Rhombohedral Ferroelectric Phase Research Articles

Reversibility in electric field-induced transitions and energy storage properties of bismuth-based perovskite ceramics

The effects of temperature and electric field-induced structural modifications on the energy storage properties of 0.95[0.94Bi0.5Na0.5TiO3–0.06BaTiO3]–0.05K0.5Na0.5NbO3 (BNT–BT–5KNN) ceramics were investigated. X-ray diffraction performed on unpoled and poled ceramics in the temperature range 25–500 °C suggested an increment in the rhombohedral phase intensity peaks and in the tetragonal distortion after electrical poling. The rhombohedral phase content reduced with increasing temperature in both unpoled and poled ceramics. In the unpoled ceramic, the rhombohedral phase eventually disappeared, while it survived in the poled specimen up to 500 °C. The stabilization of the rhombohedral ferroelectric phase by dc poling produced remarkable differences in the temperature dependence of permittivity, loss, current–polarization–electric field loops and energy density. As a consequence of a reversible transition induced by an alternating electric field, competitive energy densities (0.39–0.51 J cm−3 in the range 25–175 °C) with those of lead-based and lead-free bulk ceramics recently developed was obtained, indicating bismuth-based perovskites as potential lead-free systems for energy storage applications.

Nonlinear Ferroelectric And Dielectric Properties Of Bi(Mg0.5Ti0.5)O3-PbTiO3 Perovskite Solid Solutions

Polycrystalline samples of Bi(Mg0.5Ti0.5)O3-PbTiO3 (BMT-PT) solid solutions exhibit high ferroelectric Curie temperatures and are promising materials for high temperature piezoelectric devices. A morphotropic phase boundary (MPB) between ferroelectric rhombohedral and tetragonal phases occurs between BMT-PT ratio of 70-30 and 65-35 compositions. In the present investigation, ceramics having BMT-PT ratios in the range from 70-30 to 50-50 were prepared by a conventional solid state reaction method. The change of crystal structure as a function of composition and temperature has been studied using high temperature X-ray diffraction. Polarisation-Electric field hysteresis curves at different temperatures show antiferroelectric behaviour for x = 0.30, 0.35 and 0.40 compositions while 0.45 and 0.50 show a ferroelectric behavior.

Study of the Anelastic Behavior of PZT and PLZT Ferroelectric Ceramics

The anelastic behavior of the ferroelectric ceramics (Pb)(Zr/Ti)O3(PZT) and (Pb/La)(Zr/Ti)O3(PLZT), with Zr/Ti = 65/35, La = 5 at.% and 8 at.%, was investigated in the region of the ferroelectric phase transitions. Anelastic spectroscopy experiments were performed in an acoustic elastometer system, operating in a kilohertz bandwidth, at temperatures rising from 300 K to 770 K, at a heating rate of 1 K/min, under pressure of 10-5mbar. Anelastic measurements on PZT showed only one anomaly, associated with the occurrence of a ferroelectric-paraelectric phase transition, while the PLZT data showed two anomalies, which were associated with the following transitions: the ferroelectric-paraelectric phase transition and a ferro-ferroelectric phase transition between distinct rhombohedral ferroelectric phases. The behavior of the relative variation of the elastic moduli with temperature, near the phase transitions, which describes the change in the type of coupling between strain and the order parameter in ferroelectric-paraelectric phase transition, with the increase of lanthanum amount and, linear coupling in the strain and order parameter type to PZT ceramic and linear coupling in the strain but quadratic in order parameter type for PLZT ceramics.

Diffusion phase transition and ferroelectric properties of 0.92K0.5Na0.5NbO3–(0.08 − x)Bi0.5Li0.5TiO3–xBaZrO3 ceramics

0.92K0.5Na0.5NbO3–(0.08−x)Bi0.5Li0.5TiO3–xBaZrO3 (KNN–BLT–BZ) lead-free piezoelectric ceramics were fabricated by traditional ceramics process. XRD results showed that complete solid solutions with mixed phases of rhombohedra and tetragonal were observed. Diffusion phase transitional behavior, dielectric properties and ferroelectric properties of the KNN–BLT–BZ ceramics were investigated. It was found that the cubic-tetragonal phase transition temperature (TC) slightly decreased as the amount of BaZrO3 was increased. A morphotropic phase boundary (MPB) between rhombohedral and tetragonal ferroelectric phases was identified in the composition range of 0.04≤x≤0.06. Furthermore, good ferroelectric properties were also found in this composition range, with Pr of 14.5μCcm−2 and Ec of 1.95kVmm−1 for the sample with x=0.04.

