Dielectric Bodies in the Quaternary System BaTiO3–BaSnO3–SrSnO3–CaSnO3

Dense bodies were prepared of compositions in the quaternary system BaTiO₃–BaSnO₃–SrSnO₃–CaSnO₃ containing from 3 to 60 mole % stannate. The general effect of the stannate addition to barium titanate was to decrease the Curie temperature and broaden the peak. On a molar basis the three stannates were approximately equivalent in their effect on the dielectric properties of barium titanate, although the rate of shift of the Curie temperature was slightly greater when SrSnO₃ was used. Bodies containing calcium or strontium stannate had lower power factors than those containing barium stannate. Bodies compounded with calcium stannate matured most readily and at lower temperatures. Bodies having dielectric constants ( K ) of 2300 to 2800 at 1 kc. with low positive temperature coefficients up to about 55°C. were obtained with a 3 mole % addition of stannate. Bodies with minimum K 's of 3000 to 4000 at 1 kc. over the range 25° to 85°C. were obtained from BaTiO₃ with an addition of about 6 mole % BaSnO₃, SrSnO₃, or CaSnO₃. Bodies with negative temperature coefficients of K ranging from about 13,000 to about 1000 p.p.m. per °C. were obtained with stannate additions of from 10 to 60 mole %.     

Phase Equilibrium Studies in the System CaO-Cr2O3-SiO2

Results are presented of a study in air of mixtures in the system CaO-Cr₂O₃-SiO₂. The phase equilibrium diagram shows relations at liquidus temperatures for all but the high-lime part of the system. In this omitted part chromium in the mixtures oxidizes in air to higher valence forms. The compound Ca₃Cr₂Si₃O₁₂ (uvarovite) occurs at subsolidus temperatures, decomposing at 1370°C. to α-CaSiO₃ and Cr₂O₃. The inhibiting action of chromium oxide on the inversion of high-temperature forms of Ca₂SiO₄ to the low-temperature γ-Ca₂SiO₄ is discussed in the light of new data. Evidence is presented for the existence of a pentavalent chromium compound, Ca₃(CrO₄)₂, having solid-solution relations with Ca₃SiO₄.     

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.     

Structure-Property Phase Diagram of BaZrxTi1−xO3 System

In the course of searching environmental friendly lead-free relaxor ferroelectrics a complete phase diagram of barium zirconate titanate, Ba(Zr _x Ti_{1− x} )O₃ system with compositions 0.00≤ x ≤1.00 has been developed based on their dielectric behavior. It has been shown that BaZr _x Ti_{1− x} O₃ system depending on the composition, successively depicts the properties extending from simple dielectric (pure BaZrO₃) to polar cluster dielectric, relaxor ferroelectric, second order like diffuse phase transition, ferroelectric with pinched phase transitions and then to a proper ferroelectric (pure BaTiO₃). A comprehensive structure–property correlation of BaZr _x Ti_{1− x} O₃ ceramics has been studied to understand the various ferroelectric phenomena in the whole phase diagram.     

A-Site and B-Site Order in (Na1/2La1/2)(Mg1/3Nb2/3)O3 Perovskite

(Na_1/2La_1/2)(Mg_1/3Nb_2/3)O₃ undergoes a series of phase transitions that involve cation order on the A- and B-sites of the parent perovskite structure. At high temperatures both sites contain a random distribution of cations; below 1275°C a 〈111〉 layering of Mg and Nb leads to the formation of a 1:2 ordered structure with a monoclinic supercell. A second transition was observed at 925°C, where the Na and La cations order onto alternate A-site positions along the 〈001〉 direction of the parent subcell. By quenching samples from above 1275°C to preserve the disorder on the B-site, a fourth variant of this compound was obtained by inducing A-site order through a subsequent anneal at 900°C. Although the changes in structure do not produce significant alterations in the relative permittivity (ɛ_r∼ 35), they do have a significant effect on the value of the temperature coefficient of the capacitance.     

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.     

