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Comprehensive Summary: Recently, molecular ferroelectrics have attracted more and more attention due to their structural diversity, low cost of manufacture, and environmental‐friendly processing. However, most of the design strategies for molecular ferroelectrics have focused on the selection and modification of cations, and the use of anions has been rarely reported. Here, introducing distorted [CuBr4]2– tetrahedra, we obtained a high curie temperature (Tc) ferroelectric, [cyclopentylammonium]2CuBr4, with a Tc as high as 393 K, and exceeding commercial and most of reported molecular ferroelectric BaTiO3 (393 K). Variable‐temperature hysteresis loop shows the nonlinear P‐E loop of the crystal under a wide temperature range. Variable‐temperature single‐crystal X‐ray diffraction revealed that the Aizu expression can be written as mmmFmm2. In short, this work will inspire more creative strategies by exploiting the tunability of the anion module. [ABSTRACT FROM AUTHOR]

Ferroelectricity in fluid materials, which allows free rotation of molecules, is an unusual phenomenon raising cutting‐edge questions in science. Conventional ferroelectric liquid crystals have been found in phases with low symmetry that permit the presence of spontaneous polarization. Recently, the discovery of ferroelectricity with high symmetry in the nematic phase has attracted considerable attention. However, the physical mechanism and molecular origin of ferroelectricity are poorly understood and a large domain of macroscopically oriented spontaneous polarization is difficult to fabricate in the ferroelectric nematic phase. This study reports new fluid layered ferroelectrics with the C∞v symmetry in which nearly complete orientation of the spontaneous polarization remains stable under zero electric field without any orientation treatment. These ferroelectrics are obtained by simplifying the molecular structure of a compound with a known ferroelectric nematic phase, although the simplification reduced the dipole moment. The results provide useful insights into the mechanism of ferroelectricity due to dipole–dipole interactions in molecular assemblies. The new ferroelectric materials are promising for a wide range of applications as soft ferroelectrics. [ABSTRACT FROM AUTHOR]

Intelligent materials with adaptive response to external stimulation lay foundation to integrate functional systems at the material level. Here, with experimental observation and numerical simulation, we report a delicate nano-electro-mechanical-opto-system naturally embedded in individual multiwall tungsten disulfide nanotubes, which generates a distinct form of in-plane van der Waals sliding ferroelectricity from the unique combination of superlubricity and piezoelectricity. The sliding ferroelectricity enables programmable photovoltaic effect using the multiwall tungsten disulfide nanotube as photovoltaic random-access memory. A complete "four-in-one" artificial vision system that synchronously achieves full functions of detecting, processing, memorizing, and powering is integrated into the nanotube devices. Both labeled supervised learning and unlabeled reinforcement learning algorithms are executable in the artificial vision system to achieve self-driven image recognition. This work provides a distinct strategy to create ferroelectricity in van der Waals materials, and demonstrates how intelligent materials can push electronic system integration at the material level. Intelligent materials change their properties under external stimuli, integrating functionalities at the matter level. Here, Guo et al. report an artificial vision system based on the memory effect produced by sliding ferroelectricity in multiwalled tungsten disulfide nanotubes. [ABSTRACT FROM AUTHOR]

Hexagonal ABO3 oxides (A, B = cation) are a class of rich materials for realizing novel quantum phenomena. Their hexagonal symmetry, oxygen trigonal bipyramid coordination, and quasi-two dimensional layering give rise to properties distinct from those of the cubic ABO3 perovskites. As bulk materials, most of the focus in this class of materials has been on the rare-earth manganites, RMnO3 (R = rare earth); these materials display coupled ferroelectricity and antiferromagnetic order. In this review, we focus on the thin-film manifestations of the hexagonal ABO3 oxides. We cover the stability of the hexagonal oxides and substrates which can be used to template the hexagonal structure. We show how the thin-film geometry not only allows for further tuning of the bulk-stable manganites but also allows for the realization of metastable hexagonal oxides such as the RFeO3 that combine ferroelectricity with weak ferromagnetic order. The thin-film geometry is a promising platform to stabilize additional metastable hexagonal oxides to search for predicted high-temperature superconductivity and topological phases in this class of materials. [ABSTRACT FROM AUTHOR]

Similar to the canonical antiferroelectric (AFE) compound PbZrO3 in Pb(Zr,Ti)O3 solid‐solutions, the presence of double hysteresis loops and that of electric field–induced phase transitions are important characteristics of NaNbO3 AFE materials; yet the phase transition behavior in the latter system is typically irreversible with the related mechanisms not fully understood. Here, we explore the phase transition mechanism of ferroelectric and AFE phases in NaNbO3 based on measurements of single crystals with different directions in conjunction with density‐functional theory (DFT) calculations. The tilting and distortion behaviors of the [NbO6] octahedra are explained by DFT, and the ion displacement in the lattice is traced. The tilting and distortion behaviors of the [NbO6] octahedra with different orientations are compared. We confirm that the tilt and distortion of the [NbO6] octahedra along the [1 1 1] direction is the main reason to improve the stability of AFE phase. This conclusion is verified by the experimental characterizations of large‐size NaNbO3 single crystals successfully obtained in this study. [ABSTRACT FROM AUTHOR]

