Ferroelectric Transition Research Articles

Metallic Sliding Ferroelectricity in Trilayer Penta‐NiN2

AbstractThe experimental observation of metallic ferroelectricity has challenged the traditional understanding in recent years, as metallicity and ferroelectricity are historically considered mutually exclusive. This breakthrough has sparked significant interest in this field. However, the coexistence of metallicity and sliding ferroelectricity in 2D pentagonal materials has not been reported yet. Here, this is theoretically studied that the sliding ferroelectricity in metallic trilayer penta‐NiN2. By scanning the configuration space of stacking patterns, a pair of polarized configurations is identified with strong out‐of‐plane ferroelectric polarization of ± 5.39 × 10−13 C m−1 and a low polarization switching barrier of 14.33 meV/atom together with a non‐polarized configuration as the intermediate state for the ferroelectric transition. It is also found that there are substantial changes in the second harmonic generation of the stable configurations that would facilitate experimental observation. This work demonstrates the potential of pentagonal sheets as promising metallic sliding ferroelectrics, which would significantly expand the family of sliding ferroelectric materials.

Boosting room-temperature thermoelectricity in SrTiO3-based superlattices

Introducing tensile strain into STO-based superlattices increases the ferroelectric transition temperature, leading to phonon softening at elevated temperatures, which in turn boosts their dimensionless figure of merit (ZT) to 1.2.

Phonon Inverse Faraday Effect from Electron-Phonon Coupling.

The phonon inverse Faraday effect describes the emergence of a dc magnetization due to circularly polarized phonons. In this work we present a microscopic formalism for the phonon inverse Faraday effect. The formalism is based on time-dependent second order perturbation theory and electron phonon coupling. While our final equation is general and material independent, we provide estimates for the effective magnetic field expected for the ferroelectric soft mode in the oxide perovskite SrTiO_{3}. Our estimates are consistent with recent experiments showing a huge magnetization after a coherent excitation of circularly polarized phonons with THz laser light. Hence, the theoretical approach presented here is promising for shedding light into the microscopic mechanism of angular momentum transfer between ionic and electronic angular momentum, which is expected to play a central role in the phononic manipulation of magnetism.

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

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

Soft Mode Behavior in Transition Metal Doped SrTiO3 Thin Films on MgO Substrates

The ferroelectric soft mode in polycrystalline pristine SrTiO3 and weakly doped SrTiO3 : M (M = 2 at% Fe, Ni, Mn, Co) thin films on (001) MgO substrates has been studied using time-domain terahertz spectroscopy. Spectra of real and imaginary parts of film permittivity were determined in the frequency range of 5–100 cm–1 at temperatures between 5 and 300 K. Central frequency and dielectric contribution of the ferroelectric soft mode show Barrett-like temperature dependencies similar to crystalline SrTiO3. Large negative values of Curie temperature and enhanced positive values of Barrett quantum temperatures are discovered indicating that doped SrTiO3 thin films are farther from ferroelectric phase transition than SrTiO3 crystals.

Twofold Anisotropic Superconductivity in Bilayer T_{d}-MoTe_{2}.

Noncentrosymmetric two-dimensional superconductors with large spin-orbit coupling offer an opportunity to explore superconducting behaviors far beyond the Pauli limit. One such superconductor, few-layer T_{d}-MoTe_{2}, has large upper critical fields that can exceed the Pauli limit by up to 600%. However, the mechanisms governing this enhancement are still under debate, with theory pointing toward either spin-orbit parity coupling or tilted Ising spin-orbit coupling. Moreover, ferroelectricity concomitant with superconductivity has been recently observed in the bilayer, where strong changes to superconductivity can be observed throughout the ferroelectric transition pathway. Here, we report the superconducting behavior of bilayer T_{d}-MoTe_{2} under an in-plane magnetic field, while systematically varying magnetic field angle and out-of-plane electric field strength. We find that superconductivity in bilayer MoTe_{2} exhibits a twofold symmetry with an upper critical field maxima occurring along the b axis and minima along the a axis. The twofold rotational symmetry remains robust throughout the entire superconducting region and ferroelectric hysteresis loop. Our experimental observations of the spin-orbit coupling strength (up to 16.4meV) agree with the spin texture and spin splitting from first-principles calculations, indicating that tilted Ising spin-orbit coupling is the dominant underlying mechanism.

