Browsing by Author "Kharton, VV"
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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
- ItemFirst observation of the reversible O3↔P2 phase transition: crystal structure of the quenched high-temperature phase Na0.74Ni0.58Sb0.42O2(Elsevier, 2006-06-15) Smirnova, OA; Avdeev, M; Nalbandyan, VB; Kharton, VV; Marques, FMBAccording to thermal expansion data, O3-type phase NaxNi(1+x)/3Sb(2−x)/3O2 (x ≈ 0.8) undergoes at ca. 1270 K a reversible transition to a less dense form. The high-temperature phase quenched to liquid nitrogen belongs to P2 type, space group P63/mmc (no. 194), a = 3.0123 Å(2), c = 11.2264 Å(7) for x ≈ 0.74 at 298 K. The stabilisation of P2 versus O3-type structure at high temperatures seems to be due to alkali distribution over greater number of sites thus increasing entropy and decreasing Na+–Na+ repulsion. © 2005 Elsevier Ltd.
- ItemGeometric parameterization of the YBaCo4O7 structure type: implications for stability of the hexagonal form and oxygen uptake(Elsevier, 2010-10) Avdeev, M; Kharton, VV; Tsipis, EVWe explore the stability of the hexagonal form of MBaCo4O7 cobaltites in terms of geometric characteristics of the crystal structure and Global Instability Index (GII) based on the bond-valence considerations. Mismatch between an M3+/2+ and the three-dimensional network of CoO4 tetrahedra, whether expressed using an M ionic radii or GII, is shown to essentially determine both the temperature of structural transition to an orthorhombic modification and oxygen storage properties. A number of M cations not reported in the literature are identified to be suitable for the octahedral sites in an MBaCo4O7 structure. © 2010, Elsevier Ltd.
- ItemHigh-pressure behavior and equations of state of the cobaltates YBaCo4O7, YBaCo4O7+δ, YBaCoZn3O7 and BaCoO3−x(Elsevier, 2012-12-01) Juarez-Arellano, EA; Avdeev, M; Yakovlev, S; Lopez-de-la-Torre, L; Bayarjargal, L; Winkler, B; Friedrich, A; Kharton, VVThe compressibilities of the cobaltates YBaCo4O7, YBaCo4O7+δ, YBaCoZn3O7 and BaCoO3−x were investigated by in situ powder X-ray diffraction experiments up to 30GPa using diamond anvil cells. Pressure-induced phase transitions and amorphization were observed in all the samples. The onset of the pressure-induced phase transition and the onset of the amorphization were observed at ∼11.7 and 12.2GPa (YBaCo4O7), at ∼14.2 and 16.1GPa (YBaCo4O7+δ), and at ∼16.7 and 18.7GPa (YBaCoZn3O7), respectively. An attempt to laser anneal at high-pressure failed as it led to a decomposition of the YBaCo4O7 phase into a mixture of phases. Fits of second- and third-order Birch–Murnaghan equations-of-state to the p–V data result in B0=109(3)GPa for YBaCo4O7; B0=186(4)GPa and B′=1.5 for YBaCo4O7+δ; and B0=117(1)GPa for YBaCoZn3O7. The high-pressure behavior of the studied compounds was compared with isostructural compounds and it is shown that the oxygen-content has a very large effect on the high-pressure behavior of this class of materials. © 2012 Elsevier Inc.
- ItemIonic conductivity and thermal expansion of anion-deficient Sr11Mo4O23 perovskite(Springer Nature, 2020-07-07) Kharton, VV; Tsipis, EV; Kolotygin, VA; Avdeev, M; Kennedy, BJTransport properties of perovskite-type Sr11Mo4O23 and composite Sr11Mo4O23 - 1 wt% Al2O3 were studied at 400–1300 K in the oxygen partial pressure range from 0.21 down to 10−19 atm. The electromotive force and faradaic efficiency measurements, in combination with the energy-dispersive spectroscopy of the fractured electrochemical cells, unambiguously showed prevailing role of the oxygen ionic conductivity under oxidizing conditions. At temperatures above 600 K, protonic and cationic transport can be neglected. The oxygen ion transference numbers vary in the range of 0.95–1.00 at 973–1223 K. At temperatures lower than 550 K, the total conductivity of Sr11Mo4O23 - 1 wt% Al2O3 composite measured by impedance spectroscopy tends to increase in wet atmospheres, thus indicating that hydration and protonic transport become significant. Reducing oxygen partial pressure below 10−10–10−9 atm leads to a significant increase in the n-type electronic conduction. The average thermal expansion coefficients in oxidizing atmospheres are (14.3–15.0) × 10−6 K−1 at 340–740 K and (18.3–19.2) × 10−6 K−1 at 870–1370 K. © 2020, Springer-Verlag GmbH Germany, part of Springer Nature
- ItemMagnetic structure of Sr2Fe2O5 brownmillerite by single-crystal Mössbauer spectroscopy(Academic Press Inc Elsevier Science, 2013-09-01) Waerenborgh, JC; Tsipis, EV; Auckett, JE; Ling, CD; Kharton, VVIn order to determine orientation of the Fe3+ magnetic moments and electric field gradient (efg) axes in the brownmillerite-type strontium ferrite structure for both iron sublattices where the efg tensor is not axially symmetric, the Mössbauer spectra of powdered and oriented single-crystal Sr2Fe2O5 were analyzed by solving the complete Hamiltonian for hyperfine interactions in the excited and ground states of the 57Fe nuclei. The magnetic moments of both octahedrally and tetrahedrally coordinated iron cations lie on the ac-plane of the orthorhombic unit cell and are parallel to the shortest c-axis, whilst the main efg axes are parallel to the longest crystallographic axis, b. This orientation is similar to that in Ca2Fe2O5, in spite of the structural differences of strontium and calcium ferrite brownmillerites at low temperatures. © 2013, Elsevier Ltd.
