V. K. Fedotov

Thermal stability of Bi2O3

Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) of an α-Bi2O3 sample revealed staged phase transitions in the range 720–800°C (at 720, 780, and 800°C) and the elimination of oxygen to the composition Bi2O2.967 during heating to 895°C in air at 16 K/min. In dynamic vacuum (p = 1.33 Pa) at 780–800°C, Bi2O3 consecutively transforms to a phase with the cubic γ-Bi2O3 structure and tetragonal Bi2O2.3−2.4. In the latter, electron diffraction in a transmission electron microscope (ED/TEM) shows a superstructure with the superstructure vector q110 ≈ 1/9, which indicates an ordered arrangement of oxygen vacancies.

Existence of the homologous series of Y n Ba m Cu m + n O y ( m = 2, 3, 5; n = 1, 2) oxides with the tetragonal and orthorhombic structures of YBa 2 Cu 3 O 6 + δ

The phase composition of YxBa1−xCuOy (x = 0.29−0.40) samples annealed in air (at 930–990°C) and in an oxygen atmosphere (450–800°C, P(O2) = 101 kPa) was studied by X-ray powder diffraction, chemical analysis, electron diffraction, and elemental analysis in a transmission electron microscope. A considerable cation nonstoichiometry was discovered in particles having the tetragonal and orthorhombic structures of YBa2Cu3O6 + δ. The variation range of particle compositions comprises matrix oxides of the BamCum + nOy series with (Ba: Cu) 3: 5, 5: 8, 2: 3, and 5: 7, which in the presence of yttrium form the YnBamCum + nOy series. Tetragonal oxides Y2Ba3Cu5Oy (235), Y3Ba5Cu8Oy (358), YBa2Cu3Oy (123), and Y2Ba5Cu7Oy (257) are formed at the primary synthesis step in air and are preserved in an orthorhombic structure during short-term (1 h) oxygen annealing. Most particles of the 3: 5 and 5: 8 oxides are undersaturated with yttrium relative to the stoichiometry of the YnBamCum + nOy series, those of the 2: 3 oxide correspond to this stoichiometry, and those of the 5: 7 oxide are supersaturated with yttrium over the stoichiometry. A trend is observed for the fractions of these oxides to change during long-term (5–51 h) annealing in an oxygen atmosphere at 450°C and to the alternation of the dominant role of one of the four phases with the superconducting transition temperature Tc = 82, 85, 86, and 91 K. Each orthorhombic oxide undergoes structural transformations during oxygen annealing with a change in Tc. The coexistence of these oxides in the form of nanometer-sized domains does not allow their individual superstructures to be recognized.

BaO-BiO1.5 phase diagram in the region 80-100 mol % BiO1.5 at po2 = 21 kPa

The phase equilibria in the region from 80 to 100 mol % BiO1.5 are studied in the BaO-BiO1.5 system in air and under an argon atmosphere using visual polythermal analysis (VPA), X-ray powder diffraction, differential thermal analysis (DTA), electron diffraction (ED), and elemental analysis in a transmission electron microscope (EA/TEM). A series of discrete layered rhombohedral phases (RPs) with the composition Ba2Bi8+nOy, where n = 0, 1, 2, 4, 8, or 10, is discovered. The melting character is determined for these RPs: the 1: 4 phase melts congruently; the 2: 9, 1: 5, 1: 6, and 1: 8 phases melt incongruently. The crystallization fields are 10–30°C. At po2 = 21 kPa, the 1: 4 phase in the subsolidus region decomposes into a 4: 13 perovskite-like phase and the 2: 9 phase. The other rhombohedral phases decompose into stages to yield Bi2O3 and a neighboring RP that is richer in barium. The Ba2Bi8+nOy oxides are stable in argon and experience a first-order phase transformation at about 560°C.

