Phase Transition and Ferroelectric Characteristics of Pb[(Mg1/3Nb2/3)1-xTix]O3 Ceramics Modified with La(Mg2/3Nb1/3)O3

The effects of 0–5 mol% addition of La(Mg_2/3Nb_1/3)O₃ (LMN) on the phase transition and ferroelectric behaviors of Pb[(Mg_1/3Nb_2/3)_1-xTi_x]O₃ (PMNT) ceramics with compositions near the morphotropic phase boundary (MPB) were studied. An evolution of structure from rhombohedral to tetragonal was found with increasing PbTiO₃ (PT) content across the MPB (at ∼32.5 mol% PT), and a coexistence of both rhombohedral and tetragonal phases was also found at the MPB. The dual-phase field extended toward the lower PT content side of the MPB, and, moreover, the rhombohedrality or tetragonality was reduced, especially for the compositions near the MPB, by the addition of La in PMNT. The ferroelectric transition was found to change from normal to diffuse as the La content increased and the compositions became more rhombohedral. In accordance with the structural evolution, the change of remanent polarization ( P _r) and coercive field ( E _c) also became gradually indistinct, and both P _r and E _c were reduced. For compositions near the MPB, both PMNT and La-modified PMNT had a similar electromechanical factor ( k _p) in a range around 0.55–0.60, but the mechanical quality factor ( Q _m) was significantly reduced for the La-modified PMNT. The piezoelectric coefficient ( d ₃₃), however, was largely improved with increasing La content in PMNT of compositions at MPB. A high value of d ₃₃∼ 815 pC/N was obtained for the 5-mol%-La-modified ceramics, but it was associated with a low value of Q _m.     

Morphotropic Phase Boundary in the Pb(Zn1/3Nb2/3O3)-BaTiO3-PbTiO3 System

The morphotropic phase boundary (MPB) in the relaxor ferroelectric system Pb(Zn_1/3Nb_2/3O₃)-BaTiO₃-PbTiO₃ (PZN-BT-PT) with 15 mol% BT was investigated through dielectric permittivity and high-temperature X-ray diffraction measurements. It was revealed that MPB is a broad composition region extending from 12 to 18 mol% PT, within which the temperatures of the permittivity maximum are close to the ending temperatures for the phase transformation from coexisting rhombohedral and tetragonal phases to cubic phase on heating. When the specimen is cooled, the starting temperatures for the rhombohedral-to-tetragonal phase transition increase with increasing PT content. The large thermal hysteresis observed by X-ray diffraction is caused by the phase transformation between rhombohedral and tetragonal phases. On cooling, the MPB curves toward the PT-rich side, so that ceramics within this composition range undergoe successive phase transitions from cubic to rhombohedral and from rhombohedral to tetragonal phase. The diffuseness of the paraelectric-to-ferroelectric phase transition is remarkably decreased by the addition of PT. The enhanced dielectric permittivity peak values for the MPB compositions are correlated with the reduced lattice distortion and phase coexistence.     

PLZT Phases Near Lead Zirconate: 2. Determination by Capacitance and Polarization

The phases of PLZT, (Pb,La)(Zr,Ti)O₃, or L / Z / T for L <12 and T <10 were detected via determination of capacitance at varying temperatures at 0 field and via polarization at varying fields at room temperature and at selected elevated temperatures. It was possible to determine two different orthorhombic phases, and three rhombohedral (hexagonal) phases, one of which only exists at elevated temperatures. Two sets of phase diagrams were constructed with the phases as a function of temperature, and electric field, respectively. Temperature/field phase diagrams for 0/100/0, 2.95/5, and 4/95/5 were also constructed. A comparison was made between room-temperature and 0 field PLZT phase diagrams determined via X-ray diffraction, capacitance, and polarization.     

