Existence Of Polar Nanoregions Research Articles

Ultrafast phase‐field model of frequency‐dependent dielectric behavior in ferroelectrics

AbstractRelaxor ferroelectrics have applications in transducers and capacitors owing to their dielectric relaxation. Although the existence of polar nanoregions (PNRs) inside the relaxor ferroelectrics contributes to enhancing the dielectric response, the dielectric relaxation mechanism remains unclear in the wide frequency range from kHz to THz. In this study, we investigated the frequency‐dependent (103–1012 Hz) dielectric properties using ultrafast phase‐field simulations. The dielectric response is divided into the matrix response, the “collinear” PNR response, and the “non‐collinear” PNR response. Among them, the collinear PNR response accounts for 90% of the dielectric constant enhancement. The response behavior of collinear PNRs has a higher frequency limitation than that of non‐collinear PNRs. The different relaxation times of the collinear PNR lead to a corresponding distinct rotation for the polar vectors. Finally, we investigate the different response behaviors of the polarization in the PNR from the aspect of energy barriers. This study not only promotes the understanding of wide‐range frequency‐dependent dielectric relaxation processes but also guides the design of ultrafast relaxor ferroelectric devices in the future.

Brillouin Scattering Study of Electro-Optic KTa1-xNbxO3 Crystals.

The functionality enhancement of ferroelectrics by local polar clusters called polar nanoregions (PNRs) is one of the current interests in materials science. KTa1-xNbxO3 (KTN) with perovskite structure is a well-known electro-optic crystal with a large Kerr effect. The existence of PNRs in relaxor-like ferroelectric Nb-rich KTN with homovalent B-site cations is controversial. This paper reviews recent progress in understanding precursor dynamics in Nb-rich KTN crystals studied using Brillouin scattering. The intense central peak (CP) and significant softening of sound velocity are observed above the Curie temperature (TC) due to the polarization fluctuations in PNRs. The effects of Li-doping, defects, and electric fields on the growth and/or creation of PNRs are found using changes in acoustic properties. The electric-field-induced TC, which is shifted to higher values with increases in applied voltage, including critical endpoint (CEP) and field gradient by trapped electrons, are discussed as well. This new knowledge may give new insight into advanced functionality in perovskite ferroelectrics.

Anomalous dielectric relaxation peak in Nb-doped SrTiO3 single crystals

Anomalous dielectric relaxation appears in Nb-doped SrTiO3 (SNT) single crystal with relaxation rate displaying unusual slowing down with increasing temperature. Dielectric measurements show that the resistivity increases with temperature. Raman spectrum and the spherical aberration-corrected transmission electron microscope (STEM) show the existence of polar nano regions (PNRs) in the crystal. The temperature-dependence of DC conductivity and current-voltage (I–V) curves suggest that this anomalous behavior happens at the interfaces of PNRs. The evolution of the anomalous relaxation peak in the annealed samples indicates that this effect is simultaneously affected by the carrier concentration and the interface structure. The anomalous dielectric relaxation is reminiscent of the positive temperature coefficient of resistance (PTCR) effect in a polycrystalline material. The resistive switching effect makes such materials to have potential application value in switches, sensors, and device miniaturization.

High energy storage efficiency and fast discharge property of temperature stabilized Ba0.4Sr0.6TiO3–Bi(Mg0.5Ti0.5)O3 ceramics

(1−x)Ba0.4Sr0.6TiO3-xBi(Mg0.5Ti0.5)O3 ((1−x)BST-xBMT) relaxor ferroelectric ceramics were prepared by a conventional solid-state method. In this work, the microstructure, dielectric properties, and pulsed charge–discharge properties were investigated. The doping of BMT caused a decrease in the surface energy and grain boundary energy, and contributed to the formation of polar nanoregions (PNRs). The existence of PNRs was confirmed by piezoresponse force microscopy measurements (PFM) on the 0.8BST-0.2BMT ceramic. The PNRs responded quickly under an AC voltage, thus the pulsed charge–discharge time was short (<80 ns), aiming to realize time compression to improve the power density (PD). The PNRs were not closely connected to each other and adverse to the formation of leakage current and pinning, thus inhibiting charge transfer at the grain boundaries and contributing to the high energy storage efficiency (η ∼ 93%). In addition, the 0.8BST-0.2BMT ceramic also displayed excellent temperature stability. The capacitance-temperature dependence satisfied the requirement of X8R (−55–150 °C, ΔC/C25°C ≤ ±15%), and η had no obvious fluctuation in the temperature range from 25 °C to 150 °C. This study could provide a successful method to achieve a temperature stable and high η, and a fast charge–discharge process.