- ItemMagnetization, Mössbauer and isothermal dilatometric behavior of oxidized YBa(Co,Fe)4O7+δ(Royal Society of Chemistry, 2011-11-08) Waerenborgh, JC; Tsipis, EV; Pereira, LCJ; Avdeev, M; Naumovich, EN; Kharton, VVMössbauer spectroscopy and magnetization studies of YBaCo4-xFexO7+δ (x = 0–0.8) oxidized at 0.21 and 100 atm O2, indicate an increasing role of penta-coordinated Co3+ states when the oxygen content approaches 8–8.5 atoms per formula unit. Strong magnetic correlations are observed in YBaCo4-xFexO8.5 from 2 K up to 55–70 K, whilst the average magnetic moment of Co3+ is lower than that for δ ≤ 0.2, in correlation with the lower 57Fe3+ isomer shifts determined from Mössbauer spectra. The hypothesis on dominant five-fold coordination of cobalt cations was validated by molecular dynamics modeling of YBaCo4O8.5. The iron solubility limit in YBaCo4-xFexO7+δ corresponds to approximately x ≈ 0.7. The oxygen intercalation processes in YBaCo4O7+δ at 470–700 K, analyzed by X-ray diffraction, thermogravimetry and controlled-atmosphere dilatometry, lead to unusual volume expansion opposing to the cobalt cation radius variations. This behavior is associated with increasing cobalt coordination numbers and with rising local distortions and disorder in the crystal lattice on oxidation, predicted by the computer simulations. When the oxygen partial pressure increases from 4 × 10−5 to 1 atm, the linear strain in YBaCo4O7+δ ceramics at 598 K is as high as 0.33%. © Royal Society of Chemistry 2021
- ItemMixed conductivity and stability of CaFe2O4−δ(Electrochemical Society, 2008-03) Kharton, VV; Tsipis, EV; Kolotygin, VA; Avdeev, M; Viskup, AP; Waerenborgh, JC; Frade, JRThe total conductivity of CaFe2O4-delta, studied in the oxygen partial pressure range from 10(-17) to 0.5 atm at 1023-1223 K, is predominantly p-type electronic under oxidizing conditions. The oxygen ion transference numbers determined by the steady-state oxygen permeation and faradaic efficiency measurements vary in the range of 0.2 to 7.2 x 10(-4) at 1123-1273 K, increasing with temperature. No evidence of any significant cationic contribution to the conductivity was found. The Mossbauer spectroscopy, thermogravimetry, and X-ray diffraction (XRD) showed that the orthorhombic lattice of calcium ferrite is essentially intolerant to the oxygen vacancy formation and to doping with lower-valence cations, such as Co and Ni. The oxygen nonstoichiometry (delta) is almost negligible, 0.0046-0.0059 at 973-1223 K and p(O-2) = 10(-5)-0.21 atm, providing a substantial dimensional stability of CaFe2O4-delta ceramics. The average linear thermal expansion coefficients, calculated from the controlled-atmosphere dilatometry and high-temperature XRD data, are (9.6-13.9) x 10(-6) K-1 in the oxygen pressure range from 10(-8) to 0.21 atm at 873-1373 K. Decreasing P(02) results in a modest lattice contraction and in the p-n transition indicated by the conductivity and Seebeck coefficient variations. The phase decomposition of CaFe2O4-delta occurs at oxygen chemical potentials between the low-p(O-2) stability limit of Ca2Fe2O5-delta brownmillerite and the hematite/magnetite boundary in binary Fe-O system. © 2008, Electrochemical Society Inc.
- 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.
- ItemMossbauer spectroscopy analysis of Fe-57-doped YBaCo4O7+delta: effects of oxygen intercalation(Elsevier, 2009-03) Tsipis, EV; Waerenborgh, JC; Avdeev, M; Kharton, VVMossbauer spectroscopy of layered YBaCo3.96Fe0.04O7+delta (delta=0.02 and 0.80), where 1% cobalt is substituted With 57 Fe isotope, revealed no evidence of charge ordering at 4-293 K. The predominant state of iron cations was found trivalent, irrespective of their coordination and oxygen stoichiometry variations determined by thermogravimetric analysis. The extremely slow kinetics of isothermal oxidation at 598 K in air, and the changes of Fe3+ fractions in the alternating triangular and Kagome layers in oxidized YBaCo3.96Fe0.04O7.80, may suggest that oxygen intercalation is accompanied with a substantial structural reconstruction stagnated due to sluggish cation diffusion. Decreasing temperature below 75-80 K leads to gradual freezing of the iron magnetic moments in inverse correlation with the content of extra oxygen. The formation of metal-oxygen octahedra and resultant structural distortions extend the temperature range where the paramagnetic and frozen states co-exist, down to 45-50 K. © 2008, Elsevier Ltd.
