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.     

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 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.     

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.     

Phase developments in the Pb(Zn1/2W1/2)O3-Pb(Zn1/3Ta2/3)O3-Pb(Zn1/3Nb2/3)O3 pseudo-ternary system

The phase stability ranges in the B-site precursor (Zn_1/2W_1/2)O₂-(Zn_1/3Ta_2/3)O₂-(Zn_1/3Nb_2/3)O₂ were determined by X-ray diffraction (XRD), where wolframite, tri-αPbO₂, and columbite phases were identified. Next attempts were carried out (with the addition of PbO) for the system Pb(Zn_1/2W_1/2)O₃-Pb(Zn_1/3Ta_2/3)O₃-Pb(Zn_1/3Nb_2/3)O₃, where the perovskite phase did not develop in the entire compositions investigated. Instead, only the Pb₂WO₅ and pyrochlore phases (along with ZnO) resulted.     

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.     

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.     

Electrical properties of Pb(Mg1/3Nb2/3)O3-PbTiO3 ceramics modified with WO3

Ferroelectrics 0.67Pb (Mg_1/3Nb_2/3)O₃-0.33PbTiO₃ (PMN-PT) + x mol% WO₃ (x=0.1, 0.5, 1, 2) were prepared by columbite precursor method. Electrical properties of WO₃-modified ferroelectrics were investigated. X-ray diffraction (XRD) was used to identify crystal structure, and pyrochlore phase were observed in 0.67Pb (Mg_1/3Nb_2/3)O₃-0.33PbTiO₃+2 mol% WO₃. Dielectric peak temperature decreased with WO₃ doping, indicating that W⁶⁺ incorporated into PMN-PT lattice. Lattice constant, pyrochlore phase and grain size contribute to the variation of K_max. Both piezoelectric constant (d₃₃) and electromechanical coupling factors (k_p) were enhanced by doping 0.1 mol% WO₃, which results from the introduction of “soft” characteristics into PMN-PT, while further WO₃ addition was detrimental. We consider that the two factors, introduction of “soft” characteristics and the formation of pyrochlore phase, appear to act together to cause the variation of piezoelectric properties of 0.67PMN-0.33PT ceramics doping with WO₃.     

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.     

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.     

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.     

Dielectric responses in Mg<Subscript>1/3</Subscript>Ta<Subscript>2/3</Subscript>-replaced Pb[(Zn<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>),Ti]O<Subscript>3</Subscript> ceramics

Octahedral lattice sites of Pb[(Zn_1/3Nb_2/3),Ti]O₃ were replaced by 20 at.% Mg_1/3Ta_2/3 complex to enhance perovskite development, especially at Pb(Zn_1/3Nb_2/3)O₃-rich compositions. Resultant changes in the perovskite formation and associated dielectric responses were investigated. A perovskite structure was identified at Pb(Zn_1/3Nb_2/3)O₃-rich compositions by X-ray diffraction, although the development was rather incomplete. Phase transition modes in the dielectric constant spectra changed from diffuse to sharp ones, regardless of the introduction of Mg_1/3Ta_2/3. Dielectric maximum temperatures of the ceramics shifted linearly with the compositional change.     

TEM investigations in the PFN-PFW-PZN perovskite system

Using an analytical electron microscope, the distribution of phases and order-disorder phenomena in the Pb(Fe_1/2Nb_1/2)O₃-Pb(Fe_2/3W_1/3)O₃-Pb(Zn_1/3Nb_2/3)O₃ (PFN-PFW-PZN) perovskite system were examined. It was found that after sintering the solid solution, small cubic pyrochlore particles with chemical composition close to Pb(Fe_0.28Zn_0.01Nb_0.50W_0.21)O_3.4 anda ₀=1.05 nm were present in the 0.8 mol % Pb₂WO₅-0.2 mol % PbO liquid phase. In samples fired at 1000 °C, several 10 nm sized ordered areas with faint F spots in the selected-area electron diffraction pattern and (h+1/2k+1/2l+1/2) lattice fringes were found. Similar features were found in the PZN perovskite monocrystal, leading to the conclusion that local 11 ordering of cations (presumably Zn²⁺ and Nb⁵⁺) in the PFN-PFW-PZN system caused partial decomposition of the perovskite phase at firing temperatures equal to or higher than 850 °C.     

Phase formation and characterization of [Fe, Mg]NbO4 as a new precursor for the PMN–PFN system

An X-ray diffraction (XRD) and scanning electron microscopy (SEM) study of the phase composition and microstructure characteristics of the Mg_{(1 –x)/3}Nb_{(4 –x)/6}Fe _x/2O₂ (x = 0.5) chemical compound is presented. The samples were prepared by the conventional ceramic method and subjected to different heat treatments. Columbite (MgNb₂O₆) and iron niobium oxide (FeNbO₄, Wolframite) were identified as intermediate compounds in the reaction. A new single phase precursor for the (1 –x)Pb(Mg_1/3Nb_2/3)O₃-xPb(Fe_1/2Nb_1/2)O₃ (PMN–PFN) system identified as [Fe, Mg]NbO₄, was obtained, isostructural with the FeNbO₄ where Fe and Mg ions occupy the same crystal site (space group P1 2/a 1). From the Rietveld refinement method the cell parameters of the monoclinic structure were determined. The microstructure analysis indicates that the particles are irregular in shape and the grain size tends to increase with the calcination temperature.     