Despite the great potential of Hf0.5Zr0.5O2 (HZO) ferroelectrics, reliability issues, such as wake-up, fatigue, endurance limitations, imprint and retention loss, impede the implementation of HZO to nonvolatile memory devices. Herein, a study of the reliability properties in HZO-based stacks with the conventional TiN top electrode and Ru electrode, which is considered a promising alternative to TiN, is performed. An attempt to distinguish the mechanisms underlying the wake-up, fatigue and retention loss in both kinds of stacks is undertaken. Overall, both stacks show pronounced wake-up and retention loss. Moreover, the fatigue and retention loss were found to be worsened by Ru implementation. The huge fatigue was suggested to be because Ru does not protect HZO against oxygen vacancies generation during prolonged cycling. The vacancies generated in the presence of Ru are most likely deeper traps, as compared to the traps formed at the interface with the TiN electrode. Implementing the new procedure, which can separate the depolarization-caused retention loss from the imprint-caused one, reveal a rise in the depolarization contribution with Ru implementation, accompanied by the maintenance of similarly high imprint, as in the case with the TiN electrode. Results show that the mechanisms behind the reliability issues in HZO-based capacitors are very electrode dependent and simple approaches to replacing the TiN electrode with the one providing, for example, just higher remnant polarization or lower leakages, become irrelevant on closer examination. [ABSTRACT FROM AUTHOR]

Inorganic perovskite ferroelectric-based nanomaterials as sustainable new energy materials, due to their intrinsic ferroelectricity and environmental compatibility, are intended to play a crucial role in photoelectrochemical field as major functional materials. Because of versatile physical properties and excellent optoelectronic properties, ferroelectric-based nanomaterials attract much attention in the field of photocatalysis, photoelectrochemical water splitting and photovoltaic. The aim of this review is to cover the recent advances by stating the different kinds of ferroelectrics separately in the photoelectrochemical field as well as discussing how ferroelectric polarization will impact functioning of photo-induced carrier separation and transportation in the interface of the compounded semiconductors. In addition, the future prospects of ferroelectric-based nanomaterials are also discussed. [ABSTRACT FROM AUTHOR]

Recent realizations of ultrathin freestanding perovskite oxides offer a unique platform to probe novel properties in two-dimensional oxides. Here, we observe a giant flexoelectric response in freestanding BiFeO3 and SrTiO3 in their bent state arising from strain gradients up to 3.5 × 107 m−1, suggesting a promising approach for realizing ultra-large polarizations. Additionally, a substantial change in membrane thickness is discovered in bent freestanding BiFeO3, which implies an unusual bending-expansion/shrinkage effect in the ferroelectric membrane that has never been seen before in crystalline materials. Our theoretical model reveals that this unprecedented flexural deformation within the membrane is attributable to a flexoelectricity–piezoelectricity interplay. The finding unveils intriguing nanoscale electromechanical properties and provides guidance for their practical applications in flexible nanoelectromechanical systems. Freestanding perovskite oxides pave the way to novel flexural properties. Here, the authors observe the nanoscale strain gradient induced flexoelectric polarization with an unusual flexo-expansion/shrinkage effect in freestanding ferroelectric oxide. [ABSTRACT FROM AUTHOR]

Organic–inorganic hybrid perovskites (OIHPs) have gained tremendous interest for their rich functional properties. However, the coexistence of more than one of ferroelectricity, ferromagnetism and ferroelasticity has been rarely found in OIHPs. Herein, we report a two‐dimensional Cr2+‐based OIHP, [3,3‐difluorocyclobutylammonium]2CrCl4 ([DFCBA]2CrCl4), which shows both ferroelectricity and ferromagnetism. It undergoes a 4/mmmFm type ferroelectric phase transition at a temperature as high as 387 K and shows multiaxial ferroelectricity with a saturate polarization of 2.1 μC cm−2. It acts as a soft ferromagnet with a Curie temperature of 32.6 K. This work throws light on the exploration of OIHPs with the coexistence of ferroelectricity and ferromagnetism for applications in future multifunctional smart devices. [ABSTRACT FROM AUTHOR]

The effects of field cycling of Al0.78Sc0.22N capacitors on ferroelectric properties are investigated. In the first hundreds of switching cycles, the reduction in the switching voltage was observed, possibly due to the formation of nitrogen-vacancy to facilitate the atom displacements. With further switching cycles, fatigue effect was observed, especially for domains with low switching voltage. The leakage current analysis indicates continuous downward band bending with the number of switching cycles, effectively reducing the Schottky barrier height for electrons. The breakdown of Al0.78Sc0.22N films is triggered by the Joule heat due to excessive leakage current. The mechanism is in contrast to conventional ferroelectric materials, where the breakdown is triggered by Joule heat at the local conductive filaments. [ABSTRACT FROM AUTHOR]

Characterizing the novel properties of layered van der Waals materials is key for their application in functional devices. A better understanding of this type of material requires correlative imaging of diverse nanoscale material properties. Within this class of materials, CuInP2S6 (CIPS) has received a significant degree of interest due to its ionically mediated room temperature ferroelectricity. Moreover, it is possible to form stable self-assembled heterostructures of ferroelectric CuInP2S6 (CIPS) and non-ferroelectric (i.e., lacking Cu) In4/3P2S6 (IPS) phases, by controlling the targeted composition and kinetics of synthesis. In this work, we present a correlative nanometric imaging study of the phononic modes and piezoelectricity of the phase-separated thin heteroepitaxial CIPS/IPS flakes. We show that it is possible to isolate the different phononic modes of the two phases by spatially correlating them with their distinct ferroelectric behavior. The coupling of our experimental data with unsupervised learning statistical methods enables unraveling specific Raman peaks that are characteristic of each chemical phase (CIPS and IPS) present in the composite sample, discarding the less significant ones. [ABSTRACT FROM AUTHOR]

Methylammonium lead iodide, as related organometal halide perovskites, emerged recently as a particularly attractive material for photovoltaic applications. The origin of its appealing properties is sometimes assigned to its potential ferroelectric character, which remains however a topic of intense debate. Here, we rationalize from first-principles calculations how the spatial arrangement of methylammonium polar molecules is progressively constrained by the subtle interplay between their tendency to bond with the inorganic framework and the appearance of iodine octahedra rotations inherent to the perovskite structure. The disordered tetragonal phase observed at room temperature is paraelectric. We show that it should a priori become ferroelectric but that iodine octahedra rotations drive the system toward an antipolar orthorhombic ground state, making it a missed ferroelectric. [ABSTRACT FROM AUTHOR]