Colossal Strain Tuning of Ferroelectric Transitions in KNbO3 Thin Films.

Strong coupling between polarization (P) and strain (ɛ) in ferroelectric complex oxides offers unique opportunities to dramatically tune their properties. Here colossal strain tuning of ferroelectricity in epitaxial KNbO3 thin films grown by sub-oxide molecular beam epitaxy is demonstrated. While bulk KNbO3 exhibits three ferroelectric transitions and a Curie temperature (Tc) of ≈676K, phase-field modeling predicts that a biaxial strain of as little as -0.6% pushes its Tc >975K, its decomposition temperature in air, and for -1.4% strain, to Tc >1325K, its melting point. Furthermore, a strain of -1.5% can stabilize a single phase throughout the entire temperature range of its stability. A combination of temperature-dependent second harmonic generation measurements, synchrotron-based X-ray reciprocal space mapping, ferroelectric measurements, and transmission electron microscopy reveal a single tetragonal phase from 10 K to 975K, an enhancement of ≈46% in the tetragonal phase remanent polarization (Pr),and a ≈200% enhancementin its optical second harmonic generation coefficients over bulk values. These properties in a lead-free system, but with properties comparable or superior to lead-based systems, make it an attractive candidate for applications ranging from high-temperature ferroelectric memory to cryogenic temperature quantum computing.

Strain-Induced Tunable Enhancement of Piezoelectricity in a Novel Molecular Multiferroic Material.

Multiferroics are appealing because of application potentials in data storage devices, sensors, transducers, and energy harvesters. Molecular multiferroics emerge as a promising alternative to inorganic multiferroics due to flexibility, light weight, low toxicity, solution processing, structural diversity, and chemical tunability. While researches have predominantly focused on perovskite structures, studies on molecular ionic multiferroics remain relatively limited. It is urgent to creatively build a novel platform for studying and developing the coupling and interaction between the stress, electricity, and magnetism. Knowing this, the work focuses on a novel organic-inorganic hybrid multiferroic N-ethyl-N-(fluoromethyl)-N-methylethylammonium tetrabromoferrate (III) showing coexisting magnetic and electric orderings. It undergoes antiferromagnetic, ferroelectric, and ferroelastic transitions. Notably, under a strain of 2.0%, the piezoelectric response increases tenfold, and the coercive field of ferroelectric polarization is reduced by half. The strain-induced enhancement of piezoelectricity is rarely reported in molecular multiferroics. Density functional theory is employed to predict that the mechanism of the large piezoelectric response under strain engineering is related to the cation rotation and phase switching between the stable phase and an energetically competitive metastable phase. This study creates a new paradigm to develop molecular multiferroics and future microelectronic devices for energy conversion.

Enhancing SrBi₂Nb₂O₉ ceramics: The role of neodymium substitution in structural, dielectric, and ferroelectric properties

This study investigates the impact of neodymium substitution for bismuth in SrBi2Nb2O9 (SBN) ceramics, a potential lead-free alternative for ferroelectric applications. X-ray diffraction analysis reveals a decrease in lattice parameters and volume with increasing Nd content. Scanning electron microscopy examines the influence of neodymium on grain morphology, showing a transition from characteristic SBN plate-like grains to a mix of plates and micro-spheres. Dielectric properties are evaluated, showing an increase in room-temperature permittivity for Nd-doped samples compared to the undoped counterparts. Additionally, the influence of Nd doping on relaxor ferroelectric behavior is studied. The Curie temperature (Tc) increases with Nd concentration, while the temperature dependence of relaxation time weakens. Ferroelectric properties exhibit significant changes due to Nd doping. Inverse permittivity versus temperature (ϵr−1 vs. T) measurements suggest an unconventional response, potentially due to lattice strain induced by the larger ionic radius of Nd. The potential influence of doping concentration on the order of the ferroelectric transition is also explored. Overall, neodymium substitution in SBN ceramics enhances their ferroelectric properties, making them promising for advanced electronic applications.

Order-disorder behavior in the ferroelectric nematic phase investigated via Raman spectroscopy.

