Ferroelectric-paraelectric Transition Research Articles

Thermodynamic analysis of the inverse electrocaloric effect in ferroelectric thin films

Abstract Inverse electrocaloric (i-EC) effect occurs in ferroelectrics when the applied electric field aligns anti-parallel to polarization. In this study, the dependence of the i-EC effect on temperature and misfit strain is formulated and applied to clamped BaTiO3 thin films using the Landau–Ginzburg–Devonshire formalism. It is found that an interplay exists between the pyroelectric coefficient and the maximum possible inverse electric field. We demonstrate that the temperature change is strongly dependent on the inverse field amplitude and is maximal at lower temperatures away from the ferroelectric-paraelectric transition for a given misfit strain. Such an outcome is opposite to the direct electrocaloric effect, where it is desirable to remain near the transition temperature for the maximum electrocaloric temperature change. The fact that the i-EC effect can be maximum at lower temperatures could allow for the potential tailoring of this effect in strained films at moderate temperatures for device applications.

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.

Phase transition and polar cluster behavior above Curie temperature in ferroelectric BaTi0.8Zr0.2O3

We study the phase transition behavior of the ferroelectric BaTi0.8Zr0.2O3 in the paraelectric region. The temperature dependencies of thermal, polar, elastic, and dielectric properties indicate the presence of local structures above the paraelectric–ferroelectric transition temperature Tc = 292 K. The non-zero remnant polarization is measured up to a characteristic temperature T* ∼ 350 K, which coincides with the temperature where the dielectric constant deviates from the Curie–Weiss law. Resonant piezoelectric spectroscopy shows that DC field cooling above Tc using fields smaller than the coercive field leads to an elastic response and remnant piezoelectricity below T*, which likely corresponds to the coherence temperature associated with polar nanostructures in ferroelectrics. The observed remnant effect is attributed to the reorientation of polar nanostructures above Tc.

Structural and dielectric properties of CaSnO3-doped Sr2.1Na0.8Nb5O15 ceramics

The crystallographic, microstructural, and dielectric properties of Sr2.1Na0.8-xCaxNb5-xSnxO15 (x = 0.00, 0.01, 0.05, 0.10) polycrystalline ceramics have been studied by X-ray diffraction, scanning electron microscopy, dielectric spectroscopy (DS) and impedance spectroscopy (IS). For x = 0.00, 0.05, and 0.10, samples are single phase with P4bm symmetry at room temperature with x = 0.01 showing a small quantity of secondary phase(s). All compositions show typical ceramic microstructures and d50 grain sizes ranging from 5.1 to 26.6 μm. DS shows a clear trend in the high temperature ferroelectric-paraelectric transition with the Curie temperature, T0, decreasing from ∼ 160 to ∼ 110 °C, and an additional relaxation at approximately 120 °C with increasing CaSnO3. IS reveals all samples have a homogeneous electrical microstructure with predominantly electronic conduction. The activation energy of conduction calculated from Arrhenius plots of the conductivity increases with CaSnO3 content from 1.27 to 1.38 eV likely due to the expansion of the band gap.

Investigation of piezoelectric effect and thermally stimulated depolarization current of poly (vinyl chloride–co-vinyl acetate–co-2-hydroxypropyl acrylate)/poly (vinylidene fluoride-trifluoroethylene) polymer blend