On the structure of melt in the BaO-CuOx (50.0–80.0 mol % CuO) system at p(O2) = 21 kPa

The composition of melt from the crystallization and liquidus regions of the BaO-CuOx phase diagram was studied within the range 50.0–80.0 mol % CuO at p(O2) = 21 kPa. At 900–1050°C, melt in the range of the compositions studied is structured and consists of oxides having cubic structure BaCuO2; (Ba/Cu = 0.80−0.96), tetragonal structure BaCu2O2; (Ba/Cu = 0.50−0.62), and monoclinic structure (CuO). The off-stoichiometry of BaCu2O2 manifests itself in electron diffraction (ED) patterns taken in a transmission electron micro-scope (TEM) as diffuse scattering streaks extending in the [100] and [010] directions. These data find explanation in the existence of clusters having a suggested composition of Ba2Cu3O3.5 + x which are produced by BaCu2O2 + x disproportionation and are integrated into the BaCu2O2 structure. Thermal effects observed in the liquidus region, which are accompanied with a change in oxygen content, are associated with the cluster structure of the melt and its evolution in response to varying temperature.

New phases in the barium-rich region of the BaO-BaCuO2 system

The phase relations have been studied in the BaO-CuOx system in the range of 25.0–45.0 mol % CuO at P(O2) = 21 kPa by visual polythermal analysis (VPA), X-ray phase analysis (XPA), differential thermal analysis (DTA), thermogravimetric analysis (TGA), chemical analysis (CA), X-ray microprobe analysis (XMEA), and electron diffraction (ED), with simultaneous elemental analysis (EA) by a transmission electron microscope (TEM). A discrete deviation of the 2.10, 2.08, 2.04, 2.02 (Ba/Cu) compositions from stoichiometry in the known Ba2CuO3 + δ oxides is found. New oxides of the 2.00, l.75, 1.66, 1.15 (Ba/Cu) compositions are revealed and their unit-cell parameters are determined. The phase diagram of the BaO-CuOx system is constructed, whose structure is considered as the total projection of phase states of the system at T = f(x) in CuOx.

Primary crystallization field of the oxide EuBa2Cu3O6+δ and the existence of a homologous series EunBamCum + nOy (m = 2, 3, 4, 5; n = 1, 2)

The phase composition of Eu0.01–0.03BaxCu1 − xOy (x = 0.03–0.41) samples annealed in air at 915–1100°C was studied by X-ray powder diffraction and chemical analyses, and also electron diffraction and elemental analysis in transmission electron microscope. The primary crystallization field of the oxide with the tetragonal structure EuBa2Cu3O6+δ (Eu-123) was found to coincide with the crystallization fields of the oxides BamCum + nOy in the matrix system BaO-CuOx. A significant deviation from the nominal value was detected in the cationic composition of particles characterized by the diffraction pattern of phase 123, which suggests the existence of isostructural oxides of the homologous series EunBamCum + nOy with the matrix compositions Ba : Cu = 4 : 7, 5 : 8, and 3 : 5, and also 2 : 3, 5 : 7, 3 : 4, 4 : 5, and 5 : 6. A significant deviation of the actual cationic composition from the nominal value in the oxide Eu2CuO4 was revealed, and the impurity oxides Eu3 − xBaxCu2Oy and Eu1 − xBaxCuOy with variable cationic compositions were found to exist.

The Effect of Synthesis Conditions on the Phase Composition and Structure of EuBa 2 Cu 3 O 6 + δ Samples

AbstractThe composition and the structure of ceramic EuBa2Cu3O6 + δ (Eu-123) oxide samples annealed in steps with varying processing conditions (in air or oxygen and argon atmosphere at a temperature of 940–960°С for 1–70 h with or without homogenization) were studied by the X-ray phase and chemical analysis, electron diffraction pattern analysis, elemental analysis, and high-resolution transmission electron microscopy. Regardless of the processing conditions, Eu-123 nanostructured oxide with a tetragonal or orthorhombic structure and domains 1–20 nm in size was obtained as a result of annealing. Nanostructuring of the samples, which was revealed by high-resolution electron microscopy, is attributed to their chemical nature: the presence of identical structural elements in members of the homologous EunBamCum + nOy series of oxides allows them to intergrow coherently and create an illusion of a single crystal. Just like any other member of the EunBamCum + nOy series, oxide Eu-123 is disproportionate depending on the annealing conditions to form other members of this series located on either side of the dominant oxide. Temperature Tc of the superconducting transition of each member of the series depends on the average oxidation state of copper

Preparation of YBa2

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