Perovskite phase formation in the PbMg1/3Nb2/3 O3-PbZrO3-PbTiO3 system by the columbite route

The reaction mechanism of PbMg_1/3Nb_2/3O₃-PbZrO₃-PbTiO₃ (PMN-PZT) perovskite phase prepared by the columbite route has been studied in the temperature range from 600 to 800 °C. The effects of heating and cooling rate during the calcination of 3PbO +MgNb₂O₆+PZT powder mixtures have also been investigated. Nearly pure perovskite phase, 0.9 PMN-0.1 PZTsolid solution with no pyrochlore phase, as determined by X-ray diffraction, could be prepared at 800 °C for 2 H. From DTA/TGA, dilatometry and XRD data the reaction mechanism of PMN-PZT solid solution formation could be divided into three steps: (i) decomposition of columbite (MgNb₂O₆) by reacting with PbO at 350 to 600 °C (ii) the formation of a B-site-deficient pyrochlore phase Pb₂Nb_1.33Mg_0.17O_5.50 at close to 650 °C, and (iii) the formation of perovskite phase PMN-PZT solid solution from the reaction of Pb₂Nb_1.33Mg_0.17O_5.50 pyrochlore phase with MgO and PZT above 650 °C.     

Reaction chemistry and subsolidus phase equilibria in lead-based relaxor systems Part II The ternary system PbO-MgO-Nb2O5

Subsolidus compatibility relations in the lead-based relaxor system PbO-MgO-Nb₂O₅ were investigated by the solid-state reaction technique and the various phase assemblages that occur at 825°C in this system established. The existence of two previously reported ternary compounds, i.e. a cubic perovskite Pb₃MgNb₂O₉ [Pb(Mg_1/3Nb_2/3)O₃] and an oxygen- deficient cubic pyrochlore Pb₆MgNb₆O₂₂ [Pb_1.714(Mg_0.286Nb_1.714)O_6.286] was confirmed. A minor amount of PbO enters into the pyrochlore lattice and forms a narrow homogeneity range within the ternary system PbO-MgO-Nb₂O₅ which corresponds to a general formula Pb_{2 – x} (Mg_0.286Nb_1.714)O_{6.571 – x} , where 0 > x > 0.286. At subsolidus temperatures (<825°C), the perovskite Pb(Mg_1/3Nb_2/3)O₃ is compatible with the ternary pyrochlore solid solution and together they form a biphasic area within the system in which the perovskite and the pyrochlore phases coexist with one another. Both PbO and MgO are compatible with the perovskite Pb(Mg_1/3Nb_2/3)O₃ and these phases constitute a compatibility triangle with one another in the ternary system. However, the perovskite Pb(Mg_1/3Nb_2/3)O₃ is not compatible with Nb₂O₅ and these two phases react with one another to yield the pyrochlore Pb₆MgNb₆O₂₂ and MgO. The ternary pyrochlore solid solution is compatible with several binary lead niobates, i.e. Pb₃Nb₂O₈, Pb₅Nb₄O₁₅, Pb₂Nb₂O₇ and Pb₃Nb₄O₁₃ and forms pseudobinary tie-lines with these compounds in the ternary system PbO-MgO-Nb₂O₅. The pyrochlores Pb₃Nb₄O₁₃ [Pb_1.5Nb₂O_6.5] and Pb₆MgNb₆O₂₂ [Pb_1.714(Mg_0.286 Nb_1.714)O_6.286] are isostructural compounds and a series of continuous solid solution is formed between them.     

