New High-Permittivity AgNb1-xTaxO3 Microwave Ceramics: Part I, Crystal Structures and Phase-Decomposition Relations

The crystal structure of monophasic AgNb_1/2Ta_1/2O₃ was investigated via Rietveld refinement using powder X-ray diffractometry (XRD) data. The study revealed a monoclinic unit cell of the P 2/ m space group with the following unit-cell parameters: a = 3.9286(3) Å, b = 3.9259(2) Å, c = 3.9302(3) Å, and β= 90.49(1)°. High-temperature XRD studies of the decomposition of AgNb_1/2Ta_1/2O₃ showed that the kinetics of the decomposition are influenced by the volatilization of silver. Suppression of silver volatilization by enclosing the sample in a corundum tube and applying an oxygen atmosphere yielded sintered, almost-monophasic AgNb_1/2Ta_1/2O₃ ceramics with >97% of the theoretical density. The Ag₈(Nb,Ta)₂₆O₆₉ phase, which regularly appeared as a result of partial decomposition of Ag(Nb_{1− x} Ta _x )O₃, was studied by energy-dispersive spectroscopy analysis and XRD. The Ag₈(Nb,Ta)₂₆O₆₉ phase exhibited a bronzelike orthorhombic structure with the following unit-cell parameters: a = 37.116(5) Å, b = 12.432(3) Å, and c = 7.826(2) Å. Indexed powder diffraction data for Ag₈(Nb,Ta)₂₆O₆₉ have been reported.     

Hydrothermal Synthesis of Nanosize alpha-Al2O3 from Seeded Aluminum Hydroxide

alpha-Alumina and boehmite particles were synthesized by coprecipitation followed by a hydrothermal treatment. X-ray diffraction (XRD) indicated that alpha-Al₂O₃ was the major phase and coexisted with 4% of boehmite in the presence of the alpha-Al₂O₃ seeds. On the other hand, a single boehmite phase was obtained in the absence of the alpha-Al₂O₃ seed particles. The powder densified in the temperature range from 1050° to 1350°C. High-resolution transmission electron microscopy (HRTEM) showed that the particle size of the synthesized alpha-Al₂O₃ was 60 nm. The surface area was 245 m²/g.     

Zinc Vanadates in Vanadium Oxide-Doped Zinc Oxide Varistors

Convergent-beam electron diffraction has been used to determine the space groups of β- and γ-Zn₃(VO₄)₂ particles in vanadium oxide-doped zinc oxide varistors. The crystal structure of β-Zn₃(VO₄)₂ has been determined to be monoclinic with space group P 2₁ and lattice parameters of a = 9.80 Å, b = 8.34 Å, c = 10.27 Å, and β= 115.8°, whereas that of γ-Zn₃(VO₄)₂ is monoclinic with space group Cm and a = 10.40 Å, b = 8.59 Å, c = 9.44 Å, and β= 98.8°. Energy-dispersive X-ray microanalysis of these two phases shows significant deviations from their expected stoichiometry. It is apparent that the β-phase is, in fact, the metastable Zn₄V₂O₉ phase, whereas the γ-phase either is a new oxide that consists of zinc, vanadium, and manganese or, more likely, is a zinc vanadate phase with a Zn:V atomic ratio of 1:1 that has the ability to go into solid solution with manganese.     

Subsolidus Phase Equilibria of the CaO-B2O3-BaO System

The "subsolidus" phase relations at room temperature in the system CaO-B₂O₃-BaO are investigated. Specimens of various compositions were prepared from appropriate ratios of CaCO₃, B₂O₃, and BaCO₃, and fired from 780° to 1040°C according to their melting points. There are three ternary compounds in this system. The crystal structures of these compounds were determined by X-ray diffraction (XRD). CaBa₂(BO₃)₂ and Ca₅Ba₂B₁₀O₂₂ are monoclinic structures. The lattice constants a = 14.221 Å, b = 4.569 Å, c = 11.926 A, β= 99.947°, and V = 763.4 å³ for CaBa₂(BO₃)₂ and a = 15.714 å, b = 6.184 å, c = 10.204 å, β= 93.954°, and V = 989.29 å₃ for Ca₅Ba₂B₁₀O₂₂ are obtained. The third compound, CaBa₂(B₃O₆)₂, is isostructural with the high form of BaB₂O₄ with lattice constants a = 7.167 å and c = 35.298 å. Powder second harmonic generation efficiencies of these ternary compounds were measured using a homemade apparatus.     

