Synthesis and Dielectric Properties of Solution Sol-Gel-Derived 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 Ceramics

A solution sol-gel method has been developed to prepare 0.9Pb(Mg_1/3Nb_2/3)O₃-0.1PbTiO₃ (0.9PMN-0.1PT) ceramics. During the processing the gel first converted to cubic pyrochlore phase at a calcination temperature of 600°C followed by the formation of pure perovskite phase at 775°C. The ceramics sintered at 1250°C for 4 h showed ≈98% of the theoretical density. The room-temperature dielectric constant of the pellets sintered at 1250°C showed a maximum value of 25035 at 1 kHz. Sintering studies at different temperatures revealed that the dielectric constant increased with increasing grain size in these ceramics.     

Texture Control of Sol-Gel Derived Pb(Mg1/3Nb2/3)O3–PbTiO3 Thin Films Using Seeding Layer

Lead magnesium niobium titanate (PMNT) thin films with a composition near the morphotropic phase boundary were prepared on conventional Pt(111)/Ti/SiO₂/Si substrates using a modified sol-gel process. A PbO seeding layer was introduced to the interface between the PMNT layer and the substrate to enhance the [001]-preferential orientation of the PMNT film. Single-phase perovskite PMNT films with highly [001]-preferential orientation were obtained at reduced annealing temperatures compared with the PMNT films directly deposited on the same substrates. The dielectric and ferroelectric properties of the prepared PMNT films were evaluated as a function of annealing temperature.     

Mechanochemical Synthesis of 0.9[0.6Pb(Zn1/3Nb2/3)O3·0.4Pb(Mg1/3Nb2/3)O3]·0.1PbTiO3

Lead zinc niobate–lead magnesium niobate–lead titanate (PZN–PMN–PT) ceramic powders of perovskite structure have been prepared via a mechanochemical processing route. A single-phase perovskite powder of ultrafine particles in the nanometer range was successfully synthesized when a MZN powder (columbite precursor) was mechanically activated for 10 h together with mixed lead and titanium oxides. The following steps are involved when the ternary oxide mixture is subjected to an increasing degree of mechanical activation. First, the starting materials are significantly refined in particle size as a result of the continuous deformation, fragmentation and then partially amorphized at the initial stage of mechanical activation. This is followed by the formation of perovskite nuclei and subsequent growth of these nuclei in the activated oxide matrix with increasing activation time. When calcined at various temperatures in the range of 500–800°C, pyrochlore phase was not detected by XRD phase analysis in the mechanochemically synthesized powder. Only a minor amount (∼2%) of pyrochlore phase was observed when the calcination temperature was raised to 850°C. The PZN–PMN–PT derived from the mechanochemically synthesized powder can be sintered to ∼98% relative density at a sintering temperature of 950°C. The PZN–PMN–PT sintered at 1100°C for 1 h exhibits a dielectric constant of ∼18 600 and a dielectric loss of 0.015 at the Curie temperature of 112°C when measured at a frequency of 0.1 kHz, together with a d ₃₃ value of 323 ×10⁻¹² pC/N.     

Effect of Grain Coalescence on the Abnormal Grain Growth of Pb(Mg1/3Nb2/3)O3-35 mol% PbTiO3 Ceramics

Abnormal grain growth (AGG), which occurred during the heat treatment of Pb(Mg_1/3Nb_2/3)O₃-35 mol% PbTiO₃ (PMN-35PT) with excess PbO, was investigated. AGG has been suggested to be the consequence of grain coalescence that results in the formation of Σ3 coincidence site lattice and low angle grain boundaries. Because of reentrant edges appearing at the ends of these boundaries, the coarsening rate of grains was significantly enhanced and AGG occurred.     

Abnormal Grain Growth of Pb(Mg1/3Nb2/3)O3-35 mol% PbTiO3 Ceramics Induced by the Penetration Twin

Pb(Mg_1/3Nb_2/3)O₃-35 mol% PbTiO₃ (PMN-35PT) specimens with a 5 mol% excess PbO were prepared by excessive heat treatment at 1150°C to induce abnormal grain growth. Through electron backscatter diffraction analysis and the observation of a three-dimensional morphology, the abnormally grown PMN-35PT grains were found to be twinned crystals with penetration characteristics. The morphology of the PMN-35PT twinned crystal was crystallographically analyzed. The abnormal grain growth of PMN-35PT is suggested to be due to preferential growth at the reentrant angles formed by twins.     