Structure, ferroelectricity and piezoelectricity evolutions of Bi1−xSmxFeO3 at various temperatures

Insulated Bi1−xSmxFeO3 (x=0–0.25) were prepared to study their multiferroic properties at various temperatures. Bi1−xSmxFeO3 (x=0−0.12) shows a rhombohedral-like phase, ferroelectric polarization of >17.8 μC/cm2, piezoelectric d33 value of >35 pC/N at room temperature. Bi1−xSmxFeO3 (x=0.125–0.2) has a ferroelectric triclinic phase and an anti-polar orthorhombic phase together at room temperature, and Bi0.75Sm0.25FeO3 is mainly composed of a non-polar orthorhombic phase. After the polarized Bi1−xSmxFeO3 (x=0–0.165) was annealed at various high temperatures, the d33 evolution supports ferroelectric component exist above 500 °C. Raman spectra suggests that the main component of Bi0.875Sm0.125FeO3 does not keep a rhombohedral ferroelectric phase above 400 °C, thus its ferroelectric traits at high temperature should be due to ferroelectric clusters embedded in orthorhombic phase.

Destabilization of ferroelectric order in bismuth perovskite ceramics by A-site vacancies

This work compared the effects of isovalent Sn4+ and aliovalent Ta5+ doping on Ti-sites in lead-free perovskite type Bi1/2(Na0.82K0.18)1/2TiO3 piezoelectric ceramics on their ferroelectric and electric field-induced strain behavior. Both impurities led to a diffuse transformation from the coexistence of ferroelectric rhombohedral and tetragonal phases to a nonpolar pseudocubic symmetry. Nevertheless, Ta doping more strongly promoted the phase transition even though the ionic size difference between Ta5+ and Ti4+ was smaller than that between Sn4+ and Ti4+. The results evidenced that donor-induced A-site vacancies significantly contributed to the destabilization of ferroelectric phases in the Bi perovskite.

Stabilized antiferroelectric phase in lanthanum-doped (Na1/2Bi1/2)TiO3

Phase transition behaviour of La-modified sodium bismuth titanate ceramics [(Na0.5Bi0.5)1−1.5xV0.5xLax]TiO3 (NBLT) was investigated. The two anomalies in εr(T) and DSC analysis indicated that lower temperature phase transitions below 200 °C became pronounced with La additions. The polarization relaxation of εr(T) and double hysteresis loops showed that the intermediate region between two dielectric anomalies was the antiferroelectric modulated phase. The origin of the modulated antiferroelectric state was discussed in terms of disordering effects of the La ions and cation vacancies. With increasing La content, the long-range symmetry of the dipoles in the ferroelectric phase was disturbed in the intermediate region. The competition between rhombohedral ferroelectric phase and tetragonal paraelectric phase contributed to the formation of a modulated antiferroelectric phase in NBLT ceramics.

Large piezoelectric effect in Pb-free Ba(Ti,Sn)O3-x(Ba,Ca)TiO3 ceramics

We designed a Pb-free pseudo-binary system, Ba(Sn0.12Ti0.88)O3-x(Ba0.7Ca0.3)O3 (BTS-xBCT), characterized by a phase boundary starting from a tricritical triple point of a paraelectric cubic phase, ferroelectric rhombohedral, and tetragonal phases. The optimal composition BTS-30BCT exhibits a high piezoelectric coefficient d33 ∼ 530 pC/N at room temperature. In view of the recent report of high piezoelectricity in another Pb-free system BZT-BCT (Liu and Ren, Phys. Rev. Lett. 103, 257602 (2009)), which possesses a similar tricritical triple point in the phase diagram, it seems that forming a suitable phase boundary starting from a tricritical triple point could be an effective way to develop high-performance Pb-free piezoelectrics.

Dielectric and piezoelectric properties of PbFe1/2Nb1/2O3-PbTiO3 ceramics from the morphotropic phase boundary compositional range

Dielectric, X-ray, and piezoelectric studies of highly-resistive Li-doped (1-x)PbFe(1/2)Nb(1/2)O(3)-(x)PbTiO(3) (PFN-xPT) ceramics from the 0 ≤ x ≤ 0.2 range fabricated by solid-state synthesis and usual sintering have been carried out. Distinct anomalies of dielectric and piezoelectric parameters, corresponding to the transition between rhombohedral (monoclinic) and tetragonal ferroelectric phases, have been observed in pure PFN and PFN-xPT compositions with PbTiO(3) content up to 8 mol.%. The x,T-phase diagram of the PFN-xPT solid solution system has been constructed using these data.

