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The 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 Eu n Ba m Cum + nO y series of oxides allows them to intergrow coherently and create an illusion of a single crystal. Just like any other member of the Eu n Ba m Cum + nO y 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 $$\overline {Cu} $$ . At $$\overline {Cu} $$ < 2, all members of the series have a tetragonal structure and do not exhibit superconducting properties. At $$\overline {Cu} $$ = 2.28, five members of the Eu n Ba m Cum + nO y series with matrices (Ba : Cu) 5 : 8, 3 : 5, 2 : 3, 5 : 7, and 3 : 4 exhibit superconducting properties with Tc = 82–90 K.

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.

The phase composition of Eu0.01–0.03Ba x Cu1 − x O y (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 Ba m Cu m + n O y in the matrix system BaO-CuO x . 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 Eu n Ba m Cu m + n O y 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 − x Ba x Cu2O y and Eu1 − x Ba x CuO y with variable cationic compositions were found to exist.

Analytical and high resolution electron microscopic studies of YBa 2 Cu 3 O 7 − δ samples with the tetragonal and orthorhombic structure have revealed a deviation of cation composition from the stoichiometric ratio Y:Ba:Cu = 1:2:3 (1 2 3) and a structural inhomogeneity on a scale of a few nm. Nanostructural inhomogeneity has been related with local cation nonstoichiometry in the form of coherently intergrown nanodomains of oxides of the homologous series Y n Ba m Cu m + n O y having cation composition different from the 123 ratio.

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 existence of space inhomogeneous superconductor insulator state (SISIS) found out earlier in polycrystalline samples of high- TC system Ba0.6K0.4BiO3 (TC≈30 K) is confirmed on Ba0.6K0.4BiO3 single crystal. At T* (T*

Mass transfer during the electrolysis of melts of Y0.02Ba0.30Cu0.70Oy and Y0.02Ba0.25Cu0.75Oy samples was studied at 950°C (for 0.5 h) and currents of 5–1050 mA. YBa2Cu3O6 + δ (123) tetragonal oxide crystal boules were grown, and their cationic composition and structure were studied by X-ray powder diffraction and by electron diffraction and elemental analysis in a transmission electron microscope (ED/TEM and EA/TEM). The 123 oxide was found to have cationic off-stoichiometry and to have a domain structure with domain sizes of 20–50 A. Magnetic susceptibility versus temperature curves measured in the crystals after oxygen annealing (450°C, 1 h) feature four kinks, which indicate the occurrence of four superconducting phases with Tc = 45, 52, 75, and 86 K. Electrolysis byproducts are platinum-containing oxides Ba9Pt4Cu3Oy and Ba50Pt15Y16Al13Cu7Oy unknown hitherto; we report structure data for them.

The magnetization of Ba0.6K0.4BiO3 samples in fields up to 90 kOe in the temperature range from 2 to 30 K is investigated. It is shown that the observed increase in the width of the magnetization loop can be explained by a decrease in the phase nonuniformity upon an increase in the magnetic field. The asymmetric hysteretic dependence of magnetization with the secondary peak was successfully described by the extended critical state model taking into account the phase separation in the superconductor.