Lanthanum Dependence of Elastic and Piezoelectric Properties of PLZT Ceramics with a Zr/Ti Ratio of 65/35

The elastic and piezoelectric properties of PLZT ceramics with a 65/35 Zr/Ti ratio undergo pronounced changes as La is added to the structure. At the rhombohedral-tetragonal phase boundary, material constants plotted as functions of La concentration show strong discontinuities in slope; these discontinuities provide a precise means of locating the boundary. The radial-mode coupling factor, k_p , compliance coefficient, S₁₁ ^E , and piezoelectric coefficient, d₃₁ , attain peak values at the boundary, whereas Poisson's ratio, σ^E, and the resonance-frequency constant exhibit pronounced minima. This behavior is evidence of the increasing ferroelastic sensitivity of the material as the distortion angle of the rhombohedral phase approaches a minimum.     

Effect of MgO Doping on the Phase Transformations of BaTiO3

The successive phase transformations in MgO-doped BaTiO₃ were studied. Upon MgO doping, dielectric anomalies corresponding to lower phase transformations were broadened and depressed, while an anomaly for a cubic–tetragonal transformation remained and shifted to a lower temperature. XRD peak splitting upon tetragonality of BaTiO₃ was decreased, and the peaks exhibited abnormally broadened profiles which are different from the one for cubic BaTiO₃ above T _c. Raman spectroscopy revealed the existence of orthorhombic phase at room temperature for the solid solution with 0.5 mol% or more MgO. The temperature dependence of the Raman spectrum showed that orthorhombic and rhombohedral phases in MgO-doped BaTiO₃ were stabilized at higher temperatures than pure BaTiO₃.     

Structural Evolution and Electrical Properties of Sc2O3-Stabilized ZrO2 Aged at 850°C in Air and Wet-Forming Gas Ambients

X-ray diffraction (XRD) and electron microscopy investigations have been performed on Sc₂O₃-stabilized ZrO₂ as-sintered and after aging in air or in wet-forming gas at 850°C for 1000 h. Some tetragonal to monoclinic transformation had occurred in the near-surface regions of 4 mol% Sc₂O₃ samples after aging; the phase transition was more severe for samples aged in the forming gas ambient. A decrease of ∼20% in electrical conductivity accompanied the aging. In 6 mol% Sc₂O₃ samples, although no cubic to tetragonal transformation was detected, both the electrical conductivity and the activation energy for ionic conductivity decreased significantly during aging. Ten mole percent Sc₂O₃ samples did not show appreciable change in electrical conductivity due to aging, although some near-surface cubic to rhombohedral transformation did occur. Sharpening of the (400)_t XRD peak of Sc₂O₃-stabilzed tetragonal ZrO₂ accompanies the change(s) in the electrical conductivity.     

Massive Transformation in Gd2O3-Bi2O3 Ceramics

Gd₂O₃-doped Bi₂O₃ polycrystalline ceramic samples containing between 10 and 26 mol% Gd₂O₃ were fabricated by pressureless sintering of powder compacts. As-sintered samples were cubic (CaF₂ structure). The cubic solid solutions underwent transformation to a rhombohedral phase when annealed at lower temperatures. Under certain conditions, the cubic phase fully transformed to the rhombohedral phase of the same composition, and the kinetics were thermally activated. This suggested that the cubic → rhombohedral transformation was a massive transformation. The transition temperatures for the occurrence of a massive transformation were experimentally determined by measuring the conductivity as a function of temperature, as well as by measuring growth rate of the rhombohedral precipitates as a function of temperature. The activation enthalpy for interface motion was measured to be ∼200 kJ/mol for the samples studied. The kinetics of cubic → rhombohedral transformation could be described by the Johnson-Mehl-Avrami equation.     

Reaction Mechanisms in the Formation of Lead Zirconate Titanate Solid Solutions under Hydrothermal Conditions

Reaction mechanisms in the formation of PZT solid solution were studied under hydrothermal conditions (Pb/(Zr+Ti) = 1.0 to 1.9, Zr/Ti = 0/10 to 10/0, 1 M to 5 M KOH, 100° to 220°C, 2 h). A yellow tabular crystallite with tetragonal symmetry and Pb/Ti ∼ 2 was formed at 100° to 130°C. A PZT crystallite was formed just above 150°C. The crystallite was a mixture of Ti-rich PZT and Zr-rich PZT phases. When the temperature and KOH concentration were increased, the composition of the PZT product tended to be homogeneous. The PZT in the morphotropic phase boundary zone was formed at Zr/Ti = 5/5, 5M KOH, 220°C, 2 h. Neither PbTiO₃ nor PbZrO₃ was detected as a separate phase under the above hydrothermal conditions.     

