Syntheses and Unit-Cell Determination of Ba3V4O13 and Low-and High-Temperature Ba3P4O13

The syntheses and the results of unit-cell determinations ofBa₃V₄O₁₃ and the two forms (low- and high-temperature) of Ba₃P₄O₁₃ are presented. Ba₃V₄O₁₃ crystallizes in the monoclinic system, space group Cc or C2/c with unit-cell dimensions a=16.087, b=8.948, c=10.159 (x10nm), β=114.52° Low-Ba₃P₄O₁₃ crystallizes in the triclinic system, space group P1 or P1 with unit-cell dimensions a=5.757, b=7.243, c=8.104 (x10 nm) α=82.75°, β=73.94°, γ=70.71°. Low-Ba₃P₄O₁₃ transforms at 870°C into high-Ba₃P₄O₁₃ which crystallizes in the orthorhombic system, space group Pbcm (No. 57) (or Pbc2, No. 29) with unit-cell dimensions a =7.107, b=13.883, c=19.219 (x10 nm). No relations have been found between the structures of the tribarium tetravanadate and the tribarium tetraphosphate.     

Crystal Structure of the Microwave Dielectric Resonator Ba2Ti9O20

A single-crystal X-ray study of dibarium nonatitanate, Ba₂Ti₉O₂₀, yielded the triclinic space group P 1 with a =0.7471(1), b= 1.4081(2), c= 1.4344(2) nm, α=89.94(2)°, β= 79.43(2)°, γ= 84.45(2)°, V = 1.476 nm³ Z = 4, and D_x= 4.61 Mg/m³. A refinement of atomic coordinates and isotropic thermal parameters led to a residual of 0.03. The structure consists of hexagonally closest-packed layers of Ba and O atoms in the sequence (hch)₃. All Ti atoms reside in octahedral interstices of this closest packing. The various Ti coordination octahedra share only edges and corners with each other. One-half of the Ba atoms is twelve-coordinated by oxygen atoms, the other half is eleven-coordinated.     

Formation of AlVO4 Solid Solution from Alkoxides

Solid solutions of AlVO₄ crystallize at lower temperatures than amorphous materials between 50 and 70 mol% Al₂O₃ prepared by the simultaneous hydrolysis of aluminum and vanadyl alkoxides. They decompose into α-Al₂O₃, and V₂O₅, at 775° to 800°C. The compound AlVO₄ prepared from 50 mol% Al₂O₃ has a triclinic unit cell with a = 0.6471 nm, b = 0.7742 nm, c = 0.9084 nm, α= 96.848°, β= 105.825°, and γ= 101.399°. The volume of the unit cell increases continuously with increases in Al₂O₃ content. The structure contains tetrahedral AlO₄, octahedral AlO₆, and tetrahedral VO₄ groups.     

Tricalcium Silicate T1 and T2 Polymorphic Investigations: Rietveld Refinement at Various Temperatures Using Synchrotron Powder Diffraction

The lattice parameters, cell volume, and structure of a sample of phase pure triclinic tricalcium silicate were determined using in situ, high-temperature synchrotron powder diffraction and full-profile Rietveld refinement. The temperature range covered was from ambient to 740°C. Evidence of superstructure was found. The T₂ type structure with disordered SiO₄ tetrahedra was observed, and an average structure for the subcell ( P     , a = 11.7416(2) Å, b = 14.2785(2) Å, c = 13.7732(2) Å, α= 105.129(1)°, β= 94.415(1)°, and γ= 89.889(1)°) is presented. Differential thermal analysis and X-ray fluorescence was also performed.     