The ferroelectricity of wurtzite, Sc x Ga1âˆ' x N (x  = 0.35â€"0.44), was demonstrated in a metalâ€"ferroelectricâ€"metal capacitor. The remanent polarization (P r) obtained from positive-up negative-down measurements was high, exceeding 120 ÎĽ C cmâˆ'2. The coercive field (E c) of Sc0.44Ga0.56N was approximately 3.6 MV cmâˆ'1 at 300 K, which decreased to 3 MV cmâˆ'1 at 473 K. We observed that regardless of the host material, the Sc concentration governed the E c value in Sc x Ga1âˆ' x N and Sc x Al1âˆ' x N, suggesting that the polarization switching started in the wurtzite unit cells containing Sc. Additionally, the E c of Sc x Ga1âˆ' x N was lower than that of Sc x Al1âˆ' x N when P r was equivalent. [ABSTRACT FROM AUTHOR]

In this paper, we report on a new compound in the SrO-TiO 2 -B 2 O 3 system. For the first time, the compound was synthesized by crystallization of glass tapes (GTs) obtained from a melt of 2SrO-2TiO 2 -B 2 O 3. The crystallization was performed at 670 °C for 96 h. X-ray studies showed that the formed new phase belongs to the hexagonal system. The lattice parameters were calculated as well. It was suggested to attribute the new material to stoichiometric compound Sr 3 Ti 3 O 6 (BO 3) 2. The dielectric constant, tangent loss, ac and dc conductivities of the material were found to be almost constant in a wide range of temperatures. Investigation of varistor properties showed that the current-voltage nonlinearity coefficient is 2.05. The measurement of the hysteresis loop confirms the ferroelectricity of the material. It is found that the second-harmonic generation intensity of the new material exceeds that of KDP (by a factor of 3.2 for particles in the size range of 100 ÷ 150 μm). • A novel compound has been discovered in the SrO-TiO 2 -B 2 O 3 system. • The dielectric parameters and conductivity have been studied. • The ferroelectric and second-harmonic generation properties have been revealed. [ABSTRACT FROM AUTHOR]

In this paper, we investigated the structural, multiferroic, dielectric and optical properties of Aurivillius Bi 6 Fe 1.5 Co 0.5 Ti 3- x W x O 18 (0 ≤ x ≤ 0.07) ceramics. The doping of W6+ ions promoted the grain's growth and induced c -axis orientation. The remnant magnetization M r monotonously decreased from 0.49 to 0.13 emu/g with increasing the doping level from 0 to 0.07, which was primarily attributed to the reduced Fe2+-O-Fe3+ exchange and the improved orientation growth. The remnant polarization P r increased from 4.5 μ C/cm2 for x = 0–8.3 μ C/cm2 for x = 0.05 and then decreased to 7.2 μ C/cm2 for x = 0.07, the enhanced ferroelectricity was mainly related to the suppressed oxygen vacancy and larger grain size, while the weakened ferroelectricity was ascribed to the improved c -axis orientation. The M r (0.23 emu/g) along c axis was larger 0.13 emu/g in ab plane while the P r (7.2 μ C/cm2) along c axis was smaller than 9.3 μ C/cm2 in ab plane in x = 0.07, which demonstrated the anistropy of multiferroicity. In addition, all samples exhibited two different dielectric relaxations. The first one with activation energy ranging from 0.47 to 0.53 eV was assigned to the hopping process of electron, while the second one with activation energy varied from 1.45 to 3.63 eV was associated with the viscous motion of ferroelectric domain walls. Two direct absorption edges were demonstrated by Tauc's law in all samples. The band gap 1 increased from 1.93 to 1.95 eV due to the lower t 2 g state of Fe2+. The band gap 2 varied between 2.21 and 2.28 eV, which was proposed to the competition between TiO 6 distortion and Ti4+ content. These results indicate that doping donor W6+ in Aurivillius compounds is a feasible way to regulate the multiferroic and optical properties. • Doping W6+ ions promoted the grain's growth and the oriented growth along c -axis. • Anisotropic multiferroic properties were demonstrated in the sample x = 0.07. • Two different dielectric relaxation behaviors were observed in all samples. • Two absorption edges originated from two direct inter-band transitions were found. [ABSTRACT FROM AUTHOR]

We present a thermodynamic analysis of the recently discovered nitride ferroelectric materials using the classic Landau–Devonshire approach. Electrostrictive and dielectric stiffness coefficients of Al1−xScxN with a wurtzite structure (6 mm) are determined using a free energy density function assuming a hexagonal parent phase (6/mmm), with the first-order phase transition based on the dielectric stiffness relationships. The results of this analysis show that the strain sensitivity of the energy barrier is one order of magnitude larger than that of the spontaneous polarization in these wurtzite ferroelectrics, yet both are less sensitive to strain compared to classic perovskite ferroelectrics. These analysis results reported here explain experimentally reported sensitivity of the coercive field to elastic strain/stress in Al1−xScxN films and would enable further thermodynamic analysis via phase field simulation and related methods. [ABSTRACT FROM AUTHOR]

