S.Ya. Istomin

Synthesis and structural studies of Sr2Co2−xGaxO5, 0.3⩽x⩽0.8

The present thesis deals with the investigation of some perovskite related complex cobaltates. The phases Sr2Co2-xGaxO5 (0.3 ≤ x ≤ 0.7), Sr2Co2-xAlxO5 (0.3 ≤ x ≤ 0.5), Sr1-xBixCoO3-y (0.1 ≤ x ≤ 0.2), Sr0.75Y0.25Co1-xGaxO2.625 (0.125 ≤ x ≤ 0.375) and Sr0.75Y0.25Co1-xFexO2.625+δ (0.125 ≤ x ≤ 0.625) were synthesised and characterised. All these compounds crystallises with similar structures, they are all composed by altering layers of octahedra and tetrahedra although in the two former, the tetrahedra are organised in chains (the Brownmillerite structure), while in the latter three the tetrahedra arranges as segregated Co4O12 units (the 314 type structure). The techniques X-ray and neutron diffraction, transmission electron microscopy, thermal analysis and magnetic measurements were used to track structural and important physical properties.

Synthesis and characterization of reduced niobates CaLnNb2O7, Ln = Y, Nd with a pyrochlore structure

The first known reduced niobates with a pyrochlore structure, CaLnNb(2)O(7) Ln = Y, Nd, have been synthesized from Ca2Nb2O7, Ln(2)O(3), and NbO2 in sealed Nb ampoules heated at 1400 degrees C for 6 ...

Synthesis, structure, and properties of mixed niobium(IV,V) oxides

This review paper summarizes data on mixed oxides containing niobium in an intermediate valence state, between 4+ and 5+. The major preparation procedures are considered, and the effects of different process parameters—composition of the starting reagents, temperature, pressure, duration, and environment—are analyzed from the viewpoint of the preparation of bulk, single-phase materials with niobium in a controlled formal oxidation state. The optimal synthesis conditions are established for reduced niobates with different structures. The oxidation behavior of the niobates(IV,V) is examined as a function of the structure type. The structural and electrical properties of the reduced niobates are shown to correlate with the carrier concentration (formal oxidation state of niobium). The major factors determining the carrier concentration in the niobates are identified. The results are used to establish the conditions under which a semiconductor—metal transition is possible in reduced niobates

Synthesis, structure, and resistivity properties of K1 − xBaxNbO3 (0.2 ≤ x ≤ 0.5) and K0.5Sr0.5NbO3

The K{sub 1{minus}x}Ba{sub x}NbO{sub 3} (0.2{<=}x{<=}0.5), BaNbO{sub 3} and K{sub 0.5}Sr{sub 0.5}NbO{sub 3} compounds have been synthesized by high-pressure high-temperature technique in a Belt-type apparatus. The synthesis of the K-containing phases carried out in evacuated sealed silica tubes led to the preparation of polyphasic samples. The phases obtained have an ideal perovskite-like structure. The a parameter of Ba-containing compounds increases concomitantly with increasing Ba concentration. Studies of the K-containing phases by X-ray powder diffraction did not reveal any distortion from cubic symmetry. Electron Dispersive Spectroscopy (EDS)analysis confirmed the cation nominal stoichiometry. Resistivity measurements showed semiconducting-like type of conductivity for the K{sub 1{minus}x}Ba{sub x}NbO{sub 3} (0.2{<=} x {<=} 0.5) and K{sub 0.5}Sr{sub 0.5}NbO{sub 3} samples, but the relative increase of {rho} with the decrease of temperature became smaller for the phases with lower Nb valence, and moreover K{sub 0.5}Sr{sub 0.5}NbO{sub 3} exhibited a slight decrease of the resistivity in the 100--4.2 K temperature range. On the contrary, BaNbO{sub 3} after high-pressure treatment showed a metallic-like behavior down to 4.2 K.

Neutron diffraction studies of La1 xSrxCoO3 magnetic structure at x=0.15 and 0.3

Abstract Neutron powder diffraction and zero-fields μSR studies of La 1− x Sr x CoO 3 with x =0.15 and 0.3 has been performed. Only compound with Sr concentration x =0.3 shows ferromagnetically ordered state at low temperature. Values of Co magnetic moments have been refined.