Mechanochemical synthesis, phase composition, and properties of Pb(Zn1/3Nb2/3)O3-based ferroelectric ceramics

Pb-containing relaxor ferroelectric ceramics are prepared by mechanochemical ceramic processing. Mechanochemical reactions in binary and ternary mixtures of the PbO-ZnO-Nb₂O₅ system are studied by x-ray diffraction. Disordered compounds with the columbite, changbaiite, and pyrochlore structures are prepared. The perovskite and pyrochlore phases in 0.9Pb(Zn_1/3Nb_2/3)O₃ + 0.1ABO₃ morphotropic phase boundary materials are shown to be in mechanochemical equilibrium. Among the ABO₃ additives studied, BaMnO₃ is the most effective for stabilizing the perovskite structure. The mechanochemical synthesis path has a strong effect on the phase composition of the resulting material. Conventional synthesis through a columbite phase leads to the predominant formation of a pyrochlore phase. Firing conditions also have a profound effect on the phase composition of the ceramics, but the disordered perovskite phase retains cubic symmetry.     

Reaction chemistry and subsolidus phase equilibria in lead-based relaxor systems: Part I Formation and stability of the perovskite and pyrochlore compounds in the system PbO-MgO-Nb2O5

The reaction chemistry involved in the synthesis of perovskite Pb(Mg_1/3Nb_2/3)O₃ [Pb₃MgNb₂O₉] was studied by the solid state reaction technique using precursor oxides as reactants. At the initial stage of the reaction process, a large fraction of PbO present in the mixtures combined with Nb₂O₅ and a small amount of MgO to form an oxygen-deficient pyrochlore phase with a composition Pb_1.714(Mg_0.286Nb_1.714)O_6.286 [Pb₆MgNb₆O₂₂]. The pyrochlore phase thus formed further reacted with the remaining PbO and MgO to yield the perovskite Pb(Mg_1/3Nb_2/3)O₃. The pyrochlore Pb_1.714(Mg_0.286Nb_1.714)O_6.286 accomodates a small amount of PbO into its lattice and forms a narrow homogeneity range which extends from the composition Pb_1.714(Mg_0.286Nb_1.714)O_6.286 [Pb₆MgNb₆O₂₂] to a composition Pb₂(Mg_0.286Nb_1.714)O_6.571 [Pb₇MgNb₆O₂₃] with a corresponding increase in the lattice constant value from a = 10.586 to 10.601 Å. The pyrochlore phase melts incongruently at a temperature near 1230°C to yield Mg₄Nb₂O₉ and a liquid. Below this temperature, the perovskite Pb(Mg_1/3Nb_2/3)O₃ coexists with the pyrochlore solid solutions. However, the compound Pb(Mg_1/3Nb_2/3)O₃ is not compatible with Nb₂O₅ and these two phases react with one another to form the pyrochlore Pb_1.714(Mg_0.286Nb_1.714)O_6.286 and MgO.     

Preparation of KNbO3 by hydrothermal reaction

Perovskite-type KNbO₃ powder was prepared by hydrothermal reaction using Nb₂O₅ in KOH solution. A single phase of KNbO₃ was obtained when the molar ratio of KOH/Nb₂O₅ was above 20 and the reaction temperature was above 160 °C. Three types of KNbO₃ powder with the orthorhombic, tetragonal and cubic symmetries were obtained, depending on the reaction temperature and the ratio of KOH/Nb₂O₅. The molar ratios of K/Nb in the cubic and tetragonal phases were 0.91 and 0.94, respectively and that of the orthorhombic one was 0.98, and the mass loss was observed in the TG curves of tetragonal and cubic phases. The tetragonal and cubic phases were stabilized by OH⁻ and adsorbed water.     

New preparation method of low-temperature sinterable Pb(Mg1/3Nb2/3)O3 powder and its dielectric properties

A relaxor ferroelectric material, Pb(Mg_1/3Nb_2/3)O₃(PMN) with perovskite phase was prepared by one-step calcination in the present study. The PMN powder with >99% perovskite phase was prepared successfully by adding an aqueous Mg(NO₃)₂ solution rather than MgO to the alcoholic slurry of PbO and Nb₂O₅, followed by calcination at 950°C for 2 h. The DSC and XRD analysis showed that the pathway in the one-step calcination was different from those of the known columbite or solution processes. The PMN powder sintered to 95.6% of the theoretical density at even 900°C for 2 h. Its room temperature dielectric constant showed 13800 at 1 kHz, the loss of dielectric constant of 0.05% and the specific resistivity of 2.4 × 10¹⁰ ·cm.     