Ferroelectric and Structural Properties of the Pb(Sc1/2Nb1/2)1-x,Tix,O3 System

Extensive solid solution was observed in the system Pb(Sc_1/2/,Nb_1/2,)_1-x,Ti_x,O₃. In the range 0 ≤ x ≤ 0.425 a rhombohedral ferroelectric phase was stable at 25° C. In the range 0.45 ≤ x ≤ 1.00 a tetragonal ferroelectric phase was stable at this temperature. The phase diagram of the system below 500° C strongly resembles that of PbZrO₃−PbTiO₃. The compound Pb(Sc_1/2Nb_1/2)O₃ exhibited rhombohedral perovskite cell symmetry below the ferroelectric ↔ paraelectric transition temperature, and the angle a was acute. The radial coupling coefficient was 0.46 for the composition Sc_1/2Nb_1/2)_0.575Ti_0.4250O₃. At 25°C this composition consisted primarily of the rhombohedral phase with a small amount of the tetragonal phase present. The ferroelectric ↔ paraelectric transition occurred over a temperature range in the rhombohedral phase field. The spontaneous polarization was finite at temperatures considerably above the temperature of the permittivity maximum for a given rhombohedral solid solution.     

Effects of Amorphous and Crystalline SiO2 Additives on γ-Al2O3-to-alpha-Al2O3 Phase Transitions

This paper focused on the effects of various phases of SiO₂ additives on the γ-Al₂O₃-to-α-Al₂O₃ phase transition. In the differential thermal analysis, the exothermic peak temperature that corresponded to the theta-to-α phase transition was elevated by adding amorphous SiO₂, such as fumed silica and silica gel obtained from the hydrolysis of tetraethyl orthosilicate. In contrast, the peak temperature was reduced by adding crystalline SiO₂, such as quartz and cristobalite. Amorphous SiO₂ was considered to retard the γ-to-α phase transition by preventing γ-Al₂O₃ particles from coming into contact and suppressing heterogeneous nucleation on the γ-Al₂O₃ surface. On the other hand, crystalline SiO₂ accelerated the α-Al₂O₃ transition; thus, this SiO₂ may be considered to act as heterogeneous nucleation sites. The structural difference among the various SiO₂ additives, especially amorphous and crystalline phases, largely influenced the temperature of γ-Al₂O₃-to-α-Al₂O₃ phase transition.     

Crystal Chemistry and Dielectric Properties in the Systems BaTiO3-UO2 and BaTiO3-BaUO3

Polycrystalline barium titanate fired in nitrogen at 1300° to 1400°C accommodates up to 3 mole % UO₂ in solid solution; its structure is then cubic at room temperature. With BaUO₃ additions the structure becomes disordered and quasi-cubic. In air, about 1 mole % UO₂ goes into solid solution in BaTiO₃ but the structure remains tetragonal. Diffraction peaks of a new phase, possibly a ternary oxide of barium, uranium, and titanium, appear in patterns of specimens containing more than 2 mole % UO₂. The dielectric constant of BaTiO₃ ceramics fired in air, steam, or oxygen increases with up to about 0.5 mole % UO₂ but declines rapidly above this level. The dielectric constant of BaUO₃ is about two orders of magnitude lower than that of BaTiO₃, and additions of BaUO₃ invariably lower the dielectric constant of BaTiO₃.     

Crystallization of a PbO-BaO-TiO2-Al2O3-SiO2 Glass

The phase development, microstructure development, and Curie temperature of PbO-BaO-TiO₂-Al₂O₃-SiO₂ glass- ceramics were investigated by X-ray diffractometry, electron microscopy, and dilatometry. The primary crystalline phase was perovskite titanate, which exhibited bulk nucle- ation. When the samples were heated at low temperatures, the Curie inversion was hindered and cubic perovskite was obtained. Increased heating temperature/time resulted in tetragonal perovskite. Growth of a surface layer due to the crystallization of the secondary crystalline phase PbO- Al₂O₃-2SiO₂ resulted in the coexistence of coarse and fine perovskite particles. The increase in the relative number of coarse-to-fine perovskite particles with the heating temperature/time was the major factor responsible for increased spontaneous deformation and higher Curie temperature of the perovskite phase. Substitution of barium for lead in the lead titanate phase probably occurred and is explained by the use of SiO₂ instead of B₂O₃ as the glass former in the present study.     

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.     

