Lead-based Relaxor Ferroelectrics Research Articles

Local displacive correlation in the tetragonal relaxor ferroelectric Pb(Zn1/3Nb2/3)O3-0.15PbTiO3

Over the past decades, lead-based relaxor ferroelectrics have served as the model systems to unravel the relationship between electromechanical properties and local structure. Here, by employing pair distribution function analysis and the reverse Monte Carlo method, we investigate the local structural features and their temperature dependence in relaxor ferroelectric Pb(Zn1/3Nb2/3)O3-xPbTiO3 with x = 0.15 (PZN-15PT), which has a tetragonal average structure, but displays frequency dependence at low temperatures. The refined atomic model suggests that ordered polar nanoregions (PNRs) originate from the strong atomic displacive correlation, and their polar directions are the same as the crystallographic symmetry axis. The dipoles in the disordered matrix have weak correlations and can be averaged into a global tetragonal symmetry. These findings establish a detailed picture of the local structure of lead-based relaxor ferroelectrics and provide a deeper insight into the nature of PNRs.

Evolving Differentiated Local Polar Displacement and Relaxor Behavior in Pb(Mg1/3Nb2/3)O3–PbTiO3 Perovskites

Lead-based relaxor ferroelectrics feature exceptional dielectric and electromechanical properties. The inherent local structural inhomogeneity remains poorly characterized and impedes the understanding of their unique electrical behavior. Herein, the local structure of a technologically important 0.75Pb(Mg1/3Nb2/3)O3–0.25PbTiO3 relaxor is studied as a function of temperature, by employing neutron total scattering combined with newly advanced reverse Monte Carlo modeling. The statistical analysis of local cationic polar displacement vectors demonstrates that a prevailing order/disorder feature and strong displacive behavior are observed in Pb and Ti, but both weak components are observed in Nb/Mg. This leads to trifurcated polar behavior across the feature temperatures. Importantly, these distinct cationic polar vectors are correlated and present a microscopic picture, where sporadic correlated orthorhombic polar clusters are embedded in a long-range rhombohedral polar matrix at low temperature. Their fraction extends with increasing temperature, presenting strong ferroelectricity with dielectric relaxation macroscopically. Furthermore, the long-range rhombohedral polar ordering is disrupted by temperature, forming free-oriented polar clusters with reducing electric dipole length, gradually losing their macroscopic polarization, and resulting in a diffuse phase transition behavior. Altogether, this offers a fundamental basis in terms of local polar fluctuations in inferring the nature of the unique properties of relaxor ferroelectrics.

Designing Monoclinic Heterophase Coexistence for the Enhanced Piezoelectric Performance in Ternary Lead-Based Relaxor Ferroelectrics.

Enhanced piezoelectric, dielectric properties and thermal stability in ternary relaxor-PbTiO3 based ferroelectric crystals are expected to develop the next-generation of electromechanical devices. However, due to their increased disorder compared to other ferroelectrics, designing a controllable phase boundary structure and engineered domain remains a challenging task. Here, we construct a monoclinic heterophase coexisting in a ternary Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystal with optimized composition and an ultrahigh piezoelectric coefficient of 1400 pC N-1, to quantify the correlation between spontaneous nanopolarity and phase heterogeneity, in an attempt to understand the origin of the exceptional functionalities. By designing an in situ high-resolution spectroscopic-microscopic technique, we have observed Ma and Mc heterophase mixtures spatially separated by the monoclinic heterophase boundary (MHB), which are responsible for the ferroelectric-dominated and relaxor-ferroelectric-dominated nanodomain structure, respectively. Internal energy mapping from optical soft mode dynamics reveals the inhomogeneous polarization and local symmetry on both sides of the MHB. Various molecular polarizabilities and localized octahedral distortions correlate directly with monoclinic regions and electromechanical contribution. This work clarifies the heterogeneity between structure, energy, and polar order and provides a new design freedom for advanced relaxor ferroelectrics.