Phase evolution of electron-beam evaporated Pb(Zr,Ti)O3 thin films

Solid phase reactions among electron-beam deposited PbO, ZrO₂ and TiO₂ in the thin film state as distinct from those occurring in the bulk state are described under varied annealing conditions leading to growth of perovskite PZT phase. Loss of PbO by direct high temperature (700 °C) anneal led to the growth of cubic A₂B₂O_7−x pyrochlore as well as an AB₃O₇ phase of monoclinic structure. A lower temperature initial anneal at 600 °C in O₂ ambient minimises PbO loss through phase transformation to tetragonal Pb₃O₄ and better crystallised oxide phases partially react to form pyrochlore as well as perovskite PZT. This partial reaction is kinetically driven as it goes to completion in ∼4 h resulting in transformation of pyrochlore to perovskite phase. At high temperature (800 °C) A₂B₂O_7−x phase converts to PZT perovskite and the AB₃O₇ dissociate to yield TiO₂ secondary phase inclusion in the PZT film.     

Effect of excess Pb on formation of perovskite-type 0.67Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>-0.33PbTiO<Subscript>3</Subscript> powders synthesized through a sol–gel process

Perovskite-type 0.67Pb(Mg_1/3Nb_2/3)O₃-0.33PbTiO₃ (PMNT) powders were fabricated by using a sol–gel process. Excess Pb(CH₃COO)₂·3H₂O (0, 2, 5, 10 or 15 mol%) was added to starting materials to compensate PbO loss from volatilization during heat treatment. X-ray diffraction (XRD) was employed to investigate the effect of excess Pb on the perovksite phase formation of the PMNT powders. It was found that the optimal level of the excess Pb content is 5 mol%. When the raw materials contained 5 mol% excess Pb, the PMNT powders of purest perovskite form was obtained at the calcination temperature of 850 °C. In the PMNT powders, most part of the intermediate phase was Pb-rich pyrochlore Pb₂Nb₂O₇ which was transformed into perovskite phase after calcination at 650 °C, while the residual pyrochlore phase was Pb-deficient Pb₃Nb₄O₁₃ which required calcination at a higher temperature (650–850 °C) to transform into perovskite phase. Compared with the conventional solid-state reaction methods and the solution-based methods reported previously, the present sol–gel route is better at synthesizing PMNT powders of perovskite phase at a low temperature.     

Reaction sintering and characterization of lead magnesium niobate relaxor ferroelectric ceramics

The formation and densification of Pb(Mg_1/3Nb_2/3)O₃ ceramics prepared by reaction sintering have been investigated. Two kinds of lead sources. PbO and Pb₃O₄ are used as the starting materials for Pb(Mg_1/3Nb_2/3)O₃. During heating processes, the specimens first expand while the pyrochlore phase is formed. At elevated temperatures, the formation and rapid sintering of Pb(Mg_1/3\Nb_2/3)O₃ occur simultaneously. The starting materials of the Pb₃\O₄ system exhibit better reactivity and sinterability than those of the PbO system. With Pb₃\O₄ as the starting material, monophasic Pb(Mg_1/3\Nb_2/3)O₃ ceramics with high sintering density are successfully achieved by reaction sintering at as low as 900 °C. While for the PbO system, pure perovskite phase could not be synthesized because of the existence of residual pyrochlore phase, and the ceramics obtained have low sintering density. The dielectric permittivity of the Pb(Mg_1/3Nb_2/3)O₃ ceramics obtained in the Pb₃O₄ system is higher than that in the PbO system. This is attributed to the formation of pure perovskite phase and high sintering density in the former system.     

