Phase Transitions in the System BaTiO3—KnbO3

Solid solution formation in the system BaTiO₃—KnbO₃ was established by X-ray diffraction and dielectric measurements. Solid solutions with cubic symmetry were observed in the composition range from 4 to 90 mole % KnbO₃ at room temperature. The lattice parameter for the BaTiO₃ solid solutions increased with increasing KNbO₃; that for the KnbO₃ solid solutions decreased with the addition of BaTiO₃. A distinct discontinuity in lattice parameter was observed at the composition containing about 65 mole % BaTiO₃. Dielectric measurements were made from-195° to 400°C. The cubic-tetragonal transition temperature of BaTiO₃ was rapidly lowered with increasing addition of KNbO₃, whereas the two lower phase transition temperatures were raised. All three phase transitions of KnbO₃ were rapidly lowered with increasing addition of BaTiO₃. The observed phase transitions, lattice parameters, and electron probe data suggest a complex region in the subsolidus which extends from 35 to about 75 mole % KNbO₃.     

High-Temperature Phase Transitions in PbZrO3

A high-temperature X-ray diffraction study of a high-purity PbZrO₃ specimen at 25° to 236°C is reported. The symmetry of the perovskite sub-cell of PbZrO₃ was determined unambiguously by observing the splitting of the line groups N = 12 and N = 16 when the diffraction pattern was indexed on the basis of the perovskite cell being a unit cell for the structure. Diffractometer studies were conducted of the N = 12 line group using Fe K α radiation and of the N = 16 line group using Cu K α radiation. The ferroelectric phase of PbZrO₃ possessed a rhombohedral sub-cell with the angle α being acute. Stable two-phase mixtures and considerable thermal hysteresis were observed at the antiferroelectric α ferroelectric transition temperature. A very weak "extra" diffraction line was observed in the diffraction patterns of the ferroelectric and paraelectric phases, indicating that the unit cells of these structures may be multiples of the perovskite subcell.     

Diffuse Ferroelectric Phase Transitions in Ba(Ti1-yZry)O3 Ceramics

Very high and broad Curie maxima are observed in ceramic Ba(Ti1-yZry)O3 mixed crystals, which are often used for the preparation of ceramic dielectrics. The ferroelectric-to-paraelectric phase transition of ceramic Ba(Ti1-yZry)O3 was studied using dielectric measurements, quantitative DTA, X-ray diffraction, and determination of the remanent polarization. At higher Zr concentrations, it was found that ferroelectric and paraelectric phases coexist in a wide temperature region. Up toy =0.16, the phase transition remains first order. The diffuse character is promoted by the small energy difference between the ferroelectric and paraelectric phases appearing at higher Zr content. It can be best described by a normal distribution of Curie temperatures using the phenome-nological theory of Devonshire and the Newton-Raphson mathematical approximation.     

Phase Transitions in Pb(Mg1/3Nb2/3)O3–PbTiO3 Studied by Low-Frequency Internal Friction Measurement

The low-frequency internal friction (0.1 Hz< f <10 Hz) of (100− x %)Pb(Mg_1/3Nb_2/3)O₃− x %PbTiO₃ ( x =0, 13, 23, 33) ceramics has been measured in the temperature range between 90 and 500 K. All of the internal friction peaks show a height that is inversely proportional to the measurement frequency, thus they can be attributed to first-order phase transitions. The phase transitions between rhombohedral, monoclinic, and tetragonal phases in ceramics with x =33 occur over a very broad temperature range from 150 to 400 K, indicating that the giant piezoelectric effect in this material may be attributed to a field-induced phase transition process.     

Structural and Dielectric Investigation of the PbTiO3-BaZrO3 System

Thermal expansion measurements, dielectric measurements, and X-ray diffraction studies were made of the PbTiO₃-BaZrO₃ system. A dielectric constant maximum of 11,300 was found at the Curie temperature for the Pb_0.80-Ba_0.20Ti_0.80Zr_0.20O₃ composition. The transition from a ferroelectric tetragonal phase to a para-electric cubic phase for the 10, 20, and 30 mole % BaZrO₃ in PbTiO₃ compositions was of first order. X-ray diffraction determination of the lattice parameters (25°C) showed that this tetragonal-to-cubic phase transition occurred at a composition of 44 mole % BaZrO₃.     

