Defect Dipoles Research Articles

Improved piezoelectric and photoluminescence properties in Ce-doped PSN-PMN-PT piezoelectric ceramics by multiscale coordination

The large piezoelectric coefficient (d33) and high mechanical quality factor (Qm) are essential for piezoelectric ceramics used in high-power devices. It is pity that the two parameters are usually opposite, with higher d33 corresponding to lower Qm. In order to well-balance the d33 and Qm, the rare-earth element Ce with multi-valence was introduced into the 0.06 Pb(Sc1/2Nb1/2)O3-0.61 Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 (xCe-PSN-PMN-PT) ceramics in this work. The structure, electrical properties and optical properties of ceramics were systematically analyzed. Optimal electrical performances (d33 = 719 pC/N, Qm = 256, and dielectric loss tanδ = 0.007) were obtained in 0.02Ce-PSN-PMN-PT ceramics. These excellent performances originate from the multiscale coordination effects among defect dipoles, local structural heterogeneity, domain structure, phase structure, and grain size. The xCe-PSN-PMN-PT ceramics exhibit excellent photoluminescence properties in the blue wavelength range. Our research provides a new paradigm for deep-sea electro-optical communication.

Role of oxygen vacancy on rapid densification and giant dielectric properties of flash sintered Ca2+ and Ta5+ co-doped TiO2 ceramics

(Ca, Ta) co-doped TiO2 ceramics were successfully prepared using flash sintering technology, and the effects of oxygen vacancies on the rapid densification and giant dielectric properties were studied. The results suggest that the maximum volume fraction of oxygen vacancies in flash-sintered samples is approximately 10%. Oxygen vacancies decrease the barrier energy level of atomic diffusion and provide convenient diffusion channels, resulting in rapid densification of the sample within 24 min at 1100 °C. All flash-sintered samples exhibit high relative density, exceeding 95%. Oxygen vacancies promote the electron-pinning defect dipole effect at low temperatures. As the temperature increases, carriers such as oxygen vacancies move to the grain boundary, forming an internal barrier layer capacitance effect. The optical dielectric constant reaches 7.25 × 104, and the dielectric loss drops to 0.09. Oxygen vacancy play an important role in both flash sintering technology and the giant dielectric properties of co-doped TiO2 ceramics.

Colossal dielectric behavior of Nb-doped SrTiO3 single crystal

Crystals are usually used to explore the origin of material properties due to the fact that they have no grain boundaries and no pores. Although doped SrTiO3 crystal have remarkable dielectric properties, the origin of this improvement is still not fully understood. Herein, Nb5+ has been selected as a donor ion to study the dielectric properties of SrTiO3 due to its radius close to Ti4+. It has been found that the dielectric permittivity of the Nb-doped crystal is greater than 94,500 and the dielectric loss is maintained below 0.002 in the frequency range from 20 Hz to 0.2 MHz. X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy indicate that the dielectric properties of doped crystals were improved due to the existence of oxygen vacancies and there was no effect of interface polarization. It was beneficial for the Nb-doped crystal to be annealed in a N2 atmosphere resulting in oxygen loss more oxygen vacancies, and a higher concentration of Ti3+. The electron can be bound by VO•• and Ti4+ and form the defect dipole [Ti4+∙e − VO•• − Ti4+∙e], which showed a higher permittivity and lower dielectric loss. In summary, the origin and optimization of dielectric properties of SrTiO3-based materials can be deeply understood when from doped SrTiO3 crystals.

Oxygen vacancy migration and impact on high voltage DC polarization in 0.8BaTiO3–0.2BiZn0.5Ti0.5O3

AbstractElectrical polarization and defect transport are examined in 0.8BaTiO3–0.2BiZn0.5Ti0.5O3, an attractive capacitor material for high power electronics. Oxygen vacancies are suggested to be the majority charge carrier at or below 250°C with a grain conduction hopping activation energy of 0.97 eV and 0.92 eV for thermally stimulated depolarization current (TSDC) and impedance spectroscopy measurements, respectively. At higher temperature, thermally generated electronic conduction with an activation energy of 1.6 eV is dominant. Significant oxygen vacancy concentration is indicated (up to ∼1%) due to cation vacancy formation (i.e., acceptor defects) from observed Bi (and likely Zn) volatility. Oxygen vacancy diffusivity is estimated to be 10−12.8 cm2/s at 250°C. Low diffusivity and high activation energies are indicative of significant defect interactions. Dipolar oxygen vacancy defects are also indicated, with an activation energy of 0.59 eV from TSDC measurements. The large oxygen vacancy content leads to a short lifetime during high voltage (30 kV/cm), high temperature (250°C) direct current (DC) electrical measurements.