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 phase relations have been studied in the BaO-CuO{sub x} system in the range of 25.0-45.0 mol% CuO at 900-1100 deg. C at P(O{sub 2})=21 kPa (air) by visual polythermal analysis (VPA), powder X-ray diffraction (XRD), electron diffraction (ED) with simultaneous energy-dispersive X-ray (EDX) elemental analysis in a transmission electron microscope (TEM), and iodometric chemical analysis. The discrete deviations 2.02 (101:50), 2.04 (102:50), 2.10 (105:50) of Ba/Cu (Ba:Cu) composition from the stoichiometric ratio 2:1 have been found for the known Ba{sub 2}CuO{sub 3+{delta}} oxides in the subsolidus region 900-970 deg. C. Unit cell parameters of the 101:50 orthorhombic oxide, 102:50 tetragonal one, 105:50 orthorhombic one are, respectively, a=4.049, b=3.899, c=13.034 A; a=3.985, c=12.968 A; a=4.087, b=3.897 and c=12.950 A. ED patterns of the 101:50, 102:50, 105:50 oxides show characteristic supercell reflections with the respective vector 1/60[5 4 0], {approx}2/11 and 1/6[2 0 0]. Oxides of the 2:1, 7:4, 5:3 and 23:20 compositions have been found in the crystallization region 970-1050 deg. C. Unit cell parameters of the 2:1 orthorhombic oxide are a=4.095, b=3.795, c=13.165 A. Interplanar spacings and X-ray characteristic peak intensities of the 7:4, 5:3 and 23:20 oxides are given. Oxides 2:1 and 7:4 melt pseudocongruently atmore » 1020 and 1005 deg. C, oxides 5:3 and 23:20 melt incongruently at 995 and 980 deg. C, respectively. A diagram of the phase relations in the studied region of the BaO-CuO{sub x} system has been constructed, whose structure is considered as the total projection of phase states of the system existing for different x. - Graphical Abstract: A diagram of phase relations in the BaO-CuO{sub x} system in the range of 30.0-45.0 mol% CuO at 900-1050 deg. C at P(O{sub 2})=21 kPa (air) constructed on the data obtained by visual polythermal analysis (the liquidus line), XRD and ED with elemental analysis. Highlights: > A combination of physical-chemical and structural methods was used. > New discrete oxides have been found. > The diagram reflects the total projection of phase states at different x.« less

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.

Mass transfer during the melt electrolysis of Y0.02Ba0.30Cu0.70O y and Y0.02Ba0.25Cu0.75O y samples is investigated at a temperature of 950°C (0.5 h) and a current of 5–1050 A in a cell. Crystal deposits of YBa2CVu3O6 + δ tetragonal oxide (123) are grown, and their cation composition and structure are investigated by means of X-ray phase analysis, electron diffraction, elemental analysis, and high resolution on a transmission electron microscope. Deviation of the cation composition of oxide (123) from the stoichiometric ratio and its nanostructured state at nanocrystallite sizes of 2–5 nm are observed. The temperature dependence of magnetic susceptibility after oxygen annealing (450°C, 1 h) has four curve bends, indicating there are four superconducting phases with T s = 45, 52, 75, and 86 K.

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.

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 q 110 ≈ 1/9, which indicates an ordered arrangement of oxygen vacancies.

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 specific heat, magnetization and thermal expansion of single crystals of antiferromagnetic insulator EuTe, measured at temperatures down to 2 K and in magnetic fields up to 90 kOe, demonstrate non trivial properties. The Neel temperature, being 9.8 K at H=0, decreases with magnetic field and tends to zero at 76 kOe, therefore forming a quantum critical point. The heat capacity and thermal expansion coefficient reveal lambda-type anomalies at the second order magnetic phase transition at low magnetic fields, evolving to simple jumps at high magnetic fields and low temperatures, well described in a fluctuation free mean-field theory. The experimental data and the corresponding analysis favor the quantum concept of effective increasing space dimensionality at low temperatures that suppresses a fluctuation divergence at a second order phase transition., Comment: 5 pages, 5 figures

A study of temperature dependences of the upper critical field B_{c2}(T) and surface impedance Z(T)=R(T)+iX(T) in Ba_{1-x}K_xBiO_3 single crystals that have transition temperatures in the range 6 < T_c < 32 K (roughly 0.6>x>0.4) reveals a transition from BCS to unusual type of superconductivity. B_{c2}(T) curves corresponding to the crystals that have T_c>20 K have positive curvature (like in some HTSC), and those of the crystals with T_c, 5 pages, 5 figures

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.

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

Phase relations in the 45.0–65.0 mol % CuO region of the BaO-CuOx system at P(O2) = 21 kPa have been studied 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) and elemental analysis (EA) in a transmission electron microscope (TEM). It is found that the investigated region of the diagram consists of several crystallization fields related to barium—copper oxides of cationic (Ba: Cu) compositions of 4: 5, 5: 6, 7: 8, 12: 13, and 24: 25 with the cubic structure of BaCuO2 oxide. They can be represented as a BamCum + nOy homologous series, where m = 3, 4, 5, …, n = l, 2. The 24: 25 oxide has the highest melting point in the studied region of the phase diagram. The BaCuO2 oxide does not exist in the subsolidus region under these conditions and does not have its own crystallization field.

