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.     

Liquid Formation at the Peritectic Temperature in Superconducting YBa2Cu3O7−x—Observation of a New Phase YBa4CuAIO8

In the system Y-Ba-Cu-O, partial melting of peritectically decomposing YBa₂Cu₃O_{7- x} (123) was used to produce a bulk material of high critical current density when the material was aligned. The liquid formation mechanism and its relation to reaction with alumina refractory was studied. A previously unreported phase of the approximate stoichiometric ratio Y:Ba:Cu:Al = 1:4:1:1 (YBa₄CuAlO₈) was detected. The crystal structure was determined to be tetragonal, with lattice parameters a ₀= b ₀= 1.651 nm, c ₀= 1.793 nm. The 1411 phase bears a close structural relationship to BaCuO₂.     

Solid Phase Relations at 950°C in La-Ba-Cu-O

Subsolidus phase relations in the La₂O₃–BaO–CuO system were studied at 950°C. Three previously reported binary compounds exist (La₂CuO₄, BaLa₂O₄, and BaCuO₂) and five previously reported ternary phases occur (La_2-xBa_xCuO_4-(x/2)+δ, La_4-2xBa_2+2xCu_2-xO_10-2x, La_2-xBa_1+xCu₂O_6-(x-2), La_3-xBa_3+xCu₆O_14±δ, and La₄BaCu₅O_13+δ). Of the seven phases in the diagram, all but BaLa₂O₄, BaCuO₂, and La₄BaCu₅O_13+δ were shown to exhibit significant ranges of solubility. The diagram is important in that both >30 K (La_2-xBa_xCuO_4-(x/2)+δ) and >90 K (La_3-xBa_3+xCu₆O_14+δ, x=1) superconductors occur.     

Microstructural Analysis in Processing of the High-Tc Superconductor Ba2YCu3O7-x in Air

In the synthesis of the superconducting compound Ba₂YCu₃O_7-x from a stoichiometric mixture containing BaCO₃, Y₂O₃, and CuO In air, a low-melting liquid phase is formed at about 890°C. The liquid phase was identified as a ternary eutectic located within the compatibility triangle Ba₂YCu₃O_7-x–BaCuO₂–CuO. The implication of this finding for the processing of Ba₂YCu₃O_7-x is discussed.     

Reaction Sequencing During Processing of the 123 Superconductor

Reaction sequencing studies were conducted for two precursors to the 123 superconductor in air and helium environments. In air, the reactants in both precursor systems produce an intermediate 123 phase at 1000 K without going through the BaCuO₂ and Y₂BaCuO₅ intermediate phases, observed in earlier work with mixed powder precursors. In helium, the fully developed 123 tetragonal phase is formed at temperatures as low as 880 K, but it also decomposes to Y₂BaCuO₅, BaCu₂O₂, and BaCuO₂; the extent of decomposition depends on the temperature. The 123 tetragonal phase completely decomposes by 1075 K, but it can be reformed if the atmosphere is switched back to air at a temperature above 950 K.     

Phase Composition and Compatibilities at 930° to 950°C in the System Europium(III) Oxide-Barium Oxide-Copper(II) Oxide in Air

Phase composition and compatibilites at 930° to 950°C were determined for the system Eu₂O₃–BaO–CuO in air. The binary compound Eu₂CuO₄ dissolves Ba to the extent 0 x 0.02 in Eu_2-xBa_xCuO₄, whereas the other binary compounds, Eu₂BaO₄ and BaCuO₂, do not exhibit solid solubility. Three ternary compounds were obtained, Eu₂BaCuO₅ and two solid solution phases. The first contains the 90 K '123' superconductor and has solubility limits represented by Eu_1+xBa_2-xCu₃O_7±y, where 0 x 0.5. The second has a solubility limit represented as Eu_1+xBa_8-xCu₄O_y, where 0 x 0.44. The limited solid solution range of this phase provides insight concerning the probable solid solution range of the analogous phase in the Y₂O₃-BaO-CuO system.     