The sputtering power dependence of 40 nm thick Al0.7Sc0.3N ferroelectric properties was characterized from 200 to 300 W. X-ray rocking curve revealed higher orientated growth into the c -axis with higher sputtering power. Films formed by high power showed reduced leakage current with a higher breakdown field, enabling one to apply the high field for ferroelectric switching. A high remnant polarization (P r) of 130 μ C cm−2 was obtained with a coercive field (E c) of 6 MV cm−1. The switching cycle test revealed a wake-up effect for all the films; increasing the leakage current and modifying the E c. We anticipate the change is attributed to the existence and the generation of nitrogen vacancies (V N) in the films. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) van der Waals (vdW) materials provide the versatile playground to stack two or more vdW layers for creation of superior materials with desired properties. Here we theoretically adopt a twisted stack-engineering of two LaBr2 monolayers to break space inversion symmetry for ferroelectricity and ultimately multiferroism. The enhancement and reversal of electric polarization are accompanied with the transition from interlayer ferromagnetic and antiferromagnetic orderings, demonstrating an effective magnetoelectric coupling effect with a mechanism dissimilar to that of the conventional multiferroics. Magnetization dynamics simulations show that such magnetic phase transition can excite topologically protected bimeron, and the skyrmion Hall effect can be suppressed by bilayer-bimeron stabilized in both ferromagnetic and antiferromagnetic configurations. Moreover, in the small-angle twisted moiré superlattice, the uniform polarization will evolve into a staggered domain structure, accompanied with the appearance of bimeron, which forms a significant discrepancy with the non-twisted stack-engineered multiferroic LaBr2 bilayer. This work provides a strategy for 2D multiferroic materials by twisted stack engineering of magnetic single layers. [ABSTRACT FROM AUTHOR]

Cation ordering in solids is important for controlling physical properties and leads to ilmenite (FeTiO3) and LiNbO3 type derivatives of the corundum structure, with ferroelectricity resulting from breaking of inversion symmetry in the latter. However, a hypothetical third ABO3 derivative with R32 symmetry has never been observed. Here we show that Co2InSbO6 recovered from high pressure has a new, ordered‐R32 A2BCO6 variant of the corundum structure. Co2InSbO6 is also remarkable for showing two cation redistributions, to (Co0.5In0.5)2CoSbO6 and then Co2InSbO6 variants of the ordered‐LiNbO3 A2BCO6 structure on heating. The cation distributions change magnetic properties as the final ordered‐LiNbO3 product has a sharp ferrimagnetic transition unlike the initial ordered‐R32 phase. Future syntheses of metastable corundum derivatives at pressure are likely to reveal other cation‐redistribution pathways, and may enable ABO3 materials with the R32 structure to be discovered. [ABSTRACT FROM AUTHOR]

Cation ordering in solids is important for controlling physical properties and leads to ilmenite (FeTiO3) and LiNbO3 type derivatives of the corundum structure, with ferroelectricity resulting from breaking of inversion symmetry in the latter. However, a hypothetical third ABO3 derivative with R32 symmetry has never been observed. Here we show that Co2InSbO6 recovered from high pressure has a new, ordered‐R32 A2BCO6 variant of the corundum structure. Co2InSbO6 is also remarkable for showing two cation redistributions, to (Co0.5In0.5)2CoSbO6 and then Co2InSbO6 variants of the ordered‐LiNbO3 A2BCO6 structure on heating. The cation distributions change magnetic properties as the final ordered‐LiNbO3 product has a sharp ferrimagnetic transition unlike the initial ordered‐R32 phase. Future syntheses of metastable corundum derivatives at pressure are likely to reveal other cation‐redistribution pathways, and may enable ABO3 materials with the R32 structure to be discovered. [ABSTRACT FROM AUTHOR]

As ferroelectric Hf0.5Zr0.5O2 (HZO) thickness scales below 10 nm, the switching characteristics are severely distorted typically showing an antiferroelectric-like behavior (pinched hysteresis) with reduced remanent polarization. Using Landau-Ginsburg-Devonshire (LGD) theory for the analysis of the experimental results, it is shown here that, in thin (5 nm) HZO, depolarization fields drive the system in a stable paraelectric phase coexisting with a metastable ferroelectric one, which explains the pinched hysteresis. This state of matter resembles a first order ferroelectric above the Curie temperature which is known to result in similar double-loop behavior. Here, based on the analysis of experimental data in the framework of LGD theory, it is reported that charge injection and trapping at pre-existing interface defects during field cycling ("wake-up") screens the depolarization field stabilizing ferroelectricity. It is found in particular that a sufficiently large energy density of interface states is beneficial for the recovery of fully open ferroelectric loops. HfO2-based ferroelectric materials have immense technological potential and so significant attention has been given to improve the ferroelectric properties at low-thickness. Here, using Landau Devonshire theory, the authors show the origin of pinched hysteresis loops is connected with the existence of pronounced depolarizing fields which are minimized during field cycling recovering the full ferroelectric loops. [ABSTRACT FROM AUTHOR]

Magnetic Weyl semimetals Mn3Sn compound have appealing prospects for spintronic applications due to their various crystal structures and magnetic properties for the design of reliable high-density memories. This work presents the effect of tensile and compressive strains on the structural, electronic, magnetic, ferroelectric, optic, and magneto-optical properties of the Mn3Sn compound. Firstly, the first-principles calculations reveal an enormous ferroelectric effect with metallic behaviour. Secondly, we perform an analysis of the magneto-optical Kerr effect (MOKE) at stained thin-film, an important MOKE effect was found in an antiferromagnetic (AFM) ground state following along the direction [001] which is in good agreement with the experimental studies. Our results improve the theoretical understanding of the Dzyaloshinskii–Moriya interactions (DM) induced a large magneto-optical Kerr effect (MOKE). Also, the relationship between ferroelectric and ferromagnetic was investigated and should provide a basis for the experimental application of spintronic devices based on the anomalous Hall effect. [ABSTRACT FROM AUTHOR]

