Browsing by Author "Yaremchenko, AA"
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- ItemA (3 + 3)-dimensional “hypercubic” oxide-ionic conductor: type ii bi2o3–nb2o5(ACS Publications, 2013-04-09) Ling, CD; Schmid, S; Blanchard, PER; Petříček, V; McIntyre, GJ; Sharma, N; Maljuk, A; Yaremchenko, AA; Kharton, VV; Gutmann, MJ; Withers, RLThe high-temperature cubic form of bismuth oxide, δ-Bi2O3, is the best intermediate-temperature oxide-ionic conductor known. The most elegant way of stabilizing δ-Bi2O3 to room temperature, while preserving a large part of its conductivity, is by doping with higher valent transition metals to create wide solid-solutions fields with exceedingly rare and complex (3 + 3)-dimensional incommensurately modulated ?hypercubic? structures. These materials remain poorly understood because no such structure has ever been quantitatively solved and refined, due to both the complexity of the problem and a lack of adequate experimental data. We have addressed this by growing a large (centimeter scale) crystal using a novel refluxing floating-zone method, collecting high-quality single-crystal neutron diffraction data, and treating its structure together with X-ray diffraction data within the superspace symmetry formalism. The structure can be understood as an ?inflated? pyrochlore, in which corner-connected NbO6 octahedral chains move smoothly apart to accommodate the solid solution. While some oxide vacancies are ordered into these chains, the rest are distributed throughout a continuous three-dimensional network of wide δ-Bi2O3-like channels, explaining the high oxide-ionic conductivity compared to commensurately modulated phases in the same pseudobinary system. © 2013, American Chemical Society.
- ItemBehavior of (La,Sr)CoO3- and La2NiO4-based ceramic anodes in alkaline media: compositional and microstructural factors(Springer, 2008-01) Poznyak, SK; Kharton, VV; Frade, JR; Yaremchenko, AA; Tsipis, EV; Yakovlev, SO; Marozau, IPThe behavior of dense ceramic anodes made of perovskite-type La1-x-ySrxCo1-zAlzO3-δ (x=0.30-0.70; y=0-0.05; z=0-0.20) and K2NiF4- type La2Ni1-xMexO4+δ (Me=Co, Cu; x=0-0.20) indicates significant influence of metal hydroxide formation at the electrode surface on the oxygen evolution reaction (OER) kinetics in alkaline solutions. The overpotential of cobaltite electrodes was found to decrease with time, while cyclic voltammetry shows the appearance of redox peaks characteristic of Co(OH)(2)/CoOOH. This is accompanied with increasing effective capacitance estimated from the impedance spectroscopy data, because of roughening of the ceramic surface. The steady-state polarization curves of (La,Sr)CoO3-δ in the OER range, including the Tafel slope, are very similar to those of model Co(OH)(2)-La(OH)(3) composite films where the introduction of lanthanum hydroxide leads to decreasing electrochemical activity. La2NiO4-based anodes exhibit a low electrochemical performance and poor stability. The effects of oxygen nonstoichiometry of the perovskite-related phases are rather negligible at high overpotentials but become significant when the polarization decreases, a result of increasing role of oxygen intercalation processes. The maximum electrocatalytic activity to OER was observed for A-site-deficient (La0.3Sr0.7)(0.97)CoO3-δ, where the lanthanum content is relatively low and the Co4+ concentration determined by thermogravimetric analysis is highest compared to other cobaltites. Applying microporous layers made of template-synthesized nanocrystalline (La0.3Sr0.7)(0.97)CoO3-δ leads to an improved anode performance, although the effects of microstructure and thickness are modest, suggesting a narrow electrochemical reaction zone. Further enhancement of the OER kinetics can be achieved by electrodeposition of cobalt hydroxide- and nickel hydroxide- based films. © 2008, Springer.