DSC measurements revealed that the ferroelectric-paraelectric (F-P) transition point of PVDF-TrFE has been affected after blending with PVVH. XRD pattern and FTIR spectra of PVVH, PVDF-TrFE, and their polyblend samples are analyzed to investigate the effect of PVVH on the enhancement of crystallinity and β-fraction of PVD-TrFE. The piezoelectric coefficient (d 33) of PVVH/PVDF-TrFE polymer blend has been investigated under different parameters, such as the temperature, applied stress, and composition. It is found that the value of d 33 has been improved either with increasing temperature or increasing applied pressure for all samples. The maximum values of d33 at 0.62 MPa and 353 K are 1.01, 9.93, and 12.84 PC/N for pure PVVH, PVDF-TrFE, and 30/70 PVVH/PVDF-TrFE, respectively. TSDC of pure PVVH, PVDF-TrFE, and their polyblend samples has been studied in a wide range of temperatures at different poling electric fields. The results revealed that TSDC of pure PVVH is characterized by a dipolar relaxation peak centered at 351 K, whereas, pure PVDF-TrFE showed two peaks at 327 K, and the other is observed in the temperature range from 358 to 371 K. TS technique has been applied for decomposing TSDC spectra of all samples to its elementary peaks. It was verified that the compensation effect is operated for all samples and all thermodynamic parameters, such as ΔS, ΔH, and ΔG were calculated from the relaxation map analysis (RMA).

Abstracts

In this article, the effect of Zr addition on the crystalline structure, ferroelectric and piezoelectric properties of Barium Strontium Titanate (BST) ceramic have been studied. Zr was doped in the range of 0 ≤ x ≤ 0.20 in Ba0.90Sr0.10Ti1-xZrxO3 (BSZT) and it was prepared via solid state reaction technique. From the characterization studies, it was realized that Zr addition in BSZT ceramics results in slight modification of BSZT lattice, but due to diffusion of Zr into the lattice, the crystallite size as well as the microstructure of combined BSZT system got modified drastically. Zr addition in BSZT ceramics results in increase of lattice parameter due to larger ionic radius of Zr ions compared to Ti ions. Reduction of grain and crystallite size was also observed, which affects the ferroelectric behaviour of BSZT ceramic by influencing the polarization contribution from individual domains. This causes a slow transformation of sharp paraelectric-ferroelectric transition to diffusive ferroelectric phase transition and it also introduces relaxation phenomenon in the system at higher Zr compostion, which convert it to a relaxor ferroelectric. At relaxor phase a decreased spontaneous polarization and slimmer P–E hysteresis loop was observed with increasing Zr content. The main reason behind this conversion and modification in microstructural properties is because of the presence of strain in BSZT lattice which is due to replacement of Zr4+ (0.72 Å) inplace of smaller Ti4+ (0.609 Å) in more than a tolerable limit. Substitution of Zr4+ for Ti4+ in BaSrTiO3 introduces structural disorder, causing perturbations like local electric and strain fields. These perturbations reduce the long-range polar order resulting in relaxor behavior.

Effect of Zr substitution on electrical properties of Ba0.90Sr0.10Ti1−xZrxO3 (0 ≤ x ≤ 0.20) ceramics

In this article, the effect of Zr addition on the crystalline structure, ferroelectric and piezoelectric properties of Barium Strontium Titanate (BST) ceramic have been studied. Zr was doped in the range of 0 ≤ x ≤ 0.20 in Ba0.90Sr0.10Ti1-xZrxO3 (BSZT) and it was prepared via solid state reaction technique. From the characterization studies, it was realized that Zr addition in BSZT ceramics results in slight modification of BSZT lattice, but due to diffusion of Zr into the lattice, the crystallite size as well as the microstructure of combined BSZT system got modified drastically. Zr addition in BSZT ceramics results in increase of lattice parameter due to larger ionic radius of Zr ions compared to Ti ions. Reduction of grain and crystallite size was also observed, which affects the ferroelectric behaviour of BSZT ceramic by influencing the polarization contribution from individual domains. This causes a slow transformation of sharp paraelectric-ferroelectric transition to diffusive ferroelectric phase transition and it also introduces relaxation phenomenon in the system at higher Zr compostion, which convert it to a relaxor ferroelectric. At relaxor phase a decreased spontaneous polarization and slimmer P–E hysteresis loop was observed with increasing Zr content. The main reason behind this conversion and modification in microstructural properties is because of the presence of strain in BSZT lattice which is due to replacement of Zr4+ (0.72 Å) inplace of smaller Ti4+ (0.609 Å) in more than a tolerable limit. Substitution of Zr4+ for Ti4+ in BaSrTiO3 introduces structural disorder, causing perturbations like local electric and strain fields. These perturbations reduce the long-range polar order resulting in relaxor behavior.