Effect of Seeding and Water Vapor on the Nucleation and Growth of α-Al2O3 from γ-Al2O3

Isothermal transformation kinetics and coarsening rates were studied in unseeded and alpha-Al₂O₃-seeded γ-Al₂O₃ powders heated in dry air and water vapor. Unseeded samples heated in dry air transformed to alpha-Al₂O₃ with an activation energy of 567 kJ/mol. Seeding with alpha-Al₂O₃ increased the transformation rates and reduced incubation times by providing low-energy sites for nucleation/growth of the alpha-Al₂O₃ transformation. The activation energy for the transformation was reduced to 350 kJ/mol in seeded samples heated in dry air. Seeded samples completely transformed to alpha-Al₂O₃ after 1 h at 1050°C when heated in dry air compared to 1 h at 925°C when heated in saturated water vapor. The combined effects of a lower nucleation barrier due to seeding and the increased diffusion due to water vapor reduced the activation energy for the transformation by 390 kJ/mol and the transformation temperature by ∼225°C compared to the unseeded samples heated in dry air. The accelerated kinetics is believed to be due to increased surface diffusion.     

Synthesis and Characterization of Nanocrystalline Lanthanide Oxysulfide via a La(OH)3 Gel Solvothermal Route

A La(OH)₃ gel solvothermal process has been developed for the preparation of nanocrystalline lanthanide oxysulfide (La₂O₂S) in polar solvents at 300°C through a reaction between a La(OH)₃ gel and K₂S. X-ray powder diffraction (XRD) indicated that the product was hexagonal La₂O₂S with cell parameters a = 4.046 Å and c = 6.951 Å. Transmission electronic microscopy (TEM) showed that different morphology nanocrystallites were formed, including particles with diameters of about 10 nm and nanorods about 10 nm in diameter and 300 nm in length, depending on the solvent.     

X-ray Powder Study of 2BaO·CuO

A compound of composition 2BaO·CuO was synthesized during the phase equilibrium study of the BaO-Y₂O₃-CuO_x system. Phase characterization has been carried out by using X-ray powder diffraction. The crystal symmetry was found to be the same as that of Ca₂CuO₃ and Sr₂CuO₃. It is orthorhombic with space group Immm and lattice parameters a=12.9655(14) Å (1.29655 nm), b=4.1007(3) Å (0.41007 nm), c=3.9069(5) Å (0.39069 nm), and V=207.72(3) ų (0.20772 nm³). The experimental pattern shows good agreement, in general, with intensity values calculated by assuming Ba₂CuO₃ to have a structure similar to that of Sr₂CuO₃ and Ca₂CuO₃. Intensity discrepancy for the h00 reflections might be due to preferred orientation.     

Structural Characterization of Aluminum Phosphate Binder

The chemical structure of aluminum phosphate binder, which is used as a sealant for thermally sprayed coatings, was studied. Powder X-ray diffraction (XRD) analysis showed that the binder was a mixture of several phases. None of these phases could be identified from the powder XRD data only. Electron diffraction revealed several distinct microcrystalline phases. The predominated phase was analyzed in more detail. The three-dimensional reciprocal lattice was reconstructed from a tilt series of electron-diffraction patterns that indicated that the main phase was the cyclohexaphosphate Al₂P₆O₁₈. The similarity of the intensity distribution in the observed and simulated electron-diffraction patterns of Al₂P₆O₁₈ confirmed this observation. The following unit-cell parameters were determined from the reconstructed reciprocal lattice: a = 8.8(7) Å, b = 15.5(6) Å, and c = 6.3(9) Å, β= 108°, and Z = 2. The reflection conditions indicated that the space group was P 2₁/ a .     

