Nadezhda E. Volkova

Quintuple perovskites Ln2Ba3Fe5−xCoxO15−δ (Ln = Sm, Eu): nanoscale ordering and unconventional magnetism

Quintuple perovskites Ln2Ba3Fe5−xCoxO15−δ, with Ln = Sm, Eu have been synthesized for x values varying from 0 to 2. The HRTEM and HAADF investigations show that these oxides are ordered at the nanoscale with a tetragonal “ap × ap × 5ap” lattice, corresponding to an ordered fivefold stacking of the Ba and Ln layers. These structures are in fact chemically twinned, so that only cubic or pseudo-cubic symmetry is detected by XRD investigation. The detailed magnetic study shows the existence of short range antiferromagnetic ordering, canted in nature in the temperature range from 5 to 330 K. This particular behavior is interpreted on the basis of local antiferromagnetic M–O–M (M = Fe/Co) interactions within the nanodomains that are limited by pinning of the Co/Fe spins at the boundaries.

Specific features of phase equilibriums in Ln–Ba–Fe–O systems

Formation of five-layered Ln2–εBa3+εFe5O15–δ phases [exhibiting nanoscale ordering with layer-by-layer location of the cations in the Ln–Ba–(Ln,Ba)–(Ln,Ba)–Ba–Ln perovskite-type structure] has occurred in the Ln–Ba–Fe–O (Ln = Y, Pr, Nd, Sm, Eu, and Gd) systems at 1100°С in air. Partial substitution of iron with cobalt (Ln2–εBa3+εFe5–yCoyO15–δ, Ln = Nd, Sm, Eu) has stabilized formation of the ordered structure. The oxygen content in the complex oxides has been determined in air over a wide temperature range by means of high-temperature thermogravimetry and iodometric titration. The change in oxygen content with temperature for the phases with five-layered ordering was significantly smaller than for the disordered phases.

Coherent intergrowth of simple cubic and quintuple tetragonal perovskites in the system Nd{sub 2−ε}Ba{sub 3+ε}(Fe{sub ,}Co){sub 5}O{sub 15−δ}

Abstract Investigation of the Nd2−eBa3+e(Fe,Co)5O15−δ system, combining X-ray diffraction and electron microscopy, has allowed a tetragonal quintuple ordered perovskite “ap×ap×5ap” phasoid inter-grown within a single cubic perovskite matrix to be evidenced for e=0. This nanoscale chemically twinned perovskite is compared with other members, Ln=Sm, Eu, Pr. The unusual long range ordering of the layers develops strains due to size mismatch between Ba2+ and Ln3+ cations. Importantly, two factors allow the strains to be decreased: (i) special intergrowths of double (LnBaFe2O6−δ) and triple (LnBa2Fe3O9−δ) perovskite ribbons/layers oriented at 90°, (ii) nanoscale chemical twinning. The spin locking effect of the nano-domain boundaries upon the magnetic properties of these perovskites is discussed.

Фазовые равновесия и термодинамические свойства оксидов в системах на основе редкоземельных, щелочноземельных и 3d-переходных (Mn, Fe, Co) металлов. Краткий обзор. В. А. Черепанов, Л. Я. Гаврилова, Н. Е. Волкова, А. С. Урусова, Т. В. Аксенова, Е. А. Киселев

The kinetics of surface reactions in one-dimensional and radial (two-dimensional) distribution of diffusant MoO 3 (WO 3 ) on the surface of the substrate MeO (Me is Cd, Ni, Pb, Mn, Cu) were investigated. A kinetic equation satisfactorily describes the rate of surface reactions in the case of radial distribution of diffusant on the substrate. It’s found that when the radial distribution of diffusant the growth of layer on the substrate surface eventually slows down and stops almost completely, due to the outflow of the diffusant deeps into the substrate. When the one-dimensional distribution of diffusant the surface interaction is not slowed down and does not stop at an arbitrarily large times.In the paper, a comprehensive systematic study of different classes of bismuth containing oxide compounds was carried out. The relationship between composition, temperature and concentration regions of existence of stable, specific structure and properties of solid solutions on the basis of vanadates, molybdates, niobates and bismuth was found. The general regularities of synthesis of solid solutions with different variants of solid-phase and soluble methods were determined. On this basis, the optimal conditions for obtaining single-phase materials were formulated. For the first time the temperature and concentration boundaries of the regions of homogeneity and areas of stable existence of polymorphic modifications of solid solutions were defined and/or refined. The structural parameters of the synthesized phases were determined. By the method of electrochemical impedance spectroscopy the nature and features of impedance spectra, the temperature and concentration dependences of electrical conductivity of ceramic materials based on bismuth containing complex oxides were identified.