Atomic-resolution electron microscopy of nanoscale local structure in lead-based relaxor ferroelectrics

Relaxor ferroelectrics, which can exhibit exceptional electromechanical coupling, are some of the most important functional materials, with applications ranging from ultrasound imaging to actuators. Since their discovery, their complex nanoscale chemical and structural heterogeneity has made the origins of their electromechanical properties extremely difficult to understand. Here, we employ aberration-corrected scanning transmission electron microscopy to quantify various types of nanoscale heterogeneities and their connection to local polarization in the prototypical relaxor ferroelectric system Pb(Mg1/3Nb2/3)O3-PbTiO3. We identify three main contributions that each depend on Ti content: chemical order, oxygen octahedral tilt and oxygen octahedral distortion. These heterogeneities are found to be spatially correlated with low-angle polar domain walls, indicating their role in disrupting long-range polarization and leading to nanoscale domain formation and the relaxor response. We further locate nanoscale regions of monoclinic-like distortion that correlate directly with Ti content and electromechanical performance. Through this approach, the connections between chemical heterogeneity, structural heterogeneity and local polarization are revealed, validating models that are needed to develop the next generation of relaxor ferroelectrics.

Computational Studies of Lead-based Relaxor Ferroelectrics

Relaxor ferroelectrics have been a focus of intense attention due to their fascinating physical properties. Their diffuse phase transitions have been explained by the polar nanoregion model. Nevertheless, fundamental characterization of structure and dynamics in relaxors is still a long-standing challenge. Better scientific understanding of the microscopic origins of relaxor behavior is also required to improve efficiencies of relaxor based devices. Our molecular dynamics studies in 0.75PbMg1/3Nb2/3O3-0.25PbTiO3 showed good agreement with experimental data and revealed conflicts with the current polar nanoregion model. Here, we review our work and propose an alternate model for structure and dynamics in the relaxor phase.

Local Ordering in Lead-Based Relaxor Ferroelectrics

Lead-based ferroelectric materials are both well-studied and widely used and have a wide range of applications from ultrasonics to energy harvesting and beyond. However, the use of Pb-containing materials is environmentally undesirable, due to the toxicity of lead. This is particularly highlighted by the disposal of Pb-based devices when their lifespan is through. Because of this large drawback, chemists have been searching for Pb-free ferroic materials that can replace PZN (PbZn1/3Nb2/3O3), PMN (PbMg1/3Nb2/3O3), PZT (PbZr1-xTixO3), and all their derivatives. Underlying much of materials chemistry is the idea that function arises from structure, so if we can determine the structure of a material, we can understand how its useful properties arise. This understanding can then lead to the tuning of these properties and the development of new materials. However, the question arises: What is meant by structure? Conventionally, structure is determined by X-ray or neutron diffraction, in which the Bragg peak intensities are measured and a unit cell is determined. In many materials, local ordering, order that persists only for few unit cells or nanometers, is important in determining the physical properties. This is very much the case in the relaxor ferroelectrics, an important class of functional oxides. Indeed, disorder, randomness, and short-range order (SRO) are all invoked to help explain many of the key properties. The local order in Pb-based ferroelectrics has been extensively studied, with the most definitive probe being single-crystal diffuse scattering. In this Account, I outline the current debate on the nature of the local order and explore how this information can inform the search for lead-free materials. Local order, as distinct from the overall average order revealed by conventional techniques, relates more closely to the crystal chemistry of the individual ions and so appears to give a better insight into how the crystal chemistry leads to the ferroelectric properties.

The structural state of lead-based relaxor ferroelectrics under pressure

The exceptional properties of lead-based perovskite-type (ABO(3)) relaxor ferroelectrics are due to their structural inhomogeneities. At ambient conditions, the average structure is pseudocubic but rich in ferroic nanoregions too small to be directly studied by conventional diffraction analysis. However, combining in situ temperature and pressure diffraction and Raman scattering allows us to resolve the structural complexity of relaxors. Because of the different length and time scales of sensitivity, diffraction probes the long-range order, i.e., the structure averaged over time and space, whereas Raman spectroscopy can detect local structural deviations from the average structure via the anomalous Raman activity of the phonon modes that, when the symmetry of the average structure is considered, should not generate Raman peaks. Hence, the combined analysis of the long-range order induced at low temperatures or high pressures and of the phonon anomalies enhanced on temperature decrease or pressure increase can reveal the energetically preferred structural nanoclusters existing at ambient conditions. In this regard, high-pressure experiments are vital for understanding the nanoscale structure of relaxors. Using X-ray diffraction, neutron diffraction, and Raman scattering on stoichiometric and doped PbSc(0.5)Ta(0.5)O(3) and PbSc(0.5)Na(0.5)O(3), we demonstrate the existence of a pressure-induced cubic-to-rhombohedral continuous phase transition. The high-pressure structure has suppressed polar shifts of B-site cations, enhanced correlation of Pb-O ferroic species, and long-range ordered antiphase BO(6) octahedral tilts. The critical pressure is preceded by an intermediate pressure at which the coupling between off-centered Pb and B-cations is suppressed and octahedral tilting detectable by neutron diffraction is developed.