Bismuth containing hybrid molecular ferroelectrics are receiving tremendous attention in recent years owing to their stable and non‐toxic composition. However, these perovskite‐like structures are primarily limited to ammonium cations. Herein, we report a new phosphonium based discrete perovskite‐like hybrid ferroelectric with a formula [Me(Ph)3P]3[Bi2Br9] (MTPBB) and its mechanical energy harvesting capability. The Polarization‐Electric field (P‐E) measurements resulted in a well‐defined ferroelectric hysteresis loop with a remnant polarization value of 2.1 μC cm−2. Piezoresponse force microscopy experiments enabled visualization of the ferroelectric domain structure and evaluation of the piezoelectric strain coefficient (d33) for an MTPBB single crystal and thin film sample. Furthermore, flexible devices incorporating MTPBB in polydimethylsiloxane (PDMS) matrix at various concentrations were fabricated and explored for their mechanical energy harvesting properties. The champion device with 20 wt % of MTPBB in PDMS rendered a maximum peak‐to‐peak open‐circuit voltage of 22.9 V and a maximum power density of 7 μW cm−2 at an optimal load of 4 MΩ. Moreover, the potential of MTPBB‐based devices in low power electronics was demonstrated by storing the harvested energy in various electrolytic capacitors. [ABSTRACT FROM AUTHOR]

Plastic/ferroelectric crystals have attracted increasing attention as emerging functional molecular materials with unique features. In this study, a new plastic/ferroelectric crystal is developed from ionic molecules, which exhibits three solid phases: a paraelectric plastic crystal phase and two ferroelectric phases. Owing to the malleability in the plastic crystal phase, by applying pressure to the powder at a high temperature, transparent polycrystalline films are easily fabricated, exhibiting ferroelectric performance at room temperature. The crystal structures of the three solid phases are determined by single‐crystal X‐ray diffraction analysis. The arrangement of anions and cations in the ferroelectric crystal is significantly distorted from the CsCl‐type structure in the paraelectric cubic crystal. The large distortion results in distinct splitting of the diffraction peaks that are equivalent in the cubic phase, which enables the observation of electric‐field‐induced lattice reorientation using powder X‐ray diffraction measurements. These results broaden the range of compounds for which plastic/ferroelectric crystals can be developed and provide a solid understanding of the switching process in their bulk polycrystals, which will facilitate their application to a variety of device elements. [ABSTRACT FROM AUTHOR]

The Bi4−xErxTi3O12 films were deposited on the substrates via the spin-coating method. The doping of Er3+ does not give rise to the variation of phase composition and element valence state in the Bi4−xErxTi3O12 films. The Bi4−xErxTi3O12 films show dense surfaces and clear phase interfaces between the films and the substrates. And the surfaces of the Bi4−xErxTi3O12 films contain particles with various sizes and irregular shapes. The leakage current density and ferroelectricity of Er3+-doped Bi4Ti3O12 (Bi4−xErxTi3O12) are lower than those of Bi4Ti3O12 films. With increasing the doping concentration of Er3+, the values of leakage current density of Bi4−xErxTi3O12 films decrease at first and then increase. Meanwhile, the values of remanent polarization of Bi4−xErxTi3O12 films increase at the beginning and then decrease. Among the five films, the Bi3.4Er0.6Ti3O12 film exhibit smallest value of leakage current density (6.43 × 10−7 A·cm−2) and greatest value of remanent polarization (14.4 μC·cm−2), which indicates the better ferroelectric properties. This is beneficial for the application of ferroelectric functional devices. [ABSTRACT FROM AUTHOR]

With the urgent need to explore low‐cost, high‐efficiency solid‐state refrigeration technology, the electrocaloric effects of ferroelectric materials have attracted much attention in the past decades. With the development of modern computing technology, the phase‐field method is widely used to simulate the evolution of microstructure at mesoscale and predict the properties of different types of ferroelectric materials. In this article, we review the recent progress of electrocaloric effects from phenomenological Landau thermodynamics theory to phase‐field simulation by discussing the microcosmic composition, mesoscopic domain structures, macroscopic size/shape, and external stimulus of strain/stress. More importantly, in searching for new ferroelectric electrocaloric cooling materials, it is possible to find materials whose free energy barrier height changes rapidly with temperature, such materials have a faster change rate with polarization temperature in terms of ferroelectric macroscopic properties, from them could get superior electrocaloric effects. We compile a relatively comprehensive computational design on the high performance of electrocaloric effects in different types of ferroelectrics and offer a perspective on the computational design of electrocaloric refrigeration materials at the mesoscale microstructure level. [ABSTRACT FROM AUTHOR]

The tendency of zinc (Zn) anodes to form uncontrolled Zn electrodeposits and the occurrence of side-reactions at Zn-electrolyte interfaces are a fundamental barrier hampering broad applications of aqueous rechargeable Zn-based batteries. Herein, a ferroelectric domain-mediated strategy is proposed to manipulate the Zn plating behavior and achieve controllable Zn growth orientation by coating Zn foil with a ferroelectric tetragonal KTN (t-KTN) layer. The ferroelectric domain of t-KTN single crystals exhibits periodic distribution of upward and downward polarizations, corresponding to alternating positively and negatively charged surfaces. The charged ferroelectric surfaces can manipulate the transfer kinetics of Zn ions and the concentration distribution of anions via the interplay between ferroelectric dipoles and adsorbed ions. With the synergistic effect of the ferroelectric polarization and domain configurations, the well-aligned interlamellar arrays composed of electrodeposited Zn are formed in the initial deposition process, which enable selective deposition within interlamellar arrays and eliminate the dendrite growth during the following plating process. As a result, the t-KTN layer-modified Zn anode enables reversible Zn plating/stripping with low voltage hysteresis for over 1200 h at 1 mA cm-2 in symmetric cells, and the assembled full cell exhibits a significantly enhanced cycling stability of over 5500 cycles at 5 A g-1. [ABSTRACT FROM AUTHOR]

