Rhombohedral-tetragonal Morphotropic Phase Boundary Research Articles

Pressure-induced morphotropic phase boundary and critical points in Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 at room temperature

We find rhombohedral-tetragonal morphotropic phase boundary showing high piezoelectricity in relaxor ferroelectric Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 subjected to hydrostatic pressure based on a Landau-Ginzburg-Devonshire phenomenological modeling. Room-temperature Curie transitions driven by moderate pressures were investigated by examining evolution of free energy landscape and equations for determining critical pressures are derived. Monoclinic state was induced when applying [111]-oriented electric field to originally metastable rhombohedral state coexisting with the pressure-stabilized tetragonal state within a narrow pressure window. High piezoelectricity found at the morphotropic phase boundary can be further enhanced under electric field.

Structural Refinement, Dielectric and Electromechanical Properties of (1-x)NBT-xKBT Piezoceramics in the Morphotropic Phase Boundary Region

Lead-free ferroelectric materials of sodium-potassium bismuth titanate, (1-x)NBT-xKBT systems were synthesized by a hydrothermal process. In this way, the appropriate conditions for the hydrothermal synthesis of NBT and KBT (i.e., concentrations of synthetic precursors, solution pH and temperature) are given graphically. Ceramics of (1-x)NBT-xKBT with (x(mol.%) = 0; 12; 16; 20; 30 and 100) were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The rhombohedral-tetragonal morphotropic phase boundary (MPB) was confirmed to be in the region of 0.12 ≤ x ≤ 0.20 for (1-x)NBT-xKBT at ambient temperature. Scherrer's formula and the Williamson-Hall (W-H) analysis were used to examine the average crystallite size and lattice strain. Raman spectroscopy was effectively applied to study the structural evolution of the (1-x)NBT-xKBT phase. The ceramics exhibited a high temperature of maximum dielectric permittivity at (Tmax = 343 °C at 100 kHz) along with electromechanical coupling factors (kp = 0.34, d33 = 147 pC/N). Based on the composition of all specimens, the results indicate a diffuse phase transition, probably of second order, between ferroelectric and paraelectric phases.

Structural, dielectric and piezoelectric characterization of BFN-modified PZT-based (MPB) ceramics

ABSTRACT The solid solution of xBi(Fe1/3Nb2/3)O3–(1-x)Pb(Zr0.50Ti0.50)O3 ceramics near morphotropic phase boundary (MPB) (BFN–PZT, x = 0, 0.0025, 0.005, 0.0075, 0.01) were prepared by conventional solid-state reaction method. The samples added with different Bi2O3, Fe2O3, Nb2O5 contents were compared in terms of the phase structure, microstructure, dielectric properties, and piezoelectric properties. In particular, rhombohedral-tetragonal morphotropic phase boundary was observed in BFN–PZT crystal structure near the composition of x = 0.005. The BFN–PZT ceramics exhibited the following optimal properties at x = 0.005: dielectric constant ϵr of 847, dielectric loss tanδ of 0.034, Curie temperature TC of 390°C, electrical conductivity σ of 0.441 (Mohm.m)−1, piezoelectric charge constant d33 of 416 pC/N, electromechanical coupling factor KP of 0.654, and mechanical quality factor Qm of 426, making it a promising material for use in high-intensity ultrasound applications.

Giant piezoelectric coefficient of PNN-PZT-based relaxor piezoelectric ceramics by constructing an R-T MPB

xTa-PNN-PZT piezoelectric ceramics with [0.55 Pb(Ni1/3Nb2/3)O3 - 0.45 Pb(Zr0.3Ti0.7)O3 - xwt%Ta2O5] + 1 wt% PbO composition (0 ≤ x ≤ 0.7) were prepared by conventional solid-state reaction method. Special attention was paid to the effect of Ta2O5 concentration on structural, dielectric, piezoelectric, and ferroelectric properties of these ceramics. In particular, rhombohedral-tetragonal morphotropic phase boundary was observed in PNN-PZT crystal structure near the composition of x = 0.5. At this point, volume fraction of rhombohedral phase was almost equal to that of tetragonal phase. In addition, the ceramic exhibited the optimum values of piezoelectric coefficient, bipolar strain, unipolar strain, and inverse piezoelectric coefficient, i.e., 1090 pC/N, 0.135%, 0.165%, and 1493 pm/V, respectively. Moreover, according to in-situ X-ray diffraction, piezoelectric force microscopy, and in-situ strain measurement results, phase structure of 0.5Ta-PNN-PZT revealed relatively stable piezoelectric behavior at 60 °C.

