Browsing by Author "Yakovlev, SO"

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    Behavior 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, IP
    The 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.
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    Corrigendum to: High-pressure structural behavior and equation of state of NaZnF3
    (Elsevier, 2009-10) Yakovlev, SO; Avdeev, M; Sterer, E; Greenberg, Y; Mezouar, M
    The Authors regret that two scientists who also contributed to this work were not included as the co-authors, Eran Sterer and Yaron Greenberg. The correct author line up is listed above. © 2009 Elsevier Inc
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    High-pressure structural behavior and equation of state of NaZnF3
    (Elsevier, 2009-06) Yakovlev, SO; Avdeev, M; Mezouar, M
    We report the results of density functional theory ab-initio calculations and monochromatic synchrotron X-ray diffraction study carried out for orthorhombic NaZnF3 in the pressure range 0-40 GPa. Perovskite-to-postperovskite phase transition was anticipated by first-principles computations and then observed in high-pressure diamond anvil cell synchrotron diffraction experiment between 14 and 22 GPa. Above 25 GPa postperovskite structure (CaIrO3 type, space group Cmcm) coexists with another phase, yet unidentified. On decompression, pure postperovskite-type structure was found to be stable down to 4 GPa: below this pressure sample contained both perovskite and postperovskite modifications. Fit of experimental P-V data to the third-order Birch-Murnaghan equation of state gave bulk moduli, K-P,K-0 64.98 +/- 2.67 and 69.88 +/- 3.69 GPa for perovskite and postperovskite modifications, respectively. Both phases demonstrated strong anisotropy of compressibility. For postperovskite NaZnF3, the highest compression was observed along the direction perpendicular to the planes of ZnF6 octahedra arrangement. © 2009, Elsevier Ltd.
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    Mixed 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, JR
    This 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.
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    On the polyhedral volume ratios VA/VB in perovskites ABX³
    (International Union of Crystallography, 2007-06) Avdeev, M; Caspi, EN; Yakovlev, SO
    This paper presents analytical expressions for the calculation of ratios of cation coordination polyhedra volumes (VA/VB) for perovskites ABX³ of the Stokes–Howard diagram directly from atomic coordinates. We show the advantages of quantifying perovskite structure distortion with polyhedral volume ratios rather than with tilting angles, and discuss why space groups with multiple crystallographically inequivalent A or B sites (I4/mmm, Immm, P42/nmcetc.) are much less common than those with a single A and B site (I4/mcm, R c, Pnmaetc.). Analysis of crystallographic data for approximately 1300 perovskite structures of oxides, halides and chalcogenides from the Inorganic Crystal Structure Database revealed that the most highly distorted perovskites belong to the space group Pnma and formally lower-symmetry perovskites (I2/m, I2/a) are less distorted geometrically. Critical values of the VA/VB ratios for the most common phase transitions Pnma↔I4/mcm and Pnma↔R c are estimated to be 4.85 with the possible intermediate space group Imma stable in the very narrow range of VA/VB 4.8–4.9. Transitions to post-perovskite CaIrO3-type structures may be expected for VA/VB < 3.8. © 2007, International Union of Crystallography
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    Oxygen 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, JR
    The 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.
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    Perovskites ABX3 under pressure: transition to post-perovskite CaIrO3 type and other scenarios
    (International Union of Crystallography, 2008-08-01) Avdeev, M; Yakovlev, SO
    Recently we have performed a systematic quantitative analysis of crystal structure distortion for ~1300 ABX3 perovskites in terms of Ato B-site polyhedra volume ratios VA/VB [1]. The analysis identified a number of compositions close to the boundary of perovskite type stability which we have studied experimentally by in situ high pressure synchrotron X-ray powder diffraction. Here we will present the results of these experiments for several materials studied under pressure for the first time and will discuss our theoretical and experimental findings focusing on the following aspects of perovskite (pv) and post-perovskite CaIrO3 (ppv) structure types: (1) interplay between geometry and symmetry in distorted perovskites (is there any intermediate phase between pv and ppv?); (2) topological and geometrical constraints for ABX3 stoichiometry (what might be the structure of hypothetical structure types denser than ppv?); and (3) the effect of vacancies in ABX3 and pressure-induced amorphization as an alternative to pv-ppv transition. © International Union of Crystallography