- ItemChemically induced expansion of La2NiO4+ δ-based materials(American Chemical Society, 2007-03-21) Kharton, VV; Kovalevsky, AV; Avdeev, M; Tsipis, EV; Patrakeev, MV; Yaremchenko, AA; Naumovich, EN; Frade, JRThe equilibrium chemical strains induced by the oxygen hyperstoichiometry variations in mixed-conducting La2Ni1-xMxO4+δ (M = Fe, Co, Cu; x = 0−0.2) with K2NiF4-type structure, were studied by controlled-atmosphere dilatometry at 923−1223 K in the oxygen partial pressure range 5 × 10-4 to 0.7 atm. In combination with the oxygen content measured by coulometric titration and thermogravimetry, the results reveal a very low chemical expansivity, favorable for high-temperature electrochemical applications. Under oxidizing conditions, the isothermal expansion relative to atmospheric oxygen pressure (εC) is less than 0.02%. The ratio between these values and the corresponding nonstoichiometry increment varies from −3 × 10-3 to 6 × 10-3, which is much lower compared to most permeable mixed conductors derived from perovskite-like cobaltites and ferrites. Consequently, the chemical contribution to apparent thermal expansion coefficients at a fixed oxygen pressure, (13.7−15.1) × 10-6 K-1, does not exceed 5%. The high-temperature X-ray diffraction studies showed that this behavior results from strongly anisotropic expansion of the K2NiF4-type lattice, namely the opposing variations of the unit-cell parameters on changing oxygen stoichiometry. © 2007, American Chemical Society
- ItemMixed conductivity, thermal expansion and defect chemistry of a-site deficient LaNi(0.5)Ji(0.5)O(3-delta)(Elsevier, 2007-03-19) Yakovlev, SO; Kharton, VV; Yaremchenko, AA; Kovalevsky, AV; Naumovich, EN; Frade, JRThis work is focused on the analysis of defect chemistry and partial electronic and oxygen ionic conductivities of A-site deficient La1-xNi0.5Ti0.5O3-delta (x = 0.05 and 0.10). The orthorhombic-to-rhombohedral phase transition was monitored by means of dilatometry and high-temperature X-ray diffractometry. The average thermal expansion coefficients vary in the range (8.5-13.0) x 10(-6) K-1, increasing with temperature and A-site deficiency. The ion transference numbers determined by the Faradaic efficiency measurements are lower than 0.1% at 900-975 degrees C in air. Activation energies of the oxygen ionic conductivity at 897-977 degrees C are 430 and 220 kJ/mol for x = 0.05 and 0.10, respectively. Atomistic simulation demonstrated high stability of ternary defect clusters formed by the vacant sites in the A-sublattice, oxygen vacancies and W, cations, which leads to a very low level of mixed conductivity. © 2007, Elsevier Ltd.
- ItemOxygen permeability, stability and electrochemical behavior of Pr2NiO4+δ-based materials(Springer, 2007-08) Kovalevsky, AV; Kharton, VV; Yaremchenko, AA; Pivak, YV; Tsipis, EV; Yakovlev, SO; Markov, AA; Naumovich, EN; Frade, JRThe high-temperature electronic and ionic transport properties, thermal expansion and stability of dense Pr2NiO4+δ Pr2Ni0.9Fe0.1O4+δ ceramics have been appraised in comparison with K2NiF4-type lanthanum nickelate. Under oxidizing conditions, the extensive oxygen uptake at temperatures below 1073-1223 K leads to reversible decomposition of Pr2NiO4-based solid solutions into Ruddlesden-Popper type Pr4Ni3O10 and praseodymium oxide phases. The substitution of nickel with copper decreases the oxygen content and phase transition temperature, whilst the incorporation of iron cations has opposite effects. Both types of doping tend to decrease stability in reducing atmospheres as estimated from the oxygen partial pressure dependencies of total conductivity and Seebeck coefficient. The steady-state oxygen permeability of Pr2NiO4+δ ceramics at 1173-1223 K, limited by both surface-exchange kinetics and bulk ionic conduction, is similar to that of La2NiO4+δ. The phase transformation on cooling results in considerably higher electronic conductivity and oxygen permeation, but is associated also with significant volume changes revealed by dilatometry. At 973-1073 K, porous Pr2Ni0.8Cu0.2O4+δ electrodes deposited onto lanthanum gallate-based solid electrolyte exhibit lower anodic overpotentials compared to Pr2Ni0.8Cu0.2O4+δ, whilst cathodic reduction decreases their performance. © 2007, Springer.
- ItemOxygen permeability, thermal expansion and stability of SrCo0.8Fe0.2O3−δ–SrAl2O4 composites(Elsevier, 2007-07-15) Yaremchenko, AA; Kharton, VV; Avdeev, M; Shaula, AL; Marques, FMBAdditions of SrAl2O4 phase to mixed-conducting SrCo0.8Fe0.2O3−δ promote oxygen-vacancy ordering and brownmillerite formation at temperatures below 1050 K due to Al3+ incorporation, but also decrease thermal expansion coefficients (TECs) and improve thermal shock stability. The SrCo0.8Fe0.2O3−δ–SrAl2O4 composite membranes exhibit also a relatively high stability with respect to interaction with CO2 due to A-site deficiency of the perovskite-related phase, caused by partial SrAl2O4 dissolution. The oxygen permeability and electronic conductivity of (SrCo0.8Fe0.2O3−δ)1−x(SrAl2O4)x (x=0.3–0.7) composites are determined by the perovskite component and decrease with increasing x. Despite minor diffusion of the transition metal cations into SrAl2O4, hexagonal above 940 K and monoclinic in the low-temperature range, this phase has insulating properties. Nonetheless, at x=0.3 the oxygen permeation fluxes at 1073–1173 are similar to those through single-phase SrCo0.8Fe0.2O3−δ membranes. The average TECs of the composite materials, calculated from dilatometric data in air, vary in the ranges (10.0–11.3)×10−6 K−1 at 300–900 K and (14.7–21.1)×10−6 K−1 at 900–1100 K. The low-p(O2) stability limit and electronic transport properties of SrCo0.8Fe0.2O3−δ are briefly discussed. © 2007, Elsevier Ltd.
