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A YBa2Cu3 O 6.92 superconductor has been studied in an aberration-corrected scanning transmission electron microscope using a combination of atomic resolution imaging and elemental mapping by electron-energy loss spectroscopy (EELS). YBa2Cu3 O 6.92 has been found to be of a nanostructural state due to local variation of cation composition on a nanometer-sized scale. YBa2Cu3 O 6.92 can thus be considered as a non-single phased material formed of coherently intergrown nanodomains of phases with different cation compositions.

Samples of high-temperature superconducting oxide EuBa2Cu3O6 + δ (Eu-123) with total cationic composition Eu: Ba: Cu = 1: 2: 3 are investigated by means of local X-ray microanalysis and high-resolution transmission electron microscopy. The cationic nonstoichiometry of Eu-123 oxide is revealed. The particles of the studied samples are inhomogeneous in structure on the nanoscale, with two types of inhomogeneities: one with typical sizes of one to several nanometers, and one with typical sizes of 10 to 20 nm, respectively.

Energy dispersive X-ray spectroscopy (EDX) and electron diffraction and high-resolution electron microscopy are used to study the composition and structure of anode and cathode deposits formed during the high temperature (950°C) electrolysis of the Y-Ba-Cu-O system and low temperature (450°C) electrolysis of the Y-Ba-Cu-K-O system. It is found for the first time that an oxide with the YBa2Cu3Oy structure (123 phase) is synthesized during the high temperature electrolysis of Y0.02Ba0.30Cu0.70Oy and Y0.02Ba0.25Cu0.75Oy melts. The 123 phase is not synthesized during low temperature electrolysis, where melts are formed from Y2O3, BaO2, CuO, and KOH. Two new Pt-containing oxides with hexagonal structure are found in the products of high temperature electrolysis.

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.

The structure and cationic composition of oxides of the Ba-Cu-O system with cubic BaCuO2 and tetragonal BaCu2O2 structure and YBa2Cu3O6 + δ oxide of the Y-Ba-Cu-O system are studied. The oxides are found to be nonstoichiometric in cationic composition when the composition differs noticeably from the corresponding formula relation with conservation of the crystalline structure’s type.

The structure of barium-rich oxides of the Ba-Bi-O system was studied with the transmission electron spectroscopy (TEM) and electron diffraction techniques. The results show that TEM cannot reveal the presence of an ordered state in a material composed of irregular intergrown crystallites, if their sizes are less than several nanometers.

The composition of melt from the crystallization and liquidus regions of the BaO-CuO x 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.

The cation composition and structure of YBa2Cu3O7 − δ (YBCO, the 123 phase) oxide samples synthesized at 990°C in air and annealed in oxygen at 450°C are studied via elemental analysis in a transmission electron microscope and high-resolution electron microscopy. The occurrence of cation nonstoichiometry and nanostructured inhomogeneity of the 123 phase in tetragonal and orthorhombic (superconducting) structures is confirmed. The nanostructured inhomogeneity of the 123 phase is attributed to the presence of nanocrystallites (2–5 nm) of oxides with different cation compositions belonging to the YnBamCum + nOy series.

Phase relations have been studied in the BaO–CuO x system in the range of 42.0–83.0 mol.% CuO at P(O 2 ) = 21 kPa (air) by visual polythermal analysis (VPA), powder X-ray diffraction (XRD), differential thermal analysis (DTA), thermogravimetric analysis (TGA), chemical analysis (CA), and electron diffraction (ED) with simultaneous elemental analysis (EA) in a transmission electron microscope (TEM). The existence of discrete crystallization fields of barium–copper oxides of cation compositions Ba 4 Cu 5 O y , Ba 5 Cu 6 O y , Ba 7 Cu 8 O y , Ba 12 Cu 13 O y , and Ba 24 Cu 25 O y , which have the cubic structure of the BaCuO 2 oxide, is revealed in the studied region of the system. The oxides may be represented as members of a Ba m Cu m + n O y homologous series. The BaCuO 2 oxide does not exist in the subsolidus region and does not have its own crystallization field. The oxygen-deficient oxide BaCuO 1.78 of the cation composition (Ba:Cu) 1:1 with the BaCuO 2 cubic structure is found in melted samples of the 50.0 mol.% CuO composition quenched at 1020–1060 °С.

Three new barium bismuth oxides have been synthesized containing 2.15–2.72 mol % BaO with the integer composition BaBi39O59.5, BaBi46O70, and BaBi40O61 and a monoclinic, cubic, and triclinic structure, respectively. These double oxides are, likely, derivatives of α-, γ-, and ɛ-Bi2O3 phases. The effects of the crucible material and surrounding atmosphere on the course of the synthesis of these new barium bismuth oxides are elucidated.

