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 λ-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.

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.

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.

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.

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.

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.

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 Y n Ba m Cu m + n O y series.

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.

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 YBa2Cu3O y structure (123 phase) is synthesized during the high temperature electrolysis of Y0.02Ba0.30Cu0.70O y and Y0.02Ba0.25Cu0.75O y 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.

Analytical and high resolution electron microscopic studies of YBa2Cu3O7−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 (123) 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 YnBamCum+nOy having cation composition different from the 123 ratio.

The existence of space inhomogeneous superconductor insulator state (SISIS) found out earlier in polycrystalline samples of high- T C system Ba 0.6 K 0.4 BiO 3 ( T C ≈ 30 K) is confirmed on Ba 0.6 K 0.4 BiO 3 single crystal. At T * ( T * < T C , T * ≈ 17 K) the transition from the homogeneous superconducting state into the SISIS occurs. SISIS is characterized by the appearance of two gaps on the Fermi surface, semi- and superconducting, which are modulated in space in antiphase, the electric transport between superconducting regions being carried out due to Josephson tunneling. Thus the whole sample becomes a multiple Josephson system. Nonlinear I - V curves are observed on Ba 0.6 K 0.4 BiO 3 single crystal at temperatures below T * . Dependence of I - V curves on temperature and magnetic field, typical to a Josephson system, was found out. Besides, a step-like peculiarity at the values of voltage of the order of one and two superconducting gaps shows up. These peculiarities are suppressed by magnetic field much earlier than critical current. The new data firstly correlate with the model of SISIS and secondly permit for the first time to determining directly the energy gap between homogeneous and stratified superconductor states.

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.

Mass transfer during the electrolysis of melts of Y 0.02 Ba 0.30 Cu 0.70 O y and Y 0.02 Ba 0.25 Cu 0.75 O y samples was studied at 950°C (for 0.5 h) and currents of 5–1050 mA. YBa 2 Cu 3 O 6 + 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 Å. 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 T c = 45, 52, 75, and 86 K. Electrolysis byproducts are platinum-containing oxides Ba 9 Pt 4 Cu 3 O y and Ba 50 Pt 15 Y 16 Al 13 Cu 7 O y unknown hitherto; we report structure data for them.

Исследован процесс массопереноса при электролизе расплава образцов Y0.02Ba0.30Cu0.70Oy и Y0.02Ba0.25Cu0.75Oy при температуре 950°C (0.5 ч) и силе тока 5–1050 мА. Выращены були кристаллов тетрагонального оксида YBa2Cu3O6 + (123) и методами рентгенофазового анализа, электронной дифракции и элементного анализа в просвечивающем электронном микроскопе исследованы его катионный состав и структура. Обнаружено отклонение катионного состава оксида 123 от стехиометрического и установлена его доменная структура с размером доменов 20–50 A. На кривых температурной зависимости магнитной восприимчивости кристаллов после кислородного отжига (450°C, 1 ч) имеются четыре перегиба, свидетельствующих о присутствии четырех сверхпроводящих фаз с Tc = 45, 52, 75 и 86 К. Побочным продуктом электролиза являются ранее неизвестные Pt-содержащие оксиды Ba9Pt4Cu3Oy и Ba50Pt15Y16Al13Cu7Oy, для которых приведены структурные данные.