Strain-driven high energy storage in BaTiO3-based lead-free epitaxial thin films

In this work, the epitaxial 0.85BaTiO3-0.15Bi(Mg1/2Ti1/2)O3 (BT-BMT) films with large compressive strain were fabricated on SrTiO3 (001). The expansion of the unit cell volume and out-of-plane lattice parameter and the large built-in electric field (Ebi) in BT-BMT films indicate the existence of defect dipoles. It was found that the polarization and the breakdown strength can be optimized by the strain and the defects, respectively. Ultimately, a desirable energy density of 90.3 J/cm3 with efficiency of 62.3% was achieved. It suggests that strain can serve as a practical means to modulate the energy storage performance of ferroelectric epitaxial film capacitors.

Polar topological bubbles in (K,Na)NbO3-based single crystals via synergetic phase and defect engineering

The investigations of polar topological conformations have become one of the hotspots due to their multitudinous physical properties and potential applications in emerging electronic devices. However, constructing polar topologies in bulk form is exceedingly challenging. In this study, bubble domains with different polar topologies, including flux-closure and half-closure, are successfully achieved in Cu-doped (K,Na)(Nb,Ta)O3 single crystals by elaborately adjusting the phase structure and defect dipoles. The coexistence of orthorhombic and tetragonal phases results in a flattened free energy landscape, while the presence of a high concentration of defect dipoles in local regions leads to the generation of the strong internal electric field, further modifying the energy profile. The interplay of various types of energies gives rise to the smooth rotation of the polar vectors, facilitating the formation of topological structures. These findings provide a practical method for constructing topological bubble domains in bulk materials, thereby enriching the understanding of the physical properties arising from topological structures.

Impact of defect concentration on piezoelectricity in Mn/Fe-doped KTN crystals

Defect engineering via doping exhibits considerable potential for improving the performance of environment-friendly lead-free piezoelectric materials. Owing to the susceptibility to lattice vibrations and the micro-local chemical environment, the readily available Mn/Fe transition metal elements (TMEs) facilitate the construction of defect structures. However, the role of TMEs in shaping the domain structures and the corresponding promotional mechanism of piezoelectricity need to be further decoded. Herein, we propose the different influence mechanisms of Mn and Fe ions on the ferroelectric domain and piezoelectric properties. Different concentrations of (MnNb/Ta′-VO••)• and (FeNb/Ta″-VO••)× defect dipoles are obtained based on the synergy of Mn/Fe ions with oxygen vacancies. Diverse ferroelectric behaviors resulting from (MnNb/Ta′-VO••)• and (FeNb/Ta″-VO••)× defect dipoles are observed. Furthermore, the variation of the dielectric diffusiveness with the defect dipole concentration is investigated. Trace concentration of (MnNb/Ta′-VO••)• generates strong diffusiveness. With the characterization of the ferroelectric domain, this strong diffusiveness is attributed to the lattice-like domain structure. Thereafter, the mechanisms of Mn/Fe defect dipoles on the formation of domain structures are clarified. Macroscopically, the dielectric and piezoelectric properties are measured with Mn/Fe ion components. Trace Mn doping and the resulting lattice-like domain significantly enhance the piezoelectric coefficient, resulting in an increase of nearly 50% for K(Ta,Nb)O3 single crystals. This work highlights the tremendous potential of TME-induced defect dipoles for modifying the ferroelectric domain and provides a reference paradigm for improving piezoelectricity through defect engineering.

Modification of domain construction in PNN-PIN-PT piezoelectric ceramics for high electromechanical performance using defect engineering

Good piezoelectric properties and low mechanical losses are prerequisites for the application of high-power devices. In this study, an appropriate amount of Mn ions was introduced into a piezoelectric 0.46Pb(Ni1/3Nb2/3)O3-0.23Pb(In1/2Nb1/2)O3-0.31PbTiO3 ceramic to enhance its electromechanical properties. The results revealed that the mechanical quality factor(Qm) value of the ceramic was improved while maintaining high piezoelectric properties, and the dielectric loss was reduced. When Mn doping is 1.0 mol%, the d33 component decreased by only ∼21%, while Qm increased by ∼13 times (d33 = 750 pC/N, Qm = 385, tanδ = 0.66%). The measurement of the internal bias field (Ei) in the P-E loops of aged and nonaged samples showed that aging promoted the directional alignment of defect dipoles and improved their Qm. According to the PFM data, significant increase in Qm was mainly due to the "bulk effect" caused by low domain wall density. In turn, widening and lengthening of domains promoted the accumulation of most defective dipoles in the ceramic body, which greatly amplified the hardening effect. This work provides a good example of enhancing Qm in ceramic systems with high d33 values through the construction of low domain wall densities.