The crystal structure and cation composition of oxides of the Ba-Bi-O system in the composition range 80–100 mol % BiO1.5 have been investigated by the methods of transmission electron microscopy. Ordered phases of the compositions Ba: Bi = 2: 9, 1: 6, and 1: 15 with a rhombohedral structure have been revealed. In the range of the compositions Ba: Bi from 1: 36 to 1: 46, phases with triclinic, monoclinic, and cubic structures have been found. The monoclinic and cubic phases have structures similar to the structures of the α and γ modifications of bismuth oxide Bi2O3, respectively. Phase formation was found to be dependent on the following parameters: annealing temperature, partial oxygen pressure, oxygen content in the initial pairs of reacting components (BaO2-Bi2O3, Ba(NO3)2-Bi2O3, BaCO3-Bi2O3), and crucible material (alundum or platinum).

New barium bismuth oxides have been synthesized under an argon atmosphere at 530 and 430°C, respectively: a Ba: Bi = 1 : 4 phase has a tetragonal perovskite structure, and a 1 : 15 phase has a rhombohedral structure. For the 1 : 4 phase, the unit cell parameters are a = 4.297(2) A and c = 4.472(2) A as determined by powder X-ray diffraction (XRD). Electron diffraction (ED) patterns show superstructure reflections with the vectors q 1= 1/17[530], q 2 = 1/17[3\(\bar 5\)0], and q 3 = 1/5[002]. These reflections point to an ordered arrangement of barium and bismuth ions in the supercell with the parameters √17a and 5c. The supercell of the 1 : 4 phase includes a ten-layer stack of metal-oxygen planes along the c axis. The rhombohedral 1 : 15 phase has the following hexagonal unit cell parameters as determined by XRD: a= 6.018(2) A, c = 4.070(2) A. In view of the superstructure on the ED patterns, the supercell parameters of the rhombohedral phase are 4a and 2c. The supercell of the 1 : 15 phase contains a six-layer stack of planes along the c axis.

The phase formation in the Ba-Bi-O system is significantly affected by oxygen pressure during its synthesis. The reason for the sensitivity of this system to oxygen pressure consists in mobile coexistence of bismuth ions in different oxidation states in the same oxide (Bi5+ and Bi3+, Bi3+ and Bi2+, etc.). This circumstance determines the oxide structure type and the ability to formation of structural subtypes, i.e., oxides of an isostructural homologous series. The presence of bismuth ions in different oxidation states in the same oxide determines the variety of properties of both bismuth oxides and complex oxides on their basis (ferroelectric, semiconducting, superconducting, and other properties).

The phase composition of YzBa5Cu7Oy (1) and YzBa3Cu5Oy (2) samples with a variable percentage of yttrium up to the stoichiometric composition of the YnBamCum + nOy series is investigated by X-ray phase and elemental analyses, electron diffraction, and high resolution imaging in a transmission electron microscope at a temperature of 930°C in the crystallization field of a matrix oxide (Ba: Cu) of 5: 6 composition on the phase diagram of the BaO-CuOx system at P(O2) = 21 kPa. The substantial effect of yttrium oxide’s presence on the phase composition of both objects is found, providing evidence of a complex ionic equilibrium within the melt. The fine-domain oxide structure of the YBa2Cu3O6 tetragonal form, which is due to the coexistence of oxides of an YnBamCum+nOy homologous series of (Y: Ba: Cu) 235, 123, and 257 composition is revealed. The domain size for these phases is 20–50 A. The domains are joined coherently along axis c.

The phase composition of specimens in the primary crystallization field of the YnBamCum + nOy oxide series with compositions of (Y:Ba:Cu) 235, 123, and 257 was investigated by means of X-ray powder diffraction, X-ray microprobe analysis, electron diffraction, and high resolution imaging in a transmission electron microscope. YnBamCum + nOy oxides have the YaBa2Cu3O6 tetragonal structure and can be represented in the general form as 123 oxides. It was found that the crystallization field of 123 oxides corresponds to the crystallization fields of the BamCum + nOy oxide series with the BaCuO2 cubic structure. These crystallization fields are located in the matrix phase diagram of the BaO-CuOx system at P(O2) = 21 kPa in the range of 50–90 mol % CuO and 910–1000°C. The 123 oxides are formed according to an intercalation mechanism with the participation of barium-copper oxide matrices with the composition (Ba: Cu) 3: 5, 2: 3, and 5: 7

Similar essential properties of Ba-K-Bi-0 (BKBO) bismuthate and cuprate superconductors imply a common pairing mechanism in spite of the apparent isotropic structure of bismuthates being contradictory to the two-dimensional structures of cuprates. However, this distinction is likely not to be the case as structural studies of BKBO materials composed of nm-sized domains of two phases, superconducting and semiconducting, have not revealed true structures of BKBO superconducting phases. The thorough studies of phase relations in the Ba-Bi-O (BBO) parent system have resulted in the discovery of a large number of discrete barium-bismuth oxides. Bi-rich layered oxides are assumed to be matrices intercalated with potassium and transformed into layered superconductors with ordered ion arrangements. The structures of samples corresponding to two-phase regions formed of Ba-rich oxides and the prime perovskite BaBiO3 have been found to be disordered. We believe that phase relations in the form of discrete compounds and disordered materials are inherent in any metal-oxide systems including superconducting ones. The two-component structure of oxide superconductors is due to the synthesis of superconducting materials run under conditions of simultaneous formation of two phases.