We report the improvement of ferroelectric (FE) amorphous InGaZnO4(a-IGZO) thin film transistors (TFT) by Ar/O2 plasma treatment and subsequent thermal annealing. The a-IGZO (In2O3:Ga2O3:ZnO = 1:1:1) and HZO (Hf:Zr = 1:1) layers were deposited by DC sputtering and spray coating, respectively. The Ar/O2 plasma exposure for 10s and thermal annealing at 300°C for 15 min on the a-IGZO/HfZrO(HZO) improves the field effect mobility from 18.6 to 20.0 cm2V−1S−1, subthreshold swing from 345 to 125 mV/decade, memory window from 0.45 V to 1.0 V, and polarization from $7 ~\mu \text{C}$ /cm2 to $9 ~\mu \text{C}$ /cm2. The improvement is mainly due to the reduction of oxygen vacancies at the a-IGZO/HZO interface by Ar/O2 plasma exposure and subsequent annealing. [ABSTRACT FROM AUTHOR]

In the last decade, a surge in research on hafnium oxide (HfOx)-based ferroelectricity has become a significant part of the semiconductor research trend. In this work, we present a new factor, that is, process damage to the HfOx-based ferroelectric implementation. By reversely exploiting harmful damage in the metal-oxide-semiconductor process, the ferroelectricity in HfOx is improved by eliciting early amorphization suitable for orthorhombic phase formation. In addition, through various pulsed polarization measurements, it is found that the damaged HfOx film has a fast polarization switching speed, with an initial pinning site that is suitable for domain wall growth motion; advantageous for high-performance memory applications. [ABSTRACT FROM AUTHOR]

Densely woven highly crystallized biocompatible sodium–potassium niobate Na0.35K0.65NbO3 fibers with an average diameter of 100–200 nm and several hundreds of microns in length were sintered by the sol–gel calcination-assisted electrospinning technique. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (TEM) confirmed preferential cube-on-cube [001] orientation of nanocrystals within the fiber's body, separated by a low angle grain boundary. The Williamson–Hall method was employed to analyze the broadening of XRD reflections and to accurately determine the size and intrinsic strain of nanocrystal fiber aggregates. The main objective of this article is to test the potential capacity of direct XRD analysis to noninvasively control crystallite size and lattice distortion in core-shell coaxial nanofibers. [ABSTRACT FROM AUTHOR]

This article provides a comprehensive overview of piezo‐ and ferro‐electric materials based on organic molecules and organic–inorganic hybrids for mechanical energy harvesting. Molecular (organic and organic–inorganic hybrid) piezo‐ and ferroelectric materials exhibit significant advantages over traditional materials due to their simple solution‐phase synthesis, light‐weight nature, thermal stability, mechanical flexibility, high Curie temperature, and attractive piezo‐ and ferroelectric properties. However, the design and understanding of piezo‐ and ferroelectricity in organic and organic–inorganic hybrid materials for piezoelectric energy harvesting applications is less well developed. This review describes the fundamental characterization of piezo‐ and ferroelectricity for a range of recently reported organic and organic–inorganic hybrid materials. The limits of traditional piezoelectric harvesting materials are outlined, followed by an overview of the piezo‐ and ferroelectric properties of organic and organic–inorganic hybrid materials, and their composites, for mechanical energy harvesting. An extensive description of peptide‐based and other biomolecular piezo‐ and ferroelectric materials as a biofriendly alternative to current materials is also provided. Finally, current limitations and future perspectives in this emerging area of research are highlighted. This perspective aims to guide chemists, materials scientists, and engineers in the design of practically useful organic and organic–inorganic hybrid piezo‐ and ferroelectric materials and composites for mechanical energy harvesting. [ABSTRACT FROM AUTHOR]

2D spontaneous valley polarization attracts great interest both for its fundamental physics and for its potential applications in advanced information technology, but it can only be obtained from inversion asymmetric single-layer crystals, while the possibility to create 2D spontaneous valley polarization from inversion symmetric single-layer lattices remains unknown. Here, starting from inversion symmetric single-layer lattices, a general design principle for realizing 2D spontaneous valley polarization based on van der Waals interaction is mapped out. Using first-principles calculations, we further demonstrate the feasibility of this design principle in a real material of T-FeCl2. More remarkably, such design principle exhibits the additional exotic out-of-plane ferroelectricity, which could manifest many distinctive properties, for example, ferroelectricity-valley coupling and magnetoelectric coupling. The explored design-guideline and phenomena are applicable to a vast family of 2D materials. Our work not only opens up a platform for 2D valleytronic research but also promises the fundamental research of coupling physics in 2D lattices. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) multiferroics have attracted increasing interests in basic science and technological fields in recent years. However, most reported 2D magnetic ferroelectrics are based on the d-electron magnetism, which makes them rather rare due to the empirical d0 rule and limits their applications for low magnetic phase transition temperature. In this work, we demonstrate that the ferroelectricity can coexist with the p-electron-induced ferromagnetism without the limitation of d0 rule and metallicity in a family of stable 2D MXene-analogous oxynitrides, X2NO2 (X = In, Tl). Remarkably, the itinerant character of p electrons can lead to the strong ferromagnetic metallic states. Furthermore, a possible magnetoelectric effect is manifested in a Tl2NO2/WTe2 heterostructure through the interface engineering. Our findings provide an alternative possible route toward 2D multiferroics and enrich the concept of ferroelectric metals. [ABSTRACT FROM AUTHOR]