Pressure-induced phase transformations in lead-based relaxor ferroelectrics

Pressure-induced phase transformations in lead-based relaxor ferroelectrics

Intermediate temperature scaleT∗in lead-based relaxor systems

Via a combination of various experimental and theoretical techniques, a peculiar, identical temperature scale T* is found to exist in all complex lead-based relaxor ferroelectrics. T* corresponds to a nanoscale phase transition due to random fields. Interestingly, T* also exists in other oxides with extraordinary properties, such as giant magnetoresistivity or superconductivity. By analogy with such latter systems, the giant piezoelectricity related to relaxors might originate from proximity competing states effect.

Soft mode dispersion and ‘waterfall’ phenomenon in relaxors revisited

Results of recent inelastic neutron scattering studies of lead-based relaxor ferroelectrics by Gvasaliya et al. [J. Phys.: Condens. Matter 17, 4343 (2005); J. Phys.: Condens. Matter 19, 016219 (2007)] have put in question the existence of the “waterfall” anomaly–an apparent vertical dispersion segment joining the TA and TO branches–observed earlier in low-energy [ξ00] phonon dispersion curves of these materials. In the present article, we review the results of earlier experiments and model calculations together with the outcome of our recent measurements on PMN using the same instrumental set-up as Gvasaliya et al. to conclude that the “waterfall” feature is not an experimental artefact. We also give some hints on a possible explanation of the results of Gvasaliya et al., by exploring the fact that the reported dispersion of the underdamped transverse optic branch follows the longitudinal acoustic (LA) branch dispersion surprisingly closely.

Direct Current‐Voltage Failure of Lead‐Based Relaxor Ferroelectrics with Silver Doping

The effect of silver doping on the DC‐voltage resistance failure of lead‐based relaxor ferroelectrics was investigated via temperature‐humidity‐bias (THB) testing, scanning electron microscopy, X‐ray diffraction spectroscopy, and electrical measurements. The failure rate of silver‐doped specimens was found to increase significantly with the doping level during the THB test. However, some degraded specimens can partially recover their electrical properties after a few days of storing in natural conditions. X‐ray diffraction analysis showed that silver could be incorporated into the perovskite lattice in the range of silver contents studied. The presence of an inner‐bias field in the degraded ceramics was first demonstrated through hysteresis property measurement. Based on these results, it was inferred that the accumulation of oxygen vacancies under DC‐voltage should be responsible for the inner‐bias field, which consequently resulted in the increase of electronic defects in the ceramics.

Interface and ionic interdiffusion in cofired ferroelectric/ferrite multilayer composites

Recently, the surface mount technology (SMT) has been rapidly developed for miniaturization of electric devices such as multilayer ceramic capacitor (MLCC) and multilayer chip inductor (MLCI). The trend of electronics has been for an increasingly compact design. Some characterizations of cofired devices such as ceramic-filled glass electronic package, and varistorcapacitor cofired multilayer device have been reported [1, 2]. Multilayer chip LC filter, which is combined with several capacitors and inductors by the production process of multilayer chip components, is a type of advanced surface mount devices (SMD). Lead based relaxor ferroelectrics, such as Pb(Mg1/3Nb2/3)O3, Pb(Ni1/3Nb2/3)O3 and Pb(Zn1/3Nb2/3)O3, are being widely used as MLCC due to their high dielectric constant and low sintering temperature [3–5]. And NiCuZn ferrite, which has excellent magnetic permeability and can be densified at low temperature, is an important material using for producing low temperature sintered MLCI [6–8]. They are promising materials for the multilayer chip LC filters. The key issue of manufacturing the multilayer chip LC filters is cofiring the capacitor and inductors together at low temperature. The interfacial interactions such as chemical reaction, ionic interdiffusion, are important for the production of devices. In the present study, the multilayer components were prepared by cofiring Pb(Ni1/3Nb2/3)O3-PbTiO3 (abbreviated as PNNT) ferroelectrics and NiCuZn ferrite. The cofiring interface and ionic interdiffusion between constituents were investigated. According to the diffusioncouple model, the interfacial diffusion coefficient for Pb2+, Nb5+, Ni2+, and Fe3+ had been estimated. The specimens were prepared by the conventional oxide-mixing technique. 99% pure reagent-grade PbO, NiO, Nb2O5, TiO2, Fe2O3, CuO, and ZnO oxides were weighed in the appropriate proportions and used as starting materials. Fe2O3, NiO, CuO and ZnO powders, according to the composition (Ni0.8Cu0.1Zn0.1)Fe2O4, were calcined at 720 ◦C for 4 h to fabricate NiCuZn ferrite. The dielectric material, according to the composition 0.8 Pb(Ni1/3Nb2/3)O3-0.2PbTiO3 with little amount of CuO and ZnO, were calcined at 800 ◦C for 4 h. The calcined mixtures were ground by ball milling for 12 h to a proper surface area. The ground powders were dried and added to PVA for the disk bodies. Firstly, ferroelectic and ferrite disks were separately prepared and sintered at 850–950 ◦C for 2 h, in order to measure characteristics such as phase structure and sintered densities. Secondly, the mixed calcined powders of ferroelectric and ferrite were laminated into a mold and compressed into a plate with the diameter of 12 mm, which was sintered at 950 ◦C for 2 h. The sintered densities were measured by the Archimedes method. The phase structure for the specimens were identified using X-ray diffractometer (XRD). For lead-based ferroelectrics, perovskite phase and pyrochlore phase always coexist. The relative amounts of perovskite and pyrochlore phases were determined by measuring the major X-ray peak intensities for perovskite and pyrochlore phases, i.e. (110) and (222), respectively. The percentage of perovskite phase was calculated by the following equation [9]:

Interfacial Mismatch and Interface Structure of Epitaxial Pb(Mg1/3Nb2/3)O3 (90%)- PbTiO3 (10%) Relaxor Thin Films

Abstract Pb(Mg1/3Nb2/3)O3 (PMN)- and its solid solution with PbTiO3 (PT) is one of the lead-based relaxor ferroelectrics and has been the most widely studied materials because of their high dielectric constants and high electrostrictive coefficients. The potential impact of the thin film ferroelectric relaxors in the integrated actuators and sensing applications has stimulated research on the growth and characterization of thin films. Thin films have been made by pulsed-laser deposition (PLD), sol-gel and metalrganic chemical-vapor deposition. It is known that electrical properties may be strongly influenced by the microstructure of films and the interface structures between different phase in such heterostructure systems. In this paper, we report the investigation of interfacial mismatch and interface structure of epitaxial Pb(Mg1/3Nb2/3)O3 (90%)- PbTiO3 (10%) relaxor thin film by high resolution transmission electron microscopy (HRTEM). Thin film capacitors of Pb(Mg1/3Nb2/3)O3 (90%) - PbTiO3 (10%) (PMN-PT) were grown by PLD on (100)-oriented LaA1O3 (LAO) substrates. La0.5Sr0.5CoO3 (LSCO) layer was deposited as electrode. Cross-sectional transmission electron microscopy samples were prepared following the traditional procedures including cutting, gluing, polishing and ion milling.

Microwave processing and properties of ceramics with different dielectric loss

Microwave sintering behaviors of three kinds of ceramics with different dielectric loss [Al 2O 3, Ce–Y–ZrO 2 and lead-based relaxor ferroelectrics (PMZNT)] in 2·45 GHz microwave furnace were described. Measurement of sample densities showed an enhancement of the sintering processing for all materials studied. For PMZNT and Ce–Y–ZrO 2 with high dipolar loss or ionic conductive loss, the associated microstructure examined using scanning electron microscopy showed that microwave-sintered compacts produced much finer grain sizes at near theoretical density compared to conventional sintering. Resulting material properties, such as flexure strength and breakdown strength, were also increased due to developed microstructure in microwave processing. However, a comparable grain size and properties were observed for high pure Al 2O 3 with low dielectric loss in microwave and conventional methods. ©

Preparation and dielectric properties for temperature-stable Pb(Ni1 /3Nb2/3)O3-based composite ceramics