Epitaxial strain stabilization of a ferroelectric phase in PbZrO3thin films

PbZrO${}_{3}$/SrRuO${}_{3}$/SrTiO${}_{3}$ (100) epitaxial heterostructures with different thickness of the PbZrO${}_{3}$ (PZO) layer (d${}_{\mathrm{PZO}}$ \ensuremath{\sim} 5--160 nm) were fabricated by pulsed laser deposition. The ultrathin PZO films (d${}_{\mathrm{PZO}}$ \ensuremath{\le} 10 nm) were found to possess a rhombohedral structure. On increasing the PZO film thickness, a bulk like orthorhombic phase started forming in the film with d${}_{\mathrm{PZO}}$ \ensuremath{\sim} 22 nm and became abundant in the thicker films. Nanobeam electron diffraction and room-temperature micro-Raman measurements revealed that the stabilization of the rhombohedral phase of PZO could be attributed to the epitaxial strain accommodated by the heterostructures. Room-temperature polarization vs electric field measurements performed on different samples showed characteristic double hysteresis loops of antiferroelectric materials accompanied by a small remnant polarization for the thick PZO films (d${}_{\mathrm{PZO}}$ \ensuremath{\ge} 50 nm). The remnant polarization increased by reducing the PZO layer thickness, and a ferroelectric like hysteresis loop was observed for the sample with d${}_{\mathrm{PZO}}$ \ensuremath{\sim} 22 nm. Local ferroelectric properties measured by piezoresponse force microscopy also exhibited a similar thickness-dependent antiferroelectric-ferroelectric transition. Room-temperature electrical properties observed in the PZO thin films in correlation to their structural characteristics suggested that a ferroelectric rhombohedral phase could be stabilized in thin epitaxial PZO films experiencing large interfacial compressive stress.

Phase Transformation Behavior and Electrical Properties of Pb(Zr0.56Ti0.44)O3-Bi(Zn0.5Ti0.5)O3 Solid Solution Ceramics

The solid solutions of (1−x)Pb(Zr0.56Ti0.44)O3–xBi(Zn0.5Ti0.5)O3((1−x)PZ56T44–xBZT) were synthesized via a solid-state reaction method. X-ray diffraction results indicated that the tetragonality of (1−x)PZ56T44–xBZT was enhanced with increasing the BZT content, and a morphotropic phase boundary (MPB) between rhombohedral and tetragonal ferroelectric phases was identified to be in the range of 0.15 < x < 0.18. In addition, the dielectric diffuseness and frequency dispersion behavior were induced with increasing the BZT content, owing to increased disorder degree of both A-site and B-site cations in (1−x)PZ56T44–xBZT perovskite lattice. Contrary to the case in PT–BZT, the increased tetragonality accompanies reduced Curie temperatures (Tc) as PZ56T44 was substituted by BZT. The electrical properties of solid solutions exhibit obviously compositional dependence. The optimum dielectric and piezoelectric properties of e33T = 1430, d33 = 365 pC/N, kp = 50%, Qm = 32, and Tc = 260°C were achieved in (1−x)PZ56T44–xBZT ceramics with x = 0.18 owing to the co-existence of two ferroelectric phases near the MPB.

Phase diagram and electric properties of the (Mn, K)-modified Bi0.5Na0.5TiO3–BaTiO3 lead-free ceramics

In this study, the phase diagram and electric properties were demonstrated for a (Mn, K)-modified Bi0.5Na0.5TiO3 (BNT)-based solid solution. (0.935−x) Bi0.5Na0.5TiO3−xBi0.5K0.5TiO3−0.065BaTiO3 with 0.5% mol Mn doping was prepared by a conventional solid-state reaction method. A morphotropic phase boundary (MPB) formed between the ferroelectric rhombohedral and tetragonal phases around x of 0.04 with the MPB tolerance factor t of 0.984–0.986. The temperature and composition dependence of the dielectric, piezoelectric, ferroelectric properties along with the strain characteristics were investigated in detail and a phase diagram was presented. Around the MPB region, the maximum values of piezoelectric constant \( d_{33}^{*} \) of 290 pC/N, d33 of 155 pC/N, dielectric constant \( \varepsilon_{33}^{T} /\varepsilon_{0} \) of 1059 and low dielectric loss tangent tan δ of 0.017 were obtained. In addition, the authors also suggest that the solid solution with composition x of 0.24, exhibiting both high-depolarization temperature Td of 182 °C, \( d_{33}^{*} \) of 156 pC/N, d33 of 130 pC/N, will be favorable for high-temperature actuator and sensor applications.