In this work, atomically K1−xNaxNbO3 thin films are taken as examples to investigate the reversible and irreversible effects in a horizon plane, i.e., the changes of domain structures, phase states, free energies, etc., under a z-axis alternating current field via a phase-field method. The simulation results show the driving forces during the charging and discharging process, where there is a variation for the angles of the domain walls from 180° to 90° (and then an increase to 135°), which are the external electric field and domain wall evolution, respectively. As for the phase states, there is a transformation between the orthorhombic and rhombohedral phases which can't be explained by the traditional polarization switching theory. This work provides a reasonable understanding of the alternating current field effect, which is essential in information and energy storage. [ABSTRACT FROM AUTHOR]

Single-phase epitaxial Hf0.5Zr0.5O2 films with non-centrosymmetric orthorhombic structure have been grown directly on electrode-free corundum (α-Al2O3) substrates by pulsed laser deposition. A combination of high-resolution X-ray diffraction and X-ray absorption spectroscopy confirms the epitaxial growth of high-quality films belonging to the Pca21 space group, with [111] out-of-plane orientation. The surface of a 7-nm-thick sample exhibits an atomic step-terrace structure with a corrugation of the order of one atomic layer, as proved by atomic force microscopy. Scanning transmission electron microscopy reveals that it consists of grains with around 10 nm lateral size. The polar nature of this film has been corroborated by pyroelectric measurements. These results shed light on the mechanisms of the epitaxial stabilization of the ferroelectric phase of hafnia. [ABSTRACT FROM AUTHOR]

Bi1−xNdxFe0.99Mn0.01O3 (BNFMO, x = 0.00~0.20) films were epitaxially grown on Nb:SrTiO3 (001) substrates using pulsed laser deposition. It was found that the Nd-doping concentration has a great impact on the surface morphology, crystal structure, and electrical properties. BNFMO thin film with low Nd-doping concentration (≤16%) crystallizes into a rhombohedral structure, while the high Nd-doping (>16%) will lead to the formation of an orthogonal structure. Furthermore, to eliminate the resistive switching (RS) effect, a positive-up–negative-down (PUND) measurement was applied on two devices in series. The remnant polarization experiences an increase with the Nd-doping concentration increasing to 16%, then drops down with the further increased concentration of Nd. Finally, the ferroelectric photovoltaic effect is also regulated by the ferroelectric polarization, and the maximum photocurrent of 1758 μA/cm2 was obtained in Bi0.84Nd0.16Fe0.99Mn0.01O3 thin film. BNFMO films show great potential for ferroelectric and photovoltaic applications. [ABSTRACT FROM AUTHOR]

The present study reveals the effect of (Sr, Mn) co-doping on structural, dielectric, ferroelectric and electronic transport properties of Ba1−xSrxTi1−xMnxO3 (x = 0.0, 0.1), synthesized by the solid-state reaction route. Rietveld refinement technique ensures mono-phase nature with tetragonal crystal structure and P4mm space group. The variation in structural parameters highlights a distortion in the tetragonal phase instigated upon (Sr, Mn) co-doping. FTIR analysis reveals absorption bands in the region of 520–560 cm−1 and 425–435 cm−1 corresponding to Ti–O stretching (ν1) and bending vibrations (ν2), respectively, confirming the formation of TiO6 octahedron. As estimated from the SEM micrographs, the grain sizes were found to fall in the range of 0.2–1.4 µm. The room temperature dielectric measurements reveal high value of dielectric constant for pristine sample (ɛr = 1121), which decreases slightly upon co-doping (ɛr = 1081). The samples exhibit the universal dielectric response phenomena within the low frequency regime (42 Hz–50 kHz). The temperature dependent dielectric measurements exhibit a thermally activated peak near 400 K for the pristine sample, which shifts towards lower temperatures for the doped sample, indicating a decrease in the Curie temperature. The conduction mechanism in this system is governed by the correlated barrier hopping model. The ac conductivity data have been used to estimate the binding energies (Wm) of the polarons and activation energy in different temperature regions. The room temperature ferroelectric hysteresis (P–E loops) studies reveal that the ferroelectricity in the system is suppressed under the effect of (Sr, Mn) co-doping. The negative temperature coefficient of resistivity (NTCR) characteristics indicate the semiconductor nature of these systems. [ABSTRACT FROM AUTHOR]

The search for ultrathin and robust ferroelectrics leads to few promising two‐dimensional (2D) materials. In2Se3 has drawn special attention owing to the existence of intercoupled in‐plane (IP) and out‐of‐plane (OOP) ferroelectricity in monolayer form, which makes it a potential candidate for emerging artificial intelligence, information processing and memory applications. In addition, the high optical absorption and phase‐dependent visible to infrared bandgap become advantageous from optoelectronics point‐of‐view. This unique ferroelectric and optoelectronic coupling further leads to explore atomic‐scale multifunctional devices. In this review, we summarized the progress of non‐volatile memory (NVM) and photodetector devices, and their intercoupled applications using 2D In2Se3. We expect that this review will provide an insight towards the promising future of 2D ferroelectric‐(opto)electronics and motivate researchers for further development towards industrial scale. [ABSTRACT FROM AUTHOR]

Nonvolatile memories are in strong demand due to the desire for miniaturization, high-speed storage, and low energy consumption to fulfill the rapid developments of big data, the Internet of Things, and artificial intelligence. Hafnia (HfO2)-based materials have attracted significant interest due to the advantages of complementary-metal–oxide–semiconductor (CMOS) compatibility, large coercive voltage, and superior ferroelectricity at an ultra-thin thickness. The comparable ferroelectricity to that of traditional perovskite materials and size advantage of HfO2 result in fascinating storage performance, which can be readily applicable to the fields of integrated non-volatile memories. This Review provides a comprehensive overview of recent developments in HfO2-based ferroelectrics with attention to the origin of ferroelectricity, performance modulation, and recent achievements in the material. Moreover, potential solutions to existing challenges associated with the materials are discussed in detail, including the wake-up effect, long-term fatigue behavior, and imprint challenges, which pave the way for obtaining HfO2-based ferroelectric materials and devices with long service life and high stability. Finally, the range of potential applications for these fascinating new materials is presented and summarized, which include non-volatile memories and neuromorphic systems. This Review intends to present the state-of-the-art HfO2-based ferroelectrics and to highlight the current challenges, possible applications, and future opportunities and can act as an update for recent developments in these intriguing materials and provide guidance for future researchers in the design and optimization of HfO2-based ferroelectric materials and devices. [ABSTRACT FROM AUTHOR]