Implementing a sol-gel route to adjust the structural and dielectric characteristics of Bi and Fe co-doped BaTiO3 ceramics

The present work explores the impact of Fe insertion on the physical properties of Ba0.95Bi0.05Ti1-xFexO3 (x = 0.025, 0.050, and 0.075) prepared via sol gel method. The resulting samples crystallize in the tetragonal structure with space group P4mm and their morphological features point out the variation of the microstructure with Fe content. In turn, the dielectric constant versus temperature plot reveals the existence of two transition phases: the first one is ferroelectric-paraelectric transition phase (TF-P) and the second one is ferroelectric orthorhombic - ferroelectric tetragonal phase (TO-T). Analysis of conductivity curves using Jonscher’s augmented equation (for x = 0.025) and Jonscher’s power law (for x = 0.075) suggests the Non-Overlapping Small Polaron Tunneling (NSPT) model as a conduction mechanism.

Wrapping Assembly of BaTiO3 Nanoparticles with (Bi0.5Na0.5)TiO3 Nanosheets: A Novel Processing for Nanograined BT–BNT Complex Perovskite Relaxors

Hydrothermal syntheses of BaTiO3 (BT) nanoparticles and (Bi0.5Na0.5)TiO3 (BNT) nanosheets are explored to maximize the uniform distributions of the components in BT–BNT complex perovskite solid solution preparation. By introducing a “wrapping process,” which is a 2D nanosheet‐based process for preparing core–shell‐type assembly of the BT and BNT powder mixture, the solid solution formation and densification are accelerated effectively resulting in lowering the sintering temperature and homogeneous nanograined microstructure. The improved microstructure of the BT–BNT solid solution ceramics contributes to the enhancement in dielectric polarizations over a wide temperature range. Relaxor characteristics of the solid solutions, i.e., diffusivity of the ferroelectric–paraelectric transition, frequency dispersion of ε r′, and shift of T m with frequency deviation from the Curie–Weiss law, are determined. It is also confirmed that the BT–BNT samples prepared by the wrapping process exhibit stronger relaxor behavior compared to the conventionally prepared BT–BNT samples even in the identical chemical composition. These hydrothermal reaction‐based novel processing routes for the complex perovskite compositions are very effective to improve microstructure, sinterability, and dielectric properties.

Effect of rare earth on structural, morphological, vibrational, magnetic and dielectric properties of RFe0.5Cr0.5O3 (R = Nd, Eu) perovskites

RFe0.5Cr0.5O3 (R = Nd,Eu) perovskites were synthesized through solid-state reaction. The structural analysis evidences the orthorhombic symmetry of these materials in accordance with their tolerance factor. The lattice parameters, bond lengths, and bond angles, as obtained by Rietveld refinement, vary systematically with the size of R3+ cations. The structural stability and thermally activated expansion of the crystal lattice are affirmed by means of a high-temperature X-ray diffraction (HT-XRD) study. The microstructure of the prepared powders is investigated using scanning electron microscopy (SEM), as well as X-ray diffraction (XRD) through the calculation of the crystallite size. Diffuse reflectance spectra indicate that these materials behave as semiconductors, and the gap energies are calculated. Room-temperature Mössbauer spectroscopy highlights the presence of magnetic frustration phenomenon in both materials. The frequency-dependent dielectric study reveals a colossal dielectric constant in agreement with the Maxwell-Wagner relaxation mechanism resulting from intergrain boundaries, as asserted by impedance spectroscopy. The conductivity mechanism is provided by the correlated barrier hopping model for EuFe0.5Cr0.5O3 perovskite, and the non-overlapping small polaron tunneling mechanism for NdFe0.5Cr0.5O3 material. High-temperature dielectric measurements reveal two anomalous features attributed to the magnetodielectric effect and the onset of ferroelectric-paraelectric transition, respectively. Metallic behavior is identified at higher temperatures for both materials.