Enhanced piezoelectric and magnetic properties of Bi1−xCaxFe1−x/2Nbx/2O3 solid solutions

An investigation of the crystal structure, the magnetic and the piezoelectric properties of polycrystalline Bi1-xCaxFe1-x/2Nbx/2O3 (I) and Bi1-xPbxFe1-x/2Nbx/2O3 (II) systems was performed by x-ray diffraction, Mössbauer spectroscopy, vibrating sample magnetometry, and piezoresponse force microscopy. It is shown that an increasing niobium content induces a polar-to-nonpolar morphotropic boundary near x = 0.19 (I) and x = 0.3 (II). Within the polar region of the system (I), (0.15 ≤x ≤0.18), the solid solutions are homogeneous weak ferromagnets whereas compounds of the system (II) do not exhibit an appreciable spontaneous magnetization. It is assumed that chemical substitutions leading to a decrease of the initial volume of the unit cell favor the stabilization of the weak ferromagnetic state within the rhombohedral ferroelectric phase. The piezoresponse is significantly enhanced near the morphotropic boundary. The piezoelectric properties of the parent antiferromagnet BiFeO3, harboring a cycloidal spatially modulated spin structure, are compared with those of the polar weak ferromagnet Bi0.82Ca0.18Fe0.91Nb0.09O3.

Lead-free electrostrictive bismuth perovskite ceramics with thermally stable field-induced strains

Electric field-induced strain (EFIS) properties of Bi 1/2(Na 0.82K 0.18) 1/2TiO 3 (BNKT) ceramics modified with Sr(K 1/4Nb 3/4)O 3 (SKN) have been investigated as functions of composition and temperature. BNKT ceramics near a phase boundary revealed the coexistence of ferroelectric rhombohedral and tetragonal phases, resulting in a typical ferroelectric butterfly-shaped bipolar S– E loop at room temperature, whose normalized strain ( S max/ E max) showed a significant temperature coefficient of 0.38 pm/V/K. However, 5 mol% SKN-modified BNKT ceramics revealed a typical electrostrictive behaviour with a thermally stable electrostrictive coefficient, Q 33 = 0.021 m 4/C 2, which is almost comparable to that of Pb(Mg 1/3Nb 2/3)O 3 (PMN) ceramics that have been dominantly used as Pb-based electrostrictive materials over the last decades.

Effects of tricritical points and morphotropic phase boundaries on the piezoelectric properties of ferroelectrics

The longitudinal piezoelectric coefficient ${d}_{33}^{*}$ of a PZT-like ferroelectric is computed in the full composition-temperature parameter space using sets of parameters that control the position of the tricritical points and the degree of tilting of the morphotropic phase boundary separating the ferroelectric rhombohedral phase from the ferroelectric tetragonal phase. The system is modeled using a Ginzburg--Landau expansion of the free energy in terms of the electric polarization up to sixth order, including all the symmetry-allowed terms. We obtain two regions of the phase diagram with a large piezoelectric response. In the polar direction, ${d}_{33}^{*}$ is large in the vicinity of the paraelectric to ferroelectric line of phase transitions, whereas in a nonpolar direction ${d}_{33}^{*}$ is large in the vicinity of the morphotropic phase boundary. We find that a given degree of tilting of the morphotropic phase boundary can be obtained from free energies with different degrees of anisotropy, and therefore the titling and anisotropy are not directly related. On the other hand, the piezoelectric response is larger when the two tricritical points of the phase diagram are farther apart from each other than when they collapse onto a single tricritical point.

Large strain transduction utilizing phase transition in relaxor-ferroelectric Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 single crystals

In this work, we present experimental evidence that under relatively low level drive (&amp;lt;0.1 MV/m) the large strain (∼0.5%) associated with ferroelectric rhombohedral FR-ferroelectric orthorhombic FO phase transition in domain engineered relaxor–ferroelectric single crystals under compressive stress and bias electric field can be captured. We have demonstrated this in mechanically confined ternary Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 single crystals poised at the rhombohedral side of the morphotropic phase boundary. Experimental strain-field results, methods of mechanical confinement and drive, and a low frequency, compact, high source level sound projector design are presented. Transducers that operate on a large strain principle as a design rule will benefit from this development.