Bi(Zn0.5Ti0.5)O3 induced domain evolution and its effect on electrical property and thermal stability of 0.8Bi0.5Na0.5TiO3-0.2Bi0.5K0.5TiO3 ceramics

0.8Na0.5Bi0.5TiO3-(0.2-x)K0.5Bi0.5TiO3-xBi(Zn0.5Ti0.5)O3 (NBT-KBT-BZT, x = 0–0.05 with interval of 0.01) lead-free piezoceramics were prepared and investigated. The addition of BZT has not led to deviation of rhombohedral-tetragonal morphotropic phase boundary, but apparently composition-dependent electrical property and monotonously improved thermal stability. The x = 0.02 has the optimal electrical property with remnant polarization, bipolar total strain, piezoelectric constant of 26.5 μC/cm2, 0.30%, 121 pC/N respectively, while the depolarization temperature is increased from 93 °C for x = 0–135 °C for x = 0.05 based on dielectric measurement. Detailed investigation reveals that the domain structure transforms from labyrinthian domain for x = 0 to coexistence of nanodomain and labyrinthian domain for x = 0.02, end up with large amount of nanodomain (x = 0.04). Such domain evolution should be responsible for the composition dependent electrical property and improved thermal stability. Our work confirms that domain engineering is an alternative method to develop high performance piezoelectric materials.

Intrinsic and extrinsic electric response in Bi0.5Na0.5TiO3- BaTiO3 lead-free piezoceramics

(Bi0.5Na0.5)TiO3-BaTiO3 (BNT-BT) lead-free piezoelectric ceramics with compositions near the rhombohedral-tetragonal morphotropic phase boundary (MPB) have been prepared and investigated. In this work, the best properties was observed in BNT-0.07BT, with the piezoelectric coefficient d33 and the planar electromechanical coupling factor kp being 151 pC/N and 0.278, respectively. The dielectric response of the poled (Bi0.5Na0.5)TiO3-xBaTiO3 ceramics were investigated as a function of composition and temperature using Rayleigh analysis, dielectric Rayleigh coefficient α and initial dielectric constant εinit increase with x increasing, and has a substantial increase in the tetragonal phase. Detailed analysis of the temperature dependence of α and εinit curves of all compositions revealed different temperature in the maximum value due to the dominate contributions of domain wall motion.

Analytical calculation of saturated strain near morphotropic phase boundary of polycrystalline ferroelectrics by the generalized inverse-pole-figure model

The generalized inverse-pole-figure (IPF) model has been applied to analyze the saturated strain in polycrystalline ferroelectrics including composition near morphotropic phase boundary (MPB). Saturated strains of single-phase polycrystalline ferroelectrics with tetragonal and rhombohedral symmetry have been analytically calculated and the results have been confirmed by comparison with the results from preceding studies. In addition, the general formula to analyze relative saturated strain near MPBs of different multiple-phase polycrystalline ferroelectrics has been derived and the saturated strain near tetragonal-rhombohedral MPB has been calculated.

Pseudocubic-based polymorphic phase boundary structures and their effect on the piezoelectric properties of (Li,Na,K)(Nb,Sb)O3-SrZrO3 lead-free ceramics

CuO-added 0.96(LixNa0.5-xK0.5)(Nb1-ySby)O3-0.04SrZrO3 ceramics were sintered at 1020 °C for 6 h. Various crystal structures were synthesized in these specimens by controlling the Li2CO3 (x) and Sb2O5 (y) contents: pseudocubic, orthorhombic-pseudocubic polymorphic phase boundary (PPB), tetragonal-pseudocubic PPB, orthorhombic-tetragonal-pseudocubic PPB, and orthorhombic-tetragonal PPB structures. The pseudocubic structure developed in these specimens was similar to the R3m rhombohedral structure instead of the Pm3m cubic structure because the specimens with a pure pseudocubic structure showed good ferroelectric and piezoelectric properties. The piezoelectric properties of the specimens were influenced by their crystal structures. The specimen with the tetragonal-pseudocubic PPB structure showed the best piezoelectric properties because this structure was similar to the tetragonal-rhombohedral morphotropic phase boundary structure developed in Pb(Zr1-xTix)O3-based ceramics. In particular, the specimen with the tetragonal-pseudocubic PPB structure corresponding to x = 0.05 and y = 0.065 showed the largest d33 and kp values of 431 pC/N and 0.43, respectively.