- ItemStructures, phase transitions, hydration, and ionic conductivity of Ba4Nb2O9(American Chemical Society, 2009-08-25) Ling, CD; Avdeev, M; Kutteh, R; Kharton, VV; Yaremchenko, AA; Fialkova, S; Sharma, N; Macquart, RB; Hoelzel, M; Gutmann, MJBa4Nb2O9 is shown to have two basic polymorphs: a high-temperature γ phase, which represents an entirely new structure typed and a low-temperature (x phase, which has the rare Sr4Ru2O9 structure type. The phases are separated by a reconstructive phase transition at similar to 1370 K, the kinetics of which are sufficiently slow that the γ phase can easily be quenched to room temperature. Below similar to 950 K, both (α and γ phases absorb significant amounts of water. In the case of the γ phase, protons from absorbed water occupy ordered positions in the structure, giving rise to a stoichiometric phase γ-III-Ba4Nb2O9.1/3H(2)O at room temperature. γ-III-Ba4Nb2O9-1/3H(2)O partially dehydrates, at similar to 760 K to give another stoichiometric phase γ-II-Ba4Nb2O9.1/3H(2)O, which completely dehydrates at similar to 950 K to γ-I- Ba4Nb2O9. The hydrated γ phases exhibit faster protonic and oxide ionic transport than the hydrated (x phases because of the presence in the γ phases of 2D layers containing Nb5+ cations with unusually low oxygen coordination numbers (4 or 5) separated by discrete OH groups. Hydration appears to play an important role in stabilizing the γ phases at low temperatures, with the γ -> α transition oil reheating a quenched sample occurring at higher temperatures in humid atmospheres. © 2009, American Chemical Society
- ItemStructures, phase transitions, hydration, and ionic conductivity of Ba4Ta2O9(American Chemical Society, 2010-01-26) Ling, CD; Avdeev, M; Kharton, VV; Yaremchenko, AA; Macquart, RB; Hoelzel, MLow-temperature α-Ba4Ta2O9 is isostructural with α-Ba4Nb2O9 (Sr4Ru2O9 type), and it undergoes a reconstructive phase transition at approximately the same temperature (1400 K) to a γ form that can easily be quenched to room temperature. However, the γ forms of the two compounds are completely different. Whereas γ-Ba4Nb2O9 represents a unique structure type, γ-Ba4Ta2O9 adopts a more conventional 6H-perovskite type. The α→γ transition is virtually irreversible in the tantalate, unlike the niobate, which can be converted back to the α form by annealing slightly below the transition temperature. Quenched γ-Ba4Ta2O9 is highly strained due to the extreme size mismatch between Ba2+ (1.35 Å) and Ta5+ (0.64 Å) cations in perovskite B-sites, and undergoes a series of symmetry-lowering distortions from P63/mmc→P63/m→P21/c; the second of these transitions has not previously been observed in a 6H perovskite. Below 950 K, both α-Ba4Ta2O9 and γ-Ba4Ta2O9 hydrate to a greater extent than the corresponding phases of Ba4Nb2O9. Both hydrated forms show significant mixed protonic and oxide ionic conductivity, and electronic conductivity upon dehydration. © 2010, American Chemical Society
- ItemTransitions between P21, P63 (√ 3A), and P6322 modifications of SrAl2O4 by in situ high-temperature x-ray and neutron diffraction(Elsevier, 2007-12-01) Avdeev, M; Yakovlev, S; Yaremchenko, AA; Kharton, VVThe results of in situ high-temperature X-ray and neutron powder diffraction experiments reconcile inconsistencies in previous reports on the symmetry of high-temperature phases of SrAl2O4. The material undergoes two reversible phase transitions P21↔P63(3A) and P63(3A)↔P6322 at ∼680 and ∼860°C, respectively, and the latter one is experimentally observed and characterized for the first time. The higher symmetry above the P63(3A)↔P6322 transition is gained by disordering off-center split site of oxygen atoms around trigonal axis rather than by unbending Al–O–Al angle to the ideal value 180°. The analysis of the literature suggests that it is a common feature of the P6322 phases of stuffed tridymites.© 2007 Elsevier Inc.