Ferrielectric Phase Research Articles

Room-temperature stabilizing strongly competing ferrielectric and antiferroelectric phases in PbZrO3 by strain-mediated phase separation

PbZrO3 has been broadly considered as a prototypical antiferroelectric material for high-power energy storage. A recent theoretical study suggests that the ground state of PbZrO3 is threefold-modulated ferrielectric, which challenges the generally accepted antiferroelectric configuration. However, such a novel ferrielectric phase was predicted only to be accessible at low temperatures. Here, we successfully achieve the room-temperature construction of the strongly competing ferrielectric and antiferroelectric state by strain-mediated phase separation in PbZrO3/SrTiO3 thin film. We demonstrate that the phase separation occurs spontaneously in quasi-periodic stripe-like patterns under a compressive misfit strain and can be tailored by varying the film thickness. The ferrielectric phase strikingly exhibitsa threefold modulation period with a nearly up-up-down configuration, which could be stabilized and manipulated by the formation and evolution of interfacial defects under applied strain. The present results construct a fertile ground for further exploring the physical properties and applications based on the novel ferrielectric phase.

Ferrielectricity controlled widely-tunable magnetoelectric coupling in van der Waals multiferroics

The discovery of various primary ferroic phases in atomically-thin van der Waals crystals have created a new two-dimensional wonderland for exploring and manipulating exotic quantum phases. It may also bring technical breakthroughs in device applications, as evident by prototypical functionalities of giant tunneling magnetoresistance, gate-tunable ferromagnetism and non-volatile ferroelectric memory etc. However, two-dimensional multiferroics with effective magnetoelectric coupling, which ultimately decides the future of multiferroic-based information technology, has not been realized yet. Here, we show that an unconventional magnetoelectric coupling mechanism interlocked with heterogeneous ferrielectric transitions emerges at the two-dimensional limit in van der Waals multiferroic CuCrP2S6 with inherent antiferromagnetism and antiferroelectricity. Distinct from the homogeneous antiferroelectric bulk, thin-layer CuCrP2S6 under external electric field makes layer-dependent heterogeneous ferrielectric transitions, minimizing the depolarization effect introduced by the rearrangements of Cu+ ions within the ferromagnetic van der Waals cages of CrS6 and P2S6 octahedrons. The resulting ferrielectric phases are characterized by substantially reduced interlayer magnetic coupling energy of nearly 50% with a moderate electric field of 0.3 V nm−1, producing widely-tunable magnetoelectric coupling which can be further engineered by asymmetrical electrode work functions.

Description of CuInP<sub>2</sub>S<sub>6</sub> ferrielectrics in a mixed Ising model

The appearance of spontaneous polarization in CuInP2S6 ferrielectrics is related to the second order Jahn-Teller effect for copper cations located in a double-well local potential, the stereoactivity of indium cations located in a three-well local potential, as well as the valence fluctuations of phosphorus cations. The paraelectric to ferrielectric phase transition is primarily determined by the coupling of indium cations with their surroundings. This transition can be analyzed using the mixed Ising model with spins s = 1/2 and S = 1. The spectrum of pseudospin fluctuations at different temperatures was calculated using a mean-field approach for a set of quantum anharmonic oscillators. The results were then compared with Raman spectroscopy data for CuInP2S6 crystal. The analysis indicates that the lattice anharmonicity below 150 K, is mainly determined by the indium sublattice, leading to the coexistence of the glassy state and ferrielectric phase. Above 150 K, the anharmonicity of the copper sublattice activates the ionic conductivity and results in the existence of a long-ranged fluctuated cluster of spontaneous polarization in a temperature interval of the paraelectric phase above TC.

Translational Boundaries as Incipient Ferrielectric Domains in Antiferroelectric PbZrO_{3}.

In the archetypal antiferroelectric PbZrO_{3}, antiparallel electric dipoles cancel each other, resulting in zero spontaneous polarization at the macroscopic level. Yet in actual hysteresis loops, the cancellation is rarely perfect and some remnant polarization is often observed, suggesting the metastability of polar phases in this material. In this work, using aberration-corrected scanning transmission electron microscopy methods on a PbZrO_{3} single crystal, we uncover the coexistence of the common antiferroelectric phase and a ferrielectric phase featuring an electric dipole pattern of ↓↑↓. This dipole arrangement, predicted by Aramberri etal. to be the ground state of PbZrO_{3} at 0K, appears at room temperature in the form of translational boundaries. The dual nature of the ferrielectric phase, both a distinct phase and a translational boundary structure, places important symmetry constraints on its growth. These are overcome by sideways motion of the boundaries, which aggregate to form arbitrarily wide stripe domains of the polar phase embedded within the antiferroelectric matrix.