Morphotropic phase boundary, field-induced transitions and dielectric properties of PbxSr1-xTiO3

A low-temperature rhombohedral-tetragonal morphotropic phase boundary (MPB) is discovered using a phenomenological approach in PbxSr1-xTiO3. Monoclinic structures are readily induced under electric field around the MPB which can be up-shifted to near room temperature by applying a negative pressure. Field-induced phase transition, polarization change, and dielectric response are computed. Under stress-free conditions monoclinic-orthorhombic transition can be induced in Pb0.5Sr0.5TiO3 by applying an electric field, giving rise to stronger field dependence of room-temperature dielectric constants as compared to Pb0.2Sr0.8TiO3. Under negative pressure, however, in Pb0.2Sr0.8TiO3 cubic-tetragonal transition can be induced, around which dielectric anomaly is sensitive to electric field, enabling significant enhancement of dielectric tunability.

The dielectric, piezoelectric and ferroelectric properties of LaAlO3-doped Bi0.5Na0.5TiO3-BaTiO3 lead-free piezoceramics

ABSTRACTLaAlO3-doped Bi0.5Na0.5TiO3-BaTiO3(BNT-BT) lead-free piezoelectric ceramics with compositions near the rhombohedral–tetragonal morphotropic phase boundary (MPB) were prepared and investigated. The introduction of LaAlO3, which makes the ceramics samples avoid being fused, has a relatively large influence on the dielectric, piezoelectric and ferroelectric properties of BNT-BT ceramics. With the increase of doping concentration, these properties get weaker. But when the doping concentration of LaAlO3 is 0.2 mol%, the remnant polarization (Pr) and coercive field (Ec) are better than those in pure 0.93BNT-0.07BT ceramics. The ferroelectric properties of this ceramics appear an eccentric enhancement and the domain switch becomes more easily.

Structure and electrical behavior of unpoled and poled 0.97(Bi0.5Na0.5)0.94Ba0.06TiO3-0.03BiAlO3 ceramics

The ceramics (Bi0.5Na0.5)0.94Ba0.06TiO3 (BNBT) and 0.97BNBT-0.03BiAlO3 (BNBTA/0.03) were synthesized via a solid-state reaction method. Effects of electric poling and Al3+-doping on structure and electrical behavior of BNBT and BNBTA/0.03 were studied. The results of Raman spectroscopy and X-ray diffraction confirm lattice distortion due to the doping of Al3+. BNBT and BNBTA/0.03 exhibit pure phase and dense microstructures. The tetragonal-rhombohedral morphotropic phase boundary exists in both samples. Electric poling causes an increase in the amount of the tetragonal phase. Dielectric behavior is sensitive to the electric poling and Al3+-doping. Compared to BNBT, BNBTA/0.03 exhibits pinched polarization loops, current-electric field curves with four peaks, and higher normalized piezoelectric constant. Based on the results of ultraviolet–visible diffuse reflectance spectra, electric conductivity, and Rayleigh analysis, it is suggested that the differences in electrical behavior between BNBT and BNBTA/0.03 are related to oxygen vacancies.

Room temperature multiferrocity and magnetodielectric properties of ternary (1-x) (0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-xBiFeO3 (0 ≤x≤ 0.9) solid solutions

(1–x)(0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-xBiFeO3 (0 ≤x≤ 0.9) ternary ceramics were prepared by the modified Pechini method. X ray diffraction patterns manifest that the samples undergo a gradual structural transformation from the rhombohedral-tetragonal morphotropic phase boundary to the rhombohedral phase with the increasing content of BiFeO3. Room temperature ferromagnetism was observed in BiFeO3-rich samples with x ≥ 0.6, and this ferromagnetism can be ascribed to the suppression of the spiral spin structure with the canting of antiferromagnetically ordered spins. The BiFeO3-rich samples (x = 0.6–0.8) exhibit superior ferroelectric properties with the maximum remanent polarization of 44.7 μC/cm2, as confirmed by the positive-up negative-down measurement to exclude the contribution of leakage current. The coexistence of room-temperature ferromagnetism and ferroelectricity manifests that the lead-free BiFeO3-based solid solutions are quite promising multiferroic materials and may be important for potential applications in BFO-based magnetoelectric devices.