Thermal stability of the perovskite BaBiO[sub 3] and phase relations in the BaO-BiO[sub 1.5](BiO[sub 2.5]) system in the range of 45--55 mol%. BiO[sub 1.5] at p(O[sub 2]) = 0.21 atm have been studied by thermogravimetric analysis, differential thermal analysis, X-ray diffraction, chemical analysis, and transmission electron microscopy. It has been found that BaBiO[sub 3] loses oxygen in the temperature range between 600 and 1015 C with formation of the oxygen-deficient phases BaBiO[sub 2.88], BaBiO[sub 2.83], BaBiO[sub 2.75], and BaBiO[sub 2.55]. BaBiO[sub 3] and the oxygen-deficient phases form two-phase regions involving phases of the perovskite series Ba[sub m]Bi[sub m+n]O[sub y] at the Bi-rich side and Ba[sub m+n]Bi[sub m]O[sub y] at the Ba-rich side. At 1015 C the BaBiO[sub 2.55] phase exhibits incongruent melting. At further increasing temperature the 1:1 (Ba:Bi) sample passes through a sequence of the crystallization fields of the Ba-rich phases of the compositions 5:4, 4:3, 3:2, 9:5, and 17:9, reaching the complete melting state at 1100 C.

The effect of electron instability of heterojunctions based on Ba0.6K0.4BiO3−x single crystals has been found experimentally. This effect is shown to have the opposite sign along the electric field of the current compared to a similar effect observed in structures based on hole-doped systems.

Crystals of the BaBi x O1.5x+1 family are studied using X-ray diffraction, calorimetric, and impedance methods. It is shown that the compounds with x>5 are thermally stable in the temperature range as wide as 300–870 K, in spite of the first-order phase transition with a large heat effect. The conductivity is found to be anisotropic both in low- and high-temperature phases.

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.

The application of transmission electron microscopy in combination with physical-chemical methods has made it possible to discover a large number of discrete oxides of ordered structures in the Ba - Bi - O system, which is the parent system for the K - Ba - Bi - O superconducting system. Cation-ordered superconductors Ba 1-x K x BiO 3 with the crystalline structure of a non-cubic and layered nature have been synthesized. This result removes the crystallographic contradiction between bismuthate and cuprate superconductors that provides support to the existence of a common non-magnetic pairing mechanism.

The theory by Werthamer-Helfand-Hohenberg 1 ~WHH! predicts the universal behavior of the upper critical field Hc2(T) in superconductors with weak electron-phonon coupling. The behavior can be described by a universal function hc2(T) expressed in the reduced variables for magnetic field h5H@Tc(2dHc2 /dT)T5T c # 21 and temperature t5T/Tc . The function hc2(t) has a negative second derivative and saturates to the value hc2’0.7 at t50. The WHH theory has described successfully properties of conventional superconductors but many new superconductors discovered during the last decade exhibit behavior inconsistent with the theory. Amorphous alloys based on transition metals 2 and K 3C 60 ~Ref. 3! shows the linear dependence Hc2(T) down to low temperatures. Some organic superconductors 4 and electrondoped high-temperature superconductors @L 22x Ce x CuO 42y where L5Pr,Sm,Nd ~Refs. 5 and 6!# even have a positive second derivative. Furthermore, in stark contrast to WHH a diverging Hc2(T) was observed in overdoped Tl 2Ba 2CuO 6 films, 7 where Hc2 increased rapidly with decreasing temperature in the whole temperature range 1.t.0.001, and in

Measurements of the superconducting transition temperature of with K (sample 1) and with K (sample 2) have been performed under a pressure up to 10 GPa by the inductive method. With increasing pressure, of sample 1 initially increases, then has a maximum and finally decreases down to 6 K at 9.6 GPa. An irregularity in the dependence of versus P is observed at about 8 GPa. For sample 2, demonstrates a qualitatively similar behaviour with an additional dip in the low-pressure range. The suppression of is explained within the framework of the Bilbro - McMillan model.

