particularly at a doping concentration of x = 0.02, where the materials exhibited excellent electrical and mechanical properties.

Theoretical Study of the Multiferroic Properties of Pure and Ion-Doped Pb5M3F19, M = Fe, Cr, Al.

In a first theoretical investigation of the multiferroic properties of Pb5Fe3F19 (PFF) and Pb5Cr3F19 (PCF), we analyze their magnetic, ferroelectric, and dielectric characteristics as functions of temperature, magnetic field, and ion doping concentration using a microscopic model and Green's function theory. The temperature-dependent polarization in PFF and PCF shows a distinctive kink at the magnetic Neel temperature TN, which vanishes when an external magnetic field is applied, indicating the multiferroic behavior of these two compounds. Ion doping effectively tunes the properties of PFF and PCF. In PFF, Cr ion doping leads to a decrease in the Neel temperature TN, while Cr and Al ion doping lowers the ferroelectric Curie temperature TC. In the case of PCF, we observe the enhancement of TC by Fe ion doping and the reduction by Al ion doping. The last result coincides well quantitatively with the experimental data. Additionally, the magnetodielectric coefficient of PFF is enhanced with the increasing magnetic field.

Prominent cycling reversibility and kinetics enabled by CaTiO3 protective layer on Zn metal for aqueous Zn-ion batteries

Aqueous Zn-ion batteries (AZIBs) have received considerable attention owing to their various advantages such as safety, low cost, simple battery assembly conditions, and high ionic conductivity. However, they still suffer from serious problems, including uncontrollable dendrite growth, corrosion, hydrogen evolution reaction (HER) from water decomposition, electrode passivation, and unexpected by-products. The creation of a uniform artificial nanocrystal layer on the Zn anode surface is a promising strategy for resolving these issues. Herein, we propose the use of a perovskite CaTiO3 (CTO) protective layer on Zn (CTO@Zn) as a promising approach for improving the performance of AZIBs. The CTO artificial layer provides an efficient pathway for Zn ion diffusion towards the Zn metal because of the high dielectric constant (εr = 180) and ferroelectric characteristics that enable the alignment of dipole moments and redistribute the Zn2+ ions in the CTO layer. By avoiding the direct contact of the Zn anode with the electrolyte solution, the uneven dendrite growth, corrosion, parasitic side reactions, and HER are mitigated, while CTO retains its mechanical and chemical robustness during cycling. Consequently, CTO@Zn demonstrates an improved lifespan in a symmetric cell configuration compared with bare Zn. CTO@Zn shows steady overpotential (∼68 mV) for 1500 h at 1 mA cm−2/0.5 mA h cm−2, excelling bare Zn. Moreover, when paired with the V2O5-C cathode, the CTO@Zn//V2O5-C full battery delivers 148.4 mA h g−1 (based on the mass of the cathode) after 300 cycles. This study provides new insights into Zn metal anodes and the development of high-performance AZIBs.

Molecular Engineering Regulation Achieving Out‐of‐Plane Polarization in Rare‐Earth Hybrid Double Perovskites for Ferroelectrics and Circularly Polarized Luminescence

AbstractOut‐of‐plane polarization is a highly desired property of two‐dimensional (2D) ferroelectrics for application in vertical sandwich‐type photoferroelectric devices, especially in ultrathin ferroelectronic devices. Nevertheless, despite great advances that have been made in recent years, out‐of‐plane polarization remains unrealized in the 2D hybrid double perovskite ferroelectric family. Here, from our previous work 2D hybrid double perovskite HQERN ((S3HQ)4EuRb(NO3)8, S3HQ=S‐3‐hydroxylquinuclidinium), we designed a molecular strategy of F‐substitution on organic component to successfully obtain FQERN ((S3FQ)4EuRb(NO3)8, S3FQ=S‐3‐fluoroquinuclidinium) showing circularly polarized luminescence (CPL) response. Remarkably, compared to the monopolar axis ferroelectric HQERN, FQERN not only shows multiferroicity with the coexistence of multipolar axis ferroelectricity and ferroelasticity but also realizes out‐of‐plane ferroelectric polarization and a dramatic enhancement of Curie temperature of 94 K. This is mainly due to the introduction of F‐substituted organic cations, which leads to a change in orientation and a reduction in crystal lattice void occupancy. Our study demonstrates that F‐substitution is an efficient strategy to realize and optimize ferroelectric functional characteristics, giving more possibility of 2D ferroelectric materials for applications in micro‐nano optoelectronic devices.

Molecular Engineering Regulation Achieving Out-of-Plane Polarization in Rare-Earth Hybrid Double Perovskites for Ferroelectrics and Circularly Polarized Luminescence.

Out-of-plane polarization is a highly desired property of two-dimensional (2D) ferroelectrics for application in vertical sandwich-type photoferroelectric devices, especially in ultrathin ferroelectronic devices. Nevertheless, despite great advances that have been made in recent years, out-of-plane polarization remains unrealized in the 2D hybrid double perovskite ferroelectric family. Here, from our previous work 2D hybrid double perovskite HQERN ((S3HQ)4EuRb(NO3)8, S3HQ=S-3-hydroxylquinuclidinium), we designed a molecular strategy of F-substitution on organic component to successfully obtain FQERN ((S3FQ)4EuRb(NO3)8, S3FQ=S-3-fluoroquinuclidinium) showing circularly polarized luminescence (CPL) response. Remarkably, compared to the monopolar axis ferroelectric HQERN, FQERN not only shows multiferroicity with the coexistence of multipolar axis ferroelectricity and ferroelasticity but also realizes out-of-plane ferroelectric polarization and a dramatic enhancement of Curie temperature of 94 K. This is mainly due to the introduction of F-substituted organic cations, which leads to a change in orientation and a reduction in crystal lattice void occupancy. Our study demonstrates that F-substitution is an efficient strategy to realize and optimize ferroelectric functional characteristics, giving more possibility of 2D ferroelectric materials for applications in micro-nano optoelectronic devices.