Investigation of the evolution of thermal properties in ferrielectric CuInP2S6 and Ag0.1Cu0.9InP2S6 layered crystals by means of ac photopyroelectric calorimetry

For CuInP2S6 layered crystal the temperature anomalies of the heat capacity and entropy at the ferrielectric ordering are explained based on the theoretical studies which predict the appearance of a critical endpoint and a bicritical endpoint on the phase diagram. Thus, the splitting of the anomaly of the heat capacity in Ag0.1Cu0.9InP2S6 mixed crystal can be interpreted as a transformation of the ferrielectric first-order phase transition, that is observed for CuInP2S6, into the sequent paraelectric–ferrielectric second-order phase transition and the isostructural first-order transition with a sharp change of the polarization in the ferrielectric phase.

Morphotropic phase boundary, polymorphic phases and enhanced electrostrain/piezoelectricity in Ag1-xKxNbO3 solid-solution ceramics

Ag1−xKxNbO3(AKNx (x ≤ 0.12) ceramics were prepared to understand the relationship of structure-properties driven by compositions and temperatures. The results suggested that this binary system possessed a morphotropic phase boundary (MPB) consisted of ferrielectric and ferroelectric phases with iso-symmetry at room temperature, in which domains switching together with electric-field-induced irreversible phase transition achieved a much higher electrostrain (Smax = 0.4%) than other compositions. But this MPB was destroyed after poling, leading to inferior piezoelectricity. A phase diagram was drawn after analyzing in situ XRD and dielectric data, where an almost vertical ferrielectric/antiferroelectric ↔ polymorphic ferroelectric MPB line starting from a triple point was proposed. As temperature increased, the piezoelectricity significant enhanced near ferroelectric orthorhombic ↔ monoclinic phase boundary, while the highest piezoelectricity was achieved near the monoclinic ↔ paraelectric phase boundary with d33 = 200 pC/N. The enhanced piezoelectricity is intimately related to the ferroelectric monoclinic possessing Pm symmetry.

Negative/positive electrocaloric effect in antiferroelectric squaric acid

Using the previously developed model, we study the electrocaloric effect (ECE) in antiferroelectric crystals of squaric acid, where the low-temperature polarization reorientation by the external electric field has been earlier predicted to be a two-stage process with the intermediate ferrielectric phase. The T-E landscape of the model entropy is explored. The ECE, characterized by the electric Grüneisen parameter, is found to be negative in the antiferroelectric phase. In the ferrielectric phase, it is positive at low fields but becomes slightly negative below the transition to the ferroelectric phase. In the ferroelectric phase, the ECE is positive at all temperatures and fields. The negative EC temperature shift ΔT of the largest magnitude is predicted to be around −2.7 K at 200 kV/cm. The supercritical behavior of the Grüneisen parameter in the crossover region between two bicritical end points is studied.

Positive and Negative Electrocaloric Effect in Lead-Free Silver Niobate Antiferroelectric Ceramic Depending on Affluent Phase Transition

We prepared a dense AgNbO3 ceramic using a conventional solid-state reaction method. The phase structure, electrical properties and electrocaloric effect (ECE) were systematically investigated. Large negative and positive ECEs (−4.38 °C at 65 °C and 2.3 °C at 210 °C) under an external electric field of 180 kV·cm−1 were obtained in the eco-friendly AgNbO3 antiferroelectric (AFE) ceramic due to affluent phase transition and a high electric field. The large positive and negative ECEs originated from the phase transition between ferrielectric (FIE) phases (the orthorhombic space group (Pmc21) and AFE phases (Pbcm) tuned by an applied external field. Additionally, a probable mechanistic model was proposed to illustrate the generation of positive and negative ECEs. This study may provide guidelines for the design of high-efficiency solid-state cooling devices.