Massive Transformation in the Y2O3-Bi2O3 System

Cubic solid solutions in the Y₂O₃-Bi₂O₃ system with ∼25% Y₂O₃ undergo a transformation to a rhombohedral phase when annealed at temperatures ≤ 700°C. This transformation is composition-invariant and is thermally activated, and the product phase can propagate across matrix grain boundaries, indicating that there is no special crystallo-graphic orientation relationship between the product and the parent phases. Based on these observations, it is proposed that cubic → rhombohedral phase transformation in the Y₂O₃-Bi₂O₃ system is a massive transformation. Samples of composition 25% Y₂O₃-75% Bi₂O₃ with and without aliovalent dopants were annealed at temperatures ≤ 700°C for up to 10000 h. ZrO₂ as a dopant suppressed while CaO and SrO as dopants enhanced the kinetics of phase transformation. The rate of cubic/rhombohedra1 interface migration (growth rate or interface velocity) was also similarly affected by the additions of dopants; ZrO₂ suppressed while CaO enhanced the growth rate. Diffusion studies further showed that ZrO₂ suppressed while CaO enhanced cation interdiffusion coefficient. These observations are rationalized on the premise that cation interstitials are more mobile compared to cation vacancies in cubic bismuth oxide. The maximum growth rate measured was ∼10⁻¹⁰ m/s, which is orders of magnitude smaller than typical growth rates measured in metallic alloys. This difference is explained in terms of substantially lower diffusion coefficients in these oxide systems compared to metallic alloys.     

Dielectric Properties of Pb(Fe2/3W1/3)O3–PbTiO3 Solid-Solution Ceramics

The dielectric properties of (1− x )Pb(Fe_2/3W_1/3)O₃· x PbTiO₃ solid solutions were investigated from 10² to 10⁶ Hz in the temperature range 150–600 K. The phase transition of Pb(Fe_2/3W_1/3)O₃ (PFW) was shifted by PbTiO₃ (PT) additions to higher temperatures at a rate of 6.3 K/mol% of PT. The temperature dependence of dielectric permittivity showed a sharper transition as the PT content increased. Dielectric measurements in a wide temperature range showed the presence of a second set of dielectric peaks at higher temperatures (350–600 K), besides the ferroelectric–paraelectric phase transition. This second set of peaks vanished when the samples were annealed in nitrogen. The activation energy values for the second relaxation varied between 0.50 and 0.63 eV, in agreement with the conduction activation energy determined for each sample. This relaxation is apparently related to electron holes.     

Piezoelectricity and Phase Transitions of the Mixed-Layer Bismuth Titanate Niobate Bi7Ti4NbO21

The piezoelectiric properties of the bismuth titanate niobate Bi₇Ti₄NbO₂₁ have been studied. The two phase transitions at 677° and 856°C are anlayzed based on the measured piezoelectric properties. It is shown that the Bi₇Ti₇NbO₂₁ compound is a ferroelectric until 856°C. Bi₃TiNbO₉ clusters are shown to exist within Bi₇Ti₇NbO₂₁ granis. These clusters may be responsible for the small additional piezoelectric activity of the studied material at temperatures higher than 856°C.     

Phase Relations in the Garnet Region of the System Y2O3–Fe2O3–FeO–Al2O3

Liquidus equilibrium relations for the air isobaric section of the system Y₂O₃–Fe₂O₃–FeO–Al₂O₃ are presented. A Complete solid-solution series is found between yttrium iron garnet and yttrium aluminum garnet as well as extensive solid solutions in the spinel, hematite, orthoferrite, and corundum phases. Minimum melting temperatures are raised progressively with the addition of alumina from 1469°C in the system Y–Fe–O to a quaternary isobaric peritectic at 1547°C and composition Y _0.22 Fe _1.08 Al _0.70 O _2.83* Liquidus temperatures increase rapidly with alumina substitutions beyond this point. The thermal stability of the garnet phase is increased with alumina substitution to the extent that above composition Y _0.75 Fe _0.65 Al _0.60 O ₃ garnet melts directly to oxide liquid without the intrusion of the orthoferrite phase. Garnet solid solutions between Y _0.75 Fe _1.25 O ₃ and Y _0.75 Fe _0.32- Al _0.93 O ₃ can be crystallized from oxide liquids at minimum temperatures ranging from 1469° to 1547°C, respectively. During equilibrium crystallization of the garnet phase, large changes in composition occur through reaction with the liquid. Unless care is taken to minimize temperature fluctuations and unless growth proceeds very slowly, the crystals may show extensive compositional variation from core to exterior.     