Dielectric characteristics of bismuth-modified lead magnesium niobate ceramics

Aliovalent Bi was substituted for Pb in Pb(Mg_1/3Nb_2/3)O₃ with required alteration in the Mg/Nb ratio. Resultant changes in the perovskite developments, lattice parameters as well as dielectric characteristics were investigated. Powders were prepared via a two-step B-site precursor route to enhance the perovskite formation. The perovskite structure persisted up to the range of 30 mol% Bi(Mg_2/3Nb_1/3)O₃ substitution. Values of the maximum dielectric constant decreased drastically, while the dielectric maximum temperatures changed only moderately. Meanwhile, the diffuseness exponent values decreased continuously with the Bi modification.     

The formation and phase stability of lead magnesium niobate in the presence of a molten flux

The formation of perovskite Pb(Mg_1/3Nb_2/3)O₃ (PMN) by the molten salt synthesis method using sulphate flux has been investigated as a function of calcination temperature and mole ratio between Li₂SO₄ and Na₂SO₄. A 97% perovskite phase was synthesized at 750° C for 30 min with 0.635Li₂SO₄-0.365Na₂SO₄ flux without any sub-products affecting the formation reaction of the PMN phase. The percentage of the perovskite phase was influenced by changes in the Li₂SO₄/Na₂SO₄ mole ratio at a given temperature. The pyrochlore phase present was identified as Pb₃Nb₄O₁₃ (P₃N₂) when 0.635Li₂SO₄-0.365Na₂SO₄ flux was used. The results for other lead-based ferroelectrics are also discussed.     

Effect of MnO2 addition on the piezoelectric properties in Pb(Mg1/3Nb2/3)O3 relaxor ferroelectrics

The effects of MnO₂ addition on the piezoelectric properties in Pb(Mg_1/3Nb_2/3)O₃ relaxor ferroelectrics were studied in the ferroelectricity-dominated temperature range from −40 to 30°C. Dielectric, piezoelectric, and electrostrictive properties were examined to clarify the effect of MnO₂ addition. As an added amount of MnO₂ increases, the dielectric constant decreases and the mechanical quality factor increases in Pb(Mg_1/3Nb_2/3)O₃. From the experimental results, it has been found that Mn behaves as a ferroelectric domain pinning element.     

Diffuse transition and piezoelectric properties of Pb[(Zr1−xTix)0.74(Mg1/3Nb2/3)0.20(Zn1/3Nb2/3)0.06]O3 Ceramics

In this work, the piezoelectric ceramic system of Pb[(Zr_1−xTi_x)_0.74(Mg_1/3Nb_2/3)_0.20(Zn_1/3Nb_2/3)_0.06]O₃, 0.47≤x≤0.57, with composition close to the morphotropic phase boundary, was studied. From the results of X-ray diffraction and piezoelectric measurement, ceramics near x=0.51 were found at the morphotropic phase boundary (MPB) between the tetragonal and pseudocubic perovskite. The planar coupling factor (k_p=0.72) is high at compositions near the MPB, but the mechanical quality factor (Q_m=75) is low. The calculation of the diffuseness of phase transition shows that the region of phase coexistence of this system is broader than that of the ternary system.     

Top-cooling-solution-growth and characterization of piezoelectric 0.955Pb(Zn1/3Nb2/3)O3-0.045PbTiO3 [PZNT] single crystals

A technique of top-cooling-solution-growth (TCSG) has been developed to grow the piezo-/ferroelectric perovskite single crystals of 0.955Pb(Zn_1/3Nb_2/3)O₃-0.045PbTiO₃ [PZNT95.5/4.5]. The flux composition and concentration, and the thermal parameters have been optimized, leading to the growth of high quality PZNT crystals with a size up to 20 × 15 × 10 mm³. The perovskite crystals are found to form upon slow cooling down to 1020°C, while the undesirable pyrochlore crystals of Pb_1.5Nb₂O_6.5-type start growing upon further cooling from 1020°C to 950°C. By controlling the growth pathway, the formation of the pyrochlore phase can be avoided. The dielectric properties of the grown PZNT95.5/4.5 crystals have been measured as a function of temperature at various frequencies. Upon heating, the phase transition for the rhombohedral R3m to the tetragonal P4mm phase takes place at 132°C, while the tetragonal to cubic phase transition occurs at 160°C. The TCSG developed in this work provides an alternative technique to grow PZNT piezocrystals of medium size at low cost for transducer applications.