Tungsten-bronze-type BaNd2Ti4O12 ceramics with high permittivity and ultra-low dielectric loss under low frequency

High dielectric constant and ultra-low dielectric loss are important for electronic device applications of dielectrics. In this work, the novel ceramic BaNd2Ti4O12, synthesized via solid-state reaction, exhibits remarkable dielectric stability, with excellent dielectric properties (ε′ = 117 and tanδ = 6.27 × 10−4, at 1 kHz) and high dielectric strength (Eb = 260 kV/cm). By exploring its polarization behavior and loss mechanisms at low frequencies for the first time, we found that the sustained high dielectric constant is attributed to the synergy of defect dipole polarization and interface polarization, while the observed exponential increase of dielectric loss is primarily caused by the conduction loss, with a minor contribution from the anelastic relaxation loss. In the low-temperature range of 50∼350 °C, the relaxation behavior is governed by the aggregation and subsequent thermal ionization of the oxygen vacancies, whereas at temperatures above 350 °C, it is predominated by the long-range motion of the doubly ionized oxygen vacancies. These findings provide valuable insights for the potential electronic device applications of BaNd2Ti4O12.

Defect dipole engineering for regulating ferroelectric photovoltaic performances in BiFeO3–SrCoO2.5 solid solution films

Defect dipole engineering for regulating ferroelectric photovoltaic performances in BiFeO3–SrCoO2.5 solid solution films

Advancing Energy‐Storage Performance in Freestanding Ferroelectric Thin Films: Insights from Phase‐Field Simulations

AbstractAdvances in flexible electronics are driving the development of ferroelectric thin‐film capacitors toward flexibility and high energy storage performance. In the present work, the synergistic combination of mechanical bending and defect dipole engineering is demonstrated to significantly enhance the energy storage performance of freestanding ferroelectric thin films, achieved through the generation of a narrower and right‐shifted polarization‐electric field hysteresis loop. The recoverable energy storage density of freestanding PbZr0.52Ti0.48O3 thin films increases from 99.7 J cm−3 in the strain (defect) ‐free state to 349.6 J cm−3, marking a significant increase of 251%. The collective impact of the flexoelectric field, bending tensile strain, and defect dipoles contributes to this enhancement. The demonstrated synergistic optimization strategy has potential applicability to flexible ferroelectric thin film systems. Moreover, the energy storage properties of flexible ferroelectric thin films can be further fine‐tuned by adjusting bending angles and defect dipole concentrations, offering a versatile platform for control and performance optimization.

Modified Pb(Mg1/3Nb2/3)O3-PbZrO3-PbTiO3 ceramics with high d33，large Qm and high Tc

The development of piezoelectric ceramics with large piezoelectric coefficient (d33), high Curie temperature (Tc) and high mechanical quality factor (Qm) is still a key challenge for practical application for high-power device. Herein, a series of Sm3+ and Mn2+ ions co-doped 0.15Pb(Mg1/3Nb2/3)O3-(0.85-x)PbZrO3-xPbTiO3 ceramics were prepared by the solid-state sintering method. The optimum piezoelectric properties were achieved at x = 0.435 mol% with a large d33 ∼ 450 pC/N and a high Qm ∼ 1968 near the morphotropic phase boundary (MPB), and with high Tc ∼ 296 °C. The origin of this excellent piezoelectric properties was explained by the phase structure, piezoelectric and dielectric analysis. The composition near MPB region and the appearance of defect dipoles leaded to final excellent piezoelectric properties, and made a harmonization between d33, Tc and Qm. This work demonstrated a rational strategy to obtain large d33, high Tc and high Qm for high-power piezoceramics.