Crystal Structure of Lu4Si2O7N2 Analyzed by the Rietveld Method Using the Time-of-Flight Neutron Powder Diffraction Pattern

The crystal structure of a lutetium silicon oxynitride (Lu₄Si₂O₇N₂) was analyzed by the Rietveld method using time-of-flight (TOF) neutron powder diffraction data. The compound crystallizes in a monoclinic cell, space group P 2₁/ c (No. 14-1) with a = 7.4243(1), b = 10.2728(1), c = 10.6628(1) Å, and β= 109.773(1)° at 297 K. One nitrogen atom in Lu₄Si₂O₇N₂ occupies the bridging site between the two Si atoms, and the other one is statistically situated at the terminal sites of Si₂O₅N₂ ditetrahedra. In the local structure, Si₂O₅N₂ ditetrahedra consist of SiO₃N and SiO₂N₂ tetrahedral units sharing the N atom. Lu atoms are in sixfold, sevenfold (×2) and eightfold coordinations of O/N atoms. X-ray powder diffraction data were also analyzed with the model obtained by the neutron diffraction.     

The Structure of Yttrialite and Its Identification Using Laboratory and Synchrotron-Based Powder X-Ray Diffraction

A highly crystalline sample of the impurity stabilized phase y -Y₂Si₂O₇, generally known as yttrialite, has been formed from the melt of a glass with a nominal composition of 62(SiO₂)–10(Al₂O₃)–28(Y₂O₃) mol%. Powder X-ray diffraction patterns were collected using in-house instrumentation and the 11-BM diffractometer at the Advanced Photon Source, Argonne National Laboratory, Argonne, IL. Rietveld refinements were carried out on the patterns using two structural models. On patterns collected using in-house instrumentation the correct structure assignment was difficult to determine; however, the extremely high-quality data afforded by the 11-BM instrument showed conclusively that the sample was found to crystallize in the monoclinic system (SG= P 2₁/ m ) with lattice parameters a =5.03032(6), b =8.06892(6), c =7.33620(6) Å, and β=108.673(1). Furthermore, simulations have shown that it is likely that this structure model can be used to describe natural yttrialite or yttrialite that is formed at low temperatures, though the possibility that such materials are paracrystalline is also discussed.     

Subsolidus Phase Equilibria in the System Na2O-Bi2O3-TiO2 at 1000°C

Subsolidus phase relations in the system Na₂O-Bi₂O₃-TiO₂ at 1000°C were investigated by solid-state reaction techniques and X-ray diffraction methods. Five ternary compounds were observed in the system: Na_0.5Bi_4.5Ti₄O₁₅; Na_0.5Bi_0.5TiO₃; a cubic pyrochlore solid solution composed of xNa₂O.25Bi₂O₃.(75−;x) TiO₂ where x is 2.5 to 3.75; a new compound Na_0.5Bi_8.5Ti₇O₂₇ indexed with the orthorhombic cell of a = 5.45, b = 5.42, and c = 36.8 Å; and an unidentified phase with the probable composition NaBiTi₆O₁₄.     

High-Temperature Structural Phase Transition in Ca0.7Ti0.7La0.3Al0.3O3: Investigation by Synchrotron X-Ray Diffraction

Mixed-oxide prepared Ca_0.7Ti_0.7La_0.3Al_0.3O₃ (CTLA) ceramics (≈96% dense), grain size 6–7 μm, with dielectric properties (at 4 GHz) of ɛ_r≈46, Q × f ≈38 000 GHz, and τ_f+13 ppm/°C, were studied at 25°–1300°C using synchrotron X-ray powder diffraction. At room temperature, CTLA exhibits a distorted orthorhombic structure, with two tilt systems: a =5.40383 (4) Å, b =5.41106 (6) Å, and c =7.64114 (7) Å with space group Pbnm . At 1050°±25°C, there is a transition from orthorhombic ( Pbnm ) to tetragonal ( I 4/ mcm ), with a simpler tilt arrangement. The lattice parameters at 1100°C were: a =5.44285 (4) Å and c =7.68913 (8) Å.     