Many lead-based relaxor ferroelectrics have been developed for multilayer ceramics (MLCs) capacitors because of their excellent dielectric properties and low sintering temperature compared with BaTiO3based dielectric materials [1]. Despite these advantages in lead-based relaxor materials, temperature dependence of capacitance is relatively large, which is unfavorable to temperature-stable MLCs. Several attempts have been made to improve the temperature dependence [2±4]. Although mixed sintering of two components with different Curie temperatures is one of the methods [2±5], the two mixed lead-based constituents can very easily react to form a solid solution in the sintering process. Inhibiting the solution of two different constituents is an important problem to address. Pb(Zn1=3Nb2=3)O3-based biphasic dielectric ceramics that satisfy X7R (nC=C , 15%, ±55 to 125 8C) speci®cations of the Electronic Industries Association (EIA) standards have been prepared in our laboratory by the SiO2 ±B2O3 gel glass-coating technique [6]. A SiO2 ±B2O3 gel glass layer is useful for inhibiting the reaction between two PZN-based constituents in PZN-based composite ceramics. In preparing Pb(Ni1=3Nb2=3)O3-based composite ceramics in a Pb(Ni1=3Nb2=3)O3 ±PbTiO3 system, we also used the SiO2-B2O3 gel glass-coating technique, although it failed. To gain temperature-stable PNNbased composite ceramics, the PbTiO3 particlecoating method was used. PNN-based composite ceramics with X7R characteristics were obtained successfully. The preparation of composite ceramics and their dielectric properties are reported in this letter. Two solid solutions with different transition temperatures, y60 8C and 100 8C, were chosen in the Pb(Ni1=3Nb2=3)O3-PbTiO3 system as the lowtemperature constituent (LTC) and high-temperature constituent (HTC), respectively. Reagent grade PbO, NiO, Nb2O5, TiO2 were used. The Columbite method was employed to form NiNb2O6 by calcination of NiO and Nb2O5 at 1000 8C for 6 h. LTC and HTC were synthesized from PbO; NiNb2O6 and TiO2 powders by the conventional solid-state reaction method. The procedure for formation of PbTiO3 coating on PNN powder particles was as follows. First, the precursor for PT was prepared. Lead acetate was added to ice acetic acid, which was used as solvent. The mixture was heated, and the lead acetate was dissolved. Titanium-butoxide was mixed with acetylacetone at the same time, adding an amount of ethanol that was used as diluent to make the reaction complete. Two kinds of solutions were mixed by stirring, and the resulting solution was brown and transparent. Second, a weighed quantity of pulverized powder was mixed with PT precursor to obtain a slurry, and the mixture was blended for 4 h. After being dried, the powder was calcined at 550 8C for 1 h. The coated HTC and LTC powders were mixed following xLTCy(1y x)HTC with different (x ˆ 0:3, 0.4, 0.5) values, while at the same time, a small amount of sintering aid was added. The resultant powders were pressed using PVA binder into pellets 15 mm in diameter and 2 to 3 mm in thickness. The pellets were then ®red at 1000±1050 8C for 1±2 h in a sealed alumina crucible. To compensate for the PbO loss from pellets, a PbO-rich atmosphere was maintained by placing an equimolar mixture of PbO and ZrO2 inside the crucible. For dielectric measurements, silver paste was painted onto both sides of the pellets and ®red at 550 8C for 15 min as electrodes. Dielectric measurements were conducted on an automated system, where a temperature box and an HP4274A LCR meter were controlled by a computer. Dielectric properties were measured at 1, 10 and 100 kHz, in the temperature range of y80 to 160 8C with a heating rate of 3 8C miny1. The dielectric constants versus temperature behavior for LTC and HTC are shown in Fig. 1. The Curie temperature of LTC is y60 8C and that of HTC is 100 8C. Both exhibit relaxor ferroelectric characteristics and have a dielectric constant maximum of 6400 and 8000 at 1 kHz, respectively.

Synthesis, characterization, and properties of lead-based relaxor ferroelectrics

Lead-based relaxor ferroelectrics such as Pb(Mg1/3Nb2/3)O3 (PMN), Pb(Zn1/3Nb2/3)O3 (PZN), and their solid solutions with BaTiO3 and PbTiO3 have been prepared by a solution combustion process which involves metal nitrates/oxalate and tetraformal trisazine (TFTA) at 350 °C. Thermal evolution of perovskite relaxors has been investigated at different temperatures of calcination using the powder x-ray diffraction method. Particles are fine and sinter-active at low temperature (1050 °C). Both particulate and dielectric properties are compared.

Combustion synthesis and properties of relaxor materials

Several technologically important lead based relaxor ferroelectrics such as $Pb(Mg_{1/3}Nb_{2/3})O_3$ [PMN], Pb$(Ni_{1/3}Nb_{2/3})O_3$ [PNN], $Pb(Zn_{1/3}Nb_{2/3})O_3$ [PZN] and solid solutions of PMN and PZN with 10% $PbTiO_3$ [PT] have been prepared by a novel combustion method. The phases formed, density, particle size and surface area of the combustion derived powders have been determined. Dielectric properties of the relaxor ferroelectrics have been measured