Effect of Ta-Substitution on the Ferroelectric and Piezoelectric Properties of Bi0.5/(Na0.82K0.18)0.5TiO3Ceramics

The effect of Ta substitution on the crystal structure, ferroelectric, and piezoelectric properties of <TEX>$Bi_{0.5}(Na_{0.82}K_{0.18})_{0.5}Ti_{1-x}Ta_xO_3$</TEX> ceramics has been investigated. The Ta doping resulted in a transition from coexistence of ferroelectric tetragonal and rhombohedral phases to an electrostrictive pseudocubic phase, leading to degradations of the remnant polarization, coercive field, and piezoelectric coefficient <TEX>$d_{33}$</TEX>. However, the electricfield-induced strain was significantly enhanced by the Ta substitution-induced phase transition and reached a highest value of <TEX>$S_{max}/E_{max}$</TEX> = 566 pm/V under an applied electric field 6 kV/mm when 2% Ta was substituted on Ti sites. The abnormal enhancement in strain was attributed to the pseudocubic phase with high electrostrictive constants.

Structural disorder in the paraelectric phase of the Fe3B7O13Br boracite

A structural model of the cubic paraelectric phase of a Fe3B7O13Br crystal belonging to the boracite family has been developed using the data obtained by single-crystal X-ray diffraction with due regard for the results of extended X-ray absorption fine structure (EXAFS) spectroscopy. It has been shown that the best agreement between the data obtained by these two methods is achieved within a model assuming a disorder in the arrangement of both the Fe and Br atoms and a high degree of correlation of their displacements. It has been found that, during the phase transition from the rhombohedral ferroelectric phase to the cubic paraelectric phase, no significant transformation of the structure is observed on a local level. In this case, a change in the macroscopic symmetry occurs predominantly as a result of the variation in the set of possible spatial orientations of stable structural fragments, which is characteristic of order-disorder phase transitions.

Structural, electrical, and strain properties of stoichiometric 1−x−y(Bi0.5Na0.5)TiO3−x(Bi0.5K0.5TiO3)−y(Na0.5K0.5)NbO3 solid solutions

Results on two series of lead-free piezoelectric solid solutions of 1−x−y(Bi0.5Na0.5)TiO3−x(Bi0.5K0.5TiO3)−y(Na0.5K0.5)NbO3 (BNT-BKT-KNN) ceramics, synthesized in the vicinity of ferroelectric rhombohedral (x=0.17) and ferroelectric tetragonal (x=0.20) phase boundary of 1−x(BNT)–xBKT are presented. Orthorhombic KNN in stoichiometric proportion as third component with 0.0≤y≤0.06 is added to prepare these two series (i) with x=0.17 and (ii) with x=0.20. Addition of KNN up to a critical y is observed to improve tetragonality in both series and solid solubility of KNN was verified at least up to y=0.1 in this system. Depolarization temperature Td of these stoichiometric BNT-BKT-KNN series were found to be considerably lower than that of their parent BNT-BKT compositions. As a result of this, coexistence of polar and nonpolar phases for compositions at morphotropic phase boundary (MPB) were indicated at room temperature, resulting in pinched polarization versus electric field (P-E) loops and large strain (strain of ∼0.80% was observed for y=0.01 sample in ×20 series and strain of 0.50% was seen for y=0.04 sample in ×17 series) The MPB of ×17 and ×20 series found to be at y=0.04 and y=0.01, respectively.

Dielectric relaxation of (Pb(1-3x/2)Lax)(Zr0.5Sn0.3Ti0.2)O3 antiferroelectric ceramics induced by lanthanum doping

The (Pb(1-3x/2)Lax)(Zr0.5Sn0.3Ti0.2)O3(PLZST, 0.00≤x≤0.18)antiferroelectric ceramics were prepared by traditional ceramic process. The effect of La modification on phase structures, phase transition, and electrical properties of PLZST ceramics were investigated by X-ray diffraction, P-E hysteresis loop and dielectric spectroscopy. With the increase of La content x,the samples at room temperature are rhombohedral ferroelectric phase (x≤0.0.04),tetragonal antiferroelectric phase(AFEt, 0.06x≤0.0.08, and cubic phase (PE' cx≥0.1), respectively. The dielectric spectroscopy shows that with the increase of La content x, AFE-PE phase transformation temperature decreases, and for samples with La content x exceeding 0.12, strong dielectric frequency dispersion is observed around dielectric ε' and tanδ peaks. The peak temperatures Tm corresponding to the maxim dielectric constant shifts to higher temperatures with the increase of frequency, indicating that PLZST antiferroelectric transform to typical relaxors. The curves of dielectrics as functions of temperatures in the PLZST systems are fitted by a modified Curie–Weiss law proposed by Santos.