Synthesis, Rietveld Analysis, and Solid State Nuclear Magnetic Resonance of X2-Sc2SiO5

Compounds containing rare earths are of increasing technological interest especially because of their unique mechanical, magnetic, electrical, and optical properties. Among them, rare earth oxyorthosilicates are attractive scintillators for γ- and X-ray spectroscopy and detection. However, there are many structural aspects of those compounds that are not clear. In this research, the structure parameters for Sc₂Si₂O₅, X₂-polymorph, have been refined from powder X-ray diffraction (XRD) data and the ²⁹Si MAS NMR spectrum is reported for the first time. X₂-Sc₂SiO₅ polymorph was synthesized by the sol–gel method and characterized by XRD and ²⁹Si MAS NMR. The XRD pattern was indexed in a monoclinic unit cell with space group I 2/ c ; the resulting unit cell parameters were a =9.9674(2) Å, b =6.4264(9) Å, c =12.0636(2) Å, and β=103.938(1)°. The ²⁹Si MAS NMR spectrum showed a unique signal at −79.5 ppm, compatible with the unique Si crystallographic site in the unit cell. Finally, the band valence method has been applied to the calculation of a "shift parameter," which is correlated with the NMR chemical shift.     

Preparation of Multiple-Cation alpha-SiAlON Ceramics Containing Lanthanum

Until recently, it was accepted that Ce³⁺ cations, with an ionic radius ( r ) of 1.03 Å, were too large to form an α-SiAlON structure. However, more-recent studies have shown that cerium cations can be incorporated into α-SiAlON via quenching at a rate of 600°C/min, after sintering at 1800°C. Thus far, no α-SiAlON formation has been observed for La³⁺ cations with r = 1.06 Å. In the present work, the possibility of having the La³⁺ species as a dopant cation in α-SiAlON has been investigated by using La₂O₃ alone or in equimolar mixtures with CaO or Yb₂O₃. The resulting materials have been heat-treated at a temperature of 1450°C for up to 720 h to devitrify the grain-boundary glass into crystalline phases and also to observe the α→β SiAlON transformation. X-ray diffractometry on samples that were densified with single cations revealed that the La³⁺ cation alone does not form an α-SiAlON; rather, it forms the N-phase (La₃Si₈O₄N₁₁) with a ß-SiAlON phase. In the case of multiple cations, α-SiAlON was observed only as a matrix phase. Energy-dispersive X-ray measurements have proven that La³⁺ cations can be accommodated into the α-SiAlON structure and this structure also does not transform to β-SiAlON at lower temperatures.     

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.     

Phase Relations in the System Bi2O3-WO3

Phase relations in the system Bi₂O₃-WO₃ were studied from 500° to 1100°C. Four intermediate phases, 7Bi₂O₃· WO₃, 7Bi₂O₃· 2WO₃, Bi₂O₃· WO₃, and Bi₂O₃· 2WO₃, were found. The 7B₂O · WO₃ phase is tetragonal with a ₀= 5.52 Å and c ₀= 17.39 Å and transforms to the fcc structure at 784°C; 7Bi₂O₃· 2WO₃ has the fcc structure and forms an extensive range of solid solutions in the system. Both Bi₂O₃· WO₃ and Bi₂O₃· 2WO₃ are orthorhombic with (in Å) a ₀= 5.45, b ₀=5.46, c ₀= 16.42 and a ₀= 5.42, b ₀= 5.41, c ₀= 23.7, respectively. Two eutectic points and one peritectic exist in the system at, respectively, 905°± 3°C and 64 mol% WO₃, 907°± 3°C and 70 mol% WO₃, and 965°± 5°C and 10 mol% WO₃.     

Alkoxide Route to La0.5Sr0.5CoO3 Films and Powders

An all-alkoxide route to films and nano-phase powders of the La_0.5Sr_0.5CoO₃ perovskite is described. To our knowledge, this is the first purely alkoxide-based route to (La_{1− x} Sr _x )CoO₃, and it yields phase-pure and elementally homogeneous perovskite at 700°C by heating at 2°C/min. At 700°C, a cubic unit cell was obtained with a _c=3.853Å, and after further heating to 1000°C, a rhombohedral cell could be indexed: a _r=5.417 Å, α_r=59.94°. Ninety to 130 nm thick films of La_0.5Sr_0.5CoO₃ were obtained by spin coating. The gel-to-oxide conversion was studied in some detail, using thermo-gravimetric analysis, differential scanning calorimetry, powder X-ray diffraction, IR spectroscopy, and transmission electron microscope equipped with an energy-dispersive X-ray spectrometer.     