Three Crystalline Polymorphs of KFeSiO4, Potassium Ferrisilicate

Orthorhombic α-KFeSiO₄ ( a =0.5478, b =0.9192, c =0.8580 nm), hexagonal β-KFeSiO₄ ( a =0.5309, c =0.8873 nm), and hexagonal γ-KFeSiO₄ ( a =0.5319, c =0.8815 nm) were synthesized by devitrification of KFeSiO₄ glass. Powder X-ray diffraction data are given for all three polymorphs. Alpha KFeSiO₄, the high-temperature polymorph, melts congruently at 1197°± 2°C. Mössbauer spectroscopy of the α phase indicates that Fe³⁺ occupies two tetrahedral sites in the lattice. Beta KFeSiO₄, the low-temperature polymorph, and γ-KFeSiO₄, a metastable polymorph, appear to be isomorphous with kalsilite, KAISiO₄, and synthetic kaliophilite, KAISiO₄, respectively, and it is proposed that β- and γ -KFeSiO₄ are linked by Si-Fe order-disorder. Beta KFeSiO₄ transforms slowly into α -KFeSi0₄ above 910°C but the transformation was not shown to be reversible in the present dry-heating experiments.     

Preparation and Unit-Cell Parameters of Single Crystals of Ba2Ti9020

Ba₂Ti₉O₂₀ crystallizes in the monoclinic system with α= l.4818(5) nm, b = 1.4283(6), and c = 0.7109(2) with β = 98.37°±0.07°. The most likely space group is P 2₁/ m , Z = 4 with a calculated density 4.58 g/cm³. The powder pattern was indexed. The Ba₂Ti₉O₂₀ crystals form as stellated groups when melts of BaCl₂+ 20 to 50% TiO₂ cool from 1275°C.     

Additive-Assisted Nitridation to Synthesize Si3N4 Nanomaterials at a Low Temperature

Starting from Si powder, NaN₃ and different additives such as N -aminothiourea, iodine, or both, Si₃N₄ nanomaterials were synthesized through the nitridation of silicon powder in autoclaves at 60°–190°C. As the additive was only N -aminothiourea, β-Si₃N₄ nanorods and α-Si₃N₄ nanoparticles were prepared at 170°C. If the additive was only iodine, α-Si₃N₄ dendrites with β-Si₃N₄ nanorods were obtained at 190°C. However, when both N -aminothiourea and iodine were added to the system of Si and NaN₃, the products composed of β-Si₃N₄ nanorods and α, β-Si₃N₄ nanoparticles could be prepared at 60°C.     

Solid Solubility and Polymorphism in the System Sr2SiO4-Sr2GeO4-Ba2GeO4-Ba2SiO4

The reciprocal salt pair Sr₂SiO₄-Sr₂GeO₄-Ba₂GeO₄-Ba₂SiO₄ was investigated using X-ray powder diffraction and DTA. Unlimited solubility in the low-K₂SO₄ structure type (α') occurs throughout the system above 85°C. The nonlinear changes of some lattice constants of the solid solutions are discussed. A stable monoclinic low-temperature (β) form of Sr₂SiO₄ was found which converts reversibly to the high-temperature α'-modification at 85°C. The enthalpy of the β-α' transition is 51 cal/mol. In the reciprocal salt pair the β-form solid solutions occur in a very narrow region below 85°C.     

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.     

Low-Temperature Synthesis of Nanocrystalline α-Si3N4 Powders by the Reaction of Mg2Si with NH4Cl

Nanocrystalline α-Si₃N₄ powders have been prepared with a yield of 93% by the reaction of Mg₂Si with NH₄Cl in the temperature range of 450° to 600°C in an autoclave. X-ray diffraction patterns of the products can be indexed as the α-Si₃N₄ with the lattice constants a = 7.770 and c = 5.627 Å. X-ray photoelectron spectroscopy analysis indicates that the composition of the α-Si₃N₄ samples has a Si:N ratio of 0.756. Transmission electron microscopy images show that the α-Si₃N₄ crystallites prepared at 450°, 500°, and 550°C are particles of about 20, 40, and 70 nm in average, respectively.     