X-Ray Study of Phase Transitions in Ferroelectric PbNb2O6 and Related Materials

The phase transitions in PbNb₂O₆ and in compositions of the type Pb_1+x (B_xNb_1-x)O₆, where B = Ti⁴⁺, Zr⁴⁺, or Sn⁴⁺, have been investigated between 25° and 650°C. using X-ray and dilatometric techniques. The modified PbNb₂o₆ compositions possess orthorhombic PbNb₂O₆-type structure, with the additional Pb²⁺ ions occupying vacant lattice sites. The lattice parameters a and c expand and b contracts during heating until, at the ferroelectric Curie temperature, a and b suddenly coincide and c expands slightly. Besides this phase change at the Curie temperature, the nonstoichiometric compounds show an additional phase transition in the range 450° to 300°C. depending on composition. The intermediate phase of Pb_l+x(Ti₂Nb_1-z)₂O₆ appears to possess orthorhombic symmetry.     

Preparation of PbZrO3–PbTiO3 Ferroelectric Thin Films by the Sol–Gel Process

Ferroelectric films, PbZr _x Ti_{1− x} O₃ ( x = 0 to 0.6), have been prepared from corresponding metal alkoxides partially stabilized with acetylacetone through the sol-gel process. The films dip-coated in an ambient atmosphere were heat-treated at 400°C for decomposition of residual organics and then at temperatures between 500° and 700°C for crystallization of the films. The perovskite phase precipitated at temperatures above 560°C, accompanied by an increase in dielectric constant. The dielectric constant of the films, which was comparable with that of sintered bodies, showed a maximum (∼620) at around x = 0.52 in PbZr _x Ti_{1− x} O₃. These films showed D – E hysteresis, with slightly higher values of coercive field, compared with those of sintered bodies.     

High-Temperature Phase Transitions in Na Nb O3

An X-ray diffraction study of a high-purity NaNbO₃ specimen in the range 25α to 705α C is reported. The symmetry of the perovskite sub-cell of NaNbO₃ was determined unambiguously by observing the splitting of the line groups N = 12 and N = 16 when the diffraction pattern was indexed on the basis of the perovskite cell as a unit cell for the structure. Diffraetometter studies were conducted using Fe K α radiation for the N = 12 line group and Cu K α radiation for the N = 16 line group. Phase transitions occurred at 372°, 420°, 478°, 527°, 576°, and 640°± 5°C. The symmetry of the perovskite sub-cell changed from monoclinic to tetragonal at 372°C when heated and remained tetragonal up to 640°C when it became cubic. Two-phase mixtures were observed at 372°, 576°, and 640°C and it was concluded that these transitions are probably of first order.     

Formation of the Perovskite Phase in the PbMg1/3Nb2/3O3─PbTiO3 System

The reaction sequences during calcination of oxide mixtures were studied for the PbMg_1/3Nb_2/3O₃─PbTiO₃ (PMN-PT) system. The effect of reactivity and composition of the starting mixtures was investigated. In the present study, a B-site-deficient, cubic pyrochlore phase in the PbO-Nb₂O₅ system was formed at 500°C. The perovskite phase of PMN was formed at 7007deg;C through the diffusion of MgO into the pyrochlore phase. The lattice parameter of the pyrochlore phase decreased as this transformation to perovskite progressed.     

Hydrothermal Processing of PbTiO3 and PbTiO3/Polymer Thin Films from Titanium Metallo-organic Precursors

PbTiO₃ and PbTiO₃/polymer thin films were synthesized from metallo-organic precursors on metallized quartz substrates. Titanium dimethoxy dineodecanoate (TDD) was spin-cast onto the substrates and converted to polycrystalline TiO₂ via hydrolysis in deionized water for 5 h at 80°C. Polycrystalline PbTiO₃ films were then formed by reacting the TiO₂ films for 4 h at 200°C in aqueous solutions of KOH and Pb(CH₃COO)₂·3H₂O. Low KOH concentrations suppressed film coarsening, thereby facilitating the formation of fine-grain continuous PbTiO₃ films. PbTiO₃/polymer thin films were processed as above after first dissolving TDD and a polystyrene/polybutadiene block copolymer in p -xylene. PbTiO₃ and PbTiO₃/polymer films had relative permittivities of ≈56 and 34 and dielectric strengths of ≈250 and 850 kV/cm, respectively.     

Large Pyroelectric Effect in Relaxor-Based Ferroelectric Pb(Mg1/3Nb2/3)O3–PbTiO3 Single Crystals

The pyroelectric properties of (1− x )Pb(Mg_1/3Nb_2/3)O_{3− x} PbTiO₃ (PMN− x PT) single crystals with various compositions and orientations have been investigated using a dynamic method. Excellent pyroelectric performances can be achieved in 〈111〉-oriented rhombohedral PMN− x PT (0.24≤ x ≤0.30) crystals, where the measurement direction corresponds to the polar axis of the crystal. At room temperature, the pyroelectric coefficient and the detectivity figure of merit ( F _d ) for the 〈111〉-oriented PMN–0.28PT single crystal are 8.55 × 10⁻⁴ C·(m²·K)⁻¹ and 9.89 × 10⁻⁵ Pa^−1/2 (100 Hz), respectively, superior to those of the widely used pyroelectric materials. They are also weak temperature dependent and nearly independent of frequency. These outstanding pyroelectric performances make the single crystals a promising candidate for uncooled infrared detectors and thermal imagers.     