Distribution of pyrochlore phase in Pb(Mg1/3Nb2/3)O3-PbTiO3 films and suppression with a Pb(Zr0.52Ti0.48)O3 interfacial layer

Thin films of the relaxor ferroelectric Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT) on Pt/Ti/SiO₂/Si (Pt/Si) substrates both with and without a Pb(Zr_0.52Ti_0.48)O₃ (PZT) interfacial layer were investigated. Perovskite and pyrochlore coexistence was observed for PMN-PT thin films without a PZT interfacial layer. Interestingly, most of the pyrochlore phase was observed in single-coated films and in the first layer of multi-coated films. The pyrochlore phase exhibited grains with an average size of about 25 nm, which is smaller than those of the perovskite phase (about 90 nm). In contrast, for PMN-PT thin films grown on a PZT interfacial layer, the formation of a pyrochlore phase at the interface between PMN-PT layers and the substrate is completely suppressed. Moreover, small grains are not observed in the films with a PZT interfacial layer. The measured polarization-electric field (P-E) hysteresis loops of PMN-PT films with and without PZT layers indicate that enhanced electrical properties can be obtained when a PZT interfacial layer is used. These enhanced properties include an increase in the value of remanent polarization P_r from 2.7 to 5.8 μC/cm² and a decrease in the coercive field E_c from 60.5 to 28.0 kV/cm.     

Synthesis of Pb(Mg1/3Nb2/3) O3 perovskite by an alkoxide method

Pb(Mg_1/3Nb_2/3)O₃ materials have been synthesized using sol-gel, freeze-drying or spray-pyrolysis techniques. The as-prepared powders were of an amorphous form which could be converted into a crystalline form by calcination. The pyrochlore phase was inevitably formed with an accompanying perovskite phase. As the calcining temperature increased, greater proportions of the desired perovskite phase occurred. The residual pyrochlore phase could be completely transformed into the perovskite phase when the powders were prepared via freeze-drying or by a spray-pyrolysis method. The maximum proportion of the pyrochlore phase was, however, only 92% when the powders were synthesized by a sol-gel route. Thermal gravimetric analysis/differential thermal analysis (TGA/DTA) and infrared transmission spectroscopy (FTIR) indicated that Mg(OEt)₂ and Nb(OEt)₅ formed a double alkoxide but Pb(OAc)₂ formed separate clusters during the hydrolysis of the solution in the sol-gel process. Inhomogeneous mixing meant that the intermediate phase formed was rather difficult to eliminate completely. Homogeneous mixing was preserved when the solution was directly freeze dried or spray pyrolysed. The size of the preferentially formed pyrochlore phase was very fine and further transformation was feasible. Pb(Mg_1/3Nb_2/3)O₃ materials, free of the pyrochlore phase, could therefore be obtained.     

Preparation of lead magnesium niobate by a coprecipitation method

The ferroelectric complex perovskite lead magnesium niobate, Pb(Mg_1/3Nb_2/3)O₃ (PMN), was prepared by a coprecipitation method. As the niobium component, niobium oxalate was used. Among the various precipitants, tetramethylammonium hydroxide was effective for the formation of single-phase PMN without pyrochlore phase. The dielectric constant and the dissipation factor of PMN changed either the sintering temperature or the grain size. The maximum dielectric constant at the Curie point was obtained by sintering at 1220 °C in air for 2 h.     

Bronzoid phases in the pseudo-binary system AxNbxW1−xO3 with A = K and Cs

The systems A_xNb_xW_1?xO₃, A = K, Cs, have been studied at 1200°C for compositions x < 0.5. Phases with intergrowth tungsten bronze (ITB) structure form for x ～ 0.08 – 0.12 and with hexagonal tungsten bronze (HTB) structure for x ～ 0.20 (with a small homogeneity range). The potassium system contains a tetragonal tungsten bronze (TTB)-type phase at x ～ 0.45 – 0.60 and cesium forms a pyrochlore phase at x～0.35 – 0.40. The alkali contents of the HTB and pyrochlore phases are considerably lower than those of phases prepared at 900°C. The tunnel occupancy is approximately the same, ～ 60 %, in the ITB, HTB and pyrochlore phases prepared at 1200°C. Two types of superstructures have been observed in the HTB-type crystals.     