New anion-deficient cubic perovskites: Ba2In1 −xCo1 +xO5 +δ(0 ≤x≤ 0.8) and Ba2 −xLaxCoInO5 +δ(0.2 ≤x≤ 0.8)

The new compound Ba2InCoO5 + δ, which has a cubic perovskite structure (a = 4.1587(3) A) at high temperatures, was prepared by heating mixtures of BaCO3, In2O3 and Co3O4 in air at 1150 °C for 20 h. Heat treatment of Ba2InCoO5 + δ in air at 850 °C for 72 h revealed a phase transformation into a brownmillerite-type structure. To stabilize a cubic perovskite phase at moderate temperatures, the solid solutions Ba2In1 − xCo1 + xO5 + δ, 0 ≤ x ≤ 0.7 and Ba2 − xLaxCoInO5 + δ, 0 ≤ x ≤ 0.8 were prepared. X-Ray diffraction revealed that the cubic perovskite structure is stable only for Ba2In1 − xCo1 + xO5 + δ, 0.4 ≤ x ≤ 0.6 and Ba1.2La0.8CoInO5 + δ, Ba1.4La0.6CoInO5 + δ upon heat treatment in air at 850 °C for 72 h.

Synthesis and characterization of the reduced niobates CaLnNb2O7, Ln = La-Pr, Sm, Gd-Lu, with the pyrochlore-type structure

New reduced niobates, CaLnNb2O7, Ln = La-Pr, Sm, Gd-Lu, with the pyrochlore-type structure were synthesized in sealed Nb ampoules at 1350–1500°C for 6–10 h. The prepared compounds were characterize ...

Structure and properties of Ba6 − xLnxNb10O30, Ln = La, Ce and Nd compounds

Ba{sub 6{minus}x}Ln{sub x}Nb{sub 10}O{sub 30} compounds, Ln = La, Ce and Nd, with a tetragonal tungsten bronze-type structure have been synthesized and were characterized by X-ray powder diffraction and EDS analysis. The homogeneity regions of the obtained compounds were found to be 0 {le} x {le} 2 for La, and 0 {le} x {le} 1.5 for Ce and Nd. The crystal structures of Ba{sub 5}LaNb{sub 10}O{sub 30} and Ba{sub 4}La{sub 2}Nb{sub 10}O{sub 30} were refined using X-ray powder diffraction data. Refinement indicated the presence of vacancies in the Ba/La positions. This observation was supported by oxidation studies of the La-substituted compounds. The temperature dependence of the normalized resistivity of the compounds showed a nonmetallic behavior. The increase in the R/R{sub 273} ratio with decreasing temperature is less distinct for the phases with the higher concentration of rare-earth cation.

The effect of temperature and oxygen partial pressure on the reduction mechanism in the Pr2CuO4/Ce0.9Gd0.1O1.95 system

The electrochemical behavior of a porous electrode based on Pr2CuO4 (PCO) screen printed on the surface of Ce0.9Gd0.1O1.95 (CGO) solid electrolyte is studied by impedance spectroscopy. The rate-determining stages of the oxygen reduction reaction at the PCO/CGO interface are found for the oxygen partial pressure interval of 30–105 Pa and temperatures of 773–1173 K. Changeover of the rate-determining stage of electrode reaction is shown to occur depending on the temperature and the oxygen partial pressure. The PCO electrode polarization resistance is 1.7 Ω cm2 at 973 K in air and remains constant at thermocycling of the electrochemical cell in the temperature range of 773–1173 K. Based on the found data, PCO can be considered as the promising cathodic material for solid-oxide fuel cells operating at moderate temperatures (773–973 K).

Synthesis and crystal structure of the new complex cobalt and nickel oxide Sr2.25Y0.75Co1.25Ni0.75O6.84

The complex cobalt and nickel oxide Sr2.25Y0.75Co1.25Ni0.75O6.84 has been synthesized by the citrate method. The oxygen content of the oxide has been determined by iodometric titration. The crystal structure of the compound has been refined using X-ray powder diffraction data (a = 3.7951(2) Å, c = 19.700(1) Å, χ2 = 1.15, RF2 = 0.0586, Rp = 0.0365, Rwp = 0.0462). Sr2.25Y0.75Co1.25Ni0.75O6.84 has the structure of the second member of the Ruddlesden-Popper series An + 1BnO3n + 1.