Phase Relations in the Systems LiOH–GeO2–H2O and LiOH–SiO2–H2O at 500°C and 0.1 GPa

Crystallization in the LiOH–GeO₂–H₂O and LiOH–SiO₂–H₂O hydrothermal systems was studied at 500°C and 0.1 GPa. The systems were shown to contain both the isostructural compounds Li₂SiO₃ and Li₂GeO₃ and phases differing in crystal chemistry: Li₂Si₂O₅, Li₂Ge₃O₆(OH)₂, and Li₃HGe₇O₁₆ · 4H₂O (containing Ge in different oxygen coordinations). The crystallization fields revealed in the germanate system are (in order of increasing LiOH concentration) GeO₂ GeO₂+ Li₂Ge₃O₆(OH)₂ Li₂Ge₃O₆(OH)₂ + Li₂GeO₃ Li₃HGe₇O₁₆ · 4H₂O + Li₂GeO₃; those in the silicate system are -SiO₂ -SiO₂+ Li₂Si₂O₅ Li₂Si₂O₆ + Li₂SiO₃ Li₂SiO₃. Increasing the LiOH concentration increases the number of Li atoms per tetrahedrally coordinated Si or Ge atom in the crystallizing compounds. The high stability of Li₂Ge₃O₆(OH)₂ is interpreted in terms of the matrix assembly of the structure from cyclic invariant subpolyhedral structural units.     

Measurements of Vapor–Liquid Equilibria in the Systems NH3–H2O–NaOH and NH3–H2O–KOH at Temperatures of 303 and 318 K and Pressures 0.1 MPa<p<1.3 MPa

A static method has been used to obtain vapor–liquid equilibrium data for the systems ammonia (NH₃)–water (H₂O)–potassium hydroxide (KOH) and ammonia–water–sodium hydroxide (NaOH) at temperatures of 303 and 318 K and pressures from 0.1 to 1.3 MPa. The salt concentration in the liquid phase was chosen in the range from 2 to 60 mass% salt in water. In both systems NH₃–H₂O–NaOH and NH₃–H₂O–KOH, solid–liquid–vapor equilibria were observed. In the NH₃–H₂O–KOH system, liquid–liquid–vapor equilibrium was observed at 318 K and 1.1 MPa but at yet unknown concentrations of the liquid phases.     

Phase Relations in the Na2O–Al2O3–Nb2O5and CaO–Al2O3–Nb2O5Systems

Phase relations in the Na₂O–Al₂O₃–Nb₂O₅and CaO–Al₂O₃–Nb₂O₅systems were studied. The Na₂O system was found to contain neither ternary compounds nor niobate–aluminate solid solutions. In the CaO system, a ternary compound of composition 4CaO · Al₂O₃·Nb₂O₅was identified (cubic structure, a= 7.628 Å, Z= 2, _meas= _x= 4.43 g/cm³).     

Structure of Mechanochemical Reaction Products in the Systems PbO–Sb2O5and PbO–Sb2O3

Mechanochemical reactions in 2PbO + Sb₂O₅and 2PbO + Sb₂O₃oxide mixtures treated in a high-energy planetary mill were studied by x-ray diffraction. The reactions were found to lead to rapid formation of crystalline products with the pyrochlore and fluorite structures, in line with the reaction-zone model. The temperature ranges of the structural and chemical transformations involved were established, and the compositions of the resulting phases were accurately determined. Heating in air leads to the reduction of the sample containing the pyrochlore phase, with the formation of one crystalline product having a variable lattice parameter because of the chemical inhomogeneity of the material. Under the same conditions, the sample containing the fluorite phase oxidizes in air, yielding four phases with different Pb : Sb ratios: PbSb₂O₆, pyrochlore, a new compound Pb_1/3Sb_2/3O₂with the fluorite structure, and Pb₃Sb₂O₈. The Pb : Sb ratio is found to vary over a wide range, from 0.5 to 1.5. The morphology of the powders produced by mechanical processing at above-threshold rates is considered in terms of nonequilibrium thermodynamics: Since the powder is a dissipative structure, its inhomogeneity cannot be fully eliminated, which correlates with the formation of an intermediate, dynamic state during mechanochemical reactions.     