Phase Equilibria in the Systems NiO-Cr2,O3,-O2, MgO-Cr2O3−O2, and CdO-Cr2,O3,-O2, at High Oxygen Pressures

Phase equilibria were determined for the systems NiO-Cr₂O₃−O₂, MgO-Cr₂O₃,-O₂, and CdO-Cr₂O₃−O₂ from 450° to above 850° C and at oxygen pressures of from 2 to 3500 atm. Only two intermediate phases were found in the nickel system: NiCrO., (CrVO₄ structure) and the spinel NiCr₂O₄. The magnesium and cadmium systems are similar in that they have three analogous phases: the low-temperature α-MgCrO₄ and α-CdCrO₄ (both with the CrVO₄ structure), the high-temperature β-MgCrO₄ and β-CdCrO₄ (both with the α-MnMoO₄ structure), and the spinels MgCr₂O₄ and CdCr₂O₄. The cadmium system contains an additional phase, Cd₂CrO₅, which is primitive monoclinic.     

Studies in the System CaO-Al2O3-SiO2H2O: III, New Data on the Polymorphism of Ca2SiO2 and Its Stability in the System CaO-SiO2- H2O

The nature of the low-temperature inversions γ-α' and α'-β was investigated by various techniques: hydrothermal and "dry" quenching runs, differential thermal analysis at atmospheric and elevated nitrogen pressures, X-ray diffractometer patterns obtained at elevated temperatures, "static" pressure techniques, and infrared absorption spectrometry. A revised energy-temperature diagram is presented for Ca₂SiO₄, with the transition γ' to α' taking place at about 725°C. and the α'-β transition, although not reversible at an exact temperature, taking place at about 670° C. At low water pressures (2000 lb. per sq. in.) the inversion γ-α' was placed at 675°C. Attempts to extrapolate the value obtained at 2000 lb. per sq. in. to obtain a more accurate reversible inversion temperature at atmospheric pressure, although limited in accuracy by the reliability of heat-of-transition data, would indicate a temperature of about 725° C. at atmospheric pressure. Three new compounds, 8CaO.3SiO₂ -3H₂O (X), 6CaO 3SiO₂.H₂O (Y), and 9CaO-6SiO₂ H₂O (Z), were found to be stable above 700°C. at H₂O pressures greater than 7500 lb. per sq. in.     

The Role of Cerium Orthosilicate in the Densification of Si3N4

The existence of compounds between Si₃N₄-CeO₂ and Si₃N₄-Ce₂O₃ was investigated for firing temperatures of 1600° to 1700°C. The two new monoclinic compounds found were Ce₂O₃·2Si₃N₄ with lattice parameters a = 16.288, b = 4.848, and c =7.853 Å and β=91.54° and Ce₄Si₂O₇N₂ with lattice parameters a = 10.360, b = 10.865, and c =3.974 Å and β=90.33°. Cerium orthosilicate (Ce 4.67 (SiO₄)₃O) is present during firing as a glassy intermediate phase which promotes sintering and densification and then reacts with silicon nitride to form cerium silicon oxynitrde (CeSiO₂N).     

Formation of β-Si3N4 Coatings by Chemical Vapor Deposition

BetaSi₃N₄ coatings were obtained by chemical vapor deposition in a fused-silica reaction tube by outside heating of the system SiCl₄-NH₃-N₂ at a deposition temperature (reaction tube temperature) of 1300°C, whereas α- and α+β-phase coatings were obtained at depositon temperatures of 1150° and 1250°C, respecively. Formation of β-phase coatings at relatively low temperatures is explained in terms of the effect of a catalytic impurity, SiO vapor from the reaction tube. The X-ray diffraction patterns and sulface morphologies of the coatings were studied.     