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.

Origin of polar nanoregions from displacive correlation in relaxor ferroelectric Pb(Mg1/3Nb2/3)O3–PbTiO3

The existence of polar nanoregions (PNRs) which are widely accepted as a key factor contributing to ultrahigh piezoelectricity in relaxor-ferroelectric solid solutions is found to originate from local displacive correlations.

Investigation of the coupling effect of flexoelectricity and ferroelasticity on energy storage efficiency of relaxor ferroelectrics

The Ginzburg–Landau theory and dipole defect model have been employed to investigate the flexoelectric and ferroelastic effects on the ferroelectric and energy storage properties of relaxor ferroelectrics (RFs). The results obtained show that due to the existence of polar nanoregions (PNRs) in RFs, the elastic field of the material, which is induced by both the flexoelectric and ferroelastic effects, leads to the increase of the domain switching energy and coercive field and the decrease of the energy storage efficiency. In contrast, the short-range electric field induced by the dipole defects enhances the energy storage efficiency of the material by enhancing the material’s relaxor behavior. Hence, the energy storage efficiency of RFs can be effectively functionalized by modulating the composition ratio and the electric field of the RF materials.

Phase boundary and temperature driven enhanced piezoelectric and electrostrictive strain in (1−2x) Bi0.5Na0.5TiO3-xBaTiO3-xBa0.7Ca0.3TiO3 solid solution

Bismuth-based piezoelectric ceramics are presently of immense interest to researchers as they are believed to be Pb-free alternatives to well-known lead zirconate titanate-based piezoceramics. Herein, the author reports a lead-free ternary solid solution (1−2x)Na0.5Bi0.5TiO3-xBaTiO3-xBa0.7Ca0.3TiO3 (x = 0.01, 0.03, 0.05, 0.07, and 0.09: BNT-BT-BCT) synthesized through a standard solid state reaction route. All the samples crystallized to a complete perovskite structure studied through the powder x-ray diffraction analysis. Rietveld analysis of x-ray diffraction data revealed a structural transformation from monoclinic (Cc) to tetragonal phase (p4mm) with the co-existence of monoclinic (Cc) and tetragonal (p4mm) phases in the samples of x ≥ 0.03. The temperature-dependent dielectric analysis of (x = 0.03 and x = 0.05) systems suggests relaxor characteristics near ferroelectric–antiferroelectric phase transition temperature (Td). A changeover from relaxor to a near normal ferroelectric character was realized for x ≥ 0.07. Furthermore, the existence of polar nano-regions (PNRs) was studied through HR-TEM. Interestingly, a low electric field (±25 kV/cm) driven enhanced piezoelectric [(with 0.22% of strain; Smax/Emax = 850 pm/V for x = 0.03) and (with 0.17% strain and Smax/Emax of 714 pm/V for x = 0.07)] and an electrostrictive [with 0.20% of strain; Smax/Emax = 820 pm/V for x = 0.05] was achieved around Td. This can be attributed to the combined effects of phase boundary, ferroelectric–antiferroelectric transition, and the existence of PNRs.

Enhanced strains of Nb-doped BNKT-4ST piezoelectric ceramics via phase boundary and domain design

