T. P. Krinitsina

Fine structure and the mechanism of the low-temperature decomposition of the nonstoichiometric compounds YBa2Cu3O6.8 and YBa2Cu3O6.8 doped with Ce

The resistance of the nonstoichiometric compounds YBa2Cu3O6.8 (single crystals, ceramics) and YBa2Cu3O6.8 (Ce) (single crystal) to low-temperature decomposition (at 200°C) in air and an argon atmosphere is studied by X-ray diffraction and electron microscopy. At the first stage, the compounds are found to undergo oxygen separation into two phases, namely, oxygen-rich and oxygen-lean phases as compared to the initial state, that have different lattice parameters. After 20-to 35-h annealing, the disordering of the heavy Y, Ba, Ba, Y atoms along the c axis in ceramic pellets begins mainly because of the action of elastic stresses. This process occurs via the formation of the packets of numerous stacking faults on (001) planes. The disordering of the ceramic matrix ends in the formation of a CsCl-type cubic phase upon 100-h annealing, which is accompanied by a significant decrease in the diamagnetic response and an increase in Tc from 75 K in the initial state to 90 K. The retained superconductivity and the increase in Tc are caused by the presence of a large number of oxygenrich ortho-phase particles in a nonsuperconducting matrix; these particles are correlated with each other and form a multicoupled system of superconducting filaments.

Phase diagram of the Ba2YCu3O6-Ba2YCu3O7 system below 400°C

X-ray diffraction analysis and transmission electron microscopy have been used to study the low-temperature decomposition of the nonstoichiometric (in oxygen) HTSC compound Ba2YCu3O7 − δ. The phase diagram of the Ba2YCu3O6-Ba2YCu3O7 system below ≤400°C has been constructed. The temperature range corresponding to phase separation has been found to be divided into two portions. At T > 250°C, two orthorhombic phases characterized by different oxygen contents are formed; at the higher temperatures, the phase separation of the compound into a tetragonal and an orthorhombic phase takes place. The separation was also found to observe at T = 100°C; this indicates the possibility of natural aging for the Ba2YCu3O7 − δ compound at room temperature.

Effect of the atmosphere of low-temperature annealing on the structure and electrophysical properties of nonstoichiometric YBa2Cu3O7 − δ ceramics

The fine structure and electrophysical properties of nonstoichiometric YBa2Cu3O7 − δ ceramics and the effect of low-temperature annealing (t ⩾ 200°C) in various atmospheres on these parameters have been studied. It has been shown that, during annealing in a vacuum, the decomposition is quite sluggish; structures typical of initial stages of decomposition are observed. The decomposition in an inert-gas atmosphere occurs more actively, and structures typical of stages of deep decomposition are realized. It has been found that, during low-temperature annealing, the structure and properties are affected by two factors; these are the decomposition into phases differing in the oxygen content, and water absorption, leading to the transformation with the formation of a pseudo-cubic lattice. The annealing atmosphere substantially affects the kinetics of both processes.

Spinodal decomposition of the nonstoichiometric YBa2Cu3O7−δ compound at room temperature

Electron and polarized-light optical microscopy have been used to study the spinodal decomposition of the YBa2Cu3O7−δ compound depending on the oxygen content. Experimental data reported indicate that whatever the state —initial or preliminarily aged at a high temperature—of the YBa2Cu3O7−δ (δ ≈ 0.2 and 0.4) nonstoichiometric oxide, it undergoes natural aging typical of metallic alloys.

Restoration of the microstructure of Ba2YCu3O7 − δ at T > 900°C after low-temperature decomposition

Transmission electron microscopy and optical microscopy have been used to study the structure of the nonstoichiometric YBa2Cu3O7 − δ compound subjected to low-temperature annealing (200–300°C) and subsequent restoration annealing at 930–950°C. It was shown that the retention of part of structural defects, which were formed during decomposition and interaction with water vapor, after restoration annealing determines structural state of the compound that is typical of materials exhibiting improved electrophysical characteristics.