Electrochemical Properties of Nitride Phases in the Si3N4–Ca3N2and Si3N4–AlN–Ca3N2Systems

The electrical conductivity of silicon nitride and its solid solutions with calcium nitride and aluminum nitride was measured in the ranges 400–900 and 1000–1300°C. The conduction mechanisms were found to be substantially different in these temperature ranges. The Si₃N₄–Ca₃N₂solid solutions exhibited high ionic conductivity between 400 and 900°C. The densest and most oxidation-resistant materials were obtained in the Si₃N₄–AlN–Ca₃N₂system (Al introduced as fine powder and then nitrided).     

Intercalation of Graphite in the Ternary Systems C–HNO3–R (R = H2O, CH3COOH, H3PO4 , H2SO4)

The reactions between highly oriented pyrolytic graphite and HNO₃–R (R = H₂O, CH₃COOH, H₃PO₄, H₂SO₄) solutions were studied by x-ray diffraction and potentiometry. The results demonstrate that the nature of component R has a crucial effect on the intercalation process and phase composition of the reaction products. The ability to form ternary graphite intercalation compounds (GICs) depends on the acidity of R. It is shown that CH₃COOH, a weak protic Brönsted acid (pK = 4.76), does not form cointercalation compounds when graphite is treated chemically or electrochemically in HNO₃–CH₃COOH solutions. H₃PO₄, a weak Brönsted acid (pK = 2.12) forms ternary intercalation compounds. Stage II–IV ternary GICs with HNO₃ and H₃PO₄ (d _i = 8.05 Å) were for the first time synthesized and investigated. H₂SO₄, a strong Brönsted acid (pK = –2.8), forms stage I cointercalation compounds (I _c = 8.02 Å), independent of the HNO₃ content (5–95 wt %) of the oxidizing mixture. The potential of the HNO₃–H₂SO₄ solutions was found to be E _Ag/AgCl = 1.39 V, independent of the HNO₃ : H₂SO₄ ratio. The main relationships in the ternary systems were shown to be similar to those for the formation of binary GICs with acids. There is a perfect correlation between the redox potential of the HNO₃–R (R = H₂O, CH₃COOH, H₃PO₄, H₂SO₄) solutions and the stage index of the resulting GIC. The concentration ranges of GIC formation in nonaqueous HNO₃ solutions were extended substantially. The behavior of stage I–IV graphite nitrates in different solvents (H₂O, CH₃COOH, H₃PO₄, and H₂SO₄) was studied. Based on the experimental results, mechanisms of the processes in the systems studied were proposed.     

Phase Relations in the Systems Al2TiO5–Fe2O3 , Al2O3–TiO2–Fe2O3 , and Al2TiO5–Cr2O3

Phase relations in the systems Al₂TiO₅–Fe₂O₃, Al₂TiO₅–Cr₂O₃, and Al₂O₃–TiO₂–Fe₂O₃ are investigated, and the composition ranges of pseudobrookite Al_{2 – 2x} M_2x TiO₅ (M = Fe, Cr) solid solutions are determined.     

Solid-State Reactions in CdS–Bi2S3 Thin Films

Structural, electrical, and optical data are used to elucidate the mechanisms of solid-state reactions in thin CdS–Bi₂S₃ films prepared by spray pyrolysis. The results indicate the formation of substitutional solid solutions and a chemical compound and intercalation of Cd between layers of Bi₂S₃.     

Phase Relations in the NaF–WO3 , NaCl–Na3WO3F3 , and NaCl–NaWO3F Systems

The NaF–WO₃, NaCl–Na₃WO₃F₃, and NaCl–NaWO₃F systems were studied by thermal analysis and x-ray diffraction. Solid-state transformations were revealed, and the primary crystallization ranges were outlined. The refractive indices of the sodium fluorotungstates were measured. The thermodynamic functions of Na₃WO₃F₃were derived from emf measurements.     