Kinetics of Templated Grain Growth of 0.65Pb(Mg1/3Nb2/3)O3·0.35PbTiO3

Single-crystal layers of 0.65Pb(Mg_1/3Nb_2/3)O₃·0.35PbTiO₃ (PMN-35PT) were grown heteroepitaxially on {001}-BaTiO₃ template crystals. A {001}-BaTiO₃ crystal was embedded in a fine-grained matrix of PMN-35PT containing excess PbO and heated between 950° and 1150°C for 0–5 h. The initial growth of the PMN-35PT on the {001} surface and the growth of the matrix grains both displayed a t ^1/3 dependence which is characteristic of diffusion-controlled growth. Growth was limited to ∼100–150 μm due to the significantly reduced driving force at longer times because of matrix coarsening and porosity evolution.     

Synthesis of Pb(Mg1/3Nb2/3)O3 in Excess Lead Oxide by Mechanical Activation

Twenty hours of mechanical activation of mixed oxides at room temperature led to the formation of Pb(Mg_1/3Nb_2/3)O₃ (PMN) in excess PbO. The crystallinity of the activation-derived perovskite PMN phase was further established when the activated PMN–PbO phase mixture was subjected to calcination at 800°C. Pyrochlores, such as Pb₃Nb₄O₁₃ and Pb₂Nb₂O₇, were not observed as transitional phases on mechanical activation and subsequent calcination, although 50% excess PbO was deliberately added. The perovskite PMN phase was recovered by washing off excess PbO using acetic acid solution at room temperature. It was sintered to a relative density of 98.9% of theoretical at 1200°C for 1 h and the sintered PMN exhibited a dielectric constant of ∼14 000 at 100 Hz and a Curie temperature of −11°C.     

Electromechanical Determination of the High-Field Phase Transition of Pb(Mg1/3Nb2/3)O3–PbTiO3–(Ba,Sr)TiO3 Relaxor Ferroelectrics

Ceramics in the (1 – x )[(1 – y )Pb(Mg_1/3Nb_2/3)O₃· y PbTiO₃] · x MeTiO₃ system, where Me is Sr or Ba, exhibit very large electrostrictive strains at reasonable drive fields. However, the optimum use temperature and frequency vary with the particular composition used. As relaxor ceramics, each composition has a broad transition from electrostrictive to partially piezoelectric behavior. The transition temperature ( T _t) can be roughly determined from strain or polarization properties; however, it can be more quantitatively determined from the effective electro-mechanical Q . A plot of induced transverse strain/induced polarization squared (effective Q ₁₂) as a function of temperature shows a sharp and unmistakable change in slope—this defines T _t. The slope of induced transverse strain/polarization (effective g₃₁) also shows a change in slope at T _t, although this is more gradual than that of effective Q . The indicated T _t correlates with those found from measurement of strain and polarization.     

Direct Imaging of Atomic Ordering in Undoped and La-Doped Pb(Mg1/3Nb2/3)O3

In this paper, we report the first direct observations of local ordering in undoped and La-doped Pb(Mg_1/3Nb_2/3)O₃ (PMN) on an atomic scale by high-resolution (∼1.26 Å) Z -contrast imaging. The 1:1 ordering occurs by a variation in the occupancy of Mg and Nb cations between B^I and B^II sublattices. In ordered regions, the B^II sublattice is dominantly occupied by Nb cations, but a small Mg occupancy cannot be excluded. The Mg cations were found to dominantly occupy the B^I sublattice. Within the B^I sublattice, the local Mg/Nb ratio was found to vary among the various B^I sublattices. The results show that the random-site model, rather than the space-charge model, is a better structural model for the 1:1 ordering observed in PMN. A random distribution of Mg and Nb cations on the B^I sublattice within 1:1 ordered regions is believed to be responsible for the relaxor behavior of mixed B-site cation relaxors, such as PMN.     

