V. I. Nikolaichik

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.

Spatial Resolution of Transmission Electron Spectroscopy for the Study of Ordered Materials

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.

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.

Electron diffraction study of ordering in the Er0.715Ca0.285F2.715 tysonite phase

The structure of Er0.715Ca0.285F2.715 crystals as grown from the melt (without heat treatment) has been studied by electron microscopy. It is found that the structure is inhomogeneous, consisting of an ordered tysonite matrix with the hexagonal crystal structure (supercell reflections exhibit the vector 1/8〈203〉 in the units of small tysonite cell), in which crystallites of the other laminar phase with layers ∼10 Å thick are incorporated.

Zinc oxide nanoparticles immobilized on graphene flake

Interactions of ZnO nanoparticles with graphene oxide in isopropanol were studied; graphene oxide was shown to perform as an efficient substrate to immobilize zinc oxide nanoparticles on its surface. Interactions of nanocomposites consisting of graphene oxide-zinc oxide nanoparticles with supercritical isopropanol were studied. The conversion of graphene oxide into graphene does not appreciably changes the composition, morphology, or structure of ZnO nanoparticles.

Structural characteristics of nanomaterials based on CdS quantum dots

CdS nanoparticles with sizes where a quantum-size effect is observed are structurally characterized in a detailed way. The following complex of structural methods is used to characterize the nanoparticles: electron diffraction; analytical, diffraction, and high-resolution transmission electron microscopy; and small-angle X-ray scattering.

Effect of high-energy electron irradiation in an electron microscope column on fluorides of alkaline earth elements (CaF2, SrF2, and BaF2)

The effect of high-energy (150 eV) electron irradiation in an electron microscope column on crystals of fluorides of alkaline earth elements CaF2, SrF2, and BaF2 is studied. During structural investigations by electron diffraction and electron microscopy, the electron irradiation causes chemical changes in MF2 crystals such as the desorption of fluorine and the accumulation of oxygen in the irradiated area with the formation of oxide MO. The fluorine desorption rate increases significantly when the electron-beam density exceeds the threshold value of ∼2 × 103 pA/cm2). In BaF2 samples, the transformation of BaO into Ba(OH)2 was observed when irradiation stopped. The renewal of irradiation is accompanied by the inverse transformation of Ba(OH)2 into BaO. In the initial stage of irradiation of all MF2 compounds, the oxide phase is in the single-crystal state with a lattice highly matched with the MF2 matrix. When the irradiation dose is increased, the oxide phase passes to the polycrystalline phase. Gaseous products of MF2 destruction (in the form of bubbles several nanometers in diameter) form a rectangular array with a period of ∼20 nm in the sample.