Pyrochlore Phase Formation in the System Bi2O3–ZnO–Ta2O5

The subsolidus phase diagram of the system Bi₂O₃–ZnO–Ta₂O₅ in the region of the cubic pyrochlore phase has been determined at 1050°C. This phase forms a solid solution area that includes the ideal composition P, Bi₃Zn₂Ta₃O₁₄; possible solid solution mechanisms are proposed, supported by density measurements of Zn-deficient solid solutions. The general formula of the solid solutions is Bi_{3+ y} Zn_{2− x} Ta_{3− y} O_{14− x − y} , based on the creation of Zn²⁺, O²⁻ vacancies in Zn-deficient compositions and a variable Bi/Ta ratio.     

Pyrochlore Phases in the System ZnO–Bi2O3–Sb2O5: I. Stoichiometries and Phase Equilibria

Two cubic pyrochlore phases exist in the system ZnO–Bi₂O₃–Sb₂O₅. Neither has the supposed "ideal" stoichiometry, Zn₂Bi₃Sb₃O₁₄. One, P ₁, is a solid solution phase, Zn_{2+ x} Bi_{2.96−( x − y )}Sb_{3.04− y} O_14.04+δ where 0< x <0.13(1), 0< y <0.017(2) and a =10.4285(9)−10.451(1) Å. The other, P ₂, is a line phase, Zn₂Bi_3.08Sb_2.92O_13.92 with a =10.462(2) Å. Subsolidus phase relations at 950°C involving phases P ₁ and P ₂ in the ZnO–Bi₂O₃–Sb₂O₅ phase diagram have been determined.     

Dielectric Relaxations in Bi2O3–Nb2O5–NiO Cubic Pyrochlores

Dielectric properties of pyrochlores compositions from Bi₂O₃–Nb₂O₅–NiO system were analyzed. The dielectric properties are dominated with a low-temperature relaxation that is typical for Bi-pyrochlores. A vast pyrochlore homogeneity range that exists in this system allowed to correlate characteristics of the observed relaxations with a compositional variations within the A₂O'- and B₂O₆ pyrochlore sublattice. It was possible to make a distinction between different influences of the two sublattices, which can be satisfactorily described by the existing relaxation model for Bi_3/2ZnNb_3/2O₇. A new relaxor-like room temperature relaxation was found for Bi_1.6Ni_0.57Nb_1.43O_6.55.     

The Bi2O3–Nb2O5–NiO Phase Diagram

The Bi₂O₃–Nb₂O₅–NiO phase diagram at 1100°C was determined by means of solid-state synthesis, X-ray diffraction, and scanning electron microscopy. A ternary eutectic with a melting point below 1100°C was found to exist in the field between NiO, Bi₂O₃, and the end-member of the δBi₂O₃–Nb₂O₅ solid solution. The existence of the previously reported Bi₃Ni₂NbO₉ phase was disproved. A pyrochlore homogeneity range around Bi_1.5Ni_0.67Nb_1.33O_6.25 was determined together with all the phase relations in this phase diagram.     

Subsolidus Phase Relationships in the Ga2O3–Al2O3–TiO2 System

Subsolidus phase relationships in the Ga₂O₃–Al₂O₃–TiO₂ system at 1400°C were studied using X-ray diffraction. Phases present in the pseudoternary system include TiO₂ (rutile), Ga_{2−2 x} Al_{2 x} O₃ ( x ≤0.78 β-gallia structure), Al_{2−2 y} Ga_{2 y} O₃ ( y ≤0.12 corundum structure), Ga_{2−2 x} Al_{2 x} TiO₅ (0≤ x ≤1 pseudobrookite structure), and several β-gallia rutile intergrowths that can be expressed as Ga_{4−4 x} Al_{4 x} Ti _{n −4}O_{2 n −2} ( x ≤0.3, 15≤ n ≤33). This study showed no evidence to confirm that aluminum substitution of gallium stabilizes the n =7 β-gallia–rutile intergrowth as has been mentioned in previous work.     