Low-electric-field-induced high electrocaloric properties via defect regulation in samarium-doped BaTiO3 ceramics

A small electrocaloric effect (ECE) is usually observed in aliovalent doped ferroelectric ceramics since decreased polarization is usually present. In this work, the high ECE is achieved in a barium titanate [BaTiO3 (BT)]-based ceramic by introducing defect polarization to enhance the ferroelectric polarization and polarization change. The defect dipoles consisting of donor ion and barium vacancy, or barium and oxygen vacancies were formed in samarium-doped (Ba1−1.5xSmx)TiO3 ceramics. The phase and defect structures, and dielectric and ferroelectric properties are analyzed to confirm the evolution of charge compensation states and defect structures, and the influences of defects on polarization rotation and ECE. An enhanced ferroelectric polarization and high internal bias field are present in the ceramic with a suitable defect dipole effect, leading to a high polarization change before/after applying an electric field, causing an enhanced ECE. Finally, a high electrocaloric temperature change of ∼1.11 K and a large electrocaloric strength of ∼0.037 K·cm kV−1 were achieved under a low electric field of 30 kV/cm, indicating that forming suitable defect dipoles can optimize the ECE. This work provides a significant paradigm for tuning ECE in ferroelectric materials via defect regulation.

Significantly enhanced transmittance and EO property in PMN-PT via optimized two-step pressure-free sintering

It is a great challenge to prepare Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN-PT) ceramics of very high transparency with only oxygen atmosphere pressureless sintering process. To address this issue, this work proposed an optimal low-cost two-step oxygen atmosphere sintering process with a first step at high temperature (T1) for minutes, followed by a second step at lower temperature (T2) for hours without applied pressure. High transmittance of 70 % in the near-infrared wavelength was achieved by our novel two-step process, which far exceeds the performance of PMN–PT ceramics prepared by only oxygen atmosphere sintering process reported in the past and demonstrates comparable performance compared to the costly hot-pressing sintering process. Meanwhile, it shows a high quadratic electro-optic (EO) coefficient of 42.1 × 10−16 m2 V−2, which is meaningful for applications in EO devices. Furthermore, the effect of the temperature difference between the first and second sintering step on the microstructure, transmittance, domain structure, ferroelectric and dielectric property, and polarization switching were discussed. The enhanced transparency is mainly attributed to the higher relative density, and weaker photoluminescence effect with lesser light absorption loss at optimal T1-T2. The improved EO property is due to greater polarization response originating from the interaction of defect dipoles and nanodomains. Such a process should facilitate the cost-effective preparation of other transparent electro-optic materials for practical applications.

Defect engineering in rare-earth-doped SrTiO3 ceramics: Route to colossal capacitance material up to X9R capacitor standard

With the advent of the electronic information era, capacitors have become a hot research topic due to their economic and strategic significance. Such microelectronic components play an irreplaceable role in important fields such as scientific research and military industries. Therefore, there is an urgent need to develop dielectric materials capable of withstanding more demanding conditions. In this work, a defect engineering method was incorporating Tm3+ rare earth ions into strontium titanate were proposed, resulting in a colossal dielectric permittivity, low dielectric loss, and excellent frequency and temperature stability. For the sample with x=0.01, the dielectric permittivity exceeds 345,804, the dielectric loss is below 0.011, and the dielectric permittivity variation within the temperature range from −55–200°C is less than 15%, which meets the ELA X9R capacitor standard (X: −55 °C, 9: +200 °C, R: Δεr ≤ ± 15%). The excellent performance is ascribed to the electron pinning defect dipole effect (EPDD) and internal barrier layer capacitor effect (IBLC), which is verified by XPS and impedance spectra. A deeper understanding of these mechanisms could help to reveal the origin of the colossal capacitance and thus contribute to the design of materials with colossal capacitance.

Reversible charge injection in artificially created charged domain wall region

The impact of electric field application in the charged domain wall (DW) region on the performance of DW devices remains elusive. Here, we present our observations of substantial charge injection occurring in an artificially created charged DW region. Rapid charge injection coincides with polarization switching, contributing to the transient current collected for the integration of polarization-voltage hysteresis loops. Subsequently, a gradual injection of charges persists under externally applied voltages, diffusing into the thin film from the DW region to screen the space defect dipoles and effectively suppressing the imprint field. The process of charge injection proves reversible and is notably more pronounced in aged ferroelectric thin films, as evidenced in planar BiFeO3 nanodevices where the charge injection amount is five orders of magnitude higher than charges generated from polarization switching. This work constitutes a significant contribution to understanding the intricate process of charge injection and its influence on charged DW.