Analytical calculation of saturated polarization and equilibrium composition near tetragonal-rhombohedral morphotropic phase boundary of polycrystalline ferroelectrics

In this study, the saturated polarization and equilibrium composition near tetragonal-rhombohedral morphotropic phase boundary (MPB) of polycrystalline ferroelectrics have been analytically calculated. A generalized inverse-pole-figure (IPF) method has been suggested to estimate saturated polarization and composition of polycrystalline ferroelectrics. On the other hand, saturated polarization of monoclinic polycrystalline ferroelectrics has been analytically calculated by the generalized IPF. The results can support the results from the experimental studies which maintained that monoclinic symmetry can appear at tetragonal-rhombohedral MPB and the saturated polarization is enhanced at a certain composition.

Simultaneously enhanced ferroelectric and magnetic properties in 0.675BiFe1−x Cr x O3–0.325PbTiO3 (x = 0–0.05) ceramics

Multiferroic ceramics of 0.675BiFe1−xCrxO3–0.325PbTiO3 (x = 0, 0.01, 0.025, 0.05) were prepared and comparatively investigated. The rhombohedral–tetragonal morphotropic phase boundary (MPB) was formed in 0.675BiFeO3–0.325PbTiO3. The B-site Cr-substitution changes the structure from MPB to rhombohedral phase gradually, accompanied with monotonously decreased ferroelectric Curie temperature and average grain size. More interestingly, Such Cr-substitution enhances room temperature ferroelectric and magnetic properties simultaneously. The underlying mechanism is mainly attributed to the substitution enhanced local Fe3+–Cr3+ magnetic interaction and the suppressed oxygen vacancy effect. We believe these results are helpful supplements for optimizing structures and room temperature multiferroic properties of BiFeO3-based materials.

Combustion technique synthesis of lead-free piezoelectric bismuth sodium titanate-bismuth potassium titanate–barium titanate ceramics

ABSTRACTFabrication of lead-free piezoelectric ceramic bismuth sodium titanate–bismuth potassium titanate–barium titanate, focusing on the morphotropic phase boundary (MPB) between the rhombohedral and tetragonal phases was studied. a[0.935(Bi0.5Na0.5)TiO3-0.065BaTiO3]-(1-a)[0.8(Bi0.5Na0.5)TiO3-0.2(Bi0.5K0.5)TiO3]; BNBK ceramics with 0 ≤ a ≤ 1 were successfully fabricated using the combustion technique. All BNBK samples were sintered at 1150 °C for 2 h. The effect of a changing on the phase formation, microstructure, dielectric properties and piezoelectric properties of perovskite BNBK ceramics were investigated. XRD results of BNBK ceramics with 0 ≤ a ≤ 1 showed the rhombohedral-tetragonal morphotropic phase boundary (MPB). The SEM results showed the average grain size is between 1.18 and 2.56 µm. The effect of the a content had no influence on the tendency of grain sizes. The maximum density, shrinkage, dielectric constant at Curie temperature (ϵr), remanent polarization (Pr), coercive field (Ec) and piezoelectric constant (d33) were found from BNBK ceramic with a = 1, which is around 5.76 g/cm3, 18.43%, 6503, 15.4 μC/cm2, 20.1 kV/cm and 289 pC/N, respectively.

Variations in the morphotropic phase boundary with Bi deficiency on Ba-substituted sodium bismuth titanate

We successfully demonstrated that a slight Bi deficiency causes the rhombohedral-tetragonal morphotropic phase boundary (MPB) to shift to the considerably lower Ba content in a (Na0.5Bi0.5)1−xBaxTiO3 (Ba-substituted NBT) system. Rietveld analysis revealed that the composition yielding the MPB was located at x = 0.10 when using the polymerizable complex method to prevent evaporation of sodium and bismuth and at x = 0.068 in samples of (Na0.5Bi0.5−z)1−xBaxTiO3−δ with a Bi deficiency of z = 0.03.