Effect of Ni Substitution on Structural, Dielectric, and Ferroelectric Properties and Variation in Magnetocapacitance of Single-Phase Ba0.7Pb0.3TiO3 Ceramic

The present study demonstrates the influence of Ni substitution on the structural, dielectric, and ferroelectric properties of single-phase Pb-doped BaTiO3, that is, Ba0.7Pb0.3TiO3 (BPT) ceramic, and variations in the magnetocapacitance. The hydroxide co-precipitation method was employed to fabricate single-phase Pb-doped BaTiO3 and Ni-substituted Ba0.7Pb0.3TiO3, as Ba0.7Pb0.3TiO3-(x)Ni for x = 0.05, 0.1, and 0.15. Rietveld refinement reflects the single-phase tetragonal crystal structure of pure Ba0.7Pb0.3TiO3 and at higher Ni concentrations, the detection of rhombohedral crystal symmetry of the NiO phase confirms the biphasic structure of the composite. The ferroelectric-paraelectric transition temperature decreased from 274 °C for pure Ba0.7Pb0.3TiO3 to 162 °C for the composite with a higher Ni concentration of x = 0.15. A drastic change in the typical ferroelectric (P-E) loop of Ba0.7Pb0.3TiO3 was observed with the substitution of Ni. The composite with a higher concentration had the highest value of the magnetodielectric coupling coefficient, MD (%) = −2.86. This study reflects an easy and useful approach to achieve two-order parameter coupling with doping of acceptor ions in the ferroelectric phase. This kind of material substantially plays a vital role in the advanced electromagnetic devices for futuristic applications.

Dynamic Behavior of Polar Nanoregions in Re‐Entrant Relaxor 0.6Bi(Mg1/2Ti1/2)O3–0.4PbTiO3

The existence of polar nanoregions (PNRs) is the most important characteristic of ferroelectric relaxors; however, the size determination and the dynamic of PNRs remain uncertain. Herein, it is revealed that a re‐entrant relaxor behavior and ferroelectric–paraelectric transition coexist in complex perovskite oxide 0.6Bi(Mg1/2Ti1/2)O3‐0.4PbTiO3. Two dielectric anomalies, 1) the low‐temperature re‐entrant relaxor transition and 2) the high‐temperature diffuse phase transition (DPT), are described by the phenomenological statistical model. The sizes of the two kinds of PNRs corresponding to two ferroelectric states were obtained. The dynamics of PNRs are analyzed using isothermal electrical modulus, which shows three critical temperatures associated with the DPT, the formation, and freezing of PNRs, respectively. The temperature evolution of the PNRs evolution depends on the stoichiometry of bismuth. The results provide new insights into the dynamic behavior of PNRs and the modification way of re‐entrant relaxor behavior.

Role of B-site disorder in the properties of lead-free Na0.5Bi0.5(Mg1/3Nb2/3)O3 ceramic: A possible electrocaloric material with low leakage current

The electrocaloric (EC) effect in ferroelectric materials has been studied extensively to develop materials for solid-state cooling applications. The Na0.5Bi0.5TiO3-based ferroelectric material is selected for this particular application. The Ti4+ of the lead-free Na0.5Bi0.5TiO3 is completely replaced by (Mg1/3Nb2/3)4+ to enhance the dielectric breakdown strength and to observe the EC effect. The lead-free Na0.5Bi0.5(Mg1/3Nb2/3)O3 is synthesized via the columbite route with optimized processing conditions. The material shows a frequency-independent high ferroelectric–paraelectric transition temperature (460 °C) with a slim polarization vs electric field loop. The adiabatic temperature change (ΔT) of the compound is evaluated from the temperature-dependent polarization behavior, and calculated ΔT is ≈ 0.135 K at room temperature. The room-temperature EC strength and leakage current of the material are examined to check its suitability for solid-state refrigeration.