Crystalline Phases and YAG Laser-Induced Crystallization in Sm2O3–Bi2O3–B2O3 Glasses

The glass formation region, crystalline phases, second harmonic (SH) generation, and Nd:yttrium aluminum garnet (YAG) laser-induced crystallization in the Sm₂O₃–Bi₂O₃–B₂O₃ system were clarified. The crystalline phases of Bi₄B₂O₉, Bi₃B₅O₁₂, BiBO₃, Sm _x Bi_{1− x} BO₃, and SmB₃O₆ were formed through the usual crystallization in an electric furnace. The crystallized glasses consisting of BiBO₃ and Sm _x Bi_{1− x} BO₃ showed SH generations. The formation of the nonlinear optical BiB₃O₆ phase was not confirmed. The formation (writing) region of crystal lines consisting of Sm _x Bi_{1− x} BO₃ by YAG laser irradiation was determined, in which Sm₂O₃ contents were∼10 mol%. The present study demonstrates that Sm₂O₃–Bi₂O₃–B₂O₃ glasses are promising materials for optical functional applications.     

Phase Studies in the Binary System MgO–Ta2O5

The objective of this investigation was to determine subsolidus phase relations in the system MgO–Ta₂O₅ and to obtain accurate crystallographic data on the compounds formed. MgO and Ta₂O₅ formed three compounds: Mg₄Ta₂O₉, Mg₃Ta₂O₈, and MgTa₂O₆. Mg₄Ta₂O₉ and MgTa₂O₆ appeared to be stable up to their melting points, whereas Mg₃Ta₂O₈ was stable only between 1475° and about 1675°C. X-ray diffraction and density data are presented for the three compounds.     

Formation of Metastable Fluorite Phase by Rapid Quenching of Melts in the Y2O3-Ta2O5 System

Melts of x mol% Ta₂O₅–Y₂O₃ (x = 0–32.5) were rapidly quenched to investigate the formation of metastable fluorite solid solutions. C-type Y₂O₃, fluorite, and fergusonite phases existed in the compositional regions of 0 x 16, 8 x 32.5, and 27.5 x 32.5, respectively. Their lattice parameters were precisely measured through either Rietveld analysis or a least-squares fit of the individual X-ray diffraction peak positions. The lattice parameter of the fluorite phase decreased linearly with increasing Ta₂O₅ content, strongly suggesting the formation of compositionally homogeneous metastable solid solutions. Ta₂O₅ was almost insoluble into Y₂O₃ at 1700°C in the equilibrium state.     

The System Bi2O3-SiO2

The Bi₂O₃-rich side of the system Bi₂O₃-SiO₂ was studied with powder X-ray diffraction and differential thermal analysis. In the composition 6Bi₂O₃. x SiO₂, the metastable γ phase (bcc) was observed to exist over the range of 0 < x ≤ 1. In most of the compositions studied, metastable phases of water-quenched melts transformed into another metastable phase before reaching stable phases. A modification of the phase diagram is proposed.     

Formation and Transformation of Solid Solutions in the System Nb2O5–Ta2O5

In the system Nb₂O₅–Ta₂O₅, a continuous series of δ-Nb₂O₅ (δ-Ta₂O₅) solid solutions with a hexagonal cell is formed while heating amorphous materials prepared by the simultaneous hydrolysis of niobium and tantalum alkoxides. The lattice parameters a and c change linearly with increasing Ta₂O₅ content; the former value increases from 0.3604 to 0.3620 nm, and the latter value decreases from 0.3923 to 0.3883 nm. They transform to γ-Nb₂O₅ (β-Ta₂O₅) solid solutions with an orthorhombic cell at higher temperatures. The changes in lattice parameters a and c as functions of composition are the same as those of hexagonal solid solutions, whereas parameter b is relatively constant.     

Grain Growth of ZnO during Bi2O3 Liquid-Phase Sintering

Grain growth of ZnO during the liquid-phase sintering of binary ZnO–Bi₂O₃ ceramics has been studied for Bi₂O₃ contents from 3 to 12 wt% and sintering from 900° to 1400°C. The results are considered in combination with previously published studies of ZnO grain growth in the ZnO–Bi₂O₃ system. For the Bi₂O₃ contents of the present study, the rate of ZnO grain growth is found to decrease with increasing Bi₂O₃. Activation analysis, when combined with the results of similar analyses of the previous studies, reveals a change in the rate-controlling mechanism for ZnO grain growth. Following a low-Bi₂O₃-content region of nearly constant activation energy values of about 150 kJ/mol, further Bi₂O₃ additions cause an increase of the activation energy to about 270 kJ/mol. consistent with accepted models of liquid-phase sintering, it is concluded that the rate-controlling mechanism of ZnO grain growth during liquid-phase sintering in the presence of Bi₂O₃ changes from one of a phase-boundary reaction at low Bi₂O₃ levels to one of diffusion through the liquid phase at about the 5 to 6 wt% Bi₂O₃ level and above.     

