Physicochemical basics for using redox Cu(III)-Cu(II) system for the identification of Bi(V) and O 2 2− in Ba-Bi-O and K-Ba-Bi-O oxides

The interaction of Bi(V)-containing oxides and peroxides with Cu(II) salts in alkaline solutions of KIO₄ and K₂TeO₆ is studied by spectrophotometry, potentiometry, and potentiometric titration. It is found that the Bi₂O_4.09, NaBiO₃, BaBiO₃, and K _n Ba _m Bi _{m + n} O _y (n = 1–3, m = 1, 3–5) oxides, exhibiting superconducting properties (T _c = 28–32 K), unlike the BaO₂ and H₂O₂ peroxides, oxidize Cu(II) to complex diperiodatocuprate(III)-[Cu(IO₆) ₂ ^7? ] and ditelluratocuprate(III)-[Cu(TeO₆) ₂ ^9? ] ions of a yellow-brown color. The optimum concentrations of the reagents (CuCl₂, KIO₄, and KOH) are determined for the identification of Bi(V) and O ₂ ^2? by the drop test method.     

Linear low-temperature dependence of the surface impedance of a Ba0.6K0.4BiO3 single crystal

The temperature dependences of the real part R _s and the imaginary part X _s of the surface impedance Z _s =R _s +iX _s of the superconductor Ba_0.6K_0.4BiO₃ (T _c ≃30 K) are measured at a frequency of 9.4 GHz. Its temperature dependence _{Z s} (T) and that of the complex conductivity σ _s (T) can be described on the basis of a two-fluid model under two assumptions: The density of superconducting carriers increases linearly, and the relaxation time increases as a power law (∝1/T ⁵), with decreasing temperature T<T _c . This model also describes well the curves Z _s (T) and σs (T) recently measured for YBa₂Cu₃O_6.95 and Bi₂Sr₂CaCu₂O₈ single crystals. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 11, 783–788 (10 December 1996)     

Electrooptical parameters of uniaxial crystals

For the linear effect in classes 4, ¯4, 42, 4 · m, 6, 62, and 6 m, one of the optic axes always coincides with the high-order symmetry axis c when the electric field e lies in a plane perpendicular to that axis, while the other describes a circular cone around c as e rotates in that plane. The same rotation in classes 3, 32, 3 · m, 3 m, and m · 3 m causes both optic axes to deviate from c to opposite sides by equal angles, both describing circular cones. One of the optic axes always coincides with c for any direction of e in classes 42 and 62, while the other axis describes an elliptic cone around c as e rotates in a plane inclined to c. Both optic axes describe an elliptic cone in classes 3 m, 32, and m · 3 m.     

Two approaches toward the regio- and stereoselective synthesis of N-unsubstituted 3-aryl-4-(trifluoromethyl)-4H-spiro-[chromeno[3,4-c]pyrrolidine-1,3'-oxindoles]

Chemistry of Heterocyclic Compounds - A regio- and stereoselective method for the synthesis of N-unsubstituted 3-aryl-4-(trifluoromethyl)-4H-spiro[chromeno[3,4-c]-pyrrolidine-1,3'-oxindoles] in...     

On the thermal stability of the barium-copper oxide BaCuO2 at 900–1100°C in air

Phase relations in the 45.0–65.0 mol % CuO region of the BaO-CuO _x 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 Ba _m Cu _{m + n} O _y 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.     

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 YBa2Cu3O6 + δ

The phase composition of Y _x Ba_1?x CuO _y (x = 0.29?0.40) samples annealed in air (at 930?C990°C) and in an oxygen atmosphere (450?C800°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 YBa₂Cu₃O_{6 + ??}. The variation range of particle compositions comprises matrix oxides of the Ba _m Cu _{m + n} O _y series with (Ba: Cu) 3: 5, 5: 8, 2: 3, and 5: 7, which in the presence of yttrium form the Y _n Ba _m Cu _{m + n} O _y series. Tetragonal oxides Y₂Ba₃Cu₅O _y (235), Y₃Ba₅Cu₈O _y (358), YBa₂Cu₃O _y (123), and Y₂Ba₅Cu₇O _y (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 Y _n Ba _m Cu _{m + n} O _y 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?C51 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 T _c = 82, 85, 86, and 91 K. Each orthorhombic oxide undergoes structural transformations during oxygen annealing with a change in T _c. The coexistence of these oxides in the form of nanometer-sized domains does not allow their individual superstructures to be recognized.