A strategy that constructs the morphotropic phase boundary and manipulates the domain structure has been used to design the component of 0.96[Bi0.5(Na0.84K0.16)0.5Ti(1-x)NbxO3]-0.04SrTiO3 (BNKT-4ST-100xNb) to enhance the strain properties for actuator application. Non-equivalent Nb5+ donor doping modulates the phase transition from the mixture of rhombohedral and tetragonal phases to the pseudocubic phase and results in the coexistence of multiple phases. Moreover, the high-resolution TEM confirms the existence of polar nano regions that contribute to the macroscopic relaxor behaviour. The size of the domains is reduced with increasing Nb5+, resulting in an enhanced relaxor behaviour. The ferroelectric-relaxor transition temperature decreases from 85 to below 30 °C, implying a non-ergodic to ergodic relaxor transition. An improved strain of 0.56% and a giant normalized strain of 1120 pm/V were achieved for BNKT-4ST-1.5Nb, which were attributed to the unique domain structure in which nanodomains are embedded in an undistorted cubic matrix. Ferroelectric, antiferroelectric, and relaxor phases coexist. As the electric field is large enough, a reversible phase transition occurs. Furthermore, good temperature stability was obtained due to the stability of the nanodomains, and no degradation in strains was observed even after 104 cycles, which may originate from the reversible phase transition and dynamic domain wall. The results show that this design strategy offers a reference way to improve the strain behaviour and that BNKT-4ST-100xNb ceramics could be a potential material for high-displacement actuator applications.

Enhancement of relaxor behavior by La doping and its influence on the energy storage performance and electric breakdown strength of ferroelectric Pb(Zr0.52Ti0.48)O3 thin films

La-doped Pb1-xLax(Zr0.52Ti0.48)O3 (PLZT) thin films with dopant concentrations of up to 12% were fabricated on (111)Pt/Ti/SiO2/Si substrates by a sol-gel spin-coating method to investigate the effects of La content on the phase transition, ferroelectric and energy-storage performance properties of PZT thin films. The compositional-induced relaxor behavior was increased with the doping of La, due to the presence of polar nano-regions (PNRs). The existence of PNRs in the relaxor ferroelectric, as opposed to the microscopic long-range ordered domains in the normal ferroelectric, induced the difference between the polarizations of the normal and relaxor ferroelectrics. The results indicated that the decrease in remanent polarization and coercive field in the PLZT thin films with increasing La content was associated with the phase transition from normal ferroelectric behavior in undoped PZT to relaxor behavior in PLZT thin films due to the substitution of Pb2+ ions by La3+ ions. The maximum recoverable energy-storage density (27.5 J/cm3) and efficiency (62.2%) were obtained for the 8% La-doped film due to the large electric breakdown strength (2200 kV/cm) and slimmer polarization hysteresis loop. Because of the thinner films (250 nm), the low driven voltage of 55 V can be used to charge the thin film capacitors to obtain the above energy-storage properties. Moreover, an excellent charge-discharge cycling life with fatigue-free performance up to 1010 cycles was also realized in the thin films. All of the excellent results indicated that the relaxor PLZT thin films with 8% La doping possess excellent potential for low voltage operation advanced capacitors with high energy-storage performance.

The Contributions of Polar Nanoregions to the Dielectric and Piezoelectric Responses in Domain‐Engineered Relaxor‐PbTiO3 Crystals

The existence of polar nanoregions is the most important characteristic of relaxor‐based ferroelectric materials. Recently, the contributions of polar nanoregions to the shear piezoelectric property of relaxor‐PbTiO3 (PT) crystals are confirmed in a single domain state, accounting for 50%–80% of room temperature values. For electromechanical applications, however, the outstanding longitudinal piezoelectricity in domain‐engineered relaxor‐PT crystals is of the most significance. In this paper, the contributions of polar nanoregions to the longitudinal properties in [001]‐poled Pb(Mg1/3Nb2/3)O3‐0.30PbTiO3 and [110]‐poled Pb(Zn1/3Nb2/3)O3‐0.15PbTiO3 (PZN‐0.15PT) domain‐engineered crystals are studied. Taking the [110]‐poled tetragonal PZN‐0.15PT crystal as an example, phase‐field simulations of the domain structures and the longitudinal dielectric/piezoelectric responses are performed. According to the experimental results and phase‐field simulations, the contributions of polar nanoregions (PNRs) to the longitudinal properties of relaxor‐PT crystals are successfully explained on the mesoscale, where the PNRs behave as “seeds” to facilitate macroscopic polarization rotation and enhance electric‐field‐induced strain. The results reveal the importance of local structures to the macroscopic properties, where a modest structural variation on the nanoscale greatly impacts the macroscopic properties.