Factors Limiting Equilibrium in Fabricating a Simple Ferroelectric Oxide: BaTiO3

This paper demonstrates a number of features that can contribute to BaTiO₃ being perturbed from an equilibrium condition. The BaTiO₃ synthesis study was conducted with a modified citrate process, but the kinetic aspects limiting the equilibrium state are believed to be general to any processing route. The equilibrium conditions can be determined through a detailed analysis of the paraelectric–ferroelectric phase transition data measured by differential scanning calorimeter. Broadened and multiple latent heat peaks are found indicating a distribution of slightly different ferroelectric transition temperatures for nonequilibrated BaTiO₃ materials. A number of factors have been found that limit the equilibrium conditions, and these include time at formation conditions of temperature and oxygen partial pressure, a second phase BaCO₃, and the rapid nucleation of a BaTi₂O₅ phase on quenching to room temperature from temperatures in and around 1250°C. All of these nonequilibrium factors that occurs at high temperature lead to the ferroelectric phase transition having multiple phase transitions, owing to regions of the BaTiO₃ having different partial Schottky concentrations.     

Phase Stability of CdF2-LiF-AIF3-PbF2 Glasses

The techniques of scanning electron microscopy, energy dispersive X-ray microanalysis, and dark-field transmission electron microscopy were combined to study the phase stability of CdF₂-LiF-AlF₃-PbF₂ glasses. The selected compositions, presumed to be exceptionally stable, exhibited a liquid-liquid immiscibility phase transition; this led to the formation of two amorphous phases consisting of a matrix near the original glass composition and an isolated spherical phase which was richer in CdF₂ and poorer in PbF₂ than the matrix. Subsequent secondary-phase separation and crystallization were observed following heat treatments near the glass transition temperature.     

Dielectric-State Analysis in Solid-Solution Pb(Yb1/2Ta1/2)O3–Pb(Lu1/2Nb1/2)O3

The structure and temperature dependence of complex lead perovskite dielectrics were investigated for the system (1 − x )Pb(Yb_1/2Ta_1/2)O₃– x Pb(Lu_1/2Nb_1/2)O₃. Superlattice reflections for the compositions 0.8 < x < 1.0 were observed by X-ray diffractometry, and the temperature-composition dielectric-state diagram was determined. In the present study, the disordered middle composition, with 0.2 < x < 0.8, showed a diffuse paraelectric–ferroelectric phase transition, whereas the ordered end compositions, with 0 ≤ x < 0.2 and 0.8 < x ≤ 1.0, revealed successive sharp paraelectric–antiferroelectric and weak antiferroelectric–ferroelectric phase transitions. The dielectric state was confirmed by examining the variation of polarization ( P ) with electric field ( E ).     

Phase Relations in (Pb,La)Zr0.65Ti0.35O3

Low-temperature phase relations in (Pb,La)Zr_0.65Ti_0.35O₃ ceramics were determined as a function of La content and temperature. Dielectric and piezoelectric measurements and X-ray data were used to locate and identify phase transitions. The FE₁-FE₂ transition between rhombohedral ferroelectric phases occurs at higher temperatures as the La eontent is increased. For x >4, where x is the atom % La substituted for Pb, a phase region exists between the paraelectric and ferroelectric states. Dielectric data suggest that this region is antiferroelectric. For 6< x <9 a field-induced phase transition accompanies poling. Poled material with x ∼8 exhibits unusual mechanical properties.     

Effects of MoO3, on Phase Separation of Na2O-B2O3-SiO2 Glasses

Immiscibility temperatures of Na₂O-B₂O₃-SiO glasses, with andwithout 1 mol% MoO₃, additions, were determined and the effect of MoO₃ additions on the 65O°C immiscibility isotherms was established. In addition, immiscibility temperature and phase-separation morphology of an Na₂O-B₂O₃-SiO₂ glass with progressive additions of MoO₃, were investigated. It was found that the addition of small amounts of MoO₃ extends the immiscibility boundary of the system and raises the immiscibility temperature by ∼l8°C for each mol % MoO₃, addition. Analysis of phase-separation morphology suggests that the MoO₃, additions do not significantly alter the tie lines of phase separation in the system, although such additions cause a lowering of the viscosities and the glass-transition temperatures of these glasses.