Nanoscale structural heterogeneity enhanced temperature-stable energy storage in Bi0.5Na0.5TiO3-based relaxor ceramics via domain and defect dipoles engineering

Ceramic-based dielectric capacitors are showing great potential in advanced high-power applications. However, simultaneous achievement of high energy density (Wrec) and temperature stability is still a main bottleneck. Herein, an extraordinary nanoscale structural heterogeneity is developed to promote temperature-stable energy storage in Er/Mn co-doped Bi0.5Na0.5TiO3 based ceramics via domain and defect dipoles engineering. The composition-driven domain reconstruction by Er/Mn co-doping can induce internal random fields with rapid response to the electric field, and thus enhance ΔP and η. Meanwhile, the donor-accpetor Er/Mn doping could manipulate defect dipoles coupling to resist the applied field evolution to elevate BDS and η. Besides, the improved relaxor feature and core-shell structure give rise to the enhanced temperature stability. Ceramics with 2 mol % Er2O3 demonstrate high Wrec ∼2.79 J/cm3, large efficiency ∼89.4% and outstanding dielectric stability within 70–500 °C. The proposed nanostructural heterogeneity was verified as a promising pathway to manufact temperature-stable energy storage ceramics.

Excellent temperature stability of piezoelectric properties in PbNb2O6‐based ceramics up to 450°C

AbstractDense (Pb0.92Ba0.08)Nb2O6‐xmol% MnO2 with 0.25 wt% TiO2 ceramics with a single orthorhombic phase were synthesized through a conventional solid‐state reaction method. Excellent piezoelectric performance (d33 = 90 pC/N) and low dielectric loss (tan δ = 0.5%) are achieved in Mn‐doped samples. Interestingly, the dielectric anomalies are observed in the temperature range of 300–400°C, which could be completely suppressed through Mn doping. The dielectric anomalies are associated with thermally activated domain wall motion, which could be constrained by the defect dipoles produced by Mn doping. More importantly, all samples experience about 12% performance degradation below 300°C, attributed to an accelerated aging process at elevated temperatures. After thermal aging, the samples exhibit excellent performance stability within the temperature range of 25–450°C, showing no further degradation. The results indicate that the PbNb2O6‐based ceramics are promising high‐temperature piezoelectric materials for piezoelectric devices operating at temperatures up to 450°C.

Optimization of energy storage performance in 0.8Na0.5Bi0.5TiO3-0.2Bi3.25La0.75Ti3O12 thin films via defect dipoles

Dielectric capacitors put forward higher requirements on the energy storage density, working stability and fatigue resistance of materials with the increasing shortage of energy. Herein, 0.8Na0.5Bi0.5TiO3-0.2Bi3.25La0.75Ti3O12-x%Mn (x = 0,2,4,6) thin film capacitors are designed to address above concern. The Mn ions inhibit the valence change of Ti ions and combine with oxygen vacancies to form defect dipoles, which improve breakdown strength and energy density of the films. As a result, high energy storage density of 92.7 J cm−3 and excellent efficiency of 70.2% are synchronously achieved for the 4% Mn-doped NBT-BLT film. Besides, outstanding stability of energy storage performance also exhibit over a wide range of temperature (25–200 °C) and cycle number (105). This result provides a novel strategy for the preparation of dielectric materials with high energy storage performance and high operational stability.

Giant dielectric properties and temperature stability of triple–doped AlxIn0.05−xTa0.05Ti0.9O2 ceramics

AlxIn0.05−xTa0.05Ti0.9O2 ceramics with x ranging from 0 to 0.05 were synthesized using a conventional solid–state reaction method. All samples achieved a pure rutile–TiO2 phase. Additionally, the lattice parameters of the samples increased with higher Al doping concentrations. The ceramics exhibited a dense microstructure and uniform dispersion of dopants. Raman spectroscopy revealed a decrease in oxygen vacancies in the structure with increasing Al3+ concentration. The presence of Ti3+ in the structure was confirmed by X–ray photoelectron spectroscopy. The dielectric properties of all samples were investigated as functions of frequency and temperature. The triple–doped AITTO ceramics demonstrated a high dielectric constant (ε′) of ∼2×104 and low dielectric loss (tanδ) ranging from 0.029 to 0.04, along with excellent temperature stability of ε′ at 1 kHz, showing a Δε′/ε′30 of less than ±15% over a temperature range of −60 to 210 °C. Notably, the tanδ of the triple–doped AITTO ceramics remained below 0.04 even at 200 °C. Impedance spectroscopy was utilized to analyze the electrical properties of the grains and grain boundaries. The observed dielectric properties are attributed to the formation of electron–pinned defect–dipoles, along with the impacts of both internal and surface barrier layer capacitors.