Hafnium oxides‐based ferroelectric materials are promising for applications in nonvolatile memory devices. To control the ferroelectricity of such materials, it is necessary to tune their polymorphism, interfacial features, and defect (oxygen vacancy) distribution. A strategy is described for enhancing the ferroelectric properties of polycrystalline hafnium zirconium oxide (HZO) ultrathin films by modifying the oxygen pressure during the device preparation stage, which involves thermal annealing of TiN electrodes that serve as oxygen reservoirs. Microstructural and chemical characterizations along with theoretical analysis reveal that interfacial layers of TiO2−x (or TiOxNy) can characteristically form between the TiN electrode and the HZO thin film, depending on the oxygen treatment conditions. These interfacial layers directly affect the polymorphic distribution of the as‐deposited HZO. In particular, the engineered interfacial TiO2−x layer facilitates the generation and stabilization of ferroelectric orthorhombic phase HZO by promoting the uniform distribution of oxygen vacancies. Electric field cycling tests further highlight the enhanced ferroelectric polarization and coercive voltage following interfacial engineering. The results presented herein demonstrate successful tuning of the structural and interfacial properties of polycrystalline HZO devices, thus enabling control over their ferroelectric characteristics, which is critical for the fabrication of devices with designed functionality. [ABSTRACT FROM AUTHOR]

Tetragonal (00l)-oriented epitaxial (Bi,K)TiO3 films were grown at 240 °C on (100)cSrRuO3//(100)SrTiO3 substrates by the hydrothermal method. KOH aqueous solutions and Bi(NO3)3 · 5H2O and TiO2 powders were used as the starting materials. Film thickness was controlled from 33 to 1200 nm by changing the deposition time, and the Bi/(Bi+K) ratio in the A-site of perovskite ABO3 was almost constant for all film thicknesses. Polar-axis (00l)-oriented epitaxial (Bi,K)TiO3 films were obtained without a secondary phase and/or other orientation for all thickness ranges. Large ferroelectricity with the remanent polarization (Pr) of about 84 µC/cm2, comparable to previously reported lead-based ferroelectric films, was observed for (Bi,K)TiO3 films down to 33 nm in thickness. On the other hand, Ec increased with decreasing film thickness, but did not show strong film thickness dependence like other perovskite ferroelectric films. These data are very useful for understanding the degradation mechanism of ferroelectric thin films. [ABSTRACT FROM AUTHOR]

Ferroelectric properties of thin films can be enhanced by crystallographic texture. In this work, we report on how heat treatment of films can be designed to tailor the degree of preferential orientation in BaTiO3‐based thin films from aqueous chemical solution deposition. In situ synchrotron X‐ray diffraction in combination with Rietveld refinements was used to study the crystallization process of films from a single deposition and to give an in‐depth characterization of the crystallographic texture of the films. Transmission electron microscopy was employed to evaluate the microstructure and degree of preferred orientation in thicker films from multiple depositions. Texture was induced in the multilayer films by a repeated annealing process. Cube‐on‐cube growth was demonstrated to occur in both single and multi‐layered films provided the heating program was designed to give limited nucleation and growth below the threshold for where cube‐on‐cube growth is favoured, resulting in a very high degree of preferred orientation. The cube‐on‐cube grown films were relaxed with respect to the lattice unit cell mismatch between the film and the substrate, where the relaxation of stress depends on the film thickness. Texture and cube‐on‐cube growth were demonstrated on several types of single‐crystal oxide substrates. Calcium and zirconium substitution did not alter the crystallization process, but zirconium slowed down the texture formation kinetics. The ferroelectric response was strongest in the films with a high degree of preferred orientation. [ABSTRACT FROM AUTHOR]

In this paper, a novel ferroelectric-based electrostatic doping (Fe-ED) nanosheet tunneling field-effect transistor (TFET) is proposed and analyzed using technology computer-aided design (TCAD) Sentaurus simulation software. By inserting a ferroelectric film into the polarity gate, the electrons and holes are induced in an intrinsic silicon film to create the p-source and the n-drain regions, respectively. Device performance is largely independent of the chemical doping profile, potentially freeing it from issues related to abrupt junctions, dopant variability, and solid solubility. An improved ON-state current and ION/IOFF ratio have been demonstrated in a 3D-calibrated simulation, and the Fe-ED NSTFET’s on-state current has increased significantly. According to our study, Fe-ED can be used in versatile reconfigurable nanoscale transistors as well as highly integrated circuits as an effective doping strategy.

It has been reported that the flexoelectric effect could be dominant in the nanoscale. The discrepancy between theory and experiments on the frequency dependence of the dielectric constant of an ordered assembly of BaTiO3 nanocubes is nearly resolved by assuming the coexistence of flexo- and ferro-electric effects. Although flexoelectric polarizations perpendicular to the applied alternating electric field contribute to the dielectric constant, those parallel to the electric field do not contribute because the magnitude of the flexoelectric polarization does not change due to the mismatch of strain at the interface of the nanocubes. On the other hand, some dielectric response is possible for the ferroelectric component of the polarization parallel to the electric field. [ABSTRACT FROM AUTHOR]