Phases in the System Ba2SiO4-Ca2SiO4

The system Ba₂SiO₄-Ca₂SiO₄ was studied by heating mixtures of Ba₂SiO₄ and Ca₂SiO₄ at 1723 K. Six distinct phases resulted; they were examined by both X-ray diffraction and differential thermal analysis. The phases β -(Ba_0.05Ca_1.95)SiO₄ and α-(Ba_0.15Ca_1.85)SiO₄ are isostructural with corresponding high-temperature modifications of Ca₂SiO₄. The X phase (Ba_0.48Ca_1.52SiO₄) is orthorhombic, is a pure phase rather than a solid solution, and is defined for the first time in the present work. The T phase (Ba_1.31Ca_0.69SiO₄) is hexagonal and interpreted in terms of a unit cell with a doubled c parameter, in contrast with literature data.     

Further Consideration of Phases in the System Ba2SiO4–Ca2SiO4

The results of an investigation of the system Ba₂SiO₄–Ca₂SiO₄ by powder X-ray and electron diffraction suggest a greater complexity than supposed hitherto. The previously recognized phases α, α' h α' l , X, T, and the newly reported Y have room-temperature structures that are modulated distortions of hexagonal (or pseudohexagonal) parent structures. Each displays characteristic and distinctive modulations. The phases are more readily distinguished in this way than by their unit-cell dimensions and compositions which, for a given phase, can vary with bulk starting composition and thermal history.     

Phase Equilibria in the System Na2SiO3-Li2SiO3-SiO2

Ternary phase relations have been studied and several modifications made to Kracek's phase equilibrium diagram. These include location of the primary phase fields of Na₆Si₈O₁₉ and Li₂Si₂O₅. Metastable phases and polymorphs were often encountered, notably a primary phase, δ-Na₂Si₂O₅, and a new polymorph of Li₂Si₂O₅.     

Thermal Expansion of Pb3O4

Thermal expansion of Pb₃O₄ was investigated by high-temperature X-ray diffraction. The coefficient in the a ₀ direction is 14.6×10⁻⁶/°C. Expansion in the c₀ direction is 32% greater, with a coefficient of 19.3×10⁻⁶/°C. Coefficients of expansion are linear from 25° to 490°C and are comparable with those of tetragonal and orthorhombic PbO.     

Raman Spectroscopy of Higher Titanate Phases in the BaTiO3-TiO2 System

Polytitanates in the BaO-TiO₂ system with Ba: Ti ratios ranging from 1:2 to 1:5 were prepared employing the liquid mix technique. The samples were heated at 600° to 1300°C in oxygen, and Raman spectroscopy was used to investigate the phase relations in the system. The powders of various compounds were amorphous at temperatures less than 600°C. They crystallized into a single phase or a mixture of phases at 700°C. These mixtures underwent further phase transformation at higher temperatures to form single-phase compounds. Details of the procedure for sample preparation and characterization of the products are described. The results from the present study are compared with previously published data.     

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.     

Structural Studies of Ordering in the (Pb1-xBax)(Mg1/3Nb2/3)O3 Crystalline Solution Series

The ordered structures of the (Pb_{1- x} Ba _x )(Mg_1/3Nb_2/3)O₃crystalline solution series were investigated by selected area electron diffraction (SAED) and high-resolution electron microscopy (HREM). At low Ba contents (e.g., x < 0.40), the ordered structure was found to be isostructural with Pb(Mg_1/3Nb_2/3)O₃, with a doubled unit cell characterized by 1/3{111} superlattice reflections. At higher Ba contents (e.g., x > 0.60), the ordered structure was characterized by 1/3{111} superlattice reflections. For intermediate Ba contents (e.g., x - 0.60), diffuse scattering along the {111} between diffuse 1/2{111} and 1/3{111} reflections was observed. The ordering is attributed to the distribution of the B-site cations between multiple sublattices. Strong fluctuations in the B-site cation ratio between ordered and disordered regions are believed not to exist; however, the possibility of weak fluctuations is consistent with the observed lattice images.     

The Join Ca2SiO4-CaMgSiO4

New data obtained on the join Ca₂SiO₄-CaMgSiO₄ established a limit of crystalline solubility of Mg in α-Ca₂SiO₄ corresponding to the composition Ca_1.90Mg_0.10SiO₄ at 1575°C. The α-α'Ca₂SiO₄ inversion temperature is lowered from 1447° to 1400°C by Mg substitution in the lattice. α'-Ca₂SiO₄ takes Mg into its lattice up to the composition Ca_1.94Mg_0.06SiO₄ at 1400°C and to Ca_1.96Mg_0.04SiO₄ at 900°C. A new phase (T) reported previously by Gutt, with the approximate composition Ca_1.70Mg_0.30SiO₄, was stable between 979° and 1381°C, and should be stable at liquidus temperatures in multicomponent systems involving CaO–MgO–SiO₂.     

Crystalline Compounds and Glasses in the System B2O3-NaF-NaBF4

The system B₂O₃-NaF-NaBF, has been studied by subjecting selected compositions to thermal treatment in the range 400° to 600°C. Weight losses, chemical analyses, ir, Raman, and X-ray diffraction techniques were used to define the composition of the crystalline phases and the structural units being formed in the system. The stoichiometry of the BF₃ evolved from NaBF₄-B₂O₃ mixtures indicated that a composition corresponding to Na₂B₃F₅O₃ was formed in mixtures containing up to 33.3 mol% B₂O₃. At higher boron oxide concentrations, Na₂B₃F₅O₃ was consumed, yielding 2NaF.3B₂O₃. The crystalline compounds Na₃B₃F₆O₃, 2NaF.3B₂O₃, and phase B (apparently NaF.B₂O₃) were formed in the system. The compound Na₃B₃F₆O₃ appeared as the stable oxygen-containing species in NaBF₄-NaF mixtures of low oxide content. The main fluorine-containing structural units of the system are BF₄, (–O)₃BF, (–O)₂BF₂, (–O)₂BF, whereas the main structure for binary NaF-B₂O₃ mixtures is (–O)₃BF.     