Polar-ordered fluids are of interest both fundamentally and from an application standpoint. The recently discovered ferroelectric nematic phase is an example of a polar-ordered fluid, and while there has been extensive research interest in these materials, some of the fundamental properties are yet to be fully understood. Here, we report the order parameters of one of the first known materials that exhibit a ferroelectric nematic phase, RM734, determined via Raman spectroscopy. Raman spectroscopy been used extensively to determined order parameters in liquid crystals systems but also to probe ferroelectric behavior in solid ferroelectric systems and is therefore a powerful technique to study the ferroelectric nematic phase. A reduction and subsequent recovery of order parameters (Δ〈P_{2}〉≈0.1, Δ〈P_{4}〉≈0.06) is observed near the onset of the N to N_{F} transition, a feature that is confirmed via complementary birefringence measurements. This dip in order parameters has been attributed to splay fluctuations, which occur at the onset of the N_{F} transition; here we suggest a different explanation. A broadening of the full-width half-maxima (FWHM), of the order of 1 cm^{-1}, of the selected Raman peak is observed near the N to N_{F} phase transition, which we relate to either a change in reorientational dynamics or the onset of polar order. The N_{F} transition is analyzed using standard solid ferroelectric frameworks. An energetic barrier associated with the para- to ferroelectric transition is found to be of the order of 2.5±0.6kJ/mol, which is comparable to solid ferroelectric materials.

Prediction of multiferroicity in the layered hydrogen-bonded system α-CrOOH: Proton transfer ferroelectricity and strain-tunable magnetic transition

Ferroelectric materials with vertical polarization could provide ground-breaking device applications, compatible with electric-field switching even in the presence of a surface-depolarizing field. Herein, we present theoretical evidence that rhombohedral chromium oxide hydroxide α-CrOOH, which has been synthesized experimentally, is a type-Ⅰ multiferroic. First-principles calculations plus Monte Carlo simulations indicate that α-CrOOH is a room temperature proton transfer ferroelectricity semiconductor with robust electric polarization as large as 24.6 μC/cm2. Moreover, the system exhibits an anti-ferromagnetism ground state with a Neel temperature of 340 K, where strain can modulate the transition from antiferromagnetic to ferromagnetic. Finally, a two-dimensional (2D) heterostructure model was designed, composed of monolayer α-CrOOH and functional MXene, for various applications such as non-volatile memory (NVM). The remarkable properties may enable the system to hold great potential for the development of future multifunctional devices.

Efek Pendoping Nd3+ Pada Senyawa BaBi2-xNdxNb2O9 Terhadap Struktur, Sifat Dielektrik Dan Optik

The Aurivillius compound with formula BaBi2-xNdxNb2O9 (x = 0.05, 0.1, 0.2, and 0.4) has been successfully synthesized using the molten salt method, showing potential as a ferroelectric material. The impact of Nd3+ substitution on the structure, morphology, dielectric, and optical properties has been systematically analyzed. XRD data refinement confirms that BaBi2-xNdxNb2O9 (BBNN) exhibits an orthorhombic structure with an A21am space group. Anisotropic plate-like grains were observed across all samples, decreasing their size as Nd3+ content increases. The ferroelectric transition temperature (Tc) decreases due to structural distortion caused by the reduction of the lone pair 6s2 electron effect of Bi3+ when substituted with Nd3+. Moreover, this structural distortion also contributes to an increase in bandgap energy (Eg). The diffuse ferroelectric phase transition is characterized by a broadened Tc peak induced by Nd3+ substitution due to increased cationic disruption in the bismuth layers. The ferroelectric phase with a lower and broader Tc suggests that the x = 0.4 sample has the potential for electrocaloric applications.

Controlled Vapor-Liquid-Solid Growth of Long and Remarkably Thin Pb1-xSnxTe Nanowires with Strain-Tunable Ferroelectric Phase Transition.