Effects of Crystal-Lattice Distortion on Optical Transmittance of the (Pb, La)(Zr, Ti)O3, System

Optical transmittance and crystal-lattice distortion and their relation were studied in hot-pressed La-doped Pb zirconate-titanate (PLZT) ferroelectric ceramics at the ferroelectric-antiferroelectric morphotropic phase boundary. In the thermally annealed condition, the crystal system of PLZT at the FE-AFE morphotropic phase boundary was pseudotetragonal and similar to the antiferroelectric phase (AFE_β) appearing in Pb(Zr, Ti)O₃ and (Pb, Sr)ZrO₃. Optical transmittance and crystal-lattice distortion change monotonically with respect to the Zr/Ti ratio, and the optical transmittance increases approximately linearly with decreasing lattice distortion. In view of these relations and the temperature dependence of the optical transmittance, it is suggested that light scattering in PLZT ceramics is caused mainly by the variation in refractive index encountered as the light travels from one domain or grain into another.     

Preparation of Pb(Zr,Ti)03 Through the Use of Cupferron

An organic chelation reagent, cupferron, was used to coprecipitate Ti⁴⁺ andZr⁴⁺. After the materials were fired, they were mixed with PbO powder and fired again at high temperatures to obtain Pb(Zr,Ti)0₃ (PZT). It was confirmed that this method is useful for the preparation of homogeneous PZT having no compositional fluctuations. No coexistence range of the tetragonal and rhombohedral phases was observed in the PZT compositions near the morphotropic phase boundary.     

Stabilization of Zirconia Sintered with Titanium

Electron and X-ray diffraction studies show that zirconia sintered with 5 to 15 mol% titanium under a vacuum of 10⁻¹ to 10⁻² torr (∼13 to 1.3 Pa) was partially stabilized as cubic and tetragonal phases, whose amounts increase with increasing Ti content. The stabilization of ZrO₂ is due to the dissolution of TiO which forms as a second phase in the sintered specimen. The grain size of ZrO₂ decreases with increase in Ti. The improvements in strength and thermal shock resistance of ZrO₂ sintered with Ti are attributed to the reduction in ZrO₂ grain size and the effect of partial stabilization.     

Influence of Crystal Structure on the Fatigue Properties of Pb1−xLax(Zry', Tiz)O3 Thin Films Prepared by Pulsed-Laser Deposition Technique

Lead lanthanum zirconate titanate (Pb_{1− x} La _x (Zr _y ,Ti _z )O₃, PLZT) films containing [00 l ] preferentially oriented grains were produced successfully on YBa₂Cu₃O_{7− x} -coated (YBCOcoated) SrTiO₃ (STO) or YBCO/CeO₂-coated silicon substrates; films containing randomly oriented grains were created on platinum-coated silicon substrates. The latter possessed significantly inferior ferroelectric properties, a fact ascribed to the presence of a paraelectric phase (TiO₂) at the PLZT/platinum interface. On the other hand, the PLZT/YBCO/STO films exhibited better electrical properties than did the PLZT/YBCO/CeO₂/Si films, and this phenomenon was attributed to better alignment of the grains in normal and in-plane orientations. In terms of fatigue properties, the [00l] textured films that were deposited on YBCO/CeO₂/Si substrates possessed substantially superior polarization-switching-cycle endurance versus the randomly oriented films grown on Pt(Ti)/Si substrates. Moreover, the tetragonal films behaved much more satisfactorily than did the rhombohedral PLZT films. The ferroelectric parameters of tetragonal PLZT films showed no significant degradation up to 10⁹ polarization switching cycles, whereas the remnant polarization and coercive force of the rhombohedral PLZT films had already degraded to 80% of their initial values after 10⁸ cycles.     