Dielectric and Electrostrictive Properties of Ferroelectric Relaxors in the System Pb(Mg1/3Nb2/3)O3:PbTiO3: Ba(Zn1/3Nb2/3)O3

Ceramic dielectrics which have been fabricated in the Pb(Mg_1/3 Nb_2/3)O₃:PbTiO₃:Ba(Zn_1/3Nb_2/3)O₃ composition system are shown to exhibit two distinct dielectric maxima, both of which show the characteristic loss spectra of ferroelectrics with diffuse phase transitions. The height of the individual maxima can be controlled by the Zn:Mg ratio in the starting material and, in suitably chosen compositions, a wide range of almost temperature-independent high dielectric permittivity is possible. These dielectrics show strong electrostrictive deformations under high electric fields but the electrostrictive strain is much less temperature-sensitive than in other relaxors.     

Structure of Polymeric PbTiO3 Gels

The structural features of PbTiO₃ gels were found to be highly dependent on hydrolysis conditions. Gels formed from acid-catalyzed solutions were clear and rubbery, whereas base-catalyzed gels were translucent or cloudy with phase separation. Direct TEM observations determined that acid-catalyzed gels were fibrous in character and physically homogeneous, whereas base-catalyzed gels had a coarse texture. Analyses by EDX and SAD indicated that acidic gels were chemically homogeneous and microcrystalline, whereas basic gels were heterogeneous and amorphous.     

Phase Transitions in LiAl5O8

LiAl₅O₈ exists in many forms. The equilibrium high-temperature form (I) has a spinel structure. It undergoes a first-order reconstructive transformation at 1295°× 5°C to an equilibrium low-temperature form (II) with primitive cubic symmetry, the unit edge changing from a ₀= 7.921 A (I) to a ₀= 7.907 A (II). In addition, a whole series of phases (II-metastable) exist with the same symmetry as II but with a disorder different from I → II disorder.     

Phase Relations and Electrical Parameters in the Ferroelectric-Antiferroelectric Region of the System PbZrO3–PbTiO3-PbNbO2O6

Characteristics of temperature and field-induced ferroelectric-antiferroelectric phase transitions are reported as a function of composition. The ferroelectric compositions were found to exhibit remanent polarizations of the order of 30 μcoul per cm², coupled with low dielectric constants and piezoelectric response. Evidence is presented for the existence of a polymorphic transition in the ferroelectric rhombohedral region of the system PbZrO₃-PbTiO₃ and the existing phase diagram modified accordingly.     

Ferroelectric 90° Domain Evaluation in Tetragonal Pb(Mg1/3Nb2/3)O3–PbTiO3 Ceramics

The domain structure of ferroelectrics changes during poling has a direct influence on the macroscopic properties of the materials. The intensity variation of the different X-ray diffraction (XRD) pattern profiles was used to identify the percentage of 90° domain reorientation in the tetragonal phase of Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PMN–PT) ceramics after poling. The results are consistent with the change of piezoelectric properties. In addition, by using XRD patterns, a spatial distribution of polarization in a well-poled 0.62PMN–0.38PT ceramics has been determined and was found to be best described by the Cauchy function W _00l (φ)=1/(1+0.023φ²).     

Electrostrictive Coefficients of 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 Relaxor Ferroelectric Ceramics in the Ferroelectricity-Dominated Temperature Range

Polarization and strain induced by unipolar electric fields (P_uni, S_uni) as well as those induced by bipolar electric fields (P_bi, S_bi) were measured in 0.9Pb(Mg_l/3Nb_2/3)O₃-0.1PbTiO₃ relaxor ferroelectric ceramics in the temperature range of −50°-90°C to observe the phase transition in this region and calculate the electrostrictive coefficients from the purely electric-field-induced polarization and strain. By considering both the electrostrictive component (F_uni, S_uni) and the piezoelectric component ( P_r , S_r), it is shown quantitatively how the transition occurs from pure electrostrictive to partially piezoelectric properties across the phase transition range. P_uni represents unmixed electric-field-induced polarization.
while F_bi represents the summation of P_uni and P_r . Similarly, S_uni represents unmixed electric-field-induced strain, while S_bi represents the summation of S_uni and S_r . The effective electrostrictive coefficient (Q_eff) is calculated even in the ferroelectric region far below the phase transition temperature using S_uni and P_uni which are purely electric field induced. Q_eff significantly increases as the temperature decreases below the phase transition temperature, which was attributed to the decreased rattling space of B-site atoms.