Crystallization of BeO, Be2SiO4, and SiO2 from Li2MoO4-MoO3

Single crystals of phenacite (Be₂SiO₄), bromellite (BeO), and tridymite (SiO₂) were grown from an Li₂MoO₄-MoO₃ flux. Phenacite, with rhombohedral symmetry, grew in three distinct shapes with aspect ratios (length/width) as follows: needles (>3), rods (>1.1 to 1.5), and rhombohedral-faced crystals (=1). The latter grew as single crystals; the others were twinned on the     . For most experiments the temperature was held constant at 1165°C and the Li₂MoO₄/MoO₃ ratio at 1/16. The growth mechanism for crystallization was the evaporation of MoO₃. The system produced one to three phases, depending on the BeO/SiO₂ ratio. Bromellite grew until a BeO/SiO₂ ratio of 0.8 was attained. It grew as a hemipyramidal crystal having a short prism with a curved     top or as a hexagonal plate. The pyramid- and prism-shaped crystals were twinned, although a few hexagonal plates were single. Tridymite grew in small hexagonal plates when the BeO/SiO₂ ratio was less than 1.5. The effect of temperature, nucleation, and flux composition on crystal shape, twinning, and occurrence is discussed.     

Activity-Composition Relations in Co2SiO4-Fe2SiO4 Solid Solutions at 1180°C

Activity-composition relations of cobalt orthosilicate in cobalt-iron-orthosilicate solid solutions were determined at 1180°C by studying the equilibrium between these solid solutions, silica, metallic cobalt, and a gas phase of known oxygen pressures. The solution shows a slight positive deviation from ideality.     

Hydration in the System Ca2SiO4–Ca3(PO4)2 at 90°C

Compositions along the Ca₂SiO₄–Ca₃(PO₄)₂ join were hydrated at 90°C. Mixtures containing 15, 38, 50, 80, and 100 mol% Ca₃(PO₄)₂ were fired at 1500°C, forming nagelschmidtite + a ¹-CaSiO₄, A -phase and silicocarnotite and a -Ca₃(PO₄)₂, respectively. Hydration of these produces hydroxylapatite regardless of composition. Calcium silicate hydrate gel is produced when Ca₂SiO₄≠ 0 and portlandite when Ca₂SiO₄ is >50%. Relative hydration reactivities are a -Ca₃(PO₄)₂ > nagelschmidtite > α ¹-Ca₂SiO₄ > A -phase > silicocarnotite. Hydration in the presence of silica or lime influences the amount of portlandite produced. Hydration in NaOH solution produces 14-A tobermorite rather than calcium silicate hydrate gel.     

Thermoelastic Behavior in Ca2SiO4 Solid Solutions

Dicalcium silicate solid solutions (C₂S(ss)) doped with Na₂O, A1₂O₃, and Fe₂O₃ were examined by high-temperature optical microscopy. Surface deformation caused by a possible martensitic transformation between a'_L and β phases was observed in situ under the microscope during temperature changes, indicating that the transformation was thermoelastic. Both the start and finish temperatures of the a'_L-to-β and β-to-a'_L transformations decreased with increased Na:(Na + Ca) ratio. Because of the athermal nature of the a'_L-to-β transformation, the a'_L phase, when cooled below the finish temperature, should have been completely converted to the β phase.     

Equilibria of SiF4 with SiO2, Be2SiO4, and BeO in Molten Li2BeF4

Equilibrium partial pressures of SiF₄ were measured for the reactions 2SiO₂( c )+2BeF₂( d )⇋SiF₄( g )+Be₂SiO₄( c ) (log P _siF4(mm) = [8.790 - 7620/ T ] ±0.06(500°–640°C)) and Be₂SiO₄( c ) +2BeF₂( d )⇋SiF₄( g ) +4BeO( c )(log P _siF4(mm) = [9.530–9400/T] ±0.04 (700°–780°C)), wherein BeF₂ was present in solution with LiF as molten Li₂BeF₄. The solubility of SiF₄ was low (∼0.04 mol kg^-1 atm^-1) in the melt. The results for the first equilibrium were combined with available thermochemical data to calculate improved Δ H_f and Δ G_f values for phenacite (–497.57 ±2.2 and –470.22±2.2 kcal, respectively, at 298°K). The few measurements above 700°C for the second equilibrium are consistent with the temperature of the subsolidus decomposition of phenacite to BeO and SiO₂ and with the heat of this decomposition as determined by Holm and Kleppa. Below 700°C, the pressures of SiF₄ generated showed an increasing positive deviation from the expression given for the equilibrium involving Be₂SiO₄ and BeO. This deviation might have been caused by the formation of an unidentified phase below 700°C which replaced the BeO; it more likely resulted from a metastable equilibrium involving BeO and SiO₂.     

BaO-TiO2-WO3 Microwave Ceramics and Crystalline BaWO4

Microwave ceramic resonators composed of BaO-TiO₂-WO₃ were developed. The effect of WO₃ addition on the system of BaO·xTiO₂·(1+x)yWO₃ (x=4 and 4.5, y=0 to 0.04) was studied. The ceramics of this system are composed of crystallines including Ba₂Ti₉O₂₀, BaTi₄O₉, BaWO₄, and TiO₂. At y=0.02, the BaO·4TiO₂·0.1WO₃ ceramic was found to have excellent microwave properties such as ε=35, Q=8400 at 6 GHz, and nearly 0 ppm/°C of τ_f.