The Pb1-xSnxTe family of compounds possess a wide range of intriguing and useful physical properties, including topologically protected surface states, robust ferroelectricity, remarkable thermoelectric properties, and potential topological superconductivity. Compared to bulk crystals, one-dimensional (1D) nanowires (NWs) offer a unique platform to enhance the functional properties and enable new capabilities, e.g., to realize 1D Majorana zero modes for quantum computations. However, it has been challenging to achieve controlled synthesis of ultrathin Pb1-xSnxTe (0 ≤ x ≤ 1) nanowires in the truly 1D region. In this work, we report on a Au-catalyzed vapor-liquid-solid (VLS) growth of remarkably thin (20-30 nm) and sufficiently long (several to tens of micrometers) Pb1-xSnxTe nanowires of high single-crystalline quality in a controlled fashion. This controlled growth was achieved by enhancing the incorporation of Te into the Au catalyst particle to facilitate the precipitation of the Sn/Pb species and suppress the enlargement of the particle, which we identified as a major challenge for the growth of ultrathin nanowires. Our growth strategy can be easily extended to other compound and alloy nanowires, where the constituent elements have different incorporation rates into the catalyst particle. Furthermore, the growth of thin Pb1-xSnxTe nanowires enabled strain-dependent electrical transport measurements, which shows an enhancement of electrical resistance and ferroelectric transition temperature induced by uniaxial tensile strain along the nanowire axial direction, consistent with density functional theory calculations of the structural phase stability.

Light-Induced Complete Reversal of Ferroelectric Polarization in Sliding Ferroelectrics.

Previous experiments have provided evidence of sliding ferroelectricity and photoexcited interlayer shear displacement in two-dimensional materials, respectively. Herein, we find that a complete reversal of vertical ferroelectric polarization can be achieved within an astonishing 0.5ps in h-BN bilayer by laser illumination. Comprehensive analysis suggests that ferroelectric polarization switching originates from laser-induced interlayer sliding triggered by selective excitation of multiple phonons. The interlayer electron excitation from the p_{z} orbitals of the upper layer N atoms to the p_{z} orbitals of the lower layer B atoms produces desirable and directional interlayer forces activating the in-plane optical TO-1 and LO-1 phonon modes. The atomic motions driven by the coupling of TO-1 and LO-1 modes are coherent with ferroelectric soft mode, thus modulating the dynamical potential energy surface and resulting in ultrafast ferroelectric polarization reversal. Our work provides a novel microscopic insight into ultrafast polarization switching in sliding ferroelectrics.

Low-frequency dynamics of 0.45PMN-0.55PSN solid solution

Lead magnoniobate-scandoniobate (PMN-PSN) solid solutions are important functional materials for transducer/actuator devices due to their extraordinary dielectric and electromechanical properties. These unique properties are related to the unusual low-frequency relaxation dynamics, which has not been sufficiently understood until now. In our paper the low-frequency relaxation dynamics of 0.45Pb(Mg1/3Nb2/3)O3–0.55Pb(Sc1/2Nb1/2)O3 single crystal is studied in frequency range 10 mHz–1 MHz at temperatures near the dielectric permittivity maximum. Four relaxation processes are identified. Low-frequency mode demonstrating the critical divergence around 306 K is revealed. However, the phase transition into ferroelectric phase at this temperature does not occur. Near 306 K, the appearance of M-type superstructure is found related to the combination of oxygen octahedral rotation and anti-parallel shifts of lead ions. The appearance of the additional order parameter suppresses the slowing down of the ferroelectric mode and the phase transition into the ferroelectric phase occurs only below 295 K. In addition, two relaxation processes, similar to reorientation and “breathing” polar nanoregion (PNR) modes reported for PMN, are found. The sharp softening of “reorientation mode” is observed on cooling from 315 to 295 K with Tf ∼288 K. The fourth relaxation process is the Debye process, and we assume that it is associated with defects relaxation. Below 295 K, all four relaxation processes still exist, but their parameters are practically temperature independent. The low-temperature phase is not exactly a “normal” ferroelectric phase, the PNRs persist in the FE phase.

Multiferrocity in spin chain based polycrystalline Sm2BaCoO5

Sm2BaCoO5, a spin chain based polycrystalline compound which crystallize in orthorhombic structure (space group, Immm), is known to order antiferromagnetically close to 38 K. As motivated by 1) its Tb analogues [1] which show giant magneto-dielectric coupling and a new type-II multiferroic and 2) recently discovered multiferroicity in Sm2BaMO5 (M=Ni and Cu) compounds [2,3], detailed dielectric, ferroelectric and magneto-electric measurements have been carried out for Sm2BaCoO5. Magnetic and heat capacity measurements confirm the antiferromagnetic ordering at around 38 K. Interestingly ferroelectric transition also occurs around 38 K as revealed by frequency independent dielectric data as well as pyro-current measurements in various protocols. Therefore, the presented experimental results show that spin chain oxide Sm2BaCoO5 is new type-II multiferroic and we have identified R2BaCoO5 (R=Tb, Sm)- spin chain based cobaltates, is the new multiferroic series, so far unexplored.