Phase Relations in the Na2CO3–ZrO2–SiO2–H2O System at 0.1 and 0.05 GPa and 450°C

The phase relations in the Na₂CO₃–ZrO₂–SiO₂–H₂O system were studied at 0.1 and 0.05 GPa and 450°C using large-particle-size and nanocrystalline zirconias. Four silicates were obtained when use was made of readily soluble ZrO₂(nanocr): ZrSiO₄, Na₂ZrSi₆O₁₅ · 3H₂O, Na₂ZrSi₃O₉ · 2H₂O, and Na₄Zr₂Si₅O₁₆ · H₂O. In the system containing poorly soluble ZrO₂(cr), only Na₂ZrSi₆O₁₅ · 3H₂O was found to crystallize. It is shown that the structures of all the Na–Zr silicates contain invariant six-polyhedron structural precursors, each made up of two ZrO₆ octahedra and four SiO₄ tetrahedra, and belong to a homologous series of structures based on the silicate Na₂ZrSi₂O₇, which forms via direct packing of cyclic subpolyhedral precursors.     

Synthesis of Ternary Intercalation Compounds in the Graphite–HNO3–R (R = H2SO4 , H3PO4 , CH3COOH) Systems

The reactions of stage II–IV graphite nitrates with concentrated H₂SO₄, H₃PO₄, and CH₃COOH were studied at graphite : acid weight ratios from 3 : 1 to 1 : 1. The results demonstrate that the reactions in question follow different paths. In the graphite nitrate–H₂SO₄system, the reaction decreases the stage index and yields a ternary graphite intercalation compound. The contents of intercalated HNO₃and H₂SO₄are controlled by the amount of H₂SO₄and the stage index of the parent graphite nitrate. The reaction between graphite nitrate and H₃PO₄leads to partial replacement of HNO₃by H₃PO₄, increasing the identity period without changes in the stage index. The results for the graphite nitrate–CH₃COOH system provide no direct evidence for the formation of an intercalation compound with HNO₃and CH₃COOH. It is shown that varying the nature and amount of the second intercalate species opens up possibilities for preparing oxidized graphite with controlled physicochemical properties.     

Thermomigration Kinetics in the Si–Al–Ga and Si–Al–Sn Systems

The thermodynamics and kinetics of the thermomigration of molten zones based on Al–Ga and Al–Ga melts in the preparation of silicon epilayers have been studied in detail. We have determined the threshold thermomigration temperature for zones of various compositions. The migration onset temperature has been shown to increase monotonically with increasing Ga or Sn concentration in the liquid phase. The thermomigration rate of Si–Al–Ga zones decreases with increasing gallium concentration at temperatures below 1473 K and increases at higher temperatures. The thermomigration rate of Si–Al–Sn zones decreases with increasing Sn concentration over the entire temperature range studied. No chemical compounds have been detected in the Si–Al–Ga or Si–Al–Sn system, which simplifies the use of the thermomigration method in these systems.     

Phase Transformations in the Systems Y2BaCuO5–“Ba3Cu5O8” and Y2BaCuO5–BaCuO2

Data are presented on the sequence of phase transformations leading to the formation of YBa₂Cu₃O_{7 –} textured ceramics and single crystals in the systems Y₂BaCuO₅–Ba₃Cu₅O₈ and Y₂BaCuO₅–BaCuO₂. During cooling in the Y₂BaCuO₅–BaCuO₂ system, YBa₂Cu₃O_{7 –} crystallization in the range 1260–1210 K occurs through the intermediate phase YBa₄Cu₃O_{9 –} , without an additional oxygen source. In the Y₂BaCuO₅–Ba₃Cu₅O₈ system between 1250 and 1210 K, YBa₂Cu₃O_{7 –} crystallization is accompanied by oxygen absorption.     