Phase Equilibria in the TiO2-Rich Region of the System BaO-TiO2

Phase relations in the system BaO-TiO₂ from 67 to 100 mol% TiO₂ were investigated at 1200° to 1450°C in O₂. Data were obtained by microstructural, X-ray, and thermal analyses. The existence of the stable compounds Ba₆Ti₁₇O₄₀, Ba₄Ti₁₃O₃₀, BaTi₄O₉, and Ba₂Ti₉O₂₀ was confirmed. The compound BaTi₂O₅ is unstable and either forms as a reaction intermediate below the solidus or crystallizes from the melt. The compounds Ba₆Ti₁₇O₄₀ and Ba₄Ti₁₃O₃₀ decompose in peritectic reactions, and BaTiO₃ and Ba₆Ti₁₇O₄₀ react to form a eutectic. Special conditions are required for the formation of Ba₂Ti₉O₂₀, which decomposes in a peritectoid reaction at 1420°C. The new phase diagram is presented.     

1000°C Phase Relations and Superconductivity in the (Nd-Ce-Cu)-O Ternary System

The phase relations involving the 24 K n -type Nd_{2- x} Ce _x CuO₄ superconductor were investigated at 1000°C in air. The terminal solid solubility was confirmed to be x = 0.2. This solid solution is the only ternary phase in the Nd₂O₃–CeO₂–CuO diagram. A binary (1 − y )CeO₂– y NdO_1.5 solid solution exists out to y = 0.4. Phase diagrams for NdO_1.5–CeO₂–CuO (1000°C) and NdO_1.5–CeO₂ (900° to 1500°C) are presented.     

Alignment of YBa2Cu3O7-x and Ag─YBa2Cu3O7-x Composites at ∼930°C by Eutectic Formation

The present work describes a new technique to synthesize aligned YBa₂Cu₃O_{7- x} and Ag─YBa₂Cu₃O_{7- x} superconducting composites from Ba- and Cu-deficient compositions (relative to YBa₂Cu₃O_{7- x} ) plus BaCuO₂. For YBa₂Cu₃O_{7- x} , high transition temperature midpoint T_c (91 K), temperature of zero resistivity T ₀ (90 K), and critical current density J_c (>3000 A°Cm⁻² at 77 K) were achieved by using this technique. This procedure provides the potential for using a reliable and reproducible densification and alignment technique alternative to partial or full melting. The composite is highly aligned, with an average grain size of ∼1 to 2 mm and domains of width greater than 5 mm. The initial phase assemblage consists of YBa₂Cu₃O_{7- x} (123) as the major phase plus YBa₂CuO₅ (211) CuO as minor phases. The BaCuO₂ is added to the Ba- and Cu-deficient starting composition in order to assist in the formation of a CuO-rich liquid as well as to compensate for the Ba and Cu deficiences in 123. Since the liquid forms at ∼900°C and is compatible with 123, it can be used to facilitate alignment of 123 at ∼930°C. The addition of Ag to the system results in eutectic formation with the (solidified) liquid, substantial filling of the pores during sintering, and improved alignment.     

Barium Holmium Zirconate, A New Complex Perovskite Oxide: I, Synthesis, Characterization, and Potential Use as a Substrate for High-Critical-Temperature Superconductors

Barium holmium zirconate, a new complex perovskite ceramic oxide, has been synthesized through liquid-phase sintering for the first time. The conventional solid-state reaction method using constituent oxides and carbonates was found to be inadequate for the synthesis of Ba₂HoZrO_5.5 material. During high-temperature annealing, the development of stable BaZrO₃ and BaHoO_2.5 phases prevented the formation of Ba₂HoZrO_5.5 as a single-phase material, even at 1650°C. However, an addition of a small amount of CuO (1 wt%) in the reaction mixture has resulted in the formation of an ordered complex perovskite Ba₂HoZrO_5.5 phase during the heating process. The structure of Ba₂HoZrO_5.5 was studied by X-ray diffraction and found to have a cubic perovskite structure with a lattice constant of a = 8.482 Å. Dielectric constant and loss factor values of Ba₂HoZrO_5.5 are also in the range suitable for use as a substrate for microwave applications. The X-ray diffraction and resistivity measurements have shown that there is no detectable chemical reaction in YBa₂Cu₃O_7−δ–Ba₂HoZrO_5.5 and Bi(2212)–Ba₂HoZrO_5.5 composites, even under extreme processing conditions. Dip-coated and melt-textured YBa₂Cu₃O_7−δ and Bi(2212) thick films developed on polycrystalline Ba₂HoZrO_5.5 gave zero-resistivity transition temperatures of T _c(0) = 92 and 85 K, respectively.     