Pressure-induced phase transition and electrical properties of yttrium-doped polycrystalline BaTiO3

The electrical transport properties of undoped and 2% yttrium-doped polycrystalline BaTiO3 (BTY0 and BTY2) at high temperature and high pressure were studied using a diamond-anvil cell in conjunction with AC impedance spectroscopy up to ∼20 GPa. The BTY0 endured a ferroelectric–paraelectric transition at ∼5 GPa, and a ∼2 GPa ahead of the transition point was observed in BTY2. We found that the doping of 2% Y ions in BTY2 leads to a decrease in the Curie transition temperature. For both BTY0 and BTY2, an increase in pressure will result in a significant reduction in the Curie temperature and the decreasing rate gradually decreased under pressure. The reducing grain size of the sample and the effect of the deviatoric stresses are considered as the main factors causing the decrease of dTC/dP.

Multiferroic properties of oxygen-functionalized magnetic i-MXene

Two dimensional multiferroics inherit prominent physical properties from both low dimensional materials and magnetoelectric materials, and can go beyond their three dimensional counterparts for their unique structures. Here, based on density functional theory calculations, a MXene derivative, i.e., i-MXene (Ta$_{2/3}$Fe$_{1/3}$)$_2$CO$_2$, is predicted to be a type-I multiferroic material. Originated from the reliable $5d^0$ rule, its ferroelectricity is robust, with a moderate polarization up to $\sim12.33$ $\mu$C/cm$^2$ along the a-axis, which can be easily switched and may persist above room temperature. Its magnetic ground state is layered antiferromagnetism. Although it is a type-I multiferroic material, its N\'eel temperature can be significantly tuned by the paraelectric-ferroelectric transition, manifesting a kind of intrinsic magnetoelectric coupling. Such magnetoelectric effect is originated from the conventional magnetostriction, but unexpectedly magnified by the exchange frustration. Our work not only reveals a nontrivial magnetoelectric mechanism, but also provides a strategy to search for more multiferroics in the two dimensional limit.

0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 Phosphate Composites: Dielectric and Ferroelectric Properties.

Composite materials with 83 wt.% of the 0.7Pb(MgNb)O-0.3PbTiO distributed in phosphate-bonded ceramics were prepared at three different pressures. A phosphate matrix comprises a mixture of an aluminum phosphate binder and melted periclase, MgO. All samples demonstrate a homogeneous distribution of the ferroelectric perovskite phase and are thermally stable up to 900 K. At higher temperatures, the pyrochlore cubic phase forms. It has been found that the density of the composites non-monotonously depends on the pressure. The dielectric permittivity and losses substantially increase with the density of the samples. The fabricated composites demonstrate diffused ferroelectric–paraelectric transition and prominent piezoelectric properties.

Physics-based optimization of Landau parameters for ferroelectrics: application to BZT–50BCT

In analogy to thermochemical parameter optimization in the CALculation of PHAse Diagrams (CALPHAD) approach that relies on a wide variety of experimental measurements, a thermodynamic parameter optimization strategy to describe the polarization response with single-crystal Landau coefficients is documented herein to bridge the gap between CALPHAD, Landau theory and the generality of phase field theory. As an example, anisotropic Landau coefficients are obtained by starting from experimental parameters for Pb-free 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 (BZT–50BCT). Thermodynamic parameters are optimized separately for the rhombohedral (R) and tetragonal (T) ferroelectric phases using a combination of single-crystal and polycrystalline polarization and permittivity data. Parameters are validated against dielectric permittivity as a function of temperature from single-crystal and polycrystalline samples. The optimized parameters correctly predict the sequence of phase transitions as a function of temperature and the paraelectric–ferroelectric transition at 355.5 K is within the experimentally reported range. An upper thermodynamic limit is predicted for R + T metastable coexistence at 302.3 K in agreement with the experimentally measured transition to a stable single T phase for this chemistry at 302 K. The detailed parameter optimization procedure enables the identification of single-crystal tensor properties from experimental measurements for a wide variety of chemistries.