Физико-химические свойства сложных оксидов в системах Sm-Ba-Me-Cu-O (Me = Fe, Co) Н. Е. Волкова, К. М. Дерябина, Л. Я. Гаврилова, В. А. Черепанов

The corrosion of Hastelloy S, Hastelloy X, Haynes 230, Hastelloy N, Hastelloy G35 and Hastelloy C2000 alloys was studied in KCl–AlCl 3 melts at 550 ° С . The rates and the mechanisms of corrosion of the studied materials were determined. The processes taking place during the interaction between alloys and chloroaluminate melts were investigated.The mode and conditions of thermal sensitization in the temperature range 648-698K lead selenide films deposited from ethylendiamine acetate system were determined. The effect of heat treatment on the morphology and properties of photosensitive chemically deposited films of PbSe was showed. It is concluded that the results obtained by hydrochemical synthesis of nanostructed highly sensitive to infrared range radiation layer of lead selenide provide performance on the level of best foreign analogues.The present work was focused on the studies of crystal structure, oxygen nonstoichiometry, thermal expansion of SmBaCo 2−x Cu x O 6- δ and SmBaFe 2−x Cu x O 6- δ . Values of the oxygen content in complex oxides SmBaCo 2‑ x Cu x О 6‑ δ were determined over a wide temperature range in air using high temperature thermogravimetry and iodometric titration. Also, chemical stability with respect to Ce 0.8 Sm 0.2 O 1.9 and Zr 0.85 Y 0.15 O 1.93 two common solid electrolyte materials used for SOFCs was evaluated.The influence of PbO addition on current efficiency during the electrorefinement of lead in the KCl-PbCl 2 -PbO melt was investigated. It was shown that with PbO concentration in the KCl-PbCl 2 eqiumolar mixture increasing, the current efficiency of lead decreases. Electrode processes mechanism is proposed.Polycrystalline BaМe 1-x Y x O 3-δ (Me=Co, Fe) samples were synthesized by a conventional route and glycerol–nitrate technique. According XRD patterns of single phase solid solutions BaCo1-xYxO3-δ (0.1≤x≤0.4) and BaFe 0.9-y Y 0.1 Co y O 3-δ (0.05≤y≤0.15) were indexed using Pm3m space group. The unit cell parameters were refined using Rietveld full-profile analysis. Oxygen nonstoichiometry of these solid solutions was measured by means of thermogravimetric technique within the temperature range 298–1273 K in air.

Nanoscale ordering in oxygen deficient quintuple perovskite Sm_{2-\epsilon}Ba_{3+\epsilon}Fe_{5}O_{15-\delta} : implication for magnetism and oxygen stoichiometry

The investigation of the system Sm–Ba–Fe-O in air has allowed an oxygen deficient perovskite Sm2-eBa3+eFe5O15-δ (δ = 0.75, e = 0.125) to be synthesized. In contrast to the XRPD pattern which gives a cubic symmetry (ap = 3.934 A), the combined HREM/EELS study shows that this phase is nanoscale ordered with a quintuple tetragonal cell, “ap × ap × 5ap”. The nanodomains exhibit a unique stacking sequence of the A-site cationic layers along the crystallographic c-axis, namely “Sm–Ba–Ba/Sm–Ba/Sm–Ba–Sm”, and are chemically twinned in the three crystallographic directions. The nanoscale ordering of this perovskite explains its peculiar magnetic properties on the basis of antiferromagnetic interactions with spin blockade at the boundary between the nanodomains. The variation of electrical conductivity and oxygen content of this oxide versus temperature suggest potential SOFC applications. They may be related to the particular distribution of oxygen vacancies in the lattice and to the 3d5L configuration of iron.