Polar nano regions in lead free (Na 0.5 Bi 0.5 )TiO 3 -SrTiO 3 -BaTiO 3 relaxors: An impedance spectroscopic study

Abstract Lead free ternary (0.8− x )Na 0.5 Bi 0.5 TiO 3 -0.2SrTiO 3 - x BaTiO 3 (NBT-ST-BT) relaxor ferroelectric ceramics with dense morphology were investigated by impedance spectroscopy over a wide temperature range from 50 to 400 °C. The grain and grain boundary response as well as the relaxation processes at different frequencies and temperatures were discussed using different formalisms. The relaxation time extracted from Z″( f ) and M″( f ) spectra for x = 0.04, 0.06, 0.08 follows the Arrhenius law and reveals the existence of three different relaxation processes with different activation energies. The shoulder response in M″( f ) provides an indirect evidence of the presence of highly polarisable entities in the samples which is a signature of the existence of polar nano regions (PNRs) within the grains.

The origin of ultrahigh piezoelectricity in relaxor-ferroelectric solid solution crystals.

The discovery of ultrahigh piezoelectricity in relaxor-ferroelectric solid solution single crystals is a breakthrough in ferroelectric materials. A key signature of relaxor-ferroelectric solid solutions is the existence of polar nanoregions, a nanoscale inhomogeneity, that coexist with normal ferroelectric domains. Despite two decades of extensive studies, the contribution of polar nanoregions to the underlying piezoelectric properties of relaxor ferroelectrics has yet to be established. Here we quantitatively characterize the contribution of polar nanoregions to the dielectric/piezoelectric responses of relaxor-ferroelectric crystals using a combination of cryogenic experiments and phase-field simulations. The contribution of polar nanoregions to the room-temperature dielectric and piezoelectric properties is in the range of 50–80%. A mesoscale mechanism is proposed to reveal the origin of the high piezoelectricity in relaxor ferroelectrics, where the polar nanoregions aligned in a ferroelectric matrix can facilitate polarization rotation. This mechanism emphasizes the critical role of local structure on the macroscopic properties of ferroelectric materials.

Diffuse Phase Transition and Relaxor-Like Behavior in P(VDF-TrFE-CFE) Films Irradiated with Different Electron Dose

Poly(vinylidene fluoride-trifluoroethylene-chlorofloroethylene) films were fabricated by Langmuir-Blodgett method, and then further irradiated by low energy electron irradiation with different dose. All samples have been investigated by temperature and dc electric-field dependences of dielectric response. It was found that both the non-irradiated and irradiated samples exhibit diffuse phase transition and relaxor-like characteristics. Besides, the weak butterfly hysteresis still persists at room temperature for P(VDF-TrFE-CFE) films. These results were correlated with the existence of polar nanoregions, as well established in terms of a multipolarization mechanism model.

Temperature Dependence of Ferroelectric Properties of the Potassium Lithium Tantalate K1−xLixTaO3 Obtained with Piezoresponse Force Microscopy Technique

In the present paper we report the first results of the study of the polar state of the surface structure of single crystal of potassium tantalate doped with Li + ions (KLT-3%) by the piezoresponse force microscopy technique in the temperature range 10–295 K. The results obtained demonstrate the existence of polar nanoregions on the surface of the KLT-3% single crystal at temperatures 10 – 80 K. Spatial distribution and temperature evolution of these polar nanoregions are analyzed. Observation of the structure of polar nanoregions of KLT-3% crystal in zero field heating after zero field cooling regime gives the evidence of the existence of spontaneous polarization in the low-temperature phase.

Polar nanoregions in water: A study of the dielectric properties of TIP4P/2005, TIP4P/2005f and TTM3F

We present a critical comparison of the dielectric properties of three models of water-TIP4P/2005, TIP4P/2005f, and TTM3F. Dipole spatial correlation is measured using the distance dependent Kirkwood function along with one-dimensional and two-dimensional dipole correlation functions. We find that the introduction of flexibility alone does not significantly affect dipole correlation and only affects ɛ(ω) at high frequencies. By contrast the introduction of polarizability increases dipole correlation and yields a more accurate ɛ(ω). Additionally, the introduction of polarizability creates temperature dependence in the dipole moment even at fixed density, yielding a more accurate value for dɛ/dT compared to non-polarizable models. To better understand the physical origin of the dielectric properties of water we make analogies to the physics of polar nanoregions in relaxor ferroelectric materials. We show that ɛ(ω, T) and τD(T) for water have striking similarities with relaxor ferroelectrics, a class of materials characterized by large frequency dispersion in ɛ(ω, T), Vogel-Fulcher-Tammann behaviour in τD(T), and the existence of polar nanoregions.