Structural state of second-generation HTSCs obtained by laser ablation

The structure of buffer layers and deposited YBa2Cu3Ox (Y123) films with a high critical current density (∼106 A/cm2) has been investigated by different methods. These superconducting films and buffer layers are found to have a fine-grained structure, which, along with the high texture of Y123 films of the (001) type, is believed to be responsible for the high critical current density. An unusual texture is revealed in buffer layers, which differs from that of substrates and Y123 films. The superconducting films deposited on buffer CeO2 layers exhibit a system of orthogonal lines; in the case under consideration, this is a manifestation of a domain structure with Y123 particles at boundaries.

Mechanism of phase transformations and fine structure of a nonstoichiometric compound YBa2Cu3O7-δ at temperatures of 200 and 300°C

The spinodal decomposition of the YBa2Cu3O7-δ (δ≈0.2, 0.4, 0.6, 0.8) compound at temperatures of 200 and 300°C, which correspond to different regions under the spinodal curve in the YBa2Cu3O6-YBa2Cu3O7 phase diagram, has been studied using electron microscopy and optical microscopy in polarized light. It has been shown that at these temperatures the spinodal decomposition occurs, irrespective of the oxygen content (y = 6.8–6.22) in the 123 phase, via the formation of particles of the oxygen-enriched orthorhombic phase in an oxygen-depleted monolitic matrix. During annealing at 300°C, a new domain structure arises at the sites of clustering of oxygen-enriched particles, and a diamagnetic response appears in the sample.

Mechanism of formation, fine structure, and superconducting properties of high-temperature superconductors and superconducting composites

A review of available literature data is given and our own results (mainly obtained using transmission electron microscopy) are presented that concern some important problems in the structure of high-Tc superconducting materials, including superconducting composites based on the Bi-containing ceramics, namely, mechanisms of the formation of the basic superconducting 2223 phase from precursors (liquid and diffusion mechanisms); syntactic intergrowths of homologous 2223, 2212, and 2201 phases; mechanisms of penetration of silver atoms from the silver sheath into the ceramics; and location of carbon in the Bi-ceramics/Ag composite. Effects of all above factors on the superconducting characteristics of high-Tc superconducting materials is discussed.

X-ray photoelectron spectra and composition of YBa2Cu3O7 − δ films prepared by laser ablation

The Y3d, Ba3d5/2, Cu2p3/2, and O1s X-ray photoelectron spectra of thick (600 nm) superconducting YBa2Cu3O7 − δ films deposited on textured Ni-W substrates with Y2O3 + ZrO2 and CeO2 buffer layers have been studied. It has been established that, after the mechanical removal of surface layers with a diamond scraper (and as the analyzed region of the film approaches the interface), a decrease in the oxygen content leads to a decrease of the orthophase fraction and an increase of the tetraphase and Cu+ ion fractions. This is caused by the presence of elastic stresses in the superconducting film due to the lattice misfit between the phases making up a composite sample. These stresses prevent oxygen diffusion involved in oxidizing annealing. The spectra of the superconducting film have not revealed signals generated by elements of the substrate and buffer layers.

Oxygen atomic displacement waves in the lattice of the Bi,Pb-2223 phase composite annealed in an atmosphere with a low oxygen (O2 + N2) content

After annealing of Bi,Pb-2223/Ag composites in a nitrogen-oxygen atmosphere with a low oxygen content in the 2223 phase, cross-polarized waves of oxygen atomic displacements have been revealed by electron diffraction in the [010]2223 direction. It has been demonstrated using the methods of X-ray photo-electron spectroscopy and nuclear microanalysis that nitrogen does not interact with the 2223 lattice and that the oxygen index (9.67 ± 0.20) becomes less than the stoichiometric index (x = 10). It has been found that the generation of atomic displacement waves is due to the deficiency of oxygen in the double Bi-O layers.