Role of La0.5Sr0.5CoO3 template layers on dielectric and electrical properties of pulsed-laser ablated Pb(Nb2/3Mg1/3)O3-PbTiO3 thin films

Relaxor ferroelectric thin films of 0.7Pb(Mg_1/3Nb_2/3)O₃-0.3PbTiO₃ (PMN-PT) deposited on platinized silicon substrates with and without template layers were studied. Perovskite phase (80% by volume) was obtained through proper selection of the processing conditions on bare Pt/Ti/SiO₂/Si substrates. The films were initially grown at 300 °C using pulsed-laser ablation and subsequently annealed in a rapid thermal annealing furnace in the temperature range of 750-850 °C to induce crystallization. Comparison of microstructure of the films annealed at different temperatures showed change in perovskite phase formation and grain size etc. Results from compositional analysis of the films revealed that the films initially possessed high content of lead percentage, which subsequently decreased after annealing at temperature 750-850 °C. Films with highest perovskite content were found to form at 820-840 °C on Pt substrates where the Pb content was near stoichiometric. Further improvement in the formation of perovskite PMN-PT phase was obtained by using buffer layers of La_0.5Sr_0.5CoO₃ (LSCO) on the Pt substrate. This resulted 100% perovskite phase formation in the films deposited at 650 °C. Dielectric studies on the PMN-PT films with LSCO template layers showed high values of relative dielectric constant (3800) with a loss factor (tan δ) of 0.035 at a frequency of 1 kHz at room temperature.     

Preparation and characterization of new visible-light-driven BaCo0.5Nb0.5O3 photocatalyst

A new type of visible-light-driven photocatalyst BaCo_0.5Nb_0.5O₃ was successfully synthesized via a sol-gel process in this study. After heating the precursors at 1000 °C, a pure perovskite phase was obtained. The particle size and crystallinity of BaCo_0.5Nb_0.5O₃ powders markedly increased with a rise in the calcination temperature. The band gap of BaCo_0.5Nb_0.5O₃ calculated from the UV-visible spectra was found to be less than that of titania. BaCo_0.5Nb_0.5O₃ was demonstrated to have photocatalytic activity under visible light irradiation and this activity significantly depended on the synthesis temperature. The sol-gel derived powders were found to have better photocatalytic activity than the solid-state derived powders because of the reduced particle size and increased surface area.     

Electric-field-induced strain and piezoelectric properties of a high Curie temperature Pb(In1/2Nb1/2)O3-PbTiO3 single crystal

The temperature dependence of dielectric and piezoelectric properties, electric-field-induced strains of 0.66 Pb(In_1/2Nb_1/2)O₃-0.34 PbTiO₃ single crystals, which were grown directly from melt by using the modified Bridgman technique with the allomeric Pb(Mg_2/3Nb_1/3)O₃-PbTiO₃ seed crystals, were determined as a function of crystallographic orientation with respect to the prototypic (cubic) axes. Ultrahigh piezoelectric response (d₃₃∼2000 pC/N, k₃₃∼94%) and strain levels up to 0.8%, comparable to rhombohedral (1−x)Pb(Mg_2/3Nb_1/3)O₃-xPbTiO₃ and (1−x)Pb(Zn_2/3Nb_1/3)O₃-xPbTiO₃ single crystals, were observed for the 〈0 0 1〉-oriented crystals. Strain levels up to 0.47% and piezoelectric constant d₃₃∼1600 pC/N could be achieved being related to an electric-field-induced rhombohedral-orthorhombic phase transition for the 〈1 1 0〉-oriented crystals. In addition, high electromechanical coefficients k₃₃ (∼88%) can be achieved even heating to 110 °C. High T_C (∼200 °C), large electromechanical coefficients k₃₃ (∼94%) and low dielectric loss factor (∼1%), along with large strain make the crystals promising candidates for a wide range of electromechanical transducers.     