Facile Synthesis of Aluminum-Containing Mixed-Metal Oxides Using Doped Carboxylate Alumoxane Nanoparticles

A simple and rapid process has been developed that allows the formation of highly crystalline main-group, lanthanide, and transition-metal aluminates, including CaAl₁₂O₁₉ (hibonite), Y₃Al₅O₁₂ (yttrium aluminum garnet, YAG), LaAl₁₁O₁₈, LaAlO₃, Ce₂Al₃O₈, NdAlO₃, Er₆Al₁₀O₂₄, and Al₂TiO₅. Carboxylate-stabilized alumina nanoparticles (carboxylate alumoxanes) are reacted with the appropriate metal acetyl-acetonate complexes to form metal-doped carboxylate alumoxanes and aluminum acetylacetonate via a facile transmetalation reaction. Thermolysis of the metal-doped carboxylate alumoxanes, up to a temperature of 1400°C, yields crystalline aluminates, as confirmed by X-ray diffractometry, electron microprobe analysis, and scanning electron microscopy. The synthesis of different phases (i.e., LaAl₁₁O₁₈ versus LaAlO₃) is controlled by the stoichiometry of the transmetalation reaction. A new, distorted, perovskite phase (with lattice parameters of a = 5.47 Å and c = 13.3 Å) of LaAlO₃ is formed. The alumoxane methodology results in multicomponent aluminate materials that are obtained in shorter processing times and with greater phase purity than previously reported. This observation is undoubtedly due to the atomic-scale mixing that is inherent in the alumoxane nanoparticle approach.     

A New Low-Loss Microwave Dielectric Ceramic Sr4LaTiNb3O15

A high dielectric constant and low-loss ceramic with composition Sr₄LaTiNb₃O₁₅ has been prepared by the conventional solid-state ceramic route. This compound adopts an A₅B₄O₁₅ cation-deficient hexagonal perovskite structure and crystallizes in the trigonal system with unit cell parameters a =5.6307(2), c =11.3692(3) Å, V =312.16(2) ų, and Z =1. The dielectric properties of dense ceramics sintered in air at 1460°C have been characterized at microwave frequencies. The results show that the material affords a relatively high dielectric constant ɛ_r∼43, a high quality factor Q × f ∼44 718 GHz, and a low temperature coefficient of resonant frequency TC_f∼13 ppm/°C.     

Structural analysis of 5-[thiazol-2-yldiazenyl]pyrimidine-2,4,6(1H,3H,5H)-trione in both solid state and solution

The synthesis, crystal structure analysis and characterisation of 5-[thiazol-2-yldiazenyl]pyrimidine-2,4,6(1 H ,3 H ,5 H )-trione are reported. This dye was characterised by ultraviolet–visible, Fourier Transform–infrared, proton nuclear magnetic resonance spectroscopy, mass spectrometry techniques and elemental analysis. Both the effects of pH value and solvent polarity upon the absorption properties of the dye have been presented. In addition, the structure of the dye has been determined by a single crystal X-ray diffraction method. The dye was triclinic in P-1 with a  = 8.2419(2) Å, b  = 10.6225(3) Å, c  = 12.5106(3) Å, α = 90.543(1)°, β = 102.79(3)°, γ = 103.858(1)°, V  = 1035.26(5) Å³, D _calc = 1.634 g/cm³ and Z  = 2. The asymmetric unit contains two crystallographically independent C₇H₅N₅O₃S molecules, alongside one half of the solvent C₂H₆O₂ molecule. In the crystal structure, intramolecular N–H…N and intermolecular N–H…N, N–H…O, O–H…N, C–H…O and C–H…N hydrogen bonds link the molecules to form a supramolecular network in which they seem to be effective in the stabilisation of the structure.