Relation between structure and piezoelectric properties of (1-x-y)PbZrO3-xPbTiO3-yPb(Ni1/3Nb2/3)O3 ceramics near triple point composition

The structural and piezoelectric properties of (1-x-y)PbZrO3-xPbTiO3-yPb(Ni1/3Nb2/3)O3 [(1-x-y)PZ-xPT-yPNN] ceramics near the triple point were investigated. The 0.16PZ-0.34PT-0.5PNN ceramic with a triple point composition exhibited a large d33, kp and ε33T/εo values of 855 pC/N, 0.68 and 5800, respectively. However, its g33 and d33 x g33 values were relatively small because of its large ε33T/εo. Similar results were obtained for specimens with rhombohedral-tetragonal or pseudocubic-tetragonal morphotropic phase boundary (MPB) compositions. However, specimens with a rhombohedral structure near the rhombohedral-tetragonal MPB exhibited large g33 and d33 x g33 values because of their large d33 and small ε33T/εo that can be explained by their polarization characteristics. In particular, 0.23PZ-0.36PT-0.41PNN ceramic exhibited large g33 and d33 x g33 values of 30.2×10−3 Vm/N and 22.6×10−12m2/N, respectively, thus making it a good candidate for piezoelectric energy harvesters and multilayer actuators.

Domain Configuration and Thermal Stability of (K0.48Na0.52)(Nb0.96Sb0.04)O3-Bi0.50(Na0.82K0.18)0.50ZrO3 Piezoceramics with High d33 Coefficient.

The domain configuration of lead-free (K0.48Na0.52)(Nb0.96Sb0.04)O3-Bi0.50(Na0.82K0.18)0.50ZrO3 ceramics with rhombohedral-tetragonal morphotropic phase boundary, accounting for the high piezoelectric property and good thermal stability, were systematically studied. Short domain segments (before poling) and long domain stripes with wedge-shaped or furcated ends (after poling) were found to be typical domain configurations. The reduced elastic energy, lattice distortion, and internal stress, due to the coexistence of rhombohedral and tetragonal phases, result in much easier domain reorientation and domain wall motion, responsible for the high piezoelectric properties, being on the order of 460 pC/N, in which the extrinsic contribution from irreversible domain switching was estimated to be around 50% of the total piezoelectricity. Minor piezoelectric property variations (<6% over a temperature range from -50 to 100 °C) were observed as a function of temperature, showing a good thermal stability. In addition, nanodomains (50 ± 2 nm) were found to be assembled into domain stripes after poling, believed to benefit the high piezoelectric properties but not causing much thermal instability due to the small quantity.

Enhanced electrical properties in lead-free NBT–BT ceramics by series ST substitution

Comprehensive electrical properties of 0.94(Na1/2Bi1/2)TiO3–0.06BaTiO3 lead-free ceramics by doping series SrTiO3 were investigated. High piezoelectric constant of 205pC/N and electromechanical coupling factor of 0.34 were obtained due to the forming of the rhombohedral–tetragonal morphotropic phase boundary at x=0.02–0.06. Very large recoverable strain of 0.34% was obtained at x=0.10 due to the coexistence of ferroelectric and relaxor pseudocubic phases. A large electrocaloric effect (ΔTmax=1.71K and ΔT/ΔE=0.34KmmkV−1 at 50kVcm−1) which determined by indirect measurements method was obtained at 120°C at x=0.02, which is significantly higher than that of lead-free ferroelectric ceramics reported so far. Moreover, lower operating temperatures of 50°C and 30°C were proposed when x=0.10 and 0.20 with ΔTmax=0.79K and 0.6K, respectively. These properties added together indicate a promising material for applications in cooling systems and actuators.

Microstructure and Dielectric Properties of 0.92Na0.5Bi0.5TiO3-0.06BaTiO3-0.02K0.5Na0.5NbO3-Based Ceramics Sintered in Oxygen and Nitrogen Atmospheres

The lead-free ceramics 0.92Na0.5Bi0.5TiO3-0.06BaTiO3-0.02K0.5Na0.5NbO3 and 0.92Na0.5Bi0.5TiO3-0.06BaTiO3-0.02K0.5Na0.5NbO3 with the addition of 0.2 wt% Co2O3 were prepared by using a solid state reaction method. The ceramics were sintered in O2 and N2 atmospheres, respectively. The rhombohedral-tetragonal morphotropic phase boundary exists in all samples, with the relative amount of tetragonal phase varying with the sintering atmospheres which does not affect grain size of the ceramics. In the dielectric constant-temperature curves for all samples, two dielectric anomalies appear. The phase transition temperatures and maximum values of dielectric constant are closely related to the sintering atmospheres. The mechanism was also discussed.