Growth of Pb((Zn1/3Nb2/3)0.91Ti0.09)O3 Single Crystals Using an Allomeric Seed Crystal and Their Electrical Properties

Single crystals of Pb((Zn_1/3Nb_2/3)_0.91Ti_0.09)O₃ (PZNT 91/9), 28 mm in diameter and 30 mm in length, have been successfully grown using a modified Bridgman technique with an allomeric seed crystal. X-ray fluorescence analysis (XRFA) measurement confirms that the effect of segregation is not serious. The segregation coefficient k for PbTiO₃ content during crystal growth is 0.99, which causes some fluctuation in the composition along the growth direction. The fluctuation of composition and the complicated domain structure cause a variation of electric properties. Dielectric measurement indicates that PZNT 91/9 crystals exhibit an almost normal ferroelectric phase transition at ∼183°C from the tetragonal phase to the cubic phase. In addition, a weak frequency-dependent ferroelectric-ferroelectric phase transition is observed at ∼85°C, which is attributed to partial conversion of the rhombohedral phase to a tetragonal phase. The dielectric thermal hysteresis behavior and the existence of polarization above the Curie temperature verify that the phase transitions at ∼85° and 183°C are first order with a slight diffuse character and first order, respectively. It is demonstrated that the effects of segregation can be decreased and the homogeneity of the obtained PZNT 91/9 single crystals can be improved by optimizing growth parameters.     

Hot-Pressed (Pb,La)(Zr,Ti)O3 Ferroelectric Ceramics for Electrooptic Applications

Transparent ferroelectric ceramic materials suitable for a variety of electrooptic applications were found in the quaternary (Pb,La)(Zr,Ti)O₃ system. These PLZT materials are prepared from mixed oxides and hot-pressed typically at 1100°C for 16 h at 2000 psi. Modifying the lead zirconate-titanate system with lanthana linearly reduces the Curie point with increasing lanthana. Transmission measurements in the visible and infrared show that these materials exhibit a nearly constant response from the absorption edge of 0.37 μ to ∼6 μm. The highest transmission values, essentially 100% (neglecting reflection losses of ∼18%) for thin polished plates, were noted for compositions containing 8 at.% La or more. Specific compositions within the system display electrooptic memory or conventional linear or quadratic electrooptic effects; on the basis of the magnitude of the electrooptic effects, they compare quite favorably with single crystals.     

A Phase Diagram for Epitaxial PbZr1−xTixO3 Thin Films at the Bulk Morphotropic Boundary Composition

A phase diagram of temperature versus strain was constructed for a (001)-oriented PbZr_{1− x} Ti _x O₃ epitaxial single crystal thin film near the bulk morphotropic boundary composition ( x =0.47) on an (001)-oriented cubic substrate. The phase-field approach is employed. It is shown that a mixture of distorted rhombohedral, orthorhombic, and tetragonal phases exists under small values of strain, i.e., close to the in-plane clamped boundary condition. This result contradicts thermodynamic calculations assuming a single-domain state that predicted a single distorted rhombohedral phase under similar strains. Furthermore, it is demonstrated that under a large tensile strain current phase-field simulations showed a tetragonal phase with a ₁ /a ₂ twin structures as the stable state whereas thermodynamic calculations predicted an orthorhombic phase.     

Sinterability and Decomposition of Pb0.9175La0.055Zr0.975Ti0.025O3: Influence of Calcination and Sintering Temperature

The sinterability and decomposition of PLZT, (Pb,La)(Zr,Ti)O₃, depend on the temperature and soaking time of both the calcination and sintering temperature. They were determined from the density, linear shrinkage, weight loss, and appearance of extra phases. At moderate calcination temperatures and times, there is no escape of PbO from the PLZT. At calcination temperatures higher than 1050°C and soaking times above 3 h, PbO escapes, and ZrO₂ and La₂Zr₂O₇ can be detected. Even when sintered in a PbO-rich atmosphere, some PbO evaporates during sintering either from free PbO or from the PbO bound in the PLZT in regions in the outer surfaces of the sintered body. An aggressive depletion of PbO during sintering can result in a complete disappearance of the grain boundary phase, giving an intragranular fracture.     