- ItemOxygen nonstoichiometry, chemical expansion, mixed conductivity, and anodic behavior of Mo-substituted Sr3Fe2O7-δ(Elsevier, 2010-07-26) Kharton, VV; Patrakeev, MV; Tsipis, EV; Avdeev, M; Naumovich, EN; Anikina, PV; Waerenborgh, JCThe incorporation of molybdenum in the Ruddlesden-Popper type Sr3Fe2-xMoxO7-δ (x = 0–0.1) decreases oxygen deficiency, thermal expansion and electron-hole transport, and increases n-type electronic conductivity in reducing atmospheres. The oxygen ionic conduction remains essentially unaffected by doping. The equilibrium p(O2)–T–δ diagram of Sr3Fe1.9Mo0.1O7-δ, collected in oxygen partial pressure ranges from 10− 20 to 0.7 atm at 973–1223 K, can be adequately described by a defect model accounting for the energetic nonequivalence of apical O1 and equatorial O3 sites in the layered structure, in combination with iron disproportionation and stable octahedral coordination of Mo6+ and Mo5+ cations. The calculated enthalpy of anion exchange between the O1 and O3 positions, 0.49–0.51 eV, is in agreement with the values predicted by the atomistic computer simulation technique. The high-temperature X-ray diffraction studies showed a strongly anisotropic expansion of the Ruddlesden-Popper lattice on reduction, leading to very low chemical strains favorable for electrochemical applications. At 298–1223 K and oxygen pressures from 10− 8 to 0.21 atm, the linear thermal expansion coefficient of Sr3Fe1.9Mo0.1O7-δ varies in the narrow range (12.9–14.2) × 10− 6 K− 1. The relatively low level of n-type electronic conductivity leads, however, to a poor performance of porous Sr3Fe1.9Mo0.1O7-δ anodes in contact with lanthanum gallate-based solid electrolyte under reducing conditions. © 2010, 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.
- ItemPhase behavior and mixed ionic-electronic conductivity of Ba4Sb2O9(Elsevier Science BV., 2013-03-21) Dunstan, MT; Pavan, AF; Kharton, VV; Avdeev, M; Kimpton, JA; Kolotygin, VA; Tsipis, EV; Ling, CDThe 6H-type perovskite phase Ba4Sb2O9, which decomposes in air below 600 K, is found to survive to room temperature in a CO2-free atmosphere. It shows substantial mixed protonic, oxide ionic and electronic conductivity. Compared to Ba4Nb2O9 and Ba4Ta2O9, Ba4Sb2O9 shows higher ionic conductivity due to the relatively easy reducibility of Sb5 +, but lower electronic conductivity due to the predominantly n-type conductivity provided by the Sb5 +/Sb3 + redox couple which leads to reduced hole concentration under oxidizing conditions. Variable temperature synchrotron X-ray diffraction studies carried out in situ under controlled atmospheres reveal a strong monoclinic distortion below 1150 K. The hexagonal to monoclinic transition is slow, does not show second-order behavior, is strongly dependent on atmosphere, and coincides with the loss of ~ 0.4 molecules of H2O per formula unit of Ba4Sb2O9. All of this suggests an important structural role for protons or hydroxide ions in the monoclinic phase. © 2013, Elsevier Ltd.
- ItemPronounced impact of atmospheric conditions on Ba4Nb2O9 and Ba4Ta2O9(Elsvier Science BV, 2012-10-04) Dunstan, MT; Kimpton, JA; Kharton, VV; Ling, CDBa(4)Nb(2)O(9) and Ba(4)Ta(2)O(9) decompose upon heating in a CO(2)-rich atmosphere. Here, we report further in situ synchrotron X-ray diffraction studies under CO(2)-free atmospheres of variable humidity and under vacuum and show that this strongly influences the reconstructive phase transitions normally seen. Under normal atmospheric conditions, alpha-Ba(4)Nb(2)O(9) transforms to gamma-Ba(4)Nb(2)O(9) at 1493 K and alpha-Ba(4)Ta(2)O(9) transforms to 6H-Ba(4)Ta(2)O(9) at 1513 K. Under wet argon, these transitions shift to lower temperatures of 1453 K and 1443 K for Ba(4)Nb(2)O(9) and Ba(4)Ta(2)O(9) respectively. Upon heating both the alpha samples under vacuum, no phase transition is observed at all up to 1723 K. Heating 6H-Ba(4)Ta(2)O(9) under vacuum saw the monoclinic -> hexagonal symmetry-transition temperature increase from 1050 K under air to 1700K. The significance of these observations in terms of possible practical applications of their mixed oxide ionic, protonic and electronic conduction properties are discussed. © 2012, 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.