Perovskite Formation and Dielectric Characteristics of Pb(Zn1/3Ta2/3)O3 with PbTiO3 Substitution

Perovskite developments in the Pb(Zn_1/3Ta_2/3)O₃–PbTiO₃ system were explored. Formation yields and lattice parameters of the perovskite were determined from X-ray diffractometry results. Weak-field low-frequency dielectric properties of the system ceramics were investigated, followed by microstructure examination. Perovskite started to develop in Pb(Zn_1/3Ta_2/3)O₃ after the introduction of 30 mol% PbTiO₃, whereas complete stabilization was accomplished at 60% substitution. Dielectric relaxation behavior was not substantial across the entire composition range, whereas phase transition modes changed from diffuse to sharp with increased PbTiO₃ fraction.     

Phase Transformations in the High-Temperature Form of Pure and TiO2-Stabilized Ta2O5

The high-temperature forms of undoped tantalum pentoxide (H-100Ta₂O₅) and TiO₂–modified Ta₂O₅ (H-92Ta₂O₅–8TiO₂) were investigated by in situ synchrotron X-ray diffraction and Raman scattering measurements. Two unquenchable and reversible phase transformations were observed in pure H-Ta₂O₅, while only one was detected for TiO₂-stabilized H-Ta₂O₅. Diffraction studies were consistent with displasive transformations, but hot-stage Raman spectroscopy indicated the existence of transient intermediate forms during the transformations. Use of complementary techniques enabled the reinterpretation of phase transformations in light of a newly proposed crystal structure model for H-Ta₂O₅, ¹ and emphasized the structural contributions of the oxygen sublattice.     

Nanopowder Preparation and Dielectric Properties of a Bi2O3–Nb2O5 Binary System Prepared by the High-Energy Ball-Milling Method

The high-energy ball-milling (HEM) method was used to synthesize the compositions of BiNbO₄, Bi₅Nb₃O₁₅, and Bi₃NbO₇ in a Bi₂O₃–Nb₂O₅ binary system. Reagent Bi₂O₃ and Nb₂O₅ were chosen as the starting materials. The X-ray diffraction patterns of the three compositions milled for different times were studied. Only the cubic Bi₃NbO₇ phase, Nb₂O₅, and amorphous matters were observed in powders after being milled for 10 h. After heating at proper temperatures the amorphous matters disappeared and the proleptic phases of BiNbO₄ and Bi₅Nb₃O₁₅ could be obtained. The Scherrer formula was used to calculate the crystal size and the results of nanopowders are between 10 and 20 nm. The scanning electron microscopy photos of Bi₃NbO₇ powders showed drastic aggregation, and the particle size was about 100 nm. The dielectric properties of ceramics sintered from the nanopowders prepared by HEM at 100–1 MHz and the microwave region were measured. Bi₃NbO₇ ceramics showed a good microwave permittivity ɛ_r of about 80 and a Q × f of about 300 at 5 GHz. The triclinic phase of BiNbO₄ ceramics reached its best properties with ɛ_r=24 and Q × f =14 000 GHz at about 8 GHz.     

Ba8ZnTa6O24: A New High Q Dielectric Perovskite

The hexagonal perovskite, Ba₈ZnTa₆O₂₄, was prepared in single-phase form and was found to be a stable secondary phase, formed as a result of the loss of ZnO from Ba(Zn_1/3Ta_2/3)O₃ microwave dielectrics. The experimental and calculated X-ray patterns of Ba₈ZnTa₆O₂₄ indicate it is isostructural with Ba₈Ta₆NiO₂₄ with an 8H (cchc)₂ close-packed BaO₃ stacking sequence and the lattice parameters, a =10.0825(14), c =19.0587(38)Å. High-density ceramics of Ba₈ZnTa₆O₂₄ could be prepared at temperatures considerably lower (1400°C) than those used to sinter pure Ba(Zn_1/3Ta_2/3)O₃, and exhibit very good microwave dielectric properties with ɛ=30.5, Q _f=62 300, and τ_f=+36 ppm/°C at 8.9 GHz.     