Subsolidus Phase Equilibria and Ordering in the System ZrO2-Y2O3

The subsolidus phase relations in the entire system ZrO₂-Y₂O₃ were established using DTA, expansion measurements, and room- and high-temperature X-ray diffraction. Three eutectoid reactions were found in the system: ( a ) tetragonal zirconia solid solution→monoclinic zirconia solid solution+cubic zirconia solid solution at 4.5 mol% Y₂O₃ and ∼490°C, ( b ) cubic zirconia solid solutiow→δ-phase Y₄Zr₃O₁₂+hexagonalphase Y₆ZrO₁₁ at 45 mol% Y₂O₃ and ∼1325°±25°C, and ( c ) yttria C -type solid solution→wcubic zirconia solid solution+ hexagonal phase Y₆ZrO₁₁ at ∼72 mol% Y₂O₃ and 1650°±50°C. Two ordered phases were also found in the system, one at 40 mol% Y₂O₃ with ideal formula Y₄Zr₃O₁₂, and another, a new hexagonal phase, at 75 mol% Y₂O₃ with formula Y₆ZrO₁₁. They decompose at 1375° and >1750°C into cubic zirconia solid solution and yttria C -type solid solution, respectively. The extent of the cubic zirconia and yttria C -type solid solution fields was also redetermined. By incorporating the known tetragonal-cubic zirconia transition temperature and the liquidus temperatures in the system, a new tentative phase diagram is given for the system ZrO₂-Y₂O₃.     

The System Zirconia-Scandia

The system zirconia-scandia was investigated using X-ray diffraction analysis, differential thermal analysis, metallographic analysis, and melting point studies. Results reveal the monoclinic α₁ phase (0 to 2 mol% Sc₂O₃), the tetragonal α₂'phase (5 to 8% Sc₂O₃), the rhombohedral β phase (9 to 13% Sc₂O₃), the rhombohedral γ phase (15 to 23% Sc₂O₃), the rhombohedral δ phase (24 to 40% Sc₂O₃), and the cubic % phase (77.5 to 100% Sc₂O₃). The monoclinic α₁ phase and the tetragonal α₂'phase were found to transform to the tetragonal α₂ phase over a wide temperature range depending on composition. The β, γ, and α phases transformed to a cubic phase at temperatures of %600^%, 1100^%, and 1300^%C, respectively. A maximum melting point of %2870^%C was found at %10% Sc₂O₃ and a eutectic at %2400^%C at 55% Sc₂O₃.     

Microwave Dielectric Properties of Doped BaTi4O9

Polycrystalline BaTi₄O₉ doped with Mn, Sn, Zr, Ca, Sr, and Pb was prepared from carbonates and oxides. Single-phase ceramics with densities exceeding 97% of theoretical were made with up to 3 mol% Mn; 6 mol% Sn, Zr, and Ca; and 8 mol% Sr and Pb. Dielectric constant, k , quality factor, Q , and temperature coefficient of frequency, τ _f , of ∼37, 5675, and 15 ppm/°C, respectively, were determined at 4 GHz for undoped BaTi₄O₉. Doping did not significantly affect k and τ _f . However, doping with Mn, Sn, and Pb lowered Q , whereas doping with Zr, Ca, and Sr increased Q by up to 2000. Additions of 0.5 mol% MnO₂ as a second phase improved Q from 3675 to 7600.     

Effect of Silver Doping on the Physical and Electrical Properties of PLZT Ceramics

Additions of small amounts of silver to a PLZT dielectric with emphasis on the 88/12/70/30, Pb/La/Zr/Ti, composition were investigated. It was found that a few mole percent of Ag¹⁺ could be incorporated into the PLZT lattice as a large acceptor cation and that it tended to reduce the lead vacancies, which are normally generated by the substitution of La³⁺ in the PLZT dielectric. The addition, up to 2 mol% of silver, decreased the 25°C dielectric constant from 2300 to 1700. However, the temperature coefficient of capacitance was improved to ±5% between -55° and +125°C, and the dissipation factor was reduced from 1.5 to 0.5%. Although the gravimetrically measured lead loss appeared to depend on the relative vapor pressure of lead oxide during sintering, the compensation mechanism of Ag¹⁺ was not affected.