Ferrielectricity in the Archetypal Antiferroelectric, PbZrO3.

Antiferroelectric materials, where the transition between antipolar and polar phase is controlled by external electric fields, offer exceptional energy storage capacity with high efficiencies, giant electrocaloric effect, and superb electromechanical response. PbZrO3 is the first discovered and the archetypal antiferroelectric material. Nonetheless, substantial challenges in processing phase pure PbZrO3 have limited studies of the undoped composition, hindering understanding of the phase transitions in this material or unraveling the controversial origins of a low-field ferroelectric phase observed in lead zirconate thin films. Leveraging highly oriented PbZrO3 thin films, a room-temperature ferrielectric phase is observed in the absence of external electric fields, with modulations of amplitude and direction of the spontaneous polarization and large anisotropy for critical electric fields required for phase transition. The ferrielectric state observations are qualitatively consistent with theoretical predictions, and correlate with very high dielectric tunability, and ultrahigh strains (up to 1.1%). This work suggests a need for re-evaluation of the fundamental science of antiferroelectricity in this archetypalmaterial.

Local probing of the non-uniform distribution of ferrielectric and antiferroelectric phases

Local probing of the non-uniform distribution of ferrielectric and antiferroelectric phases

Revisiting the Kittel’s model of antiferroelectricity: phase diagrams, hysteresis loops and electrocaloric effect

We revisit the Kittel’s model of antiferroelectricity by extending the model to study the phase transitions, hysteresis loop behaviors and electrocaloric effect (ECE) of antiferroelectrics (AFEs). By considering both the first- and second-order AFEs, explicit expressions for the physical and staggered polarizations of AFEs in the stable states are derived. We also obtain the analytical solutions for describing the dielectric susceptibilities of AFEs in the AFE and paraelectric (PE) phases. Coercive fields in AFE are also derived and studied. To verify the usefulness of the Kittel’s model of antiferroelectricity, we apply the model to systematically investigate the phase transitions, hysteresis loops and ECEs of PbZrO3 (PZO). By adopting appropriate values of the Kittel’s parameters for first-order transition, analytical and numerical results are obtained and discussed. Our results show that PZO exhibits a complex temperature (T)—electric field (E) phase diagram, consisting of the AFE, ferroelectrics, ferrielectric, PE and mixed phases. The T-E phase diagram is qualitatively agreed with the new AFE model that was derived based on symmetry by Tolédano and Khalyavin (2019 Phys. Rev. B 99 024105). We found that the calculated zero-field dielectric susceptibility is qualitatively and quantitatively agreed with experimental results. We show that the polarizations and dielectric susceptibilities of PZO in heating and cooling deviate from each other, as expected for the first-order materials. Our calculated results also reveal that the ECE in PZO has an electro-heating of ΔT ≈ +6.5 °C and an electro-cooling of ΔT ≈ −4.0 °C, respectively, which are comparable to the experimental results.

Phonon spectra and phase transitions in van der Waals ferroics MM’P2X6

For CuInP2S6 ferrielectrics by Brillouin spectroscopy, the temperature dependence of the longitudinal hypersound velocity was investigated for the acoustic phonons propagated in plane of the crystal layers. Pronounced softening of acoustic phonon branch is observed in the paraelectric phase below T* ≈ 330 K at cooling to temperature Tc ≈ 312 K of the first order phase transition into ferrielectric phase. Below Tc the hypersound velocity growth in correlation with spontaneous polarization rise and its temperature anomaly is similar to earlier observed temperature behavior of longitudinal ultrasound velocity for acoustic wave propagated normally to the structural layers. Determined anomalous part of the CuInP2S6 crystal thermal conductivity in the vicinity of the ferrielectric transition also demonstrates lowering of the heat transferring phonons group velocity with cooling from T* to Tc. Observed acoustic softening obviously is induced by flexoelectric coupling of relaxational soft polar optical and acoustic branches that is related to an in ingomogeneously polarized state appearing between the paraelectric and ferrielectric phases.

Broad temperature range ferrielectric liquid crystal as a highly sensitive quadratic electro-optical material.