Preparation and Optical Properties of Glasses in the TeO2–MoO3–Pr2O3 System

Inorganic Materials - We have studied glass formation and located the boundaries of the glass-forming region in the TeO2–MoO3–Pr2O3 system. The results demonstrate the possibility of...     

Phase Relations in the LiOH–ZrO2–GeO2–H2O System at 500°C and 0.1 GPa

The phases crystallizing in the LiOH–ZrO₂–GeO₂–H₂O system at 500°C and 0.1 GPa are Zr^[8]Ge^[4]O₄, Li₂Ge^[6]Ge₂ ^[4]O₆(OH)₂, Li₃HGe₄ ^[6]Ge₃ ^[4]O₁₆ · 4H₂O, and Li₂Ge^[4]O₃. These phases differ in the oxygen coordination of Ge. At a ZrO₂ : GeO₂ molar ratio of 1 : 1, increasing the LiOH concentration leads to the crystallization of a ZrO₂ + Li₂GeO₃ mixture, instead of ZrGeO₄. At ZrO₂ : GeO₂ ratios in the range 1 : 2 to 1 : 6, Li₂Ge^[6]Ge₂ ^[4]O₆(OH)₂ crystallizes together with ZrGeO₄. The formation of the structures of ZrGeO₄ and Li₂Ge^[6]Ge₂ ^[4]O₆(OH)₂ is discussed in terms of the matrix assembly of crystal structures from cyclic subpolyhedral structural units.     

Phase Relations in the KOH–TiO2–SiO2–H2O System at 500°C and 0.1 GPa

The compounds crystallizing in the system KOH–TiO₂(rutile)–SiO₂–H₂O at 500°C, 0.1 GPa, and different TiO₂, SiO₂, and KOH concentrations are K₂TiSi₆O₁₅ (Ti davainite structure), K₂TiSi₃O₉ (Zr wadeite structure), and K₂Ti₆O₁₃ (Ti jeppeite structure). The matrix-assembly model is used to examine the formation of the K₂TiSi₆O₁₅ structure (sp. gr. P ) from subpolyhedral structural units. A centrosymmetric cyclic 12-polyhedron cluster of composition K₂ M ₂ T ₁₀ is identified, in which the K atoms lie in center positions above and below the plane of the MT ring. The precursor cluster K₂ M ₂ T ₁₀ is identical in structure to the Cs₂ M ₂ T ₁₀ cluster in Cs₂TiSi₆O₁₅, a titanosilicate which has a topologically different MT framework. The mechanisms of the assembly of the three-dimensional MT frameworks in these titanosilicates differ at the level of primary MT chains: the condensation of the clusters in K₂TiSi₆O₁₅ involves five common corners, while that in Cs₂TiSi₆O₁₅ involves only three corners.     

High-Temperature Thermodynamic Study of Micro- and Nanocrystalline SnO2–WO3 Systems

The thermodynamics of vaporization in micro- and nanocrystalline SnO₂–WO₃ systems was studied by high-temperature mass spectrometry. The results demonstrate that SnO₂ and WO₃ do not form solid solutions or compounds in the condensed phase. The vapor phases in the micro- and nanocrystalline systems contain the same species: constituent components, their associates, SnWO₄, Sn₂WO₅, and SnW₂O₇. In the nanocrystalline system, the vapor phase is supersaturated with complex molecules, whose concentrations are increased by a factor of 3–10. The standard enthalpies of formation of the SnWO₄, Sn₂WO₅, and SnW₂O₇ molecules and the partial pressures of the main vapor species are determined.     

Phase stability in Zr–H and Ti–H systems

Abstract

Enthalpy calculations, based on Miedema's model for alloy formation, show the δ–hydride (ZrH_~1·6) to be the most stable hydride in the Zr–H system. The γ–hydride (ZrH) is the least stable and, as suggested in the literature, is quite probably metastable. In addition, the ε–hydride (ZrH_2±x) is shown to be the last hydride formed in the Zr–H system. TiH_1·38 is shown to be the most stable hydride in the Ti–H system corresponding to the γ–hydride phase on the equilibrium phase diagram.

MST/187