Accurate compact nonlinear dynamical model for a volatile ferroelectric ZrO2 capacitor

We have measured the dynamical response of ZrO2 capacitors to applied triangular voltage waveforms with varying frequencies and amplitudes to determine the voltage and charge on the devices as a function of time. We have fit our experimental results to a Landau–Khalatnikov dynamical equation with a sixth order Landau–Ginzburg–Devonshire polynomial to represent the static charge-voltage behavior, and obtained coefficients of determination R2 > 0.99 for the fits. Analysis of the resulting quantitative model reveals an extremely small range of negative differential capacitance <16 mV. The hysteresis loops in the dynamical charge-voltage curves are found to result primarily from energy loss during the ferroelectric transitions, as represented by a frequency-dependent series resistance in the model.

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.

Effects of doping with magnetic cations on the hybrid improper ferroelectricity in Sr3Sn2O7

We here report the structural, magnetic and electrical properties of Sr2.9La0.1Sn1.9M0.1O7 (M= Cr, Mn or Fe) compounds. This La/M codoping of the hybrid improper ferroelectric Sr3Sn2O7, allows introducing magnetic cations into the perovskite layers of this Ruddlesden-Popper phase. The doping induces minor structural changes, preserving the polar structure with space group A21am of the undoped compound. The perovskite tolerance factor of all doped samples is slightly lower than in Sr3Sn2O7, which preserves the tilts and rotations of SnO6 octahedra. Doped samples exhibit a paramagnetic behavior down to 5 K and obey the Curie-Weiss law above 200 K. The effective paramagnetic moments agree with the expected spin-only values of the respective magnetic M3+ cations. All samples show a ferroelectric hysteresis loop at room temperature, but the doped samples show larger coercive fields. Additionally, the doping with magnetic cations has important effects on the dielectric properties: a strong decrease of the temperature of the ferroelectric transition together with a smoothing of the anomaly in the dielectric permittivity. These results suggest that the disorder produced by the two aliovalent substitutions is detrimental for the ferroelectric properties due to point charge defects but, at the same time, the prevalence of the structural distortion (tilts and rotations) preserves the ferroelectricity in the investigated doping regime.

Broad temperature range ferrielectric liquid crystal: Temperature dependencies of dielectric and electro-optical properties

Broad temperature range ferrielectric liquid crystal mixture FerriLCM-1 was previously developed in 2022 (Pozhidaev et al. 2022). In that work dielectric and electro-optical properties of the mixture were studied at room temperature. Electrically induced birefringence index reached 0.01 at a wavelength of 532 nm with response times of 300 μs. In this paper we continue the FerriLCM-1 properties investigation in a wide temperature range from the room ones till the isotropic phase transition temperature. It is found that one of the critical electric fields is almost temperature independent, while the Kerr coefficient of quadratic electro-optical effect in deformed helix ferroelectric (DHF) mode grows with temperature increase. This leads to an increase in the depth of phase modulation. Another critical electric field decreases smoothly with temperature rising. At 47–50 °C two critical electric fields merge, while temperature dependencies of all other dielectric and electro-optical parameters of the mixture are continuous and smooth functions. At about 50 °C the maximum electrically induced birefringence in DHF-mode is ΔneffE=0.022 under the applied electric field E=0.4 V/μm and with 80 μs response time. FerriLCM-1 mixture operating in the DHF-mode exhibits a giant Kerr coefficient of 1700 nm/V2 (or even up to 2900 nm/V2 at λ=400nm) at the temperature of 85 °C combined with a 50 μs response time. This is the highest value of Kkerr coefficient among liquid crystals. All these properties make ferrielectric liquid crystal mixture FerriLCM-1 suitable for solving various problems of phase modulation of light and use in low-voltage devices.