Tetragonal-to-Cubic Structural Phase Transition in Pollucite by Low-Temperature X-ray Powder Diffraction

The tetragonal-to-cubic structural phase transition in pollucite (CsAlSi₂O₆) was investigated using low-temperature X-ray powder diffraction in the temperature range of 93 to 298 K, and the lattice constants were refined with Rietveld analysis. It was found that CsAlSi₂O₆ had a tetragonal phase with a space group of I 4₁/acd in the temperature range of 93 to 248 K, a = 1.36337(4) nm, c = 1.36988(6) nm at 248 K, and underwent a phase transition from tetragonal to cubic with a space group of Ia-3 in the temperature range of 248 to 273 K, a = 1.36645(3) nm at 273 K.     

Low-Temperature Phase Equilibria in the Y-Ba-Cu-O System

Ultrasonically prepared freeze-dried nitrate precursors and high-precision solution calorimetry were used to investigate the low-temperature thermodynamic stabilities of compounds in the Y-Cu-O, Ba-Cu-O, Y-Ba-O, and Y-Ba-Cu-O pseudobinary and pseudoternary systems at 1 atm of oxygen. Y₂Cu₂O₅, Y₂BaCuO_s, and BaCuO₂ were found to be metastable below 682°, 728°, and 710°± 5°C, respectively. The only stable phases in the Y-Ba-Cu-O system at 298 K and 1 atm of oxygen are Ba₂Cu₃O₆, CuO, BaO₂, and Y₂O₃. By compiling the calorimetric and phase equilibria data, a series of Y-Ba-Cu-O isothermal phase diagrams were constructed between 25° and 900°C at 1 atm of oxygen.     

Sol-Gel Synthesis of Fine Gd2CuO4 Particles: Influence of Synthesis Variables

Fine particles of Gd₂CuO₄ were prepared by a sol-gel reaction of an aqueous solution of metal nitrates in the presence of urea, which leads to high-homogeneity polycrystalline powders of Gd₂CuO₄. We have studied the synthesis conditions, demonstrating the existence of a relationship between the calcination temperature and the heating time needed to attain the pure phase. Gd₂CuO₄ was obtained at temperatures of the order of 650°C, lower than temperatures employed in the conventional ceramic technique. The influence of the [urea]/[salts] ratio and an excess of Cu(II) in the starting solution was also studied and discussed. X-ray powder diffraction, inductively coupled plasma atomic emission spectroscopy (ICPAES), photon correlation spectroscopy (PCS), and transmission electron microscopy (TEM) were used to characterize the Gd₂CuO₄ samples obtained.     

Effects of B2O3 on the Phase Stability of Ba2Ti9O20 Microwave Ceramic

Preparation of dense and phase-pure Ba₂Ti₉O₂₀ is generally difficult using solid-state reaction, since there are several thermodynamically stable compounds in the vicinity of the desired composition and a curvature of Ba₂Ti₉O₂₀ equilibrium phase boundary in the BaO–TiO₂ system at high temperatures. In this study, the effects of B₂O₃ on the densification, microstructural evolution, and phase stability of Ba₂Ti₉O₂₀ were investigated. It was found that the densification of Ba₂Ti₉O₂₀ sintered with B₂O₃ was promoted by the transient liquid phase formed at 840°C. At sintering temperatures higher than 1100°C, the solid-state sintering became dominant because of the evaporation of B₂O₃. With the addition of 5 wt% B₂O₃, the ceramic yielded a pure Ba₂Ti₉O₂₀ phase at sintering temperatures as low as 900°C, without any solid solution additive such as SnO₂ or ZrO₂. The facilities of B₂O₃ addition to the stability of Ba₂Ti₉O₂₀ are apparently due to the eutectic liquid phase which accelerates the migration of reactant species.     