Electric-field-driven coexistence of positive and negative electrocaloric effects near room temperature for high-efficiency two-stage cooling

The electrocaloric (EC) effect is promising for the next-generation solid-state refrigeration, yet it is difficult to improve the EC response near room temperature with either the positive electrocaloric (PEC) or negative electrocaloric (NEC) effect alone. In this work, we propose and demonstrate a strategy that achieves the coexistence of PEC and NEC effects near room temperature via electric-field-induced ferroelectric–ferroelectric phase transition in Ba(ZrxTi1−x)O3 solid solutions, while the PEC effect is generally found around paraelectric–ferroelectric transition and the NEC effect is around ferroelectric–antiferroelectric phase boundaries. Utilizing such a coexistence of PEC and NEC effects, we propose a prototype two-stage cooling device, wherein both the EC cooling temperature change and efficiency are higher than its counterpart with only the PEC or NEC effect, offering a new pathway for EC cooling in practical applications near room temperature.

Critical positive temperature coefficient of resistivity of Li/Y co-doped ZnO ceramics modified by Cr-ions

ZnO-based ceramics doped with Li/Y and modified by various contents of Cr-ions, (Zn0.9475Y0.0025Li0.05)1-xCrxO (0 ≤ x ≤ 0.025), were prepared by a wet chemical route followed by a traditional ceramic sintering technology. The effect of Cr-ion content on the electronic conductivity and resistance–temperature characteristics of the prepared ceramics was investigated. All the ceramics have hexagonal wurtzite ZnO structure, and exhibit critical positive temperature coefficient of resistivity (C-PTCR) effect. With various contents of Cr-ions, the maximum temperature coefficient of resistivity (TCR) reaches 61.1%·°C−1 or the resistivity jump log(ρmax/ρmin) is up to 4.74. The critical transition temperature of resistivity can be adjusted in the temperature range of 76–147 °C for different various contents of Cr-ions. The complex impedance analysis shows that the C-PTCR effect of the ZnO ceramics resulted from both grain effect and grain boundary effect. According to the analysis of the dielectric temperature spectra, the ZnO-based ceramics have ferroelectricity at room temperature, and the ferroelectric–paraelectric transition around the critical transition temperature of resistivity during the temperature increases.

Stable piezoelectric response of 0-3 type CaBi2Nb2O9:xwt%BiFeO3 composites for high-temperature piezoelectric applications

ABSTRACT Depolarization in the ceramic materials has remained the longstanding obstacle for the materials to be utilized in high-temperature piezoelectric devices. Herein, a strategy to defer thermal depolarization is employed. To acquire the best merits of two different materials, 0–3 type CaBi2Nb2O9:xwt%BiFeO3 (CBN:xwt%BFO) composites are engineered in a unique way by embedding the isolated BiFeO3 (BFO) grains at the grain boundaries of the CaBi2Nb2O9 (CBN) matrix. The composite with 0–3 type connectivity (CaBi2Nb2O9:40wt%BiFeO3) exhibits a high density of 7.98 g/cm3, a high saturated polarization of ~19 μC/cm2, high resistivity of ~1010 Ω.cm, and an enhanced piezoelectric coefficient d33 of 29 pC/N at room temperature. Composite shows the sharp-peaked ferroelectric–paraelectric transition at high Curie temperature T C of ~881°C. Importantly, the composite maintains a very high resistivity of ~105 Ω.cm even at 500°C and the d33 of 24 pC/N after annealing at 700°C. Moreover, the poled composite has displayed strong radial and thickness dielectric resonances at 20–700°C. Concluding the results, the CBN:40 wt%BFO composite with 0–3 type connectivity shows the stable high-temperature piezoelectric response much better than either CBN or BFO and is the worthier candidate to be utilized in high-temperature piezoelectric devices.