A chiral smectic liquid crystal in which a ferrielectric phase with a helix pitch p0 less than 125 nm exists over a temperature range of at least from -3°C to +36°C has been developed. Such a wide temperature range (including room temperatures) of the ferrielectric phase with a subwavelength helix pitch has been achieved for the first time, to the best of our knowledge, which creates opportunities for the practical use of ferrielectric liquid crystals. A quadratic electro-optical effect caused by deformations of the helix in an electric field is observed in the ferrielectric phase, and the Kerr coefficient, which reaches almost 200 nm/V2, is significantly higher than the same coefficient for the blue phase and for the smectic C* phase, which means a higher sensitivity of the developed ferrielectric liquid crystal to the electric field. The electro-optical response time does not exceed 300μs at room temperatures for this ferrielectric liquid crystal.

Nanoscale Control of Polar Surface Phases in Layered van der Waals CuInP2S6.

Antiferroelectric (AFE) materials, in which alternating dipole moments cancel out to a zero net macroscopic polarization, can be used for high-density energy storage and memory applications. The AFE phase can exist in bulk CuInP2Se6, CuBiP2S6, and a few other transition-metal thiophosphates below 200 K. The required low temperature poses challenges for practical applications. In this work, we report the coexistence of ferrielectric (FE) states and a stable surface phase that does not show piezoelectric response ("zero-response phase") in bulk CuInP2S6 at room temperature. Using piezoresponse force microscopy (PFM) tomographic imaging together with density functional theory, we find that direct and alternating voltages can locally and stably convert FE to zero-response phases and vice versa. While PFM loops show pinched hystereses reminiscent of antiferroelectricity, PFM tomography reveals that the zero-response areas form only on top of the FE phase in which the polarization vector is pointing up. Theoretical calculations suggest that the zero-response phase may correspond to AFE ordering where stacked CuInP2S6 layers have alternating polarization orientations thereby leading to a net-zero polarization. Consistent with experimental findings, theory predicts that the FE polarization pointing down is robust up to the top surface, whereas FE polarization pointing up energetically favors the formation of an AFE surface layer, whose thickness is likely to be sensitive to local strains. AFE order is likely to be more robust against detrimental size effects than polar order, therefore providing additional opportunities to create multifunctional heterostructures with 2D electronic materials.

Phase transition and piezoelectric property of (Ag,K)NbO3 ceramics

AgNbO3 is an antiferroelectric (AFE) material with double hysteresis loop. Both the antiferroelectricity and ferroelectricity can be enhanced by doping. Herein, the ferroelectricity of AgNbO3 ceramics was enhanced via K-doping and the phase diagram of the (Ag1-xKx)NbO3 ceramics was upgraded. In details, (Ag1-xKx)NbO3 ceramics are ferrielectric (FIE) M1 phase as x = 5.00–5.50 mol% and ferroelectric (FE) O phase as x = 5.75–6.00 mol% before poling, and FE O phase as x = 5.00–6.00 mol% after poling at room temperature. With increasing temperature, (Ag1-xKx)NbO3 ceramics show the phase evolutions from FIE M1, AFE M2 to paraelectric (PE) T phase at x = 5.00–5.50 mol% and from FE O, FE T to PE T phase at x = 5.75–6.00 mol% before poling, and from FE O, FE T to PE T phase at x = 5.00–6.00 mol% after poling. High d33 values of 180 pC/N and 285 pC/N are obtained at the FE O-FE T and FE T-PE T phase boundaries. This work sheds light on a novel and promising lead-free piezoelectric system.

Stabilization of the ferrielectric phase in NaNbO3-based lead-free ceramics for a wide-temperature large electrocaloric effect

The newly discovered high-temperature ferrielectric phase in NaNbO3is stabilized at room temperature by compositional modification, providing the basis for obtaining a giant electrocaloric effect in a wide temperature range.

On the possibility that PbZrO3 not be antiferroelectric

Lead zirconate (PbZrO3) is considered the prototypical antiferroelectric material with an antipolar ground state. Yet, several experimental and theoretical works hint at a partially polar behaviour in this compound, indicating that the polarization may not be completely compensated. In this work, we propose a simple ferrielectric structure for lead zirconate. First-principles calculations reveal this state to be more stable than the commonly accepted antiferroelectric phase at low temperatures, possibly up to room temperature, suggesting that PbZrO3 may not be antiferroelectric at ambient conditions. We discuss the implications of our discovery, how it can be reconciled with experimental observations and how the ferrielectric phase could be obtained in practice.