Molten-Salt Synthesis of Ba1–xPbxTiO3 in the System BaTiO3–PbCl2

Ba_{1– x} Pb _x TiO₃ powder with a fixed composition was prepared by the reaction of BaTiO₃ powders with molten PbCl₂at various PbCl₂/BaTiO₃ molar ratios at 600° and 800°C in a nitrogen atmosphere. When 0.1 μm powder was used, the reaction was finished when x = 0.9. Two phases of BaTiO₃and a solid solution of Ba_{1– x} Pb _x TiO₃ coexisted, but the final phase gave a solid solution of Ba_{1– x} Pb _x TiO₃ at 800°C. When 0.5 μm powder was used, the two phases coexisted in the products at 600°C at PbCl₂/BaTiO₃= 1.0. A sintered compact of Ba_{1– x} Pb _x TiO₃ powders solid solution was prepared by hot isostatic pressing, and its dielectric constant was measured in the temperature range 20°–550°C.     

Temperature-Dependent Phase Boundaries for BaTi409, Ba4Ti13030, and Ba6Ti17040

The phase boundaries of BaTi₄O₉ and Ba₄Ti₁₃O₄₀ are temperature dependent and curve toward BaTiO₃ above 1250°C. The appearance of a barium-rich surface phase during slow cooling is a sensitive indicator of a temperature-dependent boundary. For both compounds the surface phase which is barium-rich and has u strong X-ray peak at d=0.232 nm. No surface phase was detected on Ba₆Ti₁₇O₄₀; therefore, its phase boundary is independent of temperature.     

Fabrication and Characterization of Pyroelectric Ba0.8Sr0.2TiO3 Thin Films by a Sol-Gel Process

A sol-gel process was used to prepare pyroelectric Ba_0.8Sr_0.2TiO₃ thin films with large columnar grains (100–200 nm in diameter) on Pt/Ti/SiO₂/Si substrates, via using a 0.05 M solution precursor. The relationship between dielectric constant and temperature (ɛ_r- T ) showed two distinctive phase transitions in the Ba_0.8Sr_0.2TiO₃ thin films. Both the remnant polarization and the coercive field decreased as the temperature increased from −73°C to 40°C. Its low dissipation factor (tan δ= 2.6%) and high pyroelectric coefficient ( p = 4.6 × 10⁻⁴ C·(m²·K)⁻¹ at 33°C), together with its good insulating properties, made the prepared Ba_0.8Sr_0.2TiO₃ thin films promising for use in uncooled infrared detectors and thermal imaging applications.     

Defect-Chemical Aspects in the Synthesis of Metallic Conductors La1–xBaxCuO3 (x= 0.0 to 0.5)

Compositions of La_{1- x} Ba _x CuO₃, where x ranges from 0.0 to 0.5, were fired in air, oxygen-enriched air, and oxygen. Studies show that BaO cannot make a solid solution with LaCuO₃ without changing the basic structure. The resulting phases, in all attempts, were the binary compounds La₂CuO₄, La_{1- x} Ba _x CuO_3-δ ( x = 0.2 to 0.5), or their mixtures. All samples showed metallic conductivity. Extra oxygen in the reaction atmosphere appeared to encourage the formation of the LaCuO₃-based phases of La_{1- x} Ba _x CuO_3-δ ( x = 0.2 or 0.5). We provide a defect-chemical and thermodynamical explanation for this observation.     

Existence of the Ba2Ti5O12 Phase in Ca-Doped TiO2-Rich BaTiO3

A TEM investigation was conducted on the structure of a second phase precipitated between the grains of a polycrystalline TiO₂-rich BaTiO₃ which was doped with 8 mol% Ca. This phase was identified as Ca-stabilized Ba₂Ti₅O₁₂ with a 10-layer orthorhombic structure and unit-cell parameters a=0.990, b=1.131, and c=2.330 nm.     

Subsolidus Equilibria in the System BaO-TiO2-GeO2

Subsolidus phase relations in the system Ba0-Ti02-Ge02 were investigated using conventional solid-state reaction techniques and X-ray powder diffraction. The existence of 2 ternary compounds, BaTiGe309 and BazTiGeZ08, was confirmed and their X-ray crystallographic data are presented. The compound BaTiGe₃0₉ has a lower limit of stability at 1135°C and melts incongruently at 1232°C; Ba₂TiGe₂O₂ melts congruently at 1228°C. Subsolidus compatibility relations in the ternary system were established and tie lines between the various phases which constitute a total of 12 compatibility triangles at 1000°C are shown in a subsolidus phase diagram.