Analysis of Ferroelectric Domain Switching in 0.7Pb(Mg1/3Nb2/3)O3·0.3PbTiO3: Heterogeneous Nucleation

Polarization reversal and domain dynamics were investigated in 0.7Pb(Mg_1/3Nb_2/3)O₃·0.3PbTiO₃ using a method of current transients. Investigations were performed as a function of applied electric field. The kinetics of the transients were modeled to a stretched exponential-type function.     

Preparation of Dense Lead Magnesium Niobate–Lead Titanate (Pb(Mg1/3Nb2/3)O3–PbTiO3) Ceramics by Spark Plasma Sintering

The effect of spark plasma sintering (SPS) on the densification behavior of Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ ceramics has been investigated. Specimens with a density of >99% of the theoretical density (TD) were obtained using SPS treatment at 900°C. Through normal sintering at 1200°C, however, the density of the specimen was only ∼92% of TD.     

Solid-State Epitaxial Effects in Structurally Diphasic Xerogel of Pb(Mg1/3Nb2/3)O3

Lead magnesium niobate (PMN), Pb(Mg_1/3Nb_2/3)O₃, with perovskite structure has been prepared using structurally diphasic PMN gels. The diphasic gels were made using various concentrations of perovskite PMN seeds. The unseeded gel calcined at 775°C for 2 h gave ∼98% of perovskite PMN phase. The use of 1% PMN perovskite seed not only led to a pure perovskite phase but also lowered the crystallization temperature of these gels by about 75°C. These results show that isostructural seeding helps to lower the crystallization temperature of perovskite PMN phase.     

Structure Evolution from Pb(Mg1/3Nb2/3)O3 to La(Mg2/3Nb1/3)O3

The crystal structure of lanthanum-modified lead magnesium niobates having composition (Pb_{1− x} La _x ) (Mg_{(1+ x )/3}-Nb_{(2− x )/3})O₃ with X = 0 to 1 was investigated by X-ray powder diffraction. It was found that the fundamental reflections from perovskite structure remain in the whole range of composition. The superlattice reflections from the A(B'_1/2-B"_1/2)O₃ ordered structure are also well preserved for La content greater than 50 at.%; however, a series of extra peaks of mixing indices appears, with intensities gradually enhanced with the increase of La content. For the complete substitution of Pb by La, a splitting of some reflections can be observed in the diffraction pattern. The results indicate that the crystal structure evolves continuously with the La content, from disordered cubic perovskite of space group Pm 3 m for X = 0, to ordered cubic perovskite of space group Fm 3 m for X = 0.5, distorted cubic perovskite of space group Pa 3 for 0.5 < X < 0.9, and finally to a rhombohedral perovskite, possibly belonging to the space group R 3 , for X ≥ 0.9. In the evolution of structure, a linear reduction of the lattice constant of the perovskite cell from 4.048 to 3.964 Å was observed.     

Synthesis of Pyrochlore-Free 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 Ceramics via a Soft Mechanochemical Route

Single-phase perovskite 0.9Pb(Mg_1/3Nb_2/3)O₃-0.1PbTiO₃ (0.9PMN–0.1 PT) from a stoichiometric mixture of starting materials was synthesized by applying a mechanochemical technique to the stage of a precursor. A stoichiometric mixture of PbO, TiO₂, Mg(OH)₂, and Nb₂O₅ was milled for 60 min and heated at temperatures as low as 850°C for 4 h to obtain a single phase. The maximum dielectric constant of the samples from the milled mixture increased as the sintering temperature increased, with the remarkable grain growth, and attained 24600 at 1200°C. In contrast, poor densification and coexistence of the pyrochlore phase were observed on the samples from the nonmilled mixture. Further observation suggested that the pyrochlore phase concentrated near the surface during sintering and then migrated into the PbZrO₃ packing powder, leading to a pyrochlore–free phase at 1250°C. The dielectric constant of the latter ceramics was explained by the series mixing rule for the dielectric constant of a diphasic solid.     