Synthesis and structural characterization of some Pb(B 1 3/t’ Nb2/3)O3 type materials by two-stage solid-state route

Two-stage columbite solid state reaction route has been used for the preparation of Pb(B ₁ ^3/t’ Nb_2/3)O₃ materials (B′ = Mg, Ni and Cd). The columbite precursor phase was structurally characterized using diffraction data. MgNb₂O₆, NiNb₂O₆ and CdNb₂O₆ show orthorhombic structures i.e. pure columbite phase. Final phase materials get stabilized in mixed phase. The diffraction pattern shows that it is a mixture of cubic pyrochlore and perovskite phase. Percentage of perovskite phase was calculated using the band intensities of (110) perovskite and (222) pyrochlore peaks. The calculated percentages show the dominant perovskite phase. Possible reasons for mixed phase are discussed.     

Effect of gadolinia on phase structure and thermal conductivity of ZrO2-4.5 mol%Y2O3 ceramics

This paper deals with the effect of gadolinia on the phase structure and thermal conductivity of ZrO₂-4.5 mol%Y₂O₃ (YSZ) ceramics for thermal barrier coatings. The YSZ-Gd₂O₃ ceramics were synthesized by solid state reaction at 1500 °C for 2 h in air. The relative density, structure of different YSZ-Gd₂O₃ ceramics and thermal diffusivity in a temperature range of room temperature to 1200 °C were investigated by the Archimedes method, X-ray diffraction and laser-flash method. The ZrO₂-4.5 mol%Y₂O₃ (YSZ) ceramics consist of tetragonal, cubic and a small amount of monoclinic phase, and the YSZ-1.5 mol%Gd₂O₃ ceramics consist of both tetragonal and cubic phases. However, the YSZ-3.0 mol%Gd₂O₃, YSZ-4.5 mol%Gd₂O₃ and YSZ-6.0 mol%Gd₂O₃ ceramics only exhibit a cubic structure. The thermal conductivity of YSZ-Gd₂O₃ ceramics decreases with the increase of gadolinia content under identical temperature conditions. The thermal conductivities of the YSZ and YSZ-1.5 mol%Gd₂O₃ ceramics first decrease gradually with the increase of temperature below 800 °C and then increase slightly above 800 °C. The thermal conductivities of the YSZ-3.0 mol%Gd₂O₃, YSZ-4.5 mol%Gd₂O₃ and YSZ-6.0 mol%Gd₂O₃ ceramics are almost constants from room temperature to 1200 °C.     

Acoustic emission study of phase relations in low-Y2O3 portion of ZrO2-Y2O3 system

The (metastable) tetragonal phase in 3–4 mol% Y₂O₃-ZrO₂ alloys undergoes a transition to the monoclinic form in the 200–300 °C temperature range. Microcracking due to the volume change at this transition has been detected in these compositions by sharp acoustic emission during heating. The phase change was confirmed by X-ray diffraction, dilatometry and scanning electron microscopy. The monoclinic tetragonal transition in ZrO₂-1 mol% Y₂O₃ alloy at 850–750 °C and the same phase change in 2, 3, 4 and 6 mol% Y₂O₃ compositions at the eutectoid temperature of about 560 °C was also clearly signalled by the acoustic emission counts during heating and cooling. There was no significant acoustic emission activity on heating and cooling the 9 and 12 mol% Y₂O₃ compositions, which are cubic. The acoustic emission data thus confirm the phase relations in the 1–12 mol% Y₂O₃ region, established by conventional methods such as differential thermal analysis, dilatometry and X-ray diffraction.     

Stoichiometric Pb(Mg1/3Nb2/3)O3 perovskite ceramics produced by reaction-sintering process

Stoichiometric lead magnesium niobate, Pb(Mg_1/3Nb_2/3)O₃ (PMN), perovskite ceramics produced by reaction-sintering process were investigated. Without calcination, a mixture of PbO, Nb₂O₅, and Mg(NO₃)₂ was pressed and sintered directly. Stoichiometric PMN ceramics of 100% perovskite phase were obtained for 1, 2, and 4 h sintering at 1250 and 1270 °C. PMN ceramics with density 8.09 g/cm³ (99.5% of theoretical density 8.13 g/cm³) and K_max 19,900 under 1 kHz were obtained.