Effect of Bi2O3 Addition on the Sintering Temperature and Microwave Dielectric Properties of Zn2SiO4 Ceramics

Bi₂O₃ was added to a nominal composition of Zn_1.8SiO_3.8 (ZS) ceramics to decrease their sintering temperature. When the Bi₂O₃ content was <8.0 mol%, a porous microstructure with Bi₄(SiO₄)₃ and SiO₂ second phases was developed in the specimen sintered at 885°C. However, when the Bi₂O₃ content exceeded 8.0 mol%, a liquid phase, which formed during sintering at temperatures below 900°C, assisted the densification of the ZS ceramics. Good microwave dielectric properties of Q × f =12,600 GHz, ɛ_r=7.6, and τ_f=−22 ppm/°C were obtained from the specimen with 8.0 mol% Bi₂O₃ sintered at 885°C for 2 h.     

Effect of Sb2O3 on Thermal Properties of Glasses in Bi2O3–B2O3–SiO2 System

Glasses with compositions 50Bi₂O₃– x Sb₂O₃–10B₂O₃–(40– x ) SiO₂ ( x =0, 1, 3, 5, 8, 10) have been prepared by conventional melt quench technique. Substitution of Sb₂O₃ for SiO₂ exerted an obvious effect on properties of glasses, especially, increased glass transition temperature ( T _g) and crystalline temperature ( T _c) greatly. Results of infrared transmission spectra attributed the effect to the formation of new bridging bonds of Sb–O–B and Sb–O–Si in glass network.     

Microwave Dielectric Properties and Chemical Resistance of Low-Temperature-Sintered CaZrB2O6 Ceramics

Dolomite-type borate ceramics consisting of CaZrB₂O₆ were synthesized via a conventional solid-state reaction route; low-temperature sintering was explored using Bi₂O₃–CuO additives of 1–7 wt% for low-temperature co-fired ceramics applications. For several sintering temperatures, the microwave dielectric properties and chemical resistance of the ceramics were investigated. The CaZrB₂O₆ ceramics with 3 wt% Bi₂O₃–CuO addition could be sintered below 925°C, and the microwave dielectric properties of the low-temperature samples were ɛ_r=10.55, Q × f =87,350 GHz, and τ_f=+2 ppm/°C. The chemical resistance test result showed that both CaZrB₂O₆- and Bi₂O₃–CuO-added CaZrB₂O₆ ceramics were durable in basic solution but were degraded in acid solution.     

Properties and Structure of Glasses in the System Bi2O3-SiO2-GeO2

In the system Bi₂O₃-SiO₂-GeO₂, good glasses can be formed only from limited compositional regions consisting of 2 narrow strips along the lines x Bi₂O₃-(100-:t) GeO₂ ( x ≤40) and 40Bi₂O₃ y SiO₂ (60- y )GeO₂ (mol%); such glass is dark brown. Compositions from a large region (Bi₂O₃ content <40 mol%) showed immiscibility. In the binary system Bi₂O₃-GeO₂, density and refractive index vary linearly with composition (mol%). Negative deviations of molar volume from ideality suggest that the coordination of a significant number of Ge ions is changing from 4-fold to 6-fold. Thermal expansion and electrical resistivity data are also reported.     

Phase Relationships in the Si3N4–SiO2–Lu2O3 System

Phase relationships in the Si₃N₄–SiO₂–Lu₂O₃ system were investigated at 1850°C in 1 MPa N₂. Only J-phase, Lu₄Si₂O₇N₂ (monoclinic, space group P 2₁/ c , a = 0.74235(8) nm, b = 1.02649(10) nm, c = 1.06595(12) nm, and β= 109.793(6)°) exists as a lutetium silicon oxynitride phase in the Si₃N₄–SiO₂–Lu₂O₃ system. The Si₃N₄/Lu₂O₃ ratio is 1, corresponding to the M-phase composition, resulted in a mixture of Lu–J-phase, β-Si₃N₄, and a new phase of Lu₃Si₅ON₉, having orthorhombic symmetry, space group Pbcm (No. 57), with a = 0.49361(5) nm, b = 1.60622(16) nm, and c = 1.05143(11) nm. The new phase is best represented in the new Si₃N₄–LuN–Lu₂O₃ system. The phase diagram suggests that Lu₄Si₂O₇N₂ is an excellent grain-boundary phase of silicon nitride ceramics for high-temperature applications.