A combined theoretical and experimental study of the phase coexistence and morphotropic boundaries in ferroelectric-antiferroelectric-antiferrodistortive multiferroics

The physical nature of the ferroelectric (FE), ferrielectric (FEI) and antiferroelectric (AFE) phases, their coexistence and spatial distributions underpins the functionality of antiferrodistortive (AFD) multiferroics in the vicinity of morphotropic phase transitions. Using Landau-Ginzburg-Devonshire (LGD) phenomenology and a semi-microscopic four sublattice model (FSM), we explore the behavior of different AFE, FEI, and FE long-range orderings and their coexistence at the morphotropic phase boundaries in FE-AFE-AFD multiferroics. These theoretical predictions are compared with the experimental observations for dense Bi1-yRyFeO3 ceramics, where R is Sm or La atoms with the fraction 0 ≤ y ≤ 0.25, as confirmed by the X-ray diffraction (XRD) and Piezoresponse Force Microscopy (PFM). These complementary measurements were used to study the macroscopic and nanoscopic transformation of the crystal structure with doping. The comparison of the measured and calculated AFE/FE phase fractions demonstrate that the LGD-FSM approach well describes the experimental results obtained by XRD and PFM for Bi1-yRyFeO3. Hence, this combined theoretical and experimental approach provides further insight into the origin of the morphotropic boundaries and coexisting FE and AFE states in model rare-earth doped multiferroics.

Mechanism of enhanced energy storage density in AgNbO3-based lead-free antiferroelectrics

The mechanisms underpinning high energy storage density in lead-free Ag 1–3 x Nd x Ta y Nb 1- y O 3 antiferroelectric (AFE) ceramics have been investigated. Rietveld refinements of in-situ synchrotron X-ray data reveal that the structure remains quadrupled and orthorhombic under electric field ( E ) but adopts a non-centrosymmetric space group, Pmc2 1 , in which the cations exhibit a ferrielectric configuration. Nd and Ta doping both stabilize the AFE structure, thereby increasing the AFE-ferrielectric switching field from 150 to 350 kV cm −1 . Domain size and correlation length of AFE/ferrielectric coupling reduce with Nd doping, leading to slimmer hysteresis loops. The maximum polarization (P max ) is optimized through A-site aliovalent doping which also decreases electrical conductivity, permitting the application of a larger E . These effects combine to enhance energy storage density to give W rec = 6.5 J cm −3 for Ag 0.97 Nd 0.01 Ta 0.20 Nb 0.80 O 3 . Schematic diagram illustrating how energy storage density is optimized through doping in AgNbO 3 and shedding light on the design of novel antiferroelectric (AFE) materials with excellent energy storage density. ● Excellent performance achieved for AgNbO 3 -based antiferroelectric lead-free energy storage ceramics. ● The first observation of a field induced ferrielectric phase. ● 4 principles proposed for the design of high energy density properties of AgNbO 3 -based antiferroelectric ceramics: i) increased resistivity, ii) optimization of maximum polarization, iii) decreased antipolar/polar coupling, and iv) enhanced AFE stability.

Rich antiferroelectric phase diagram of antiferroelectric–ferroelectric superlattices: internal electric field- and interface induced phase transitions

We propose a thermodynamic model to the study the antiferroelectric (AFE) phase transitions in antiferroelectric–ferroelectric (AFE–FE) superlattices in which the coupling at the interface between two layers is mediated by local polarizations. Phase diagram of the AFE layer in term of the degree of interfacial effect λ and temperature T involving ferrielectric (FI) and ferroelectric (FE) phases is investigated. These two phases are stabilized by the interfacial effect and internal electric field. AFE thickness LAFE versus T phase diagram is also constructed. Intermediate regions of two-phase coexistence (IM) emerge in the λ–T and LAFE–T phase diagrams, if certain interface properties λ and layer thickness LAFE criteria are met. These IM regions are metastable states, which exist as a transition state between two phases. A tricritical point locates at the boundaries across the FI, IM and FE phases is found in the LAFE–T phase diagram. Competition among the internal electric field due to the electrostatic coupling, the FE ordering arises from the interfacial effect and the antiferroelectric ordering within the AFE layer giving rises to the rich AFE phase diagram.