Studies in Lithium Oxide Systems: XIII, Li,O.Al2O3.2SiO2-Li2O.Al2O3.2GeO2

Complete solid solubility was demonstrated to occur between LiAlGeO₄ and the low temperature form of Li AlSiO₂ (a-eucryptite). Hydrother-mal preparation was necessary for the silicate-rich compositions. Under atmospheric pressure, about 65 mole % LiAlGeO₄ entered the β-eucryβ-tite phase at 1150°C, but solid solutions containing more than 25 mole % LiAlGeO4 exsolved if held at lower temperatures. Directional thermal expansion data were obtained by X-ray diffraction methods on both α- and β-eucryptite and their solid solutions. Substitution of Ge⁴⁺ for Si⁴⁺ produced no significant difference in the thermal expansion coefficients in the α and β phases. An increase in the lattice parameters in the a and c directions took place as expected when Ge⁴⁺ (0.53 A) was substituted for Si⁴⁺ (0.39 A).     

Synthesis of β-Si3N4 by Chemical Vapor Deposition

Beta-type CVD-Si₃N₄ plates (up to 1.1 mm thick) have been prepared by adding TiCl₄ vapor to the system SiCl₄-NH₃-H₂ at deposition temperatures of 1350° to 1450°C, while α-type or amorphous CVD-Si₃N₄ was obtained without TiCl₄ vapor at the same deposition temperature. Three to four wt % 777V was included in the β-type CVD-Si₃N₄ matrix. The density, preferred orientation, and lattice parameters of β-type CVD-Si₃N₄ were examined.     

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.     

Preparation of Porous Ca10(PO4)6(OH)2 and β-Ca3(PO4)2 Bioceramics

Submicrometer-sized, pure calcium hydroxyapatite (HA, (Ca₁₀(PO₄)₆(OH)₂)) and β-tricalcium phosphate (β-TCP, Ca₃(PO₄)₂) bioceramic powders, that have been synthesized via chemical precipitation techniques, were used in the preparation of aqueous slurries that contained methyl cellulose to manufacture porous (70%–95% porosity) HA or β-TCP ceramics. The pore sizes in HA bioceramics of this study were 200–400 μm, whereas those of β-TCP bioceramics were 100–300 μm. The pore morphology and total porosity of the HA and β-TCP samples were investigated via scanning electron microscopy, water absorption, and computerized tomography.     

Polymorphism and Thermodynamic Stability of Zn7Sb2O12

The composition Zn_2.33Sb_0.67O₄ (or Zn₇Sb₂O₁₂) exists in two polymorphic forms. The thermodynamically stable, low-temperature orthorhombic β form transforms to the high-temperature cubic α-polymorph with a spinel structure at 1225°±25°C. The transformation is fully reversible but slower in the α→β direction and therefore, it is easy to preserve the high-temperature α-polymorph to lower temperatures where it is kinetically stable but thermodynamically metastable. It is also possible to synthesize the α-polymorph directly at low temperatures, e.g., 900°C. This synthesis, of a phase that is thermodynamically stable only at high temperatures, but which has sufficient kinetic stability to exist metastably at low temperatures, represents an example of Ostwald's law of successive reactions in which the first phase to crystallize from a reaction mixture is not necessarily the equilibrium phase of lowest free energy. The crystal structure of the α-polymorph has been confirmed by Rietveld refinement of X-ray powder diffraction data to be an inverse spinel, (Zn)[Sb_2/3Zn_4/3]O₄, in which octahedral sites contain a disordered, random mixture of Zn and Sb and tetrahedral sites are fully occupied by Zn.     

Solvothermal Synthesis of Si3N4 Nanomaterials at a Low Temperature

Silicon nitride nanowires or nanorods have been synthesized from SiCl₄, NaN₃, and metallic Mg at temperatures ranging from 200° to 300°C. X-ray powder diffraction patterns indicated that the as-obtained products were mainly β-Si₃N₄. Scanning electron microscope and high-resolution transmission electronic microscopy showed that the samples mostly consisted of Si₃N₄ nanowires or nanorods. As metallic iron powder was used, α-Si₃N₄ was mainly formed at 250°C.