Field history dependence of nonlinear dielectric properties of Ba 0.6Sr 0.4TiO 3 ceramics under bias electric field: Polarization behavior of polar nano-regions

Nonlinear dielectric properties of Ba 0.6Sr 0.4TiO 3 ceramics prepared by citrate method were investigated under bias electric field with respect to field history. X-ray diffraction analysis and temperature dependence of the dielectric constant ( ε r ) confirmed a macroscopically paraelectric state for the specimen at room temperature. A slim polarization versus electric field ( P– E) hysteresis loop of the specimen at room temperature indicated the existence of polar nano-regions (PNRs) superimposed on the paraelectric background. The nonlinear dielectric properties in continuous cycles of bias field sweep displayed a strong sensitivity to the field history. This phenomenon was qualitatively explained in terms of an irreversible polarization evolution of the PNRs under the bias fields. A considerable decline of the tunability with the cycle number suggests an appreciable contribution of the PNRs to the dielectric nonlinearity. The polarization and size of the PNRs were determined by fitting the dielectric constants to a multipolarization mechanism model.

SIZE AND STRAIN EFFECTS ON RELAXOR FERROELECTRIC PROPERTIES OF PBSC0.5NB(1 −x)/2TAx/2O3 THIN FILMS AND NANOCERAMICS

ABSTRACT PbSc0.5Nb(1 −x)/2Tax/2O3(PSNT for x = 0.3, PST for x = 1) nanoceramics and thin films were prepared by high energy ball mill with solid state reaction technique and pulse laser deposition (PLD) respectively. A comparative study of the microstructure, dielectric properties, and polarization of PSNT and PST thin films and nano ceramics were carried out over a wide range of temperature (100 K-600 K) and frequency (100 Hz to 1 MHz). High Resolution Transmission Electron Microscopy (HRTEM) image of PSNT and PST thin films indicate the presence of small polar nano regions (PNRs) and long range ordered regions in crystal lattice plane respectively. PST nanoceramics (PST-N) showed normal ferroelectric behavior where as its thin film form (PST-F) illustrated relaxor behavior. PSNT nanoceramics (PSNT-N) and its films (PSNT-F) demonstrated weak frequency dispersion and strong frequency dispersion in dielectric spectra respectively. A shift in dielectric maximum temperature (Tm) towards lower temperature side i.e. 65 K and 85 K were observed for PSNT-F and PST-F compare to their nanoceramices indicates greater role of substrate-electrode-interface strain on dielectric properties. Well behaved hysteresis at room temperature for PSNT-F point toward strong competition between short range order and long range order. A weak slim hesteresis for PST-F were observed at room temperature suggesting the existence of polar nano regions far above dielectric maximum temperature.

Dielectric evidences of core-shell-like effects in nanosized relaxor PbMg1∕3Nb2∕3O3

Temperature and frequency dependence of dielectric permittivity in nanotextured ceramics of PbMg1∕3Nb2∕3O3 has been performed in a wide range of grain sizes and shown to display strong variation. In particular, frequency relaxation follows the Vögel Fulcher law for the high grain sizes, whereas for intermediate sizes, two Debye processes are evidenced. This behavior as well as a diminution of the maximum of the real dielectric constant εmax could be explained within the mechanism of relaxation based on the existence of polar nanoregions with a core-shell structure for each grain.

Analysis of diffuse phase transition and relaxorlike behaviors in Pb0.5Sr0.5TiO3 films through dc electric-field dependence of dielectric response

The pulsed-laser deposited Pb0.5Sr0.5TiO3 (PST) films with typical diffuse phase transition (DPT) and relaxorlike characteristics have been investigated by dc electric-field dependence of dielectric response. It was found that the butterfly hysteresis still persists above the Curie temperature on the curves of dielectric constant changing with the dc electric field. This result was correlated with the existence of polar nanoregions (PNRs) in the PST films, as well established in terms of a multipolarization mechanism model. Consequently, the existence of PNRs may be one of the crucial causes of the DPT and relaxorlike behaviors in PST ferroelectric films.