Relaxor Behavior of the 0.9BaTiO3–0.1La(Mg1/2Ti1/2)O3 Solid Solution

Low-frequency dielectric response of air- and oxygen-sintered ceramics with the composition 0.9BaTiO₃–0.1La(Mg_1/2Ti_1/2) O₃ (0.9BT–0.1LMT) has been studied in the temperature range of 12–550 K. In comparison with pure BT, in 0.9BT–0.1LMT the dielectric permittivity maximum is shifted by almost 300 K toward lower temperatures. Both real and imaginary parts of dielectric permittivity of the solid solution, in the range 12–150 K, show a strong frequency-dependent behavior, which is typical of relaxors. On the basis of the model of exponential cluster size distribution and the Cole–Cole equation, the degree of interaction between the polar clusters was estimated. It was shown that the oxygen vacancies arising during sintering at high temperatures did not affect noticeably the relaxor properties of the material. The role of heterovalent La³⁺/Ba²⁺ and Mg²⁺/Ti⁴⁺ substitutions in the relaxor behavior formation is discussed.     

Electromechanical Properties of Some Pb (Mg1/3Nb2/3–PbTiO3–(Ba,Sr)TiO3 Ceramics: I

Compositional variation within the Pb(Mg_1/3Nb_2/3) O₃–PbTiO₃–(Ba, Sr)TiO₃ (hereafter PMN–PT–BT,ST) ternary (6.4% PT% 14.1%, 1.25% BT,ST% 2.5%) results in major changes in induced strain and hysteresis. For the 1.25% BT family, the increase in strain correlates with an increase in T _max, while the dielectric loss is uncorrelated with hysteresis and strain. In addition, weak field aging (which is not reset by application of field) shows little effect on strain and hysteresis for drive fields of > 0.2 MV/m. The vary narrow polarization-fields loops (virgin curvesnearly indistinguishable from subsequent cycles) show that weak-field permittivity is a good approximation to the high-field permittivity. is a good approximation to the high-field Permittivity. Although these data clarify the frequency ( T _max is linearly dependent on the logarithm of the frequency) effect on weak-field dielectric behavior, they do not directly address the question of meaningful extrapolation of high-field strain with frequency. In particular, the question remains as to whether the high-field permittivity and strain are frequency dependent. In future papers we will address this question by a combination of measurement techniques as functions of frequency.     

Kinetics of {001} Pb(Mg1/3Nb2/3)O3–35 mol% PbTiO3 Single Crystals Grown by Seeded Polycrystal Conversion

The kinetics of {001}-oriented Pb(Mg_1/3Nb_2/3)O₃–35 mol% PbTiO₃ (PMN–35PT) single crystals grown by seeded polycrystal conversion were systematically quantified as a function of excess PbO liquid phase. The coarsening behavior of the corresponding matrix grains was similarly quantified. Single-crystal seed plates were embedded in a matrix of PMN-35PT with varying amounts of liquid phase (PbO) content in the range of 0 to 5 vol% and annealed at 1150°C for 0–10 h. Apparent maxima in the growth rates were observed at a PbO content of ∼3 vol% for both the single crystal and matrix grains. In both cases, the growth data were found to most closely follow cubic growth kinetics. Implications regarding the effect of PbO volume fraction on the matrix and single-crystal growth mechanisms are discussed.     

Sequential Combination of Constituent Oxides in the Synthesis of Pb(Fe1/2Nb1/2)O3by Mechanical Activation

Pb(Fe_1/2Nb_1/2)O₃(PFN) has been successfully synthesized via a novel mechanical activation of mixed oxides and columbite precursor consisting of lead oxide and FeNbO₄. A nanocrystalline perovskite phase 5–15 nm in crystallite size was formed after 30 h of mechanical activation at room temperature for both types of starting materials. However, the nanocrystalline PFN phase derived from the mixed oxides of PbO, Fe₂O₃, and Nb₂O₅is unstable, and develops pyrochlore phases when calcined at 500°–900°C, while no pyrochlore phase is observed for the material derived from the columbite precursor consisting of PbO and FeNbO₅. Different sintering behavior and dielectric properties were also observed between the two types of PFN. These differences are accounted for by the compositional inhomogeneity in the material derived from the mixed oxides, as was revealed by Raman spectroscopic studies. This suggests that mechanical activation is analogous to thermal activation, where the phase development is strongly dependent on the sequence of combining the constituent oxides.