The phase relations have been studied in the BaO-CuO{sub x} system in the range of 25.0-45.0 mol% CuO at 900-1100 deg. C at P(O{sub 2})=21 kPa (air) by visual polythermal analysis (VPA), powder X-ray diffraction (XRD), electron diffraction (ED) with simultaneous energy-dispersive X-ray (EDX) elemental analysis in a transmission electron microscope (TEM), and iodometric chemical analysis. The discrete deviations 2.02 (101:50), 2.04 (102:50), 2.10 (105:50) of Ba/Cu (Ba:Cu) composition from the stoichiometric ratio 2:1 have been found for the known Ba{sub 2}CuO{sub 3+{delta}} oxides in the subsolidus region 900-970 deg. C. Unit cell parameters of the 101:50 orthorhombic oxide, 102:50 tetragonal one, 105:50 orthorhombic one are, respectively, a=4.049, b=3.899, c=13.034 A; a=3.985, c=12.968 A; a=4.087, b=3.897 and c=12.950 A. ED patterns of the 101:50, 102:50, 105:50 oxides show characteristic supercell reflections with the respective vector 1/60[5 4 0], {approx}2/11 and 1/6[2 0 0]. Oxides of the 2:1, 7:4, 5:3 and 23:20 compositions have been found in the crystallization region 970-1050 deg. C. Unit cell parameters of the 2:1 orthorhombic oxide are a=4.095, b=3.795, c=13.165 A. Interplanar spacings and X-ray characteristic peak intensities of the 7:4, 5:3 and 23:20 oxides are given. Oxides 2:1 and 7:4 melt pseudocongruently atmore » 1020 and 1005 deg. C, oxides 5:3 and 23:20 melt incongruently at 995 and 980 deg. C, respectively. A diagram of the phase relations in the studied region of the BaO-CuO{sub x} system has been constructed, whose structure is considered as the total projection of phase states of the system existing for different x. - Graphical Abstract: A diagram of phase relations in the BaO-CuO{sub x} system in the range of 30.0-45.0 mol% CuO at 900-1050 deg. C at P(O{sub 2})=21 kPa (air) constructed on the data obtained by visual polythermal analysis (the liquidus line), XRD and ED with elemental analysis. Highlights: > A combination of physical-chemical and structural methods was used. > New discrete oxides have been found. > The diagram reflects the total projection of phase states at different x.« less

Bismuthate superconductor ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{BiO}}_{3}$ $(x=0.27\ensuremath{-}0.49,{T}_{c}=25\ensuremath{-}32\mathrm{K})$ grown by an electrolysis technique was studied by electron diffraction and high-resolution electron microscopy. The crystalline structure thereof has been found to be noncubic, noncentrosymmetric and of the layered nature, with the lattice parameters $a\ensuremath{\approx}{\mathrm{a}}_{p},$ $c\ensuremath{\approx}2{\mathrm{a}}_{p}$ $({\mathrm{a}}_{p}$ is a simple cubic perovskite cell parameter) containing an ordered arrangement of barium and potassium. The evidence for the layered nature of the bismuthate superconductor removes the principal crystallographic contradiction between bismuthate and cuprate high-${T}_{c}$ superconductors.

A series of perovskite-like compounds with general composition BanBin+mOy(m, n—integer) has been studied by means of electron diffraction and high-resolution electron microscopy. For a number of simple, discrete values of the cation composition ratio Ba:Bi the structures were found to be Ruddlesden–Popper–like derivatives of the perovskite structures of BaBiO3, with composition-dependent long periods. For more complicated values of the cation composition ratio the perovskite blocks are further fragmented by planar interfaces into a two-dimensional periodic island structure. This fragmentation gives rise to complicated local structures formed by a mosaic of square or (and) rectangular islands, which often produce incommensurate diffraction patterns. Such diffraction patterns are interpreted in the framework of the “fractional shift” method for interface modulated structure. Structure models and compositions can be made compatible by assuming that substitution of barium ions by bismuth ions must take place, predominantly along the island boundaries.

The existence of space inhomogeneous superconductor insulator state (SISIS) found out earlier in polycrystalline samples of high-TC system Ba0.6K0.4BiO3 (TC30 K) is confirmed on Ba0.6K0.4BiO3 single crystal. At T* (T*

The Brillouin-Mandel’stam scattering (BMS) of light is one of